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Geomorphic consequences of volcanic eruptions in Alaska: A review
Geomorphic consequences of volcanic eruptions in Alaska: A review Christopher F. Waythomas PII: DOI: Reference:
S0169-555X(15)30023-4 doi: 10.1016/j.geomorph.2015.06.004 GEOMOR 5245
To appear in:
Geomorphology
Received date: Revised date: Accepted date:
18 November 2014 29 May 2015 2 June 2015
Please cite this article as: Waythomas, Christopher F., Geomorphic consequences of volcanic eruptions in Alaska: A review, Geomorphology (2015), doi: 10.1016/j.geomorph.2015.06.004
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ACCEPTED MANUSCRIPT Geomorphic consequences of volcanic eruptions in
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Christopher F. Waythomas* U.S. Geological Survey, Alaska Volcano Observatory 4210 University Drive, Anchorage, AK 99508 E-mail: [email protected].
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Alaska: a review
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Abstract
Eruptions of Alaska volcanoes have significant and sometimes profound
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geomorphic consequences on surrounding landscapes and ecosystems. The effects of eruptions on the landscape can range from complete burial of surface vegetation and
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preexisting topography to subtle, short-term perturbations of geomorphic and ecological
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systems. In some cases, an eruption will allow for new landscapes to form in response to
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the accumulation and erosion of recently deposited volcaniclastic material. In other cases, the geomorphic response to a major eruptive event may set in motion a series of landscape changes that could take centuries to millennia to be realized. The effects of volcanic eruptions on the landscape and how these effects influence surface processes has not been a specific focus of most studies concerned with the physical volcanology of Alaska volcanoes. Thus, what is needed is a review of eruptive activity in Alaska in the context of how this activity influences the geomorphology of affected areas. To illustrate the relationship between geomorphology and volcanic activity in Alaska, several eruptions and their geomorphic impacts will be reviewed. These eruptions include the 1912 Novarupta-Katmai eruption, the 1989–1990 and 2009 eruptions of Redoubt volcano, the 2008 eruption of Kasatochi volcano, and the recent historical eruptions of Pavlof volcano. The geomorphic consequences of eruptive activity associated with these 1
ACCEPTED MANUSCRIPT eruptions are described, and where possible, information about surface processes, rates of landscape change, and the temporal and spatial scale of impacts are discussed.
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A common feature of volcanoes in Alaska is their extensive cover of glacier ice, seasonal snow, or both. As a result, the generation of meltwater and a variety of
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sediment-water mass flows, including debris-flow lahars, hyperconcentrated-flow lahars,
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and sediment-laden water floods, are typical outcomes of most types of eruptive activity.
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Occasionally, such flows can be quite large, with flow volumes in the range of 107–109 m3. A review of the lahars generated during the 2009 eruption of Redoubt volcano will
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illustrate the geomorphic impacts of lahars on stream channels and riparian habitat. Although much work is needed to develop a comprehensive understanding of the
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geomorphic consequences of volcanic activity in Alaska, this review provides a synthesis
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work on this topic.
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of some of the best-studied eruptions and perhaps will serve as a starting point for future
Keywords: Alaska eruptions, geomorphology, impacts, lahars
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ACCEPTED MANUSCRIPT 1. Introduction Volcanic eruptions in the Aleutian arc of Alaska have had a profound impact on
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the landscape, and throughout this region (Fig. 1) volcanic activity of various magnitudes and frequencies have exerted an important influence on hydrologic, biologic, atmospheric,
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and geomorphic systems (Vanderhoek and Nelson, 2007; DeGange et al., 2010).
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Volcanic activity over long time scales (tens to hundreds of thousands of years) has
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resulted in the growth of many of the major mountains in parts of the Alaska Range, the Aleutian Range, and the Wrangell Mountains. It could be argued that eruptions and their
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aftermath have played a dominant role in shaping the ecosystems and landscapes of southern and southwestern Alaska, and the Aleutian Islands, as volcanoes and their
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eruptive products are the principal landforms of these regions. Volcanoes within the
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Aleutian arc (Fig. 1) have exhibited a wide variety of eruptive styles, ranging from mild
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effusive eruptions to catastrophic caldera-forming events associated with voluminous ash flow sheets and tephra fallout over broad areas of Alaska and the Yukon (Miller and Smith, 1987; Preece et al., 2000). Some of the largest documented eruptions of late Quaternary age worldwide have occurred in Alaska, and some of the most historically active volcanoes in North America also are found here. Given the significant magnitude of many Alaska eruptions and the high frequency of occurrence of eruptive activity, it is worthwhile to examine how eruptive activity and the products of this activity have affected the geomorphic evolution of landscapes throughout the Aleutian arc. This task is practical and academic because of the obvious implications for hazards to people, infrastructure, and the environment and for understanding how volcanic systems evolve in an area that is as geologically dynamic as Alaska.
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ACCEPTED MANUSCRIPT Although much of Alaska and the Aleutian arc are remote and unpopulated, the frequency of modern eruptions (average of 1–2 per year) presents a unique opportunity to
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observe and evaluate geomorphic processes that operate on newly formed volcanic deposits. In addition to their frequent eruptive activity, most active volcanoes in Alaska
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are large stratocones with significant relief (several thousand meters), and therefore,
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ample potential energy associated with steep flanking slopes is available as a driving
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mechanism for geomorphic processes. As a result of their high relief and northern location, nearly every volcano in Alaska has some amount of ice and snow cover. This
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means that water is readily available, and as a result, a variety of flowing sediment-water mixtures (lahars) can form on the flanks of volcanoes and in surrounding valleys during
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eruptive activity.
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Volcanoes that are characterized by frequent explosive eruptions typically have a
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mantling cover of loose, fragmental pyroclastic debris that is easily eroded by water-rich mass flows. Eruption-related introduction of large volumes of water and sediment into river systems associated with volcanoes in Alaska results in flows that can be several orders of magnitude larger than the largest meteorologically generated floods in respective drainage basins. As will be discussed later, the size, characteristics, and unpredictable occurrence of such flows presents significant challenges for incorporating large lahars into conventional flood-hazard analyses.
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ACCEPTED MANUSCRIPT Most explosive eruptions in Alaska result in significant impacts to proximal areas surrounding the volcano (roughly 5–10 km from vent) as well as producing significant
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changes to the edifice itself. Throughout the Holocene epoch, a number of caldera- and crater-forming eruptions have been significant, primarily on the Alaska Peninsula (Fig. 1),
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that in addition to producing far-traveled, highly mobile pyroclastic flows (Miller and
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Smith, 1987) and tephra fallout, have resulted in large circular caldera structures and
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volcanic craters. The ca. 8–10-km-diameter caldera’s at Aniakchak, Okmok, and Veniaminof volcanoes all formed during major Holocene eruptions, but afterward, the
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calderas filled with water (Aniakchak and Okmok) or ice (Veniaminof), which set the stage for future geomorphic events associated with the catastrophic drainage of these
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caldera lakes and lava–ice interactions (Waythomas et al., 1996; Wolfe, 2001;
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Waythomas, 2013). Summit craters, such as those at Chiginagak and Kaguyak volcanoes
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(Fig. 1), impound smaller amounts of water, but release of relatively small amounts of water may be devastating to riparian areas downstream, especially if the waters issue from acidic crater lakes (Schaefer et al., 2008). Several Alaska eruptions are the focus of this article, including the 1912 Novarupta-Katmai eruption, the 2008 eruption of Kasatochi volcano in the Aleutian Islands, historical eruptions of Pavlof volcano, and the 2009 eruption of Redoubt volcano. These eruptions and their products are reviewed from the perspective of their geomorphic impacts to surrounding environments, although the geomorphic impacts of these events have been addressed in varying degrees of detail. Some eruptions, such as the 1912 Novarupta-Katmai eruption and the Redoubt eruptions of 1989–1990 and 2009, have been evaluated comprehensively, whereas others, such as the recent eruptions of Pavlof
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ACCEPTED MANUSCRIPT volcano have received more limited study because of the remote location and logistical problems accessing field sites. Given the constraints imposed by location, weather, and
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cost of fieldwork, it is not possible to provide the same level of analysis for all of the eruptions described in this article. With these considerations in mind, the objectives of
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this review are (i) to address the geomorphic impacts associated with large infrequent
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eruptions, (ii) to address the geomorphic impacts of small, frequent eruptions, (iii) to
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address the geomorphic impacts of large lahars on streams and channel networks, and (iv) to briefly describe the geomorphic impacts of tephra fallout. The events discussed
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illustrate how eruptions of various sizes and characteristics can have significant impacts
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2. Geologic setting
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on the landscape that may persist for decades to millennia.
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The primary late Quaternary volcanic province of Alaska is the Aleutian arc (Fig. 1). The arc is a major geologic feature of the North Pacific region and includes the Aleutian Trench and numerous volcanoes and volcanic islands that extend about 4000 km from Cook Inlet in the east (~152°W) to west of Attu Island (~173°E). The arc consists of two distinct segments, the Aleutian Ridge segment (which includes the Aleutian Islands west of Unimak Pass) and the Alaska Peninsula–Kodiak Island segment that extends east from Unimak Pass (Fig. 1; Vallier et al., 1994). The Aleutian arc is a classic volcanic island arc that is the result of northwestward subduction of the Pacific plate beneath the North American plate at an average convergence rate of 2–9 cm/y (DeMets et al., 1990; Freymueller et al., 2008). In the eastern part of the arc, the direction of convergence is normal to the trench; but in the central Aleutian Islands the direction is oblique. In the
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ACCEPTED MANUSCRIPT western Aleutian Islands, west of 175°E, the direction of relative plate motion is parallel to the trench.
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The oldest volcanic rocks in the arc are middle Eocene age (ca. 38 Ma, Jicha et al., 2006), and there have been three significant periods of arc-wide magmatism at 38–29,
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16–11, and 6–0 Ma (Fournelle et al., 1994; Jicha et al., 2006). Volcanic rocks and
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deposits within the arc range in composition from basalt to dacite and occasionally
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rhyolite (Kay et al., 1982; Fournelle et al., 1994), but basaltic lavas are the most abundant rock composition (Myers, 1988).
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Most of the major stratocones and calderas in the arc are Quaternary in age, and they include some of the highest, largest, and most explosive volcanoes of the entire
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circum-Pacific rim (Miller and Richter, 1994). In the eastern part of the arc, the
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volcanoes and volcanic fields are built on continental crust; whereas the volcanoes of the
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central part of the arc overlie mostly marine sedimentary rocks, and in the western part of the arc, the basement rocks consist mostly of older volcanic rocks (Miller and Richter, 1994: Vallier et al., 1994).
3. Aleutian arc eruptions
The earliest written records of eruptive activity in the Aleutian arc date to the late 1700s, and the first arc-wide synthesis of volcanic activity was compiled by Coats (1950) for the period between 1760 and 1948. Although several volcanoes were explored during scientific expeditions to the region in the early 1900s (Jaggar, 1908; Finch, 1934, 1935), the first systematic evaluation of volcanic activity was that of the U.S. Geological Survey (USGS) from 1945 to 1954. This work was published in a series of USGS Bulletins
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ACCEPTED MANUSCRIPT known as the 1028 Series. Since then, geological investigations of volcanoes were generally limited to site-specific studies until the advent of seismic monitoring of
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volcanoes that began in the 1970s and the formation of the Alaska Volcano Observatory in the late 1980s, which provided impetus for a more regional understanding of Aleutian
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arc volcanism and its implications for hazards.
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3.1. Historical eruptions
The Aleutian arc contains at least 52 volcanoes that have had some type of
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documented or suspected eruptive activity in about the past 300 years (Fig. 1). These volcanoes are generally known as the historically active volcanoes of Alaska, and 257
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eruptions are documented over this period. An additional 30 volcanoes have had
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documented eruptive activity within the Holocene epoch (Fig. 1). The average frequency
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of historical eruptions is 1–2 per year, and the majority of events (about 78 %) rank as 2 or 3 on the Volcanic Explosivity Index scale of Newhall and Self (1982). The VEI scale is based primarily on the volume of material erupted and the maximum height above sea level (asl) attained by the eruption cloud. The scale ranges from 0, for effusive lavaproducing eruptions, to 8 for catastrophic caldera-forming eruptions such as the Toba and Taupo super eruptions (Ninkovich et al., 1978; Wilson and Walker, 1985). The VEI scale is logarithmic, and each interval on the scale indicates a tenfold increase in erupted volume, except for VEI 0–2 events. Roughly 20 % of documented historical eruptions in Alaska are effusive events of VEI 0–1 in magnitude. The VEI 2 eruptions produce > 1,000,000 m³ of ejecta, and plume heights range from 1–5 km asl. About half of the documented historical eruptions in Alaska are VEI 2 magnitude events. The VEI 3 events
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ACCEPTED MANUSCRIPT produce > 10,000,000 m³ of ejecta and have associated ash cloud heights of 3–15 km asl. About 25 % of the historical eruptions in Alaska are VEI 3 in magnitude. The VEI 4
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events erupt >0.1 km3 of ejecta and generate ash clouds that reach 10–25 km asl. Historical VEI 4 eruptions in Alaska are rare, and only about 5 % of documented
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historical eruptions are of this magnitude. Only one historical eruption was larger than
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VEI 4 and that was the VEI 6 1912 Novarupta-Katmai eruption (Hildreth and Fierstein,
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2000).
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3.2. Eruptions of Holocene age
The Holocene eruptive history of volcanoes in the Aleutian arc is known
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primarily from study of volcanic ash deposits and proximal unconsolidated volcaniclastic
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deposits on the flanks of volcanoes (Riehle, 1985; Miller and Smith, 1987; Waythomas,
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1999; Stelling et al., 2005; Fierstein, 2007). Some of the largest Holocene eruptions on Earth have occurred in the Aleutian arc and resulted in the formation of several noteworthy calderas (Table 1). Many of these eruptions generated >50 km3 of bulk eruptive material (Miller and Smith, 1987) and had substantial and long-lasting impacts on the surrounding landscape (Vanderhoek and Nelson, 2007). Although the status of the Holocene eruptive record continues to evolve as new work is accomplished throughout the arc, at present more than half of the volcanoes with known or suspected Holocene eruptive activity (Fig. 1) have no published studies documenting this eruptive history. As a result, addressing long-term (thousands of years) volcanic unrest from a frequency – magnitude perspective is difficult and may lead to an over emphasis of the significance of the historical eruptive record.
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ACCEPTED MANUSCRIPT 3.3. Caldera-forming eruptions Major eruptions associated with caldera formation have occurred during the
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Holocene epoch at a number of volcanoes throughout the Aleutian arc. Aniakchak, Veniaminof, and possibly Fisher have experienced multiple caldera-forming eruptions
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within the late Quaternary, although the chronology of these events remains somewhat
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uncertain. Here the focus is on the most recent of these eruptions, which all have
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occurred in Holocene time (Table 1). Of the eruptions listed in Table 1, little is known about the physical volcanology and chronology of the caldera-forming eruptions at
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Semisopochnoi and Yunaska volcanoes, and the most recent published work at these locations dates to the 1950s and 1980s (Coats, 1950, 1959; DeLong et al., 1985). All of
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the other volcanoes have received some study within the past 15 years, although a
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number of questions and uncertainties remain about the chronology and volcanological
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characteristics of the caldera-forming eruptions at Hayes, Black Peak, and Semisopochnoi.
The twelve calderas described in Table 1 are roughly circular in planview shape (Fig. 2A) and fall into two groups when ranked according to planview caldera area (Fig. 2B). Semisopochnoi, Okmok, Fisher, Veniaminof, and Aniakchak all have caldera areas >40 km2; Fisher caldera clearly has the largest area at about 140 km2. The calderas at Yunaska, Makushin, Akutan, Black Peak, Katmai, Kaguyak, and Hayes are all much smaller and have planview areas of about 10 km2 or less.
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ACCEPTED MANUSCRIPT In general, caldera size is a proxy for bulk eruptive volume produced during the most recent caldera-forming eruption. However, in many cases the volume of material
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produced by these eruptions is uncertain and estimates can vary by at least an order of magnitude (Table 1). Furthermore, the present caldera structure may not be the result of
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the most-recent caldera-forming eruption, and parts of the caldera could have formed
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during older eruptions. Thus, it is not possible to estimate the volume of eruptive material
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based on caldera area alone as the relation is poorly constrained because of large uncertainties in the eruptive volume estimates.
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Because of their large eruptive magnitude (>VEI 5), caldera-forming eruptions have significant geomorphic impacts on surrounding landscapes due to the emplacement
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of pyroclastic fall and flow deposits. Among the main effects of these types of eruptions
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is the generation of highly mobile ash-flow sheets that extend for many tens of kilometers
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beyond the caldera (Miller and Smith, 1977, 1987) and widespread tephra fall out (Preece et al., 2000). The eruption of aerially extensive pyroclastic flows during large calderaforming eruptions mantles the topography with sometimes very thick (tens of meters), valley-filling, pyroclastic-flow deposits (Fig. 3). Emplacement of these deposits reshapes the landscape and results in broad, flat-floored valleys and sediment aprons abutting higher terrain. At some volcanoes, such as Veniaminof, the ash-flow deposits were emplaced hot enough to weld and form indurated columnar jointed accumulations along pre-eruptive channels and valleys. Welded ash-flow deposits are resistant to fluvial erosion and have resulted in the formation of narrow, slot canyons in a number of areas where post-eruption incision has occurred (Fig. 3). Because stream channels and preexisting valleys are typically filled with pyroclastic debris during these large eruptions,
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ACCEPTED MANUSCRIPT a subsequent period of erosion occurs and this results in channel incision, removal of loose pyroclastic debris, and transport of this material to the ocean. Although not well
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documented, the mouths of larger rivers that drain these pyroclastic-flow affected areas are characterized by substantial volumes of deltaic material derived from erosion of the
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pyroclastic debris.
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The emplacement of these large volume ash-flow deposits creates extensive areas
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of bare ground that are highly susceptible to wind erosion. Resuspension of volcanic ash from fresh pyroclastic-flow surfaces results in localized ash fallout of reworked ash and
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visible ash plumes that are similar to primary ash clouds (Hadley et al., 2004). If a disturbance regime persists it may be centuries to millennia before pyroclastic flow
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surfaces become sufficiently stabilized by vegetation to inhibit eolian erosion.
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Tephra fallout during caldera-forming and other large explosive eruptions can
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cover many thousands of square kilometers with millimeters to centimeters of fine ash that typically decreases in thickness and particle size with distance from source (Preece, et al., 1999; Westgate et al., 2000; Pearce et al., 2004; Wallace et al., 2013). Because the aerial extent of ash fall associated with caldera-forming eruptions is typically widespread, the geomorphic effects should be significant (Jensen et al., 2014). Aerially extensive ash fall deposits associated with caldera-forming eruptions, such as the Dawson and Old Crow tephras of late Pleistocene age (Froese et al., 2002) and the Aniakchak tephra of late Holocene age (Begét et al., 1992), likely had significant effects on the landscape across Alaska resulting from burial of vegetation, changes in surface albedo, and the introduction of acidic volcanic aerosols into surface water (Blackford et al., 2014). To
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ACCEPTED MANUSCRIPT date however, the specific effects of widespread ash fall associated with caldera-forming eruptions in Alaska are not known.
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The process of caldera formation also results in substantial physical changes to the volcanic edifice itself (Table 2). Chief among these physical changes is the removal
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of the upper third or so of the volcanic edifice resulting in the circular, basin-like caldera
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structure. Several Holocene age calderas in Alaska (Aniakchak, Okmok) supported large
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temporary lakes that were removed when their caldera rim dams failed, resulting in spectacular dam-break floods (Waythomas et al., 1996; Wolfe, 2001). Other calderas,
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such as Veniaminof, became filled with glacier ice.
All calderas of late Holocene age in Alaska have experienced significant post-
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caldera volcanism, ranging from mild, effusive, lava-producing events, to more explosive
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eruptions involving silicic magmas (Bacon et al., 2014). As a result of this activity, a
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diverse assemblage of cones, flows, and vents characterizes the floors of most Alaska calderas. Many post-caldera eruptive products reflect magma-water interaction, as the calderas themselves are basin-like structures that contain shallow groundwater, glacier ice, or in some cases, small lakes. All of the caldera structures described above formed as a result of substantial magma withdrawal, which led to the eventual downward collapse of the remaining volcanic edifice and thus are known as collapse calderas. Less common in Alaska are caldera structures that formed as a result of large (>1 km3) volcanic landslides where the headscarp of the landslide is known as an avalanche caldera. The caldera at Mount Spurr volcano is such a structure, and it formed during a major sector collapse of the volcano possibly in mid- to early Holocene time (Waythomas, 2007).
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ACCEPTED MANUSCRIPT 4. Geomorphic effects at frequently active volcanoes As discussed previously, Alaska contains some of the most frequently active
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volcanoes in North America. The volcanoes with the greatest number of conclusively documented historical eruptions are Pavlof (>40 eruptions), Akutan (32 eruptions),
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Cleveland (>25 eruptions), and Shishaldin (>25 eruptions). Pavlof, Cleveland, and
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Shishaldin are steep-sided stratocones that are mantled by loose pyroclastic debris up to
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several meters thick and rubbly aa lava flows that are largely unvegetated. Of these, Pavlof has received the most study and its eruptions are good examples of the types of
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geomorphic processes that occur at Aleutian arc volcanoes that erupt frequently. Pavlof has erupted more than 40 times since the early 1800s, and its most recent
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eruption occurred in November 2014 (http://www.avo.alaska.edu/volcanoes/ volcinfo.
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php? volcname=Pavlof). The volcano is a 2518-m-high, ice- and snow-covered
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stratocone within the Emmons Lake volcanic center (Mangan et al., 2009) and is located on the southwestern part of the Alaska Peninsula (Fig. 1), a remote, uninhabited area near Izembek National Wildlife Refuge. The volcano is situated on the outer rim of one of the large mid-Pleistocene caldera structures that characterize the Emmons Lake volcanic center, and most of the edifice is a post-caldera feature (Mangan et al., 2009). Historical eruptions of Pavlof have been characterized by moderate amounts of ash emission, lava fountaining, and spatter accumulation on the upper flanks of the volcano and the generation of watery hyperconcentrated-flow lahars (Waythomas et al., 2014). Since the early 1970s, when seismic instruments were first installed on the volcano, eruptive episodes characterized by periods of lava fountaining and strombolian explosions have been more quantitatively documented (McNutt, 1987; McNutt et al., 1991), and eruptive
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ACCEPTED MANUSCRIPT products have been studied to a limited degree during brief field visits to the volcano (Waythomas et al., 2008, 2014; Mangan et al., 2009).
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Typical Pavlof eruptions are characterized by moderate levels of intermittent VEI 2 strombolian activity and occasional more energetic vulcanian explosions (VEI 3) that
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have generated ash plumes reaching as high as 16 km asl (McNutt et al., 1991). Eruptions
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typically last from a week up to several months, although the 1986–1988 eruption lasted
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850 days (McNutt et al., 1991). The intensity of eruptive activity can be quite variable and episodic. Typically, sustained bursts of activity lasting for several hours are followed
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by periods of lower level unrest, which can last for days to weeks (McNutt et al., 1991; Waythomas et al., 2014).
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Most of the material erupted from Pavlof is andesite to basaltic andesite in
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composition (Mangan et al., 2009), and lava flows on the upper flanks of the cone consist
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almost entirely of fountain-fed flows and nondescript piles of agglutinate, spatter, and coarse tephra. The volcano supports a relatively extensive cover of glacier ice and perennial snow, and the combined snow and ice volume is about 1–2 km3. The lower slopes of the volcano are largely unvegetated, and a several-meter-thick mantle of loose, easily erodible pyroclastic debris is present around the volcano, which obscures the underlying glacier ice in many areas (Fig. 4). Meltwater produced during eruptions by the interaction of hot granular rock debris with ice and snow initiates lahars, and all of the drainages that originate on the volcano have been inundated repeatedly by lahars during historical eruptions (Fig. 5). Some of these lahars have been relatively extensive, at times reaching the Bering Sea about 40 km north of the volcano. Lahar deposits associated with older Holocene eruptions form prominent terraces along many of the main drainages (Fig.
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ACCEPTED MANUSCRIPT 6). The frequency of eruption-induced lahars and tephra fallout has provided a steady supply of volcaniclastic sediment to the drainages on the volcano, which has affected
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riparian habitat, limited vegetation growth, and resulted in episodic pulses of aggradation and incision. Meltwater generated by eruptive activity results in the reactivation of
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ephemeral channel networks and distributary channels on the distal slopes of the volcano.
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This can result in minor erosion of stream channels and progradation of lahar deposits
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over vegetated areas or across perennial streams causing short-term blockages and debris dams. Drainages affected by the combined input of tephra fallout and lahar inundation
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are in a long-term disturbance regime, and drainage systems may not reach a condition of relative channel stability until eruptive activity declines dramatically or when the volcano
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becomes dormant.
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5. Geomorphic consequences of lahar inundation of stream channels and valleys Volcanic mass flows of all types are commonly emplaced in valleys and stream channels on the flanks of many Alaska volcanoes. The generation of lahars and other types of gravity-driven mass flows (pyroclastic flows, debris avalanches) are typical outcomes of eruptive activity, and most of the major drainages on volcanoes contain mass-flow deposits. Typically, the most voluminous lahars that form during eruptions in Alaska are those that are generated by pyroclastic flow interaction with ice and snow. Such flows can have volumes in the range of 107–108 m3 (Waythomas et al., 2013), and lahars of this size can have dramatic impacts on the valleys they inundate. The most significant impacts include complete burial of valley bottoms with sometimes thick accumulations of unconsolidated deposits of gravel, sand, and silt; the diversion of water
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ACCEPTED MANUSCRIPT flow paths; and the destruction of riparian habitat (Dorava and Meyer, 1994; Waythomas et al., 2013). Lahars at Alaska volcanoes may form by other mechanisms, such as
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drainage of crater lakes (Schaefer et al., 2008) or as a result of intense rainfall; however, these types of events occur rarely in comparison to lahars that form during eruptions.
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Large, valley-filling lahars change the valley-bottom morphology and produce flat,
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gently sloping surfaces underlain by meters to tens of meters of volcaniclastic fill. As the
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lahars are emplaced, they can displace or bury stream networks, and in some settings will cause complete or partial blockage of streams and rivers resulting in the formation of
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lahar-dammed lakes (Fig. 7; Waythomas, 2001). In some locations lahars are significant hazards, but because of the remote setting of volcanoes in Alaska, little infrastructure is
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located in lahar hazard zones (with a few exceptions); and thus the potential threat of
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lahars is much lower than in many other areas of the world where lahars develop. Lahar
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inundation does affect riparian habitat, and drainages on volcanoes that support significant runs of anadromous fish are vulnerable to the effects of lahars (Schaefer et al., 2008).
5.1. Eruption-induced lahars
Most of the large hazardous lahars that develop during eruptive activity in Alaska are initiated by the interaction of pyroclastic flows with snow and ice on the volcano. Pyroclastic density currents produced by collapse of unstable lava domes or collapse of particle-laden eruption columns can scour, entrain, and melt snow and ice and usually result in the formation of large amounts of meltwater. Meltwater generated by this process will entrain loose pyroclastic debris and other types of unconsolidated sediment
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ACCEPTED MANUSCRIPT from the flanking slopes of the volcano and form lahars that may travel many tens of kilometers from the eruption site (Pierson, 1998). Of the Aleutian arc volcanoes that have
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been active within the Holocene (roughly 70 volcanoes), about 50 % of them support glacier ice and all of them are seasonally snow covered (Fig. 8). At volcanoes where ice
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volume estimates have been made, the ice cover typically exceeds 1 km3 (Trabant and
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Hawkins, 1997; Welch et al., 2007). The ubiquitous nature of snow and ice means that
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lahars are a common outcome of explosive eruptive activity throughout the Aleutian arc.
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5.1.1. Eruption-induced lahars at Redoubt volcano
The best-studied eruption-induced lahars in Alaska are the lahars and associated
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deposits in the Drift River valley generated during eruptions of Redoubt volcano.
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Redoubt volcano is located in the Cook Inlet region of south-central Alaska (Fig. 1) and
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is an andesite to dacite, glaciated stratocone that erupted most recently from March–July 2009 (Schaefer, 2012). The volcano supports about 4 km3 of glacier ice and perennial snow (Trabant and Hawkins, 1997), about 1 km3 of which makes up the Drift glacier on the north flank of the volcano (Fig. 9). All of the known historical eruptive activity, and probably several prehistoric eruptions, have occurred from vents within the breached, 1 x 2 km diameter, ice-filled summit crater at the head of Drift glacier (Fig. 10). During the past three historical eruptions (1966–1968, 1989–1990, 2009), multiple large lahars flowed through the Drift River valley on the north side of the volcano (Sturm et al., 1986; Dorava and Meyer, 1994; Waythomas et al., 2013) and inundated significant parts of the valley floor and associated distributary channels in the lower part of the drainage (Fig. 15). The lahars produced during the 1989–1990 and 2009 eruptions posed a significant
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ACCEPTED MANUSCRIPT hazard to oil-production infrastructure located near the mouth of Drift River and directly threatened the Drift River Marine Terminal (DRMT; Fig. 9) and a buried pipeline that
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delivers oil to the terminal from the north. These facilities are important oil storage and transfer infrastructure in Cook Inlet, and more than 100,000 barrels of crude oil are
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typically stored at the terminal at any given time (Dorava and Meyer, 1994; Waythomas
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et al., 2013).
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Extensive inundation of the Drift River valley by lahars was first documented during an explosive eruption on 25 January 1966 when a lahar, assumed to be at least 106
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m3 in volume (Riehle et al., 1981; Sturm et al., 1986) was likely triggered by a pyroclastic flow. This lahar inundated the Drift River valley floor (actual extent of
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inundation not known), transporting blocks of ice many meters in diameter and forcing
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the evacuation of a survey crew working at the future site of the DRMT (Waythomas et
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al., 2013). A second large lahar was generated on 9 February 1966, but apparently it contained little or no ice (Riehle et al., 1981). Both lahars had local flow depths of at least 4–6 m, but their volumes and peak discharges are unknown. During the 1989–1990 eruption, at least 18 lahars were generated by vigorous vent explosions or pyroclastic flows associated with the collapse of lava domes (Brantley, 1990). Three of these lahars were large enough to inundate the Drift River valley floor (about 100 km2), including the DRMT and vicinity (Dorava and Meyer, 1994). The lahar of 2 January 2 1990 was the largest of the 1989–1990 eruption and was initiated by a meltwater flood of about 25 million m3 (Dorava and Meyer, 1994; Trabant et al., 1994). The meltwater entrained pyroclastic debris, supraglacial debris, and alluvium and transformed to a watery debris-flow lahar with a volume of about 107–108 m3 (Dorava
19
ACCEPTED MANUSCRIPT and Meyer, 1994; Gardner et al., 1994). The estimated peak discharge of the 2 January 1990 lahar was 16,000–80,000 m3s-1 at a valley cross section in the upper Drift River
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valley located about 2.5 km downstream from the terminus of Drift glacier (Dorava and Meyer, 1994). This lahar transported ice blocks up to 8 m in diameter as far as Cook Inlet
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and inundated the entire Drift River valley and most of the distributary channel network
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of the lower Drift River valley (Fig. 9). A lahar generated on 15 February 1990 was
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smaller and more dilute than the January 2 flow, but it also inundated much of the Drift River valley and the DRMT area and vicinity (Dorava and Meyer, 1994).
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The sediment delivered to the lower Drift River valley by the 1990 lahars caused aggradation of the valley floor and lateral shifts in the position of the active channel.
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During July–August 1990, the Drift River avulsed northward from its main channel into
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adjacent Montana Bill Creek (Fig. 11B), which thereafter conveyed an estimated 70–
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90 % of the flow of Drift River (Dorava and Meyer, 1994). This resulted in several meters of bed scour along lower Montana Bill Creek, exposing the buried oil pipeline that connects with the DRMT.
During the most recent eruption of Redoubt Volcano between 22 March and 4 April 2009, explosive eruptive activity destroyed several lava domes and triggered four large lahars in the Drift River valley (two on March 23, one on March 26, and one on April 4) and several smaller lahars between March 23 and March 30 (Schaefer, 2012; Waythomas et al., 2013). The lahars generated on March 22–23 and April 4 inundated the upper and middle reaches of the Drift River valley and a substantial area of the lower Drift River valley (Fig. 15D). The lahars of March 22–23 and April 4 also reached the DRMT and overtopped and flowed around protective dikes surrounding the oil storage
20
ACCEPTED MANUSCRIPT tanks. The lahars deposited significant amounts of sediment, ice, and debris that severely impacted operations at the facility but did not result in any oil spills (Schaefer, 2012).
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All of the lahars triggered by the 2009 eruption were detected in seismic data (Buurman et al., 2013), but because of the remote location of the volcano, none of the
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lahars were observed directly. Aerial observations and occasional remote camera images
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provided confirmation of several lahar events, which are described in greater detail in
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Waythomas et al. (2013).
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5.1.2. Lahars and lahar deposits associated with the 2009 Redoubt eruption Twenty discrete lahars were identified in seismic data during the roughly three-
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week long period of explosive eruptive activity that characterized the 2009 Redoubt
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eruption. The lahars that formed on March 22–23 and April 4 were the largest of the 2009
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eruption, and deposits produced by these flows were easily recognized throughout the Drift River valley. Although other smaller lahars formed during the eruption, associated deposits could not be identified in the field because they were either eroded away by subsequent flows or were indistinguishable from the April 4 deposits. The final lahar of the eruption on April 4 covered all previously emplaced deposits with sand to fine gravel more than 1 m thick.
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ACCEPTED MANUSCRIPT Ice-rich lahar deposits generated by the March 22–23 eruptive activity were deposited throughout the Drift River valley. At the DRMT, the lahar deposits were about
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2 m thick and reached the top of the protective levees on the northwestern side of the DRMT tank farm (Waythomas et al., 2013). Elsewhere in the Drift River valley, the
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March 22–23 lahars produced ice-rich deposits, 1–4 m thick that also contained variable
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amounts of vegetation, trees, and minor amounts of sediment. Clasts of ice, some up to
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several meters in diameter, formed the bulk of the lahar deposit (Fig. 12; Waythomas et al., 2013). In many parts of the Drift River valley, the ice-rich lahars abraded and
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removed bark from alders, willows, and the trunks of mature cottonwood trees to a height of several meters above the top of resulting deposits. Many 2–3 m high stands of willow
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and alder were knocked over and buried by the ice-rich flows. As the lahar passed
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through the Drift River valley, it left prominent horizontal mud and debris lines many
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meters above the active channel that were clearly evident along the valley walls and indicated flow depths of 6–8 m above the valley floor (Waythomas et al., 2013). At Dumbbell Hill, a bedrock knob in the Drift River channel about 3 km downstream from the terminus of Drift glacier (Fig. 9), ice-rich lahar deposits, including frozen blocks of alluvium, were emplaced on the upstream (west) side of the hill about 13–15 m above the valley floor. In this area, blocks of ice up to 10 m in length were scattered about on the valley floor (Waythomas et al., 2013). As a result of the March 22–23 lahars, the main channel of the Drift River along the north side of the DRMT became choked with ice-rich lahar deposits, and the active channel of the Drift River was diverted southward into the Rust Slough-Cannery Creek drainage (Fig. 11E).
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ACCEPTED MANUSCRIPT Although seismic data and aerial observations indicated that at least three lahars developed on March 22–23, later field studies revealed only a single, ice-rich deposit (Fig.
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12). The deposits produced by the March 22–23 lahars primarily consisted of 2–4 m thick accumulations of interlocking, framework-supported, tabular ice blocks (some several
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meters in length) and subangular to rounded ice cobbles (Fig. 12). Lithic material in these
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deposits (including river gravel from the valley floor) was rare. Only minor amounts of
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silt, sand, gravel, and rock debris were evident in the lahar deposits examined, no juvenile rock clasts were observed, and the composite deposit was 80–100 % ice (Waythomas et
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al., 2013).
The lahars that formed on March 22–23 were probably initiated by meltwater
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surges during the initial period of eruptive activity that marked the beginning of the
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explosive phase of the 2009 eruption (Waythomas et al., 2013). The scenario envisioned
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is that meltwater ran out over a frozen and snow-covered Drift River valley floor, and as a result, the flow must have rapidly bulked up with ice and snow. Such a flow probably behaved like a large icy slurry, possibly analogous to the ice-rich flows that develop during ice-jam floods associated with breakup of river ice (Jasek, 2003). Visits to the Drift River valley in 2010, 2011, and 2012 revealed that complete melting of the 22–23 March 2009 ice-rich lahar deposit resulted in a 1–6 cm thick bed of silt to fine sand (Fig. 12C) that exhibited no obvious textures or characteristics indicative of a very large lahar. At Redoubt and other volcanoes where similar icy flows develop, the meltout deposits of ice-rich lahars may be difficult to recognize in the geologic record because they are likely to be only a few centimeters in thickness and inconspicuous relative to the deposits produced by more typical lahars with high sediment
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ACCEPTED MANUSCRIPT concentrations, whose deposits are thicker (several meters or greater) and more likely to be preserved. The thin meltout deposits derived from ice-rich lahars may be difficult to
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identify and easy to overlook, or possibly may not be preserved at all in the geologic record despite being the products of significant and very voluminous lahars.
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The final lahar of the 2009 Redoubt eruption formed on April 4 and was the most
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extensive lahar of the eruption. This lahar inundated about 125 km2 of the Drift River
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valley, an area roughly 20 % greater than that inundated by the March 22–23 lahar (Waythomas et al., 2013). The April 4 lahar was initiated by a major explosive event that
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resulted in the destruction of the lava dome that had grown in the summit crater of the volcano (Bull et al., 2013). The April 4 lahar completely inundated the upper and middle
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reaches of the Drift River valley floor and about 80 % of the lower Drift River valley
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including the DRMT area (Fig. 13). The April 4 lahar deposits were primarily sandy,
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hyperconcentrated-flow deposits but also contained trees, scattered blocks of ice, and other vegetal debris. At peak flow, the April 4 lahar reached the top of the containment levees on the northwest side of the DRMT tank farm and deposits in this area were about 1 m thick. Most of the flow was conveyed by channels of the Montana Bill Creek drainage north of the DRMT and by the Rust Slough drainage south of the DRMT (Fig. 13). Minor flow along the main channel of the Drift River was reestablished temporarily, but nearly all of the active flow subsequent to the emplacement of the April 4 lahar was within the Rust Slough drainage (Fig. 13).
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ACCEPTED MANUSCRIPT In contrast to the March 22–23 lahar deposits, the April 4 deposits contained only minor amounts of ice (Fig. 14). Most of the ice in the deposit was present only in the
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upper part of the Drift River valley where large subrounded blocks of glacier ice scoured from Drift glacier (some as large as 200–300 m3), and some meter-sized tabular clasts of
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river ice (possibly reworked from the March 22–23 deposits) were evident on the surface
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of the lahar deposit. In most areas of the Drift River valley, the April 4 lahar deposits
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consisted almost entirely of massive to horizontally stratified, poorly sorted sand to fine gravel. Rock material in the deposit was primarily juvenile andesite derived from the lava
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dome that was destroyed by the April 4 explosion (Coombs et al., 2013). The lahar deposits emplaced on April 4 ranged in thickness from 1–6 m and were thickest in slack
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water areas along the valley margin. Massive, clast-supported, gravel-rich lahar deposits
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1–2 m thick were observed in the upper Drift River valley, indicating that at least locally
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the lahar had achieved debris-flow sediment concentrations prior to transformation to hyperconcentrated flow (Waythomas et al., 2013). Many of the April 4 lahar deposits examined in the lower Drift River valley contained water-escape structures and soft sediment deformation features indicative of rapid deposition by water-rich hyperconcentrated flows (Pierson, 2005).
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ACCEPTED MANUSCRIPT 5.1.3. Flow characteristics of the 2009 Redoubt lahars The volumes of the largest lahars of the 2009 Redoubt eruption range from 107–
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108 m3, and the estimated peak discharges were in the range of 104–105 m3s-1 (Waythomas et al., 2013). The 2009 lahars all had volumes similar to the largest lahar of
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the 1989–1990 eruption. Lahars of this magnitude or possibly greater probably occurred
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several times during prehistoric eruptions of Redoubt volcano. Stratigraphic data from the
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Drift River valley indicates at least two older lahar deposits of widespread extent suggesting a similar degree of inundation during eruptions of late Holocene age (Begét
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and Nye, 1994; Waythomas, unpub. data). The two largest lahars of the 2009 Redoubt eruption occurred at the beginning and at the end of the period of explosive eruptive
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activity, whereas the largest lahars of the 1989–1990 eruption occurred only at the
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beginning of explosive activity and became smaller in volume as the eruption progressed.
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The flow characteristics of the March 22–23 and April 4 lahars were significantly different. These lahars all originated from meltwater surges generated by explosive eruptive activity; the March 22–23 lahars being composed of >80 % ice and leaving a deposit that today is barely recognizable as the product of a large volume lahar, whereas the April 4 lahar was a water-rich hyperconcentrated flow lahar with <10 % ice content. The April 4 lahar produced a deposit of sand and fine gravel several meters in thickness that is evident throughout the Drift River valley today.
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ACCEPTED MANUSCRIPT 5.1.4. Erosion, channel changes, and context of the 2009 Redoubt lahars Substantial quantities of sediment were mobilized by lahars in the Drift River
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valley during the 2009 eruption, and this resulted in significant changes to the valley bottom geomorphology. By the summer of 2010, as much as 10 m of incision through the
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2009 lahar deposits and the underlying alluvium had occurred in the upper Drift River
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valley. The upstream channel erosion contributed additional sediment to the lower Drift
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River valley, which caused active channels in the lower part of the valley to aggrade by several meters. As erosion of the 2009 deposits in the upper and middle reaches of the
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Drift River valley continues, downstream aggradation and lateral shifting of channels will likely continue for many years. At the time of this writing, all of the flow in the Drift
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River is within the Rust Slough drainage south of the DRMT. The main Drift River
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and debris (Fig. 11E).
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channel on the north side of the DRMT remains abandoned and choked with sediment
The lahars that formed in the Drift River valley during eruptions of Redoubt volcano are considerably larger than flows that result from rainfall or snowmelt runoff. Documented flood peaks in a variety of watersheds throughout south-central Alaska provide a general estimate of the largest meteorologically generated flows possible in the region (Jones and Fahl, 1994). An envelope curve for the relation between peak discharge and drainage basin area (Waythomas et al., 2013; Fig. 20) indicates that for the Drift River drainage (drainage area = 570 km2) the maximum peak discharge is about 2000 m3s-1. The estimated 100-year and 500-year-flood peak discharges for the Drift River drainage are about 1000 and 1300 m3s-1, based on regional flood-frequency equations described in Curran et al. (2003). These values are tens to hundreds of times smaller than
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ACCEPTED MANUSCRIPT the peak flow estimates of the largest lahars of the 1989–1990 and 2009 eruptions. The estimated peak discharge of the 4 April 2009 lahar near the DRMT was 3000–33000
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m3s-1; these values are 3–30 times larger than the estimated 100-year flood peak
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constrained by regional flood-frequency relations (Waythomas et al., 2013).
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6. Geomorphic effects at volcanoes that erupt infrequently
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Another category of volcanoes in Alaska are those that have erupted after a period of repose of hundreds to thousands of years, including volcanoes with a limited or no
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known record of historical eruptive activity. An example of a volcano that has had a major eruption after a long repose period is Kasatochi volcano, a 3-km-diameter, 300-m-
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high, isolated island volcano located in the southern Bering Sea in the central Aleutian
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Islands (Fig. 1). The eruption of Kasatochi volcano in 2008 has provided an opportunity
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to observe erosion-related changes in the geomorphology of Kasatochi Island following a significant landscape-altering geologic event. The effects of the eruption are significant because the ecosystem of the island was drastically altered by the accumulation of thick pyroclastic deposits over the entire island (Fig. 15). The opportunity to document the erosion and redistribution of volcaniclastic deposits and to address ecosystem recovery on Kasatochi using modern techniques is unprecedented for Alaska (DeGange et al., 2010; Walker et al., 2013).
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ACCEPTED MANUSCRIPT Prior to its 2008 eruption, little was known about the eruptive history of the volcano. Pyroclastic-flow and -surge deposits, exposed in sea cliffs and in the upper walls
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of the crater (Figs. 15A, B) probably record at least one or more explosive eruptions within the past 1000 years that may have been similar in scale to the 2008 eruption
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(Waythomas et al., 2010b). The only known historical eruption of Kasatochi occurred on
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7–8 August 2008 (Waythomas et al., 2010a). The VEI 4 eruption covered all of
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Kasatochi Island with tens of meters of pyroclastic debris consisting of granular pyroclastic-flow, -surge, and fine ash deposits (Fig. 15C). The emplacement of
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pyroclastic-flow deposits off the flanks of the volcano extended the coastline into the sea by as much as 400 m, increasing the area of the island by about 30 % (Waythomas et al.,
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2010a).
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The pre-eruption vegetation on Kasatochi Island consisted of a mosaic of mesic
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herbs, grasses, and alpine heath and dense dune grass meadows along the shoreline (Walker et al., 2013). The near-shore marine environment included abundant brown, red, and green algae and a mature kelp canopy (Jewett et al., 2010). All of the vegetation on the island and in the near shore zone to about the 20-m isobath was buried by pyroclasticflow, -surge, and tephra deposits (Waythomas et al., 2010a). Pyroclastic-flow deposits examined about two weeks after the eruption were clearly hot (Fig. 16A), but in some areas the deposits contained unburned organic matter, twigs, roots, and chunks of turf (Fig. 16B) indicating that local emplacement temperatures were relatively cool. The emplacement of low-temperature pyroclastic flows may have been the result of mixing of eruptive material with water that was resident in the Kasatochi crater lake (Fig. 15B), all of which was likely expelled during the 21-hour-long eruption. Where the 2008 deposits
29
ACCEPTED MANUSCRIPT were thin and emplaced cool, pockets of the underlying vegetation survived and are now serving as nuclei for regrowth of vegetation (Walker et al., 2013). These areas are very
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restricted around the island and the amount of new vegetation emerging in these areas is limited.
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Because of the amount of pyroclastic-flow material and ash emplaced by the
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eruption, all preexisting stream channels, valleys, talus, beaches, and other geomorphic
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surfaces were buried. Following the eruption, surface erosion processes were free to operate on a landscape devoid of vegetation and characterized by thick accumulations of
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loose, easily erodible volcaniclastic deposits (Fig. 16). Gully erosion on the flanks of the volcano began immediately after the eruption ended and led to the generation of
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numerous narrow, several-meter-deep gullies (Fig. 17) on most parts of the island, and
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the drainage density increased by a factor of about 20 in one year (Waythomas et al.,
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2010b). Wave erosion of pyroclastic material along the coastline occurred rapidly, and coastal erosion rates, as defined by the landward retreat of wave-cut cliffs, were as high as 150 my-1 (Fig. 17; Waythomas et al., 2010b). The main impact of surface erosion by gullying and wave erosion of the coastline was the introduction of sediment into the nearshore zone, which suppressed the growth of kelp and other marine biota for several years after the eruption. Continued erosion of the pyroclastic deposits on the island is expected to continue for some time, possibly for decades to centuries, until the gullies progress to a stable configuration and vegetation is able to colonize the surface (Fig. 18). As of 2014, the coastline along the northern half of the island has nearly returned to its pre-eruption configuration, and rates of coastal erosion have declined markedly or ceased in some areas where the coastline has reached bedrock. The exhumation of talus buried by
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ACCEPTED MANUSCRIPT pyroclastic debris is continuing, and seabirds that nest within accumulations of talus, such as Auklets, have not abandoned the island, although it is difficult to know how the loss of
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nesting habitat has affected their population. Formation of new talus at the base of steep bedrock cliffs is occurring locally (Figs. 17C, D) and such areas are being highly utilized
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7. Geomorphic impacts of tephra deposits
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by nesting seabirds (G. Drew, USGS, pers. comm, 2014).
Tephra deposits are common products of explosive eruptions and result from the
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fallout of particles in the eruption cloud through the atmosphere to the ground surface where they typically form a blanketing cover over the landscape. Tephra deposits can
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possess a range of particle sizes from extremely fine ash (<63 μm diameter) to ballistic
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particles up to several meters in diameter (White and Houghton, 2006). The accumulation
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of tephra generated by sedimentation from volcanic eruption clouds typically exhibits roughly exponential changes in thickness and particle size with distance from source (Bonadonna and Costa, 2013), and because of this phenomena the most significant impacts from tephra fallout occur in the proximal to medial areas around the volcano (generally 5–10 km from the vent). The volume of tephra produced by a given eruption is a key parameter in determining eruption magnitude and assigning a VEI value (Newhall and Self, 1982). As discussed previously, about 78 % of the documented historical eruptions in Alaska have been VEI 2 and 3 events. Such eruptions typically produce 106– 107 m3 of tephra and typically more than 50 % of this amount falls in the proximal areas around the volcano (Scott and McGimsey, 1994; McGimsey et al., 2001; Wallace et al., 2013). Thus, the spatial footprint of geomorphically significant tephra fallout for
31
ACCEPTED MANUSCRIPT eruptions this size is generally limited to areas within about 10 km of the vent. Larger eruptions, of course, produce more tephra and have a much greater spatial extent of
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fallout. The 1912 Novarupta-Katmai eruption is a good example of the types of the geomorphic impacts associated with a thick and widespread ash fall produced by a large
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historically rare eruption.
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7.1. Tephra-fall impacts from the Novarupta-Katmai eruption of 1912 The 6–8 June 1912 eruption of Novarupta-Katmai produced a cumulative tephra
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fallout volume of roughly 17 km3 that covered an area of about 120,000 m2 with >1-cmthick deposits of light-colored rhyolitic, dacitic, and some andesitic ash (Fig. 19; Fierstein
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and Hildreth, 1992). Locations within about 5 km of the Novarupta vent received more
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than several meters of ash fall and some areas were covered by >12 m of ash as far as 4
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km from the vent (Fierstein and Hildreth, 1992). The early summer eruption occurred when local snow cover was still relatively extensive and the accumulation of warm pumiceous tephra fall on snow and ice led to the generation of hyperconcentrated- and debris-flow lahars in many of the drainages that head on Mt. Katmai (Hildreth and Fierstein, 2012). In addition, many tephra-loaded slopes failed during the eruption, probably because of strong attendant seismicity, which led to a number of blocked drainages where lakes were impounded (Griggs, 1922; Hildreth and Fierstein, 2012). One such lake was the source of a significant dam-break flood in Katmai Canyon (Griggs, 1922) that caused substantial damage at the now abandoned village of Katmai and inundated the Katmai River valley to depths of 3–8 m (Griggs, 1922; Hildreth and Fierstein, 2012).
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ACCEPTED MANUSCRIPT Erosion and incision of the 1912 deposits began during brief pauses in the eruption, and several-meter-thick sequences of pumiceous alluvium within the primary
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eruptive deposits have been recognized in several locations (Hildreth, 1983; Hildreth and Fierstein, 2012). After the eruption ended, erosion of the thick and widespread eruptive
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products commenced rapidly, and in some areas more than 20 m of vertical incision
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occurred by 1917 as documented by Griggs (1922). Systematic documentation of the
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geomorphic impacts and erosion of the 1912 deposits was not a primary focus of the many investigations that followed the initial studies by Griggs (1922). However, it is
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clear from many of the photographs obtained from the Griggs-led expeditions from 1915–1919, and from observations made since then, that rates of erosion were initially
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rapid (Fig. 20), but after about 1917 further changes were less significant (Hildreth and
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Fierstein, 2012). The rapid accumulation and erosion of pyroclastic fall and flow material
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initiated significant aggradation of several drainages that continues today. The accumulation of pumiceous fluvial deposits in the lower Katmai River drainage initially filled in parts of Katmai Bay and caused minor progradation of the coastline (Griggs, 1922), and as reported in Hildreth and Fierstein (2012), tidal mudflats had apparently advanced ~1 km seaward. However, significant geomorphic changes to the lower Katmai River appear to have been minimal since the early 1950s (Hildreth and Fierstein, 2012). In addition to erosion and redistribution of the 1912 tephra deposits by water, the characteristic high winds of the Katmai region have reworked and in many places stripped the landscape of pyroclastic fall deposits. Thick accumulations of pumiceous eolian sand are common in many areas, and intense dust storms occur regularly today. Occasionally such storms are severe enough to loft ash 1–4 km asl and transport it several
33
ACCEPTED MANUSCRIPT hundred kilometers from the source area (Hadley et al., 2004) where it interferes with
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local air travel and may pose a public health hazard.
7.2. Tephra-fall impacts associated with other Alaskan eruptions
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Only a few studies have documented the extent, characteristics, and volume of
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tephra fallout associated with historical eruptions in Alaska, but none of these were
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focused specifically on the geomorphic impacts of ash fall. The ash fallout produced during recent eruptions of Redoubt volcano in 1989–1990 and 2009, and during the
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Crater Peak eruption of 1992, has been well enough documented (Scott and McGimsey, 1994; McGimsey et al., 2001; Wallace et al., 2013) to comment on the geomorphic
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impacts of tephra fall associated with these events. Both Redoubt eruptions occurred in
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winter; and by the following summer, rainfall and snowmelt runoff had largely removed
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most of the tephra deposits except for areas within a few kilometers of the vent. Observations made in May 2009 indicated that nearly all of the 19 tephra-fall layers produced by the explosive phase of the 2009 Redoubt eruption had coalesced into a single merged layer of fine ash that lacked any evidence of having originated as multiple fall deposits (Wallace et al., 2013). The amount of tephra removed by erosion following the 2009 Redoubt eruption is not known. Visits to the volcano since the eruption ended revealed that millimeter- to centimeter-thick primary fall deposits are preserved only in areas protected from wind and water erosion. The thickness of proximal ashfall was insufficient to impair vegetation growth, and in many areas around the volcano, plants have grown up through the thin tephra cover.
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ACCEPTED MANUSCRIPT The 1992 eruption of the Crater Peak vent at Mount Spurr volcano consisted of three explosive eruptions on 27 June, 18 August,, and 16–17 September 1992 (Keith,
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1995). The ash clouds from these eruptive events resulted in ash fallout over large parts of south-central Alaska (McGimsey et al., 2001). Most of the distal ash deposits were
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covered by winter snowfall, and then removed the following spring during snowmelt.
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Coarse lapilli tephra deposits from the 1992 eruption formed a blanketing cover in
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proximal areas, and these deposits have been little modified since they accumulated. The geomorphic effect of tephra fallout on distal areas has not been addressed in a
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systematic manner. Some recent studies of distal ash fall from the ca. 1600 yBP eruption of Aniakchak volcano suggest that sites as far as 1100 km from the volcano may have
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experienced a period of retarded vegetation growth following ash deposition possibly
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lasting as long as 100 years (Blackford et al., 2014). Griggs (1920) found that vegetation
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growth on Kodiak Island after the 1912 Katmai eruption increased for several years following the eruption and attributed this to the mechanical effects of the ash fall (Griggs, 1915).
The deposition of volcanic ash in lakes has long been recognized in many parts of Alaska, and most lake core records obtained throughout the state contain recognizable ash layers (Begét et al., 1994; Schiff et al., 2008; Kaufman et al., 2012; Krawiec et al., 2013). The ash beds are typically amenable to dating with radiocarbon techniques and high-resolution records of ash fall based on age–depth models that have been developed (Schiff et al., 2008). Although much emphasis has been placed on tephra correlation and analysis of the frequency and magnitude of ash fall for hazard assessment and paleoclimatic reconstructions, only a few studies aimed at determining the physical and
35
ACCEPTED MANUSCRIPT geochemical changes associated with ash fall in Alaska lakes and how this might affect primary productivity have been completed (Eicher and Rounsefell, 1957; Barsdate and
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Dugdale, 1972). Many lakes in Alaska provide unique rearing habitats for salmon and other species of fish that play an important economic role in subsistence and commercial
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fisheries.
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8. Concluding remarks
Clearly from this review, much work remains on the topic of geomorphic impacts
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of eruptions in Alaska. Continued studies at Kasatochi volcano hold promise for providing insight about how an island volcano responds geomorphically to the effects of
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a large explosive eruption that essentially created a new, pristine landscape. The
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Kasatochi example is unique because the island is somewhat isolated geographically
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making it nearly impossible for vegetation to encroach from unaffected areas. In general, the degree of impact scales with eruption magnitude, and thus further analysis of the effects of significant eruptions, such as the 1912 Novarupta-Katmai eruption, would yield the most insight about how such events geomorphically alter affected environments. The most common historical eruptions in Alaska are VEI 2–3 events, and with few exceptions, these types of eruptions have impacts that are restricted to the area immediately surrounding the volcanic edifice. This does not mean that these impacts are unimportant; they clearly are for the particular sector of the volcano that is disturbed. However, for smaller eruptions, the scope of impact is typically at the drainage basin scale, and the effects can be quite limited and transient.
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ACCEPTED MANUSCRIPT At volcanoes that experience frequent eruptions, such as Pavlof volcano, the most significant geomorphic impacts are related to the introduction of significant volumes of
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loose, easily erodible fragmental sediment primarily generated by tephra fallout and small pyroclastic flows. As a result, affected drainages become choked with pyroclastic silt,
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sand, and gravel; and aggrading, unstable reaches of active channels are common. This
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severely limits the development of riparian vegetation, which further promotes channel
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instability.
The role of glacier ice and the nature of the interaction between eruptive products
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and ice is important because this governs the generation of meltwater that can lead to large voluminous (and hazardous) lahars. The emplacement of large lahars and their
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associated deposits can completely alter the geomorphic regime of the valleys they
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inundate, leading to major changes in sediment flux to the sea, altered drainage patterns,
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development of lahar-dammed lakes, and sometimes highly unstable and rapidly shifting stream channels.
Additional research is needed to document how geomorphic, hydrologic, and biologic systems in the Aleutian arc respond to volcanic events. The hazards associated with explosive eruptions at volcanoes in Alaska are generally understood, especially at those volcanoes that pose the greatest risk to people and infrastructure. What is lacking however, is a more comprehensive understanding of the secondary effects that result from eruptive activity and the time scales over which these effects are significant.
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ACCEPTED MANUSCRIPT Acknowledgements The author wishes to thank and acknowledge the support and encouragement of
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his colleagues at the Alaska Volcano Observatory. The author is grateful for the helpful reviews provided by K. Wallace (USGS-AVO), three anonymous reviewers, and for the
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help and support provided by the journal editor, R. Marston, and the review paper
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coordinator, T. Horscoft.
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Geophysical survey of the intra-caldera icefield of Mt Veniaminof, Alaska. Annals of Glaciology 45, 58–65. Westgate, J. A., Preece, S. J., Kotler, E., Hall, S., 2000. Dawson tephra: a prominent stratigraphic marker of Late Wisconsinan age in west-central Yukon, Canada. Canadian Journal of Earth Sciences, 37(4), 621-627. White, J. D. L., Houghton, B. F., 2006. Primary volcaniclastic rocks. Geology 34(8), 677–680. Wilson, C. J. N., Walker, G. P. L., 1985. The Taupo eruption, New Zealand I. General aspects. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, pp. 199–228.
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ACCEPTED MANUSCRIPT Wolfe, B. A., 2001. Paleohydrology of a catastrophic flood release from Okmok caldera and post-flood eruption history at Okmok Volcano, Umnak Island, Alaska:
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University of Alaska Fairbanks unpublished M.S. thesis, 100 p. Figure Captions
Major volcanoes and volcanic fields of the Aleutian arc. The red triangles
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Fig. 1.
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indicate volcanoes that have experienced eruptive activity since A.D. 1700
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and are considered historically active. The yellow triangles indicate volcanoes that have had confirmed eruptive activity within the past 10,000
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years. Also shown are the location of the Aleutian trench and the rates of plate motion for the Pacific Plate (from Freymueller et al., 2008).
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Size of calderas of Holocene age in the Aleutian arc. (A)Plot of caldera
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length versus width. (B)Bar graph of caldera area from west (Semisopochnoi) to east (Hayes).
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Fig. 2.
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ACCEPTED MANUSCRIPT Fig. 3.
Valley-filling pyroclastic-flow deposits associated with the most recent caldera-forming eruption at Veniaminof volcano. (A)View down
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Blueberry Creek valley (the northeast flank of the volcano) showing loose (pf) and welded (w) pyroclastic-flow deposits incised by Blueberry Creek.
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The pyroclastic deposits are nearly 100 m thick in this location and formed
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a continuous, surface-mantling cover that obscured the underlying surface
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morphology. (B)Valley-filling pyroclastic-flow deposits on the southeast flank of Veniaminof volcano. In this area, valleys that were glaciated
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during the late Pleistocene have been partially filled with pyroclastic-flow deposits erupted during the last caldera-forming eruption about 3700 yBP.
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(C)Sketch of valley-filling pyroclastic-flow deposits at Veniaminof
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volcano. Not drawn to scale, but pyroclastic-flow deposits in such areas
Fig. 4.
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are on the order of 25–100 m thick. Generalized surficial geology of the north flank of Pavlof volcano. (A)Shaded relief perspective view of the north flank of Pavlof volcano showing the extent of debris-covered glacier ice, lahar deposits, and the general extent of fountain-fed lava flows and spatter accumulations. View is toward the southwest. (B)Generalized cross section of the north flank of Pavlof volcano showing the relationship between spatter accumulations, lava flows, lahar deposits, and glacier ice.
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ACCEPTED MANUSCRIPT Fig. 5.
Satellite image of Pavlof volcano and vicinity showing areas around the volcano that have been inundated by lahars during historical eruptions,
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and generalized extent of proximal pyroclastic fall and flow deposits. Fountain-fed lava flows on the upper flanks of the cone also shown. Photograph and schematic cross section of lahar deposits along the
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Fig. 6.
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Leontovich River, emplaced by the 1996 eruption of Pavlof volcano. The
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helicopter in the photograph is parked on the surface of a 1–2 m thick lahar deposit emplaced during the 1996 eruption.
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Examples of large lahars blocking drainages and leading to the formation of lahar-dammed lakes. (A)Northwest flank of Veniaminof volcano and
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lahar inundation along the Muddy and Sandy river drainages by lahars
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initiated during the most recent caldera-forming eruption 4100–4400 years ago. (B)South flank of Redoubt volcano and lahar deposits filling the
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Fig. 7.
Crescent River drainage. The Crescent River lahar was emplaced during eruptive activity about 3700 yBP and resulted in the formation of Crescent Lake as the Lake Fork of Crescent River was blocked by the lahar.
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ACCEPTED MANUSCRIPT Fig. 8.
Plot of volcano summit altitude versus distance along the axis of the Aleutian arc from west (Buldir Island) to east (Hayes volcano) for all
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volcanoes with confirmed Holocene activity. The 71 points are plotted with respect to the presence or absence of modern glacier ice on the
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edifice. The dashed line indicates the approximate glaciation threshold and
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shows that volcanoes in the eastern part of the Aleutian arc can support
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glacier ice at lower altitudes than volcanoes in the western part of the arc. All volcanoes indicted are seasonally snow covered, although the amounts
Fig. 9.
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are not known with certainty and vary significantly by location. Shaded-relief map of Redoubt volcano and vicinity showing Drift glacier,
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the Drift River valley, and the Drift River Marine Terminal. Also
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indicated are the Drift River fan and its distributary channels, Montana
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Bill Creek (M), Rust Slough (R), and Cannery Creek (C). The main channel of the Drift River (D) on the fan is also shown.
Fig. 10.
The summit crater of Redoubt volcano, final lava dome of the 2009 eruption (D) and Drift glacier gorge downstream from the lava dome. The summit crater is about 2 km wide, and the volume of the lava dome is about 70 million m3..
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Maps of the lower Drift River valley showing the extent of the active channel of the Drift River during eruptive and noneruptive periods.
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(A)Lower Drift River channel, 13 August 1988, 1 year and four months prior to the start of the 1989–1990 eruption. (B)Lower Drift River channel,
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4 June 1990, showing maximum extent of inundation by lahars during the
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1989–1990 eruption. (C)Lower Drift River channels, 16 October 2005,
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about 15 years after the 1989–1990 eruption. (D)Lower Drift River, 4 April 2009, showing extent of largest lahar of the 2009 eruption.
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(E)Lower Drift River, 30 July 2014, about 5 years after the 2009 eruption, showing abandoned main channel of the Drift River.
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Photographs of the 22–23 March and 4 April 2009 lahar deposits.
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(A)Outcrop of frozen March 23 lahar deposit showing framework assemblage of tabular ice clasts. Scale in photograph is 2 m. (B)March 22–
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Fig. 12.
23 lahar deposits along the main channel of the Drift River and on the upstream part of the protective levees surrounding the DRMT tank farm. In this area the lahar deposits contain numerous rounded to subrounded cobble and smaller-sized ice clasts in a granular ice matrix and many tabular ice blocks several meters in length. The approximate width of the Drift River channel choked with ice-rich lahar debris is about 450 m. (C)Eight-centimeter-thick bed of silt and fine sand that is the deposit resulting from complete melting of the ice within the March 23 lahar deposit. This deposit is overlain by sandy hyperconcentrated-flow deposits
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Fig. 13.
the extent of lahar inundation associated with eruptive activity on April 4. Cut bank exposure of March 23 and April 4 lahar deposits in the middle
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Fig. 14.
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Drift River valley, May 2009. Combined thickness about 3 m. Note
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contrast in thickness of lahar deposits attributable to the significant ice content of the March 23 deposit. Arrows indicate the position of high-
Fig. 15.
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water mark mud lines on trees in the area. Photographs of Kasatochi Island in 2005 before the 2008 eruption (A, B)
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and (C) on 23 October 2008, 76 days after the 7-8 August 2008 eruption.
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Photos (A) and (B) show pyroclastic deposits associated with young pre-
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historical eruptions of Kasatochi. Photo (C) shows Kasatochi Island covered with pyroclastic-flow and -fall deposits generated by the 2008 eruption.
Fig. 16.
(A)East flank of Kasatochi Island, 22 August 2008, two weeks after the 2008 eruption. The red arrows indicate still-steaming fumaroles, and the areas labeled (p) show zones of sulfurous precipitate on the surface. The distance from the photographer to the fumaroles is about 50 m. (B)Coastal exposure of pyroclastic-flow deposits containing juvenile pumice, lithic material, and unburned vegetal matter (circled). Divisions on scale are in centimeters.
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Photographs of the flanks of Kasatochi volcano taken on 22 August 2008, 14 days after the 2008 eruption ended, and on 12 June 2009, 308 days
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after the eruption ended. (A)Northeast flank of Kasatochi volcano, 22 August 2008. Very little post-emplacement erosion of pyroclastic deposits
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evident in this photograph. Photographer is standing on recently formed
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beach at coastline. Note location of large block labeled (R) compared to its
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location on Fig. 13B. (B)Approximately the same view as Fig. 13A; note gully development and coastline erosion relative to Fig. 13A <1 year after
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the 2008 eruption. Feature labeled (R) is the same in both photos. Wavecut cliff in foreground about 15 m high. (C)Northwest flank of Kasatochi
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volcano on 22 August 2008 showing smooth cover of pyroclastic debris
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and ash. Note accumulation of rock fall talus (T) at the base of steep
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bedrock cliff. (D)Northwest flank of Kasatochi volcano on 12 June 2009 showing gullies developed in pyroclastic fan. After the eruption ended, these pyroclastic deposits were graded to sea level (Fig. 13C) and within <1 year were eroded by waves to form cliffs about 15 m high. Circled area shows an accumulation of rockfall talus that formed within two weeks after the 2008 eruption ended. This talus accumulation has been utilized by crevice-nesting seabirds (least Auklets) that used Kasatochi as prime nesting habitat all of which was destroyed by the 2008 eruption. Fig. 18.
Generalized time scale of geomorphic processes operating on the posteruption landscape of Kasatochi Island.
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(A)Cumulative tephra fallout over the lower Cook Inlet–Shelikof Strait area from the three plinian eruptions at Novarupta, June 1912 (from
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Fierstien and Hildreth, 1992). Contours in centimeters. (B)Extent of proximal tephra fallout and areas containing significant accumulations of
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reworked ash and pyroclastic debris (from Fierstien and Hildreth, 1992).
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Example of fluvial erosion within the proximal tephra fallout zone of the 1912 Novarupta-Katmai eruption. Top of tree to right of men was at the
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ground surface in 1917. By 1919, the channel bed in the foreground had been eroded vertically by about 3 m. Photo by W.L. Henning, National
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Geographic Society Expedition, 1919.
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Fig. 20.
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KV, Katmai Village site.
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Width (km) Semisopochnoi
Unknown
Unknown
6.5
Yunaska
Unknown
Unknown
3.8
Okmok
2140 – 1900
25 – 50
Makushin
9091 – 9284
4–5
Akutan
1554 – 1711
Unknown
Fisher
10,264 – 11,067
10 – 100 (DRE)
Veniaminof
4103 – 4405
>50
Length (km) 9
Area (km2)
3.5
12.4
45
9.4
97
Caldera-forming eruption likely Holocene age. Only proximal eruptive products preserved on Semisopochnoi Island, all other products entered the ocean. Age of caldera-forming eruption not known, but likely Holocene. Only proximal eruptive products preserved on Yunaska Island, all other products entered the ocean. Complete burial of nearly all of eastern Umnak Island by pyroclasticflow deposits meters to tens of meters thick. (2, 3, 6)
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10.3
Geomorphic impacts (references)
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Caldera dimensions
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Bulk eruptive volume (km3)
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Eruption age (Cal. 2 σ age range in yBP)
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Volcano
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Table 1 Holocene calderas and caldera-forming eruptions of the Aleutian arc; eruption ages pertain to the most recent caldera-forming eruption only
2.7
5.5
Extensive pyroclastic flows, but largely confined to glacial valleys on the volcano. Some pyroclastic flows may have crossed water and reached small islands east of the volcano. (1)
2.7
2.6
4.8
Extensive pyroclastic flows, but largely confined to glacial valleys on the volcano. Thick ash fall over most of Akutan Island and the surrounding ocean. (12)
17
137
Extensive pyroclastic flow sheets on western Unimak Island. Flows were highly mobile and rode up and over high topography in the vicinity of Fisher caldera. (10)
10.4
65.5
Extensive, valley-filling pyroclastic flows on all flanks of the volcano extending 40–60 km beyond the caldera. (7, 13)
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2.3
11
8.5
83
7.3
3721 – 3957
>50
10.4
10.4
78.7
Katmai/ Novarupta
1912 AD
10 – 50
2.5
4.2
9
Kaguyak
6311 – 6632
3.5a
2.5
3.4
Hayes
4259 – 4824b
Unknown
3
4.3
Thick pyroclastic deposits in the vicinity of the volcano and extensive ash fallout that formed a prominent regional tephra layer. (7, 13)
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3.4
Extensive, valley-filling pyroclastic flows and sheets on all flanks of the volcano extending 40–60 km beyond the caldera. Extensive ash fallout that formed a prominent regional tephra layer. (7, 11)
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Aniakchak
a
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Pyroclastic flows extending 10–20 km beyond vent. Extensive and thick ash fallout on areas east and southeast of the vent. Resuspension of ash occurs today during times of high winds and dry conditions. (5)
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10 – 50
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4444 – 4784
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Black Peak
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6.5
Pyroclastic flows and tephra fallout in the vicinity of the caldera. (4)
10.7
Pyroclastic flows and tephra fallout in the vicinity of the caldera and extensive ash fallout over parts of south-central Alaska. (9)
Tephra fallout bulk volume. Modeled age from lake core data. Multiple tephra deposits from Hayes volcano spanning an age range of 3500 to 3800 C-14 yBP are known in south-central Alaska.
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b
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References (1) Bean, K. W., 1999, The Holocene eruptive history of Makushin Volcano, Alaska: University of Alaska Fairbanks unpublished M.S. thesis, Fairbanks, AK, 130 p. (2) Beget, J.E., Larsen, J.F., Neal, C.A., Nye, C.J., and Schaefer, J.R., 2005, Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2004-3, 32 p. (3) Burgisser, A., 2005, Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska: Bulletin of Volcanology, v. 67, n. 6, p. 497-525. (4) Fierstein, J., 2007. Explosive eruptive record in the Katmai Region, Alaska Peninsula: an overview. Bull. Volcanol. 69, 469–509. (5) Fierstein, Judy, and Hildreth, Wes, 2008, Kaguyak dome field and its Holocene caldera, Alaska Peninsula: Journal of Volcanology and Geothermal Research, v. 177, n. 2, p. 340-366
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(6) Larsen, J. F., Neal, Christina, Schaefer, Janet, Beget, Jim, and Nye, Chris, 2007, Late Pleistocene and Holocene caldera-forming eruptions of Okmok Caldera, Aleutian Islands, Alaska, in Eichelberger, John, Gordeev, Evgenii, Izbekov, Pavel, Kasahara, Minoru, and Lees, Jonathan, eds., Volcanism and Subduction: The Kamchatka Region: Geophysical Monograph 172, American Geophysical Union, p. 343-364.
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(7) Miller, T. P., and Smith, R. L., 1987, Late Quaternary caldera-forming eruptions in the eastern Aleutian arc, Alaska: Geology, v. 15, n. 5, p. 434-438.
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(8) Miller, T. P., 2004, Geology of the Ugashik-Mount Peulik volcanic center, Alaska: U.S. Geological Survey Open-File Report OF 2004-1009, 19 p.
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(9) Schiff, C.J., Kaufman, D.S., Wallace, K.L., Werner, A., Ku, T.L., and Brown, T.A., 2008, Modeled tephra ages from lake sediments, base of Redoubt Volcano, Alaska: Quaternary Geochronology, v. 3, p. 56-67.
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(10) Stelling, P., Gardner, J. E., and Beget, J., 2005, Eruptive history of Fisher Caldera, Alaska, USA: Journal of Volcanology and Geothermal Research, v. 139, no. 3-4, p. 163-183. (11) Waythomas, C. F., and Neal, C. A., 1998, Tsunami generation by pyroclastic flow during the 3500-year B.P. caldera-forming eruption of Aniakchak Volcano, Alaska: Bulletin of Volcanology, v. 60, n. 2, p. 110-124.
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(13) Waythomas, C.F., unpublished data.
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(12) Waythomas, C. F., 1999, Stratigraphic framework of Holocene volcaniclastic deposits, Akutan Volcano, east-central Aleutian Islands, Alaska: Bulletin of Volcanology, v. 61, n. 3, p. 141-161.
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Table 2 Principal geomorphic effects of caldera-forming eruptions in Alaska Effects on glaciers
Effects on edifice morphology
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Tsunamis generated where thick, voluminous pyroclastic flows entered the ocean New water bodies generated where drainages are blocked Temporary dams form that may fail catastrophically
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Upper edifice truncated by circular caldera structure Large lakes or glaciers develop in caldera
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Glacier ice destroyed Upper parts of glaciers truncated by eruptive activity Glacier ice heavily mantled by pyroclastic debris
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Valleys filled with loose pyroclastic debris; drainage networks altered, riparian habitat destroyed Post-eruption incision and erosion contributes substantial amounts of sediment to newly developing stream channels. Sediment flux to the ocean increased significantly
Effects on water bodies
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Effects on stream channels and drainages
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Highlights
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Several Alaska eruptions are the focus of this article, including the 1912 Novarupta-Katmai eruption, the 2008 eruption of Kasatochi volcano in the Aleutian Islands, historical eruptions of Pavlof Volcano, and the 2009 eruption of Redoubt Volcano. These eruptions and their products are reviewed from the perspective of their geomorphic impacts to surrounding environments, although the geomorphic impacts of these events have been addressed in varying degrees of detail. Some eruptions, such as the 1912 Novarupta-Katmai eruption and the Redoubt eruptions of 1989–90 and 2009, have been evaluated comprehensively, whereas others, such as the recent eruptions of Pavlof Volcano have received more limited study due to remote location and logistical problems accessing field sites. Given the constraints imposed by location, weather, and cost of fieldwork, it is not possible to provide the same level of analysis for all of the eruptions described in this article. With these considerations in mind, the objectives of this review are: (1) to address the geomorphic impacts associated with large infrequent eruptions, (2) to address the geomorphic impacts of small, frequent eruptions, (3) to address the geomorphic impacts of large lahars on streams and channel networks, and (4) to briefly describe the geomorphic impacts of tephra fallout. The events discussed illustrate how eruptions of various sizes and characteristics can have significant impacts on the landscape that may persist for decades to millennia.
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S0169-555X(15)30023-4 doi: 10.1016/j.geomorph.2015.06.004 GEOMOR 5245
To appear in:
Geomorphology
Received date: Revised date: Accepted date:
18 November 2014 29 May 2015 2 June 2015
Please cite this article as: Waythomas, Christopher F., Geomorphic consequences of volcanic eruptions in Alaska: A review, Geomorphology (2015), doi: 10.1016/j.geomorph.2015.06.004
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ACCEPTED MANUSCRIPT Geomorphic consequences of volcanic eruptions in
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Christopher F. Waythomas* U.S. Geological Survey, Alaska Volcano Observatory 4210 University Drive, Anchorage, AK 99508 E-mail: [email protected].
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Alaska: a review
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Abstract
Eruptions of Alaska volcanoes have significant and sometimes profound
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geomorphic consequences on surrounding landscapes and ecosystems. The effects of eruptions on the landscape can range from complete burial of surface vegetation and
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preexisting topography to subtle, short-term perturbations of geomorphic and ecological
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systems. In some cases, an eruption will allow for new landscapes to form in response to
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the accumulation and erosion of recently deposited volcaniclastic material. In other cases, the geomorphic response to a major eruptive event may set in motion a series of landscape changes that could take centuries to millennia to be realized. The effects of volcanic eruptions on the landscape and how these effects influence surface processes has not been a specific focus of most studies concerned with the physical volcanology of Alaska volcanoes. Thus, what is needed is a review of eruptive activity in Alaska in the context of how this activity influences the geomorphology of affected areas. To illustrate the relationship between geomorphology and volcanic activity in Alaska, several eruptions and their geomorphic impacts will be reviewed. These eruptions include the 1912 Novarupta-Katmai eruption, the 1989–1990 and 2009 eruptions of Redoubt volcano, the 2008 eruption of Kasatochi volcano, and the recent historical eruptions of Pavlof volcano. The geomorphic consequences of eruptive activity associated with these 1
ACCEPTED MANUSCRIPT eruptions are described, and where possible, information about surface processes, rates of landscape change, and the temporal and spatial scale of impacts are discussed.
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A common feature of volcanoes in Alaska is their extensive cover of glacier ice, seasonal snow, or both. As a result, the generation of meltwater and a variety of
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sediment-water mass flows, including debris-flow lahars, hyperconcentrated-flow lahars,
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and sediment-laden water floods, are typical outcomes of most types of eruptive activity.
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Occasionally, such flows can be quite large, with flow volumes in the range of 107–109 m3. A review of the lahars generated during the 2009 eruption of Redoubt volcano will
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illustrate the geomorphic impacts of lahars on stream channels and riparian habitat. Although much work is needed to develop a comprehensive understanding of the
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geomorphic consequences of volcanic activity in Alaska, this review provides a synthesis
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work on this topic.
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of some of the best-studied eruptions and perhaps will serve as a starting point for future
Keywords: Alaska eruptions, geomorphology, impacts, lahars
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ACCEPTED MANUSCRIPT 1. Introduction Volcanic eruptions in the Aleutian arc of Alaska have had a profound impact on
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the landscape, and throughout this region (Fig. 1) volcanic activity of various magnitudes and frequencies have exerted an important influence on hydrologic, biologic, atmospheric,
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and geomorphic systems (Vanderhoek and Nelson, 2007; DeGange et al., 2010).
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Volcanic activity over long time scales (tens to hundreds of thousands of years) has
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resulted in the growth of many of the major mountains in parts of the Alaska Range, the Aleutian Range, and the Wrangell Mountains. It could be argued that eruptions and their
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aftermath have played a dominant role in shaping the ecosystems and landscapes of southern and southwestern Alaska, and the Aleutian Islands, as volcanoes and their
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eruptive products are the principal landforms of these regions. Volcanoes within the
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Aleutian arc (Fig. 1) have exhibited a wide variety of eruptive styles, ranging from mild
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effusive eruptions to catastrophic caldera-forming events associated with voluminous ash flow sheets and tephra fallout over broad areas of Alaska and the Yukon (Miller and Smith, 1987; Preece et al., 2000). Some of the largest documented eruptions of late Quaternary age worldwide have occurred in Alaska, and some of the most historically active volcanoes in North America also are found here. Given the significant magnitude of many Alaska eruptions and the high frequency of occurrence of eruptive activity, it is worthwhile to examine how eruptive activity and the products of this activity have affected the geomorphic evolution of landscapes throughout the Aleutian arc. This task is practical and academic because of the obvious implications for hazards to people, infrastructure, and the environment and for understanding how volcanic systems evolve in an area that is as geologically dynamic as Alaska.
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observe and evaluate geomorphic processes that operate on newly formed volcanic deposits. In addition to their frequent eruptive activity, most active volcanoes in Alaska
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are large stratocones with significant relief (several thousand meters), and therefore,
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ample potential energy associated with steep flanking slopes is available as a driving
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mechanism for geomorphic processes. As a result of their high relief and northern location, nearly every volcano in Alaska has some amount of ice and snow cover. This
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means that water is readily available, and as a result, a variety of flowing sediment-water mixtures (lahars) can form on the flanks of volcanoes and in surrounding valleys during
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eruptive activity.
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Volcanoes that are characterized by frequent explosive eruptions typically have a
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mantling cover of loose, fragmental pyroclastic debris that is easily eroded by water-rich mass flows. Eruption-related introduction of large volumes of water and sediment into river systems associated with volcanoes in Alaska results in flows that can be several orders of magnitude larger than the largest meteorologically generated floods in respective drainage basins. As will be discussed later, the size, characteristics, and unpredictable occurrence of such flows presents significant challenges for incorporating large lahars into conventional flood-hazard analyses.
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changes to the edifice itself. Throughout the Holocene epoch, a number of caldera- and crater-forming eruptions have been significant, primarily on the Alaska Peninsula (Fig. 1),
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that in addition to producing far-traveled, highly mobile pyroclastic flows (Miller and
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Smith, 1987) and tephra fallout, have resulted in large circular caldera structures and
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volcanic craters. The ca. 8–10-km-diameter caldera’s at Aniakchak, Okmok, and Veniaminof volcanoes all formed during major Holocene eruptions, but afterward, the
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calderas filled with water (Aniakchak and Okmok) or ice (Veniaminof), which set the stage for future geomorphic events associated with the catastrophic drainage of these
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caldera lakes and lava–ice interactions (Waythomas et al., 1996; Wolfe, 2001;
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Waythomas, 2013). Summit craters, such as those at Chiginagak and Kaguyak volcanoes
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(Fig. 1), impound smaller amounts of water, but release of relatively small amounts of water may be devastating to riparian areas downstream, especially if the waters issue from acidic crater lakes (Schaefer et al., 2008). Several Alaska eruptions are the focus of this article, including the 1912 Novarupta-Katmai eruption, the 2008 eruption of Kasatochi volcano in the Aleutian Islands, historical eruptions of Pavlof volcano, and the 2009 eruption of Redoubt volcano. These eruptions and their products are reviewed from the perspective of their geomorphic impacts to surrounding environments, although the geomorphic impacts of these events have been addressed in varying degrees of detail. Some eruptions, such as the 1912 Novarupta-Katmai eruption and the Redoubt eruptions of 1989–1990 and 2009, have been evaluated comprehensively, whereas others, such as the recent eruptions of Pavlof
5
ACCEPTED MANUSCRIPT volcano have received more limited study because of the remote location and logistical problems accessing field sites. Given the constraints imposed by location, weather, and
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cost of fieldwork, it is not possible to provide the same level of analysis for all of the eruptions described in this article. With these considerations in mind, the objectives of
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this review are (i) to address the geomorphic impacts associated with large infrequent
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eruptions, (ii) to address the geomorphic impacts of small, frequent eruptions, (iii) to
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address the geomorphic impacts of large lahars on streams and channel networks, and (iv) to briefly describe the geomorphic impacts of tephra fallout. The events discussed
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illustrate how eruptions of various sizes and characteristics can have significant impacts
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2. Geologic setting
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on the landscape that may persist for decades to millennia.
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The primary late Quaternary volcanic province of Alaska is the Aleutian arc (Fig. 1). The arc is a major geologic feature of the North Pacific region and includes the Aleutian Trench and numerous volcanoes and volcanic islands that extend about 4000 km from Cook Inlet in the east (~152°W) to west of Attu Island (~173°E). The arc consists of two distinct segments, the Aleutian Ridge segment (which includes the Aleutian Islands west of Unimak Pass) and the Alaska Peninsula–Kodiak Island segment that extends east from Unimak Pass (Fig. 1; Vallier et al., 1994). The Aleutian arc is a classic volcanic island arc that is the result of northwestward subduction of the Pacific plate beneath the North American plate at an average convergence rate of 2–9 cm/y (DeMets et al., 1990; Freymueller et al., 2008). In the eastern part of the arc, the direction of convergence is normal to the trench; but in the central Aleutian Islands the direction is oblique. In the
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The oldest volcanic rocks in the arc are middle Eocene age (ca. 38 Ma, Jicha et al., 2006), and there have been three significant periods of arc-wide magmatism at 38–29,
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16–11, and 6–0 Ma (Fournelle et al., 1994; Jicha et al., 2006). Volcanic rocks and
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deposits within the arc range in composition from basalt to dacite and occasionally
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rhyolite (Kay et al., 1982; Fournelle et al., 1994), but basaltic lavas are the most abundant rock composition (Myers, 1988).
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Most of the major stratocones and calderas in the arc are Quaternary in age, and they include some of the highest, largest, and most explosive volcanoes of the entire
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circum-Pacific rim (Miller and Richter, 1994). In the eastern part of the arc, the
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volcanoes and volcanic fields are built on continental crust; whereas the volcanoes of the
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central part of the arc overlie mostly marine sedimentary rocks, and in the western part of the arc, the basement rocks consist mostly of older volcanic rocks (Miller and Richter, 1994: Vallier et al., 1994).
3. Aleutian arc eruptions
The earliest written records of eruptive activity in the Aleutian arc date to the late 1700s, and the first arc-wide synthesis of volcanic activity was compiled by Coats (1950) for the period between 1760 and 1948. Although several volcanoes were explored during scientific expeditions to the region in the early 1900s (Jaggar, 1908; Finch, 1934, 1935), the first systematic evaluation of volcanic activity was that of the U.S. Geological Survey (USGS) from 1945 to 1954. This work was published in a series of USGS Bulletins
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volcanoes that began in the 1970s and the formation of the Alaska Volcano Observatory in the late 1980s, which provided impetus for a more regional understanding of Aleutian
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arc volcanism and its implications for hazards.
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3.1. Historical eruptions
The Aleutian arc contains at least 52 volcanoes that have had some type of
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documented or suspected eruptive activity in about the past 300 years (Fig. 1). These volcanoes are generally known as the historically active volcanoes of Alaska, and 257
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eruptions are documented over this period. An additional 30 volcanoes have had
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documented eruptive activity within the Holocene epoch (Fig. 1). The average frequency
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of historical eruptions is 1–2 per year, and the majority of events (about 78 %) rank as 2 or 3 on the Volcanic Explosivity Index scale of Newhall and Self (1982). The VEI scale is based primarily on the volume of material erupted and the maximum height above sea level (asl) attained by the eruption cloud. The scale ranges from 0, for effusive lavaproducing eruptions, to 8 for catastrophic caldera-forming eruptions such as the Toba and Taupo super eruptions (Ninkovich et al., 1978; Wilson and Walker, 1985). The VEI scale is logarithmic, and each interval on the scale indicates a tenfold increase in erupted volume, except for VEI 0–2 events. Roughly 20 % of documented historical eruptions in Alaska are effusive events of VEI 0–1 in magnitude. The VEI 2 eruptions produce > 1,000,000 m³ of ejecta, and plume heights range from 1–5 km asl. About half of the documented historical eruptions in Alaska are VEI 2 magnitude events. The VEI 3 events
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ACCEPTED MANUSCRIPT produce > 10,000,000 m³ of ejecta and have associated ash cloud heights of 3–15 km asl. About 25 % of the historical eruptions in Alaska are VEI 3 in magnitude. The VEI 4
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events erupt >0.1 km3 of ejecta and generate ash clouds that reach 10–25 km asl. Historical VEI 4 eruptions in Alaska are rare, and only about 5 % of documented
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historical eruptions are of this magnitude. Only one historical eruption was larger than
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VEI 4 and that was the VEI 6 1912 Novarupta-Katmai eruption (Hildreth and Fierstein,
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2000).
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3.2. Eruptions of Holocene age
The Holocene eruptive history of volcanoes in the Aleutian arc is known
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primarily from study of volcanic ash deposits and proximal unconsolidated volcaniclastic
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deposits on the flanks of volcanoes (Riehle, 1985; Miller and Smith, 1987; Waythomas,
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1999; Stelling et al., 2005; Fierstein, 2007). Some of the largest Holocene eruptions on Earth have occurred in the Aleutian arc and resulted in the formation of several noteworthy calderas (Table 1). Many of these eruptions generated >50 km3 of bulk eruptive material (Miller and Smith, 1987) and had substantial and long-lasting impacts on the surrounding landscape (Vanderhoek and Nelson, 2007). Although the status of the Holocene eruptive record continues to evolve as new work is accomplished throughout the arc, at present more than half of the volcanoes with known or suspected Holocene eruptive activity (Fig. 1) have no published studies documenting this eruptive history. As a result, addressing long-term (thousands of years) volcanic unrest from a frequency – magnitude perspective is difficult and may lead to an over emphasis of the significance of the historical eruptive record.
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ACCEPTED MANUSCRIPT 3.3. Caldera-forming eruptions Major eruptions associated with caldera formation have occurred during the
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Holocene epoch at a number of volcanoes throughout the Aleutian arc. Aniakchak, Veniaminof, and possibly Fisher have experienced multiple caldera-forming eruptions
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within the late Quaternary, although the chronology of these events remains somewhat
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uncertain. Here the focus is on the most recent of these eruptions, which all have
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occurred in Holocene time (Table 1). Of the eruptions listed in Table 1, little is known about the physical volcanology and chronology of the caldera-forming eruptions at
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Semisopochnoi and Yunaska volcanoes, and the most recent published work at these locations dates to the 1950s and 1980s (Coats, 1950, 1959; DeLong et al., 1985). All of
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the other volcanoes have received some study within the past 15 years, although a
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number of questions and uncertainties remain about the chronology and volcanological
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characteristics of the caldera-forming eruptions at Hayes, Black Peak, and Semisopochnoi.
The twelve calderas described in Table 1 are roughly circular in planview shape (Fig. 2A) and fall into two groups when ranked according to planview caldera area (Fig. 2B). Semisopochnoi, Okmok, Fisher, Veniaminof, and Aniakchak all have caldera areas >40 km2; Fisher caldera clearly has the largest area at about 140 km2. The calderas at Yunaska, Makushin, Akutan, Black Peak, Katmai, Kaguyak, and Hayes are all much smaller and have planview areas of about 10 km2 or less.
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ACCEPTED MANUSCRIPT In general, caldera size is a proxy for bulk eruptive volume produced during the most recent caldera-forming eruption. However, in many cases the volume of material
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produced by these eruptions is uncertain and estimates can vary by at least an order of magnitude (Table 1). Furthermore, the present caldera structure may not be the result of
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the most-recent caldera-forming eruption, and parts of the caldera could have formed
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during older eruptions. Thus, it is not possible to estimate the volume of eruptive material
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based on caldera area alone as the relation is poorly constrained because of large uncertainties in the eruptive volume estimates.
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Because of their large eruptive magnitude (>VEI 5), caldera-forming eruptions have significant geomorphic impacts on surrounding landscapes due to the emplacement
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of pyroclastic fall and flow deposits. Among the main effects of these types of eruptions
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is the generation of highly mobile ash-flow sheets that extend for many tens of kilometers
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beyond the caldera (Miller and Smith, 1977, 1987) and widespread tephra fall out (Preece et al., 2000). The eruption of aerially extensive pyroclastic flows during large calderaforming eruptions mantles the topography with sometimes very thick (tens of meters), valley-filling, pyroclastic-flow deposits (Fig. 3). Emplacement of these deposits reshapes the landscape and results in broad, flat-floored valleys and sediment aprons abutting higher terrain. At some volcanoes, such as Veniaminof, the ash-flow deposits were emplaced hot enough to weld and form indurated columnar jointed accumulations along pre-eruptive channels and valleys. Welded ash-flow deposits are resistant to fluvial erosion and have resulted in the formation of narrow, slot canyons in a number of areas where post-eruption incision has occurred (Fig. 3). Because stream channels and preexisting valleys are typically filled with pyroclastic debris during these large eruptions,
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ACCEPTED MANUSCRIPT a subsequent period of erosion occurs and this results in channel incision, removal of loose pyroclastic debris, and transport of this material to the ocean. Although not well
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documented, the mouths of larger rivers that drain these pyroclastic-flow affected areas are characterized by substantial volumes of deltaic material derived from erosion of the
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pyroclastic debris.
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The emplacement of these large volume ash-flow deposits creates extensive areas
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of bare ground that are highly susceptible to wind erosion. Resuspension of volcanic ash from fresh pyroclastic-flow surfaces results in localized ash fallout of reworked ash and
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visible ash plumes that are similar to primary ash clouds (Hadley et al., 2004). If a disturbance regime persists it may be centuries to millennia before pyroclastic flow
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surfaces become sufficiently stabilized by vegetation to inhibit eolian erosion.
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Tephra fallout during caldera-forming and other large explosive eruptions can
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cover many thousands of square kilometers with millimeters to centimeters of fine ash that typically decreases in thickness and particle size with distance from source (Preece, et al., 1999; Westgate et al., 2000; Pearce et al., 2004; Wallace et al., 2013). Because the aerial extent of ash fall associated with caldera-forming eruptions is typically widespread, the geomorphic effects should be significant (Jensen et al., 2014). Aerially extensive ash fall deposits associated with caldera-forming eruptions, such as the Dawson and Old Crow tephras of late Pleistocene age (Froese et al., 2002) and the Aniakchak tephra of late Holocene age (Begét et al., 1992), likely had significant effects on the landscape across Alaska resulting from burial of vegetation, changes in surface albedo, and the introduction of acidic volcanic aerosols into surface water (Blackford et al., 2014). To
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ACCEPTED MANUSCRIPT date however, the specific effects of widespread ash fall associated with caldera-forming eruptions in Alaska are not known.
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The process of caldera formation also results in substantial physical changes to the volcanic edifice itself (Table 2). Chief among these physical changes is the removal
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of the upper third or so of the volcanic edifice resulting in the circular, basin-like caldera
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structure. Several Holocene age calderas in Alaska (Aniakchak, Okmok) supported large
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temporary lakes that were removed when their caldera rim dams failed, resulting in spectacular dam-break floods (Waythomas et al., 1996; Wolfe, 2001). Other calderas,
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such as Veniaminof, became filled with glacier ice.
All calderas of late Holocene age in Alaska have experienced significant post-
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caldera volcanism, ranging from mild, effusive, lava-producing events, to more explosive
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eruptions involving silicic magmas (Bacon et al., 2014). As a result of this activity, a
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diverse assemblage of cones, flows, and vents characterizes the floors of most Alaska calderas. Many post-caldera eruptive products reflect magma-water interaction, as the calderas themselves are basin-like structures that contain shallow groundwater, glacier ice, or in some cases, small lakes. All of the caldera structures described above formed as a result of substantial magma withdrawal, which led to the eventual downward collapse of the remaining volcanic edifice and thus are known as collapse calderas. Less common in Alaska are caldera structures that formed as a result of large (>1 km3) volcanic landslides where the headscarp of the landslide is known as an avalanche caldera. The caldera at Mount Spurr volcano is such a structure, and it formed during a major sector collapse of the volcano possibly in mid- to early Holocene time (Waythomas, 2007).
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ACCEPTED MANUSCRIPT 4. Geomorphic effects at frequently active volcanoes As discussed previously, Alaska contains some of the most frequently active
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volcanoes in North America. The volcanoes with the greatest number of conclusively documented historical eruptions are Pavlof (>40 eruptions), Akutan (32 eruptions),
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Cleveland (>25 eruptions), and Shishaldin (>25 eruptions). Pavlof, Cleveland, and
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Shishaldin are steep-sided stratocones that are mantled by loose pyroclastic debris up to
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several meters thick and rubbly aa lava flows that are largely unvegetated. Of these, Pavlof has received the most study and its eruptions are good examples of the types of
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geomorphic processes that occur at Aleutian arc volcanoes that erupt frequently. Pavlof has erupted more than 40 times since the early 1800s, and its most recent
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eruption occurred in November 2014 (http://www.avo.alaska.edu/volcanoes/ volcinfo.
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php? volcname=Pavlof). The volcano is a 2518-m-high, ice- and snow-covered
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stratocone within the Emmons Lake volcanic center (Mangan et al., 2009) and is located on the southwestern part of the Alaska Peninsula (Fig. 1), a remote, uninhabited area near Izembek National Wildlife Refuge. The volcano is situated on the outer rim of one of the large mid-Pleistocene caldera structures that characterize the Emmons Lake volcanic center, and most of the edifice is a post-caldera feature (Mangan et al., 2009). Historical eruptions of Pavlof have been characterized by moderate amounts of ash emission, lava fountaining, and spatter accumulation on the upper flanks of the volcano and the generation of watery hyperconcentrated-flow lahars (Waythomas et al., 2014). Since the early 1970s, when seismic instruments were first installed on the volcano, eruptive episodes characterized by periods of lava fountaining and strombolian explosions have been more quantitatively documented (McNutt, 1987; McNutt et al., 1991), and eruptive
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ACCEPTED MANUSCRIPT products have been studied to a limited degree during brief field visits to the volcano (Waythomas et al., 2008, 2014; Mangan et al., 2009).
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Typical Pavlof eruptions are characterized by moderate levels of intermittent VEI 2 strombolian activity and occasional more energetic vulcanian explosions (VEI 3) that
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have generated ash plumes reaching as high as 16 km asl (McNutt et al., 1991). Eruptions
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typically last from a week up to several months, although the 1986–1988 eruption lasted
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850 days (McNutt et al., 1991). The intensity of eruptive activity can be quite variable and episodic. Typically, sustained bursts of activity lasting for several hours are followed
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by periods of lower level unrest, which can last for days to weeks (McNutt et al., 1991; Waythomas et al., 2014).
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Most of the material erupted from Pavlof is andesite to basaltic andesite in
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composition (Mangan et al., 2009), and lava flows on the upper flanks of the cone consist
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almost entirely of fountain-fed flows and nondescript piles of agglutinate, spatter, and coarse tephra. The volcano supports a relatively extensive cover of glacier ice and perennial snow, and the combined snow and ice volume is about 1–2 km3. The lower slopes of the volcano are largely unvegetated, and a several-meter-thick mantle of loose, easily erodible pyroclastic debris is present around the volcano, which obscures the underlying glacier ice in many areas (Fig. 4). Meltwater produced during eruptions by the interaction of hot granular rock debris with ice and snow initiates lahars, and all of the drainages that originate on the volcano have been inundated repeatedly by lahars during historical eruptions (Fig. 5). Some of these lahars have been relatively extensive, at times reaching the Bering Sea about 40 km north of the volcano. Lahar deposits associated with older Holocene eruptions form prominent terraces along many of the main drainages (Fig.
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ACCEPTED MANUSCRIPT 6). The frequency of eruption-induced lahars and tephra fallout has provided a steady supply of volcaniclastic sediment to the drainages on the volcano, which has affected
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riparian habitat, limited vegetation growth, and resulted in episodic pulses of aggradation and incision. Meltwater generated by eruptive activity results in the reactivation of
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ephemeral channel networks and distributary channels on the distal slopes of the volcano.
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This can result in minor erosion of stream channels and progradation of lahar deposits
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over vegetated areas or across perennial streams causing short-term blockages and debris dams. Drainages affected by the combined input of tephra fallout and lahar inundation
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are in a long-term disturbance regime, and drainage systems may not reach a condition of relative channel stability until eruptive activity declines dramatically or when the volcano
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becomes dormant.
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5. Geomorphic consequences of lahar inundation of stream channels and valleys Volcanic mass flows of all types are commonly emplaced in valleys and stream channels on the flanks of many Alaska volcanoes. The generation of lahars and other types of gravity-driven mass flows (pyroclastic flows, debris avalanches) are typical outcomes of eruptive activity, and most of the major drainages on volcanoes contain mass-flow deposits. Typically, the most voluminous lahars that form during eruptions in Alaska are those that are generated by pyroclastic flow interaction with ice and snow. Such flows can have volumes in the range of 107–108 m3 (Waythomas et al., 2013), and lahars of this size can have dramatic impacts on the valleys they inundate. The most significant impacts include complete burial of valley bottoms with sometimes thick accumulations of unconsolidated deposits of gravel, sand, and silt; the diversion of water
16
ACCEPTED MANUSCRIPT flow paths; and the destruction of riparian habitat (Dorava and Meyer, 1994; Waythomas et al., 2013). Lahars at Alaska volcanoes may form by other mechanisms, such as
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drainage of crater lakes (Schaefer et al., 2008) or as a result of intense rainfall; however, these types of events occur rarely in comparison to lahars that form during eruptions.
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Large, valley-filling lahars change the valley-bottom morphology and produce flat,
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gently sloping surfaces underlain by meters to tens of meters of volcaniclastic fill. As the
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lahars are emplaced, they can displace or bury stream networks, and in some settings will cause complete or partial blockage of streams and rivers resulting in the formation of
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lahar-dammed lakes (Fig. 7; Waythomas, 2001). In some locations lahars are significant hazards, but because of the remote setting of volcanoes in Alaska, little infrastructure is
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located in lahar hazard zones (with a few exceptions); and thus the potential threat of
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lahars is much lower than in many other areas of the world where lahars develop. Lahar
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inundation does affect riparian habitat, and drainages on volcanoes that support significant runs of anadromous fish are vulnerable to the effects of lahars (Schaefer et al., 2008).
5.1. Eruption-induced lahars
Most of the large hazardous lahars that develop during eruptive activity in Alaska are initiated by the interaction of pyroclastic flows with snow and ice on the volcano. Pyroclastic density currents produced by collapse of unstable lava domes or collapse of particle-laden eruption columns can scour, entrain, and melt snow and ice and usually result in the formation of large amounts of meltwater. Meltwater generated by this process will entrain loose pyroclastic debris and other types of unconsolidated sediment
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ACCEPTED MANUSCRIPT from the flanking slopes of the volcano and form lahars that may travel many tens of kilometers from the eruption site (Pierson, 1998). Of the Aleutian arc volcanoes that have
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been active within the Holocene (roughly 70 volcanoes), about 50 % of them support glacier ice and all of them are seasonally snow covered (Fig. 8). At volcanoes where ice
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volume estimates have been made, the ice cover typically exceeds 1 km3 (Trabant and
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Hawkins, 1997; Welch et al., 2007). The ubiquitous nature of snow and ice means that
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lahars are a common outcome of explosive eruptive activity throughout the Aleutian arc.
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5.1.1. Eruption-induced lahars at Redoubt volcano
The best-studied eruption-induced lahars in Alaska are the lahars and associated
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deposits in the Drift River valley generated during eruptions of Redoubt volcano.
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Redoubt volcano is located in the Cook Inlet region of south-central Alaska (Fig. 1) and
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is an andesite to dacite, glaciated stratocone that erupted most recently from March–July 2009 (Schaefer, 2012). The volcano supports about 4 km3 of glacier ice and perennial snow (Trabant and Hawkins, 1997), about 1 km3 of which makes up the Drift glacier on the north flank of the volcano (Fig. 9). All of the known historical eruptive activity, and probably several prehistoric eruptions, have occurred from vents within the breached, 1 x 2 km diameter, ice-filled summit crater at the head of Drift glacier (Fig. 10). During the past three historical eruptions (1966–1968, 1989–1990, 2009), multiple large lahars flowed through the Drift River valley on the north side of the volcano (Sturm et al., 1986; Dorava and Meyer, 1994; Waythomas et al., 2013) and inundated significant parts of the valley floor and associated distributary channels in the lower part of the drainage (Fig. 15). The lahars produced during the 1989–1990 and 2009 eruptions posed a significant
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ACCEPTED MANUSCRIPT hazard to oil-production infrastructure located near the mouth of Drift River and directly threatened the Drift River Marine Terminal (DRMT; Fig. 9) and a buried pipeline that
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delivers oil to the terminal from the north. These facilities are important oil storage and transfer infrastructure in Cook Inlet, and more than 100,000 barrels of crude oil are
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typically stored at the terminal at any given time (Dorava and Meyer, 1994; Waythomas
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et al., 2013).
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Extensive inundation of the Drift River valley by lahars was first documented during an explosive eruption on 25 January 1966 when a lahar, assumed to be at least 106
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m3 in volume (Riehle et al., 1981; Sturm et al., 1986) was likely triggered by a pyroclastic flow. This lahar inundated the Drift River valley floor (actual extent of
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inundation not known), transporting blocks of ice many meters in diameter and forcing
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the evacuation of a survey crew working at the future site of the DRMT (Waythomas et
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al., 2013). A second large lahar was generated on 9 February 1966, but apparently it contained little or no ice (Riehle et al., 1981). Both lahars had local flow depths of at least 4–6 m, but their volumes and peak discharges are unknown. During the 1989–1990 eruption, at least 18 lahars were generated by vigorous vent explosions or pyroclastic flows associated with the collapse of lava domes (Brantley, 1990). Three of these lahars were large enough to inundate the Drift River valley floor (about 100 km2), including the DRMT and vicinity (Dorava and Meyer, 1994). The lahar of 2 January 2 1990 was the largest of the 1989–1990 eruption and was initiated by a meltwater flood of about 25 million m3 (Dorava and Meyer, 1994; Trabant et al., 1994). The meltwater entrained pyroclastic debris, supraglacial debris, and alluvium and transformed to a watery debris-flow lahar with a volume of about 107–108 m3 (Dorava
19
ACCEPTED MANUSCRIPT and Meyer, 1994; Gardner et al., 1994). The estimated peak discharge of the 2 January 1990 lahar was 16,000–80,000 m3s-1 at a valley cross section in the upper Drift River
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valley located about 2.5 km downstream from the terminus of Drift glacier (Dorava and Meyer, 1994). This lahar transported ice blocks up to 8 m in diameter as far as Cook Inlet
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and inundated the entire Drift River valley and most of the distributary channel network
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of the lower Drift River valley (Fig. 9). A lahar generated on 15 February 1990 was
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smaller and more dilute than the January 2 flow, but it also inundated much of the Drift River valley and the DRMT area and vicinity (Dorava and Meyer, 1994).
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The sediment delivered to the lower Drift River valley by the 1990 lahars caused aggradation of the valley floor and lateral shifts in the position of the active channel.
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During July–August 1990, the Drift River avulsed northward from its main channel into
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adjacent Montana Bill Creek (Fig. 11B), which thereafter conveyed an estimated 70–
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90 % of the flow of Drift River (Dorava and Meyer, 1994). This resulted in several meters of bed scour along lower Montana Bill Creek, exposing the buried oil pipeline that connects with the DRMT.
During the most recent eruption of Redoubt Volcano between 22 March and 4 April 2009, explosive eruptive activity destroyed several lava domes and triggered four large lahars in the Drift River valley (two on March 23, one on March 26, and one on April 4) and several smaller lahars between March 23 and March 30 (Schaefer, 2012; Waythomas et al., 2013). The lahars generated on March 22–23 and April 4 inundated the upper and middle reaches of the Drift River valley and a substantial area of the lower Drift River valley (Fig. 15D). The lahars of March 22–23 and April 4 also reached the DRMT and overtopped and flowed around protective dikes surrounding the oil storage
20
ACCEPTED MANUSCRIPT tanks. The lahars deposited significant amounts of sediment, ice, and debris that severely impacted operations at the facility but did not result in any oil spills (Schaefer, 2012).
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All of the lahars triggered by the 2009 eruption were detected in seismic data (Buurman et al., 2013), but because of the remote location of the volcano, none of the
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lahars were observed directly. Aerial observations and occasional remote camera images
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provided confirmation of several lahar events, which are described in greater detail in
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Waythomas et al. (2013).
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5.1.2. Lahars and lahar deposits associated with the 2009 Redoubt eruption Twenty discrete lahars were identified in seismic data during the roughly three-
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week long period of explosive eruptive activity that characterized the 2009 Redoubt
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eruption. The lahars that formed on March 22–23 and April 4 were the largest of the 2009
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eruption, and deposits produced by these flows were easily recognized throughout the Drift River valley. Although other smaller lahars formed during the eruption, associated deposits could not be identified in the field because they were either eroded away by subsequent flows or were indistinguishable from the April 4 deposits. The final lahar of the eruption on April 4 covered all previously emplaced deposits with sand to fine gravel more than 1 m thick.
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ACCEPTED MANUSCRIPT Ice-rich lahar deposits generated by the March 22–23 eruptive activity were deposited throughout the Drift River valley. At the DRMT, the lahar deposits were about
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2 m thick and reached the top of the protective levees on the northwestern side of the DRMT tank farm (Waythomas et al., 2013). Elsewhere in the Drift River valley, the
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March 22–23 lahars produced ice-rich deposits, 1–4 m thick that also contained variable
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amounts of vegetation, trees, and minor amounts of sediment. Clasts of ice, some up to
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several meters in diameter, formed the bulk of the lahar deposit (Fig. 12; Waythomas et al., 2013). In many parts of the Drift River valley, the ice-rich lahars abraded and
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removed bark from alders, willows, and the trunks of mature cottonwood trees to a height of several meters above the top of resulting deposits. Many 2–3 m high stands of willow
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and alder were knocked over and buried by the ice-rich flows. As the lahar passed
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through the Drift River valley, it left prominent horizontal mud and debris lines many
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meters above the active channel that were clearly evident along the valley walls and indicated flow depths of 6–8 m above the valley floor (Waythomas et al., 2013). At Dumbbell Hill, a bedrock knob in the Drift River channel about 3 km downstream from the terminus of Drift glacier (Fig. 9), ice-rich lahar deposits, including frozen blocks of alluvium, were emplaced on the upstream (west) side of the hill about 13–15 m above the valley floor. In this area, blocks of ice up to 10 m in length were scattered about on the valley floor (Waythomas et al., 2013). As a result of the March 22–23 lahars, the main channel of the Drift River along the north side of the DRMT became choked with ice-rich lahar deposits, and the active channel of the Drift River was diverted southward into the Rust Slough-Cannery Creek drainage (Fig. 11E).
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ACCEPTED MANUSCRIPT Although seismic data and aerial observations indicated that at least three lahars developed on March 22–23, later field studies revealed only a single, ice-rich deposit (Fig.
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12). The deposits produced by the March 22–23 lahars primarily consisted of 2–4 m thick accumulations of interlocking, framework-supported, tabular ice blocks (some several
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meters in length) and subangular to rounded ice cobbles (Fig. 12). Lithic material in these
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deposits (including river gravel from the valley floor) was rare. Only minor amounts of
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silt, sand, gravel, and rock debris were evident in the lahar deposits examined, no juvenile rock clasts were observed, and the composite deposit was 80–100 % ice (Waythomas et
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al., 2013).
The lahars that formed on March 22–23 were probably initiated by meltwater
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surges during the initial period of eruptive activity that marked the beginning of the
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explosive phase of the 2009 eruption (Waythomas et al., 2013). The scenario envisioned
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is that meltwater ran out over a frozen and snow-covered Drift River valley floor, and as a result, the flow must have rapidly bulked up with ice and snow. Such a flow probably behaved like a large icy slurry, possibly analogous to the ice-rich flows that develop during ice-jam floods associated with breakup of river ice (Jasek, 2003). Visits to the Drift River valley in 2010, 2011, and 2012 revealed that complete melting of the 22–23 March 2009 ice-rich lahar deposit resulted in a 1–6 cm thick bed of silt to fine sand (Fig. 12C) that exhibited no obvious textures or characteristics indicative of a very large lahar. At Redoubt and other volcanoes where similar icy flows develop, the meltout deposits of ice-rich lahars may be difficult to recognize in the geologic record because they are likely to be only a few centimeters in thickness and inconspicuous relative to the deposits produced by more typical lahars with high sediment
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ACCEPTED MANUSCRIPT concentrations, whose deposits are thicker (several meters or greater) and more likely to be preserved. The thin meltout deposits derived from ice-rich lahars may be difficult to
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identify and easy to overlook, or possibly may not be preserved at all in the geologic record despite being the products of significant and very voluminous lahars.
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The final lahar of the 2009 Redoubt eruption formed on April 4 and was the most
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extensive lahar of the eruption. This lahar inundated about 125 km2 of the Drift River
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valley, an area roughly 20 % greater than that inundated by the March 22–23 lahar (Waythomas et al., 2013). The April 4 lahar was initiated by a major explosive event that
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resulted in the destruction of the lava dome that had grown in the summit crater of the volcano (Bull et al., 2013). The April 4 lahar completely inundated the upper and middle
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reaches of the Drift River valley floor and about 80 % of the lower Drift River valley
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including the DRMT area (Fig. 13). The April 4 lahar deposits were primarily sandy,
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hyperconcentrated-flow deposits but also contained trees, scattered blocks of ice, and other vegetal debris. At peak flow, the April 4 lahar reached the top of the containment levees on the northwest side of the DRMT tank farm and deposits in this area were about 1 m thick. Most of the flow was conveyed by channels of the Montana Bill Creek drainage north of the DRMT and by the Rust Slough drainage south of the DRMT (Fig. 13). Minor flow along the main channel of the Drift River was reestablished temporarily, but nearly all of the active flow subsequent to the emplacement of the April 4 lahar was within the Rust Slough drainage (Fig. 13).
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ACCEPTED MANUSCRIPT In contrast to the March 22–23 lahar deposits, the April 4 deposits contained only minor amounts of ice (Fig. 14). Most of the ice in the deposit was present only in the
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upper part of the Drift River valley where large subrounded blocks of glacier ice scoured from Drift glacier (some as large as 200–300 m3), and some meter-sized tabular clasts of
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river ice (possibly reworked from the March 22–23 deposits) were evident on the surface
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of the lahar deposit. In most areas of the Drift River valley, the April 4 lahar deposits
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consisted almost entirely of massive to horizontally stratified, poorly sorted sand to fine gravel. Rock material in the deposit was primarily juvenile andesite derived from the lava
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dome that was destroyed by the April 4 explosion (Coombs et al., 2013). The lahar deposits emplaced on April 4 ranged in thickness from 1–6 m and were thickest in slack
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water areas along the valley margin. Massive, clast-supported, gravel-rich lahar deposits
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1–2 m thick were observed in the upper Drift River valley, indicating that at least locally
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the lahar had achieved debris-flow sediment concentrations prior to transformation to hyperconcentrated flow (Waythomas et al., 2013). Many of the April 4 lahar deposits examined in the lower Drift River valley contained water-escape structures and soft sediment deformation features indicative of rapid deposition by water-rich hyperconcentrated flows (Pierson, 2005).
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ACCEPTED MANUSCRIPT 5.1.3. Flow characteristics of the 2009 Redoubt lahars The volumes of the largest lahars of the 2009 Redoubt eruption range from 107–
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108 m3, and the estimated peak discharges were in the range of 104–105 m3s-1 (Waythomas et al., 2013). The 2009 lahars all had volumes similar to the largest lahar of
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the 1989–1990 eruption. Lahars of this magnitude or possibly greater probably occurred
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several times during prehistoric eruptions of Redoubt volcano. Stratigraphic data from the
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Drift River valley indicates at least two older lahar deposits of widespread extent suggesting a similar degree of inundation during eruptions of late Holocene age (Begét
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and Nye, 1994; Waythomas, unpub. data). The two largest lahars of the 2009 Redoubt eruption occurred at the beginning and at the end of the period of explosive eruptive
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activity, whereas the largest lahars of the 1989–1990 eruption occurred only at the
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beginning of explosive activity and became smaller in volume as the eruption progressed.
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The flow characteristics of the March 22–23 and April 4 lahars were significantly different. These lahars all originated from meltwater surges generated by explosive eruptive activity; the March 22–23 lahars being composed of >80 % ice and leaving a deposit that today is barely recognizable as the product of a large volume lahar, whereas the April 4 lahar was a water-rich hyperconcentrated flow lahar with <10 % ice content. The April 4 lahar produced a deposit of sand and fine gravel several meters in thickness that is evident throughout the Drift River valley today.
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ACCEPTED MANUSCRIPT 5.1.4. Erosion, channel changes, and context of the 2009 Redoubt lahars Substantial quantities of sediment were mobilized by lahars in the Drift River
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valley during the 2009 eruption, and this resulted in significant changes to the valley bottom geomorphology. By the summer of 2010, as much as 10 m of incision through the
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2009 lahar deposits and the underlying alluvium had occurred in the upper Drift River
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valley. The upstream channel erosion contributed additional sediment to the lower Drift
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River valley, which caused active channels in the lower part of the valley to aggrade by several meters. As erosion of the 2009 deposits in the upper and middle reaches of the
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Drift River valley continues, downstream aggradation and lateral shifting of channels will likely continue for many years. At the time of this writing, all of the flow in the Drift
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River is within the Rust Slough drainage south of the DRMT. The main Drift River
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and debris (Fig. 11E).
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channel on the north side of the DRMT remains abandoned and choked with sediment
The lahars that formed in the Drift River valley during eruptions of Redoubt volcano are considerably larger than flows that result from rainfall or snowmelt runoff. Documented flood peaks in a variety of watersheds throughout south-central Alaska provide a general estimate of the largest meteorologically generated flows possible in the region (Jones and Fahl, 1994). An envelope curve for the relation between peak discharge and drainage basin area (Waythomas et al., 2013; Fig. 20) indicates that for the Drift River drainage (drainage area = 570 km2) the maximum peak discharge is about 2000 m3s-1. The estimated 100-year and 500-year-flood peak discharges for the Drift River drainage are about 1000 and 1300 m3s-1, based on regional flood-frequency equations described in Curran et al. (2003). These values are tens to hundreds of times smaller than
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ACCEPTED MANUSCRIPT the peak flow estimates of the largest lahars of the 1989–1990 and 2009 eruptions. The estimated peak discharge of the 4 April 2009 lahar near the DRMT was 3000–33000
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m3s-1; these values are 3–30 times larger than the estimated 100-year flood peak
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constrained by regional flood-frequency relations (Waythomas et al., 2013).
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6. Geomorphic effects at volcanoes that erupt infrequently
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Another category of volcanoes in Alaska are those that have erupted after a period of repose of hundreds to thousands of years, including volcanoes with a limited or no
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known record of historical eruptive activity. An example of a volcano that has had a major eruption after a long repose period is Kasatochi volcano, a 3-km-diameter, 300-m-
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high, isolated island volcano located in the southern Bering Sea in the central Aleutian
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Islands (Fig. 1). The eruption of Kasatochi volcano in 2008 has provided an opportunity
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to observe erosion-related changes in the geomorphology of Kasatochi Island following a significant landscape-altering geologic event. The effects of the eruption are significant because the ecosystem of the island was drastically altered by the accumulation of thick pyroclastic deposits over the entire island (Fig. 15). The opportunity to document the erosion and redistribution of volcaniclastic deposits and to address ecosystem recovery on Kasatochi using modern techniques is unprecedented for Alaska (DeGange et al., 2010; Walker et al., 2013).
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ACCEPTED MANUSCRIPT Prior to its 2008 eruption, little was known about the eruptive history of the volcano. Pyroclastic-flow and -surge deposits, exposed in sea cliffs and in the upper walls
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of the crater (Figs. 15A, B) probably record at least one or more explosive eruptions within the past 1000 years that may have been similar in scale to the 2008 eruption
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(Waythomas et al., 2010b). The only known historical eruption of Kasatochi occurred on
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7–8 August 2008 (Waythomas et al., 2010a). The VEI 4 eruption covered all of
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Kasatochi Island with tens of meters of pyroclastic debris consisting of granular pyroclastic-flow, -surge, and fine ash deposits (Fig. 15C). The emplacement of
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pyroclastic-flow deposits off the flanks of the volcano extended the coastline into the sea by as much as 400 m, increasing the area of the island by about 30 % (Waythomas et al.,
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2010a).
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The pre-eruption vegetation on Kasatochi Island consisted of a mosaic of mesic
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herbs, grasses, and alpine heath and dense dune grass meadows along the shoreline (Walker et al., 2013). The near-shore marine environment included abundant brown, red, and green algae and a mature kelp canopy (Jewett et al., 2010). All of the vegetation on the island and in the near shore zone to about the 20-m isobath was buried by pyroclasticflow, -surge, and tephra deposits (Waythomas et al., 2010a). Pyroclastic-flow deposits examined about two weeks after the eruption were clearly hot (Fig. 16A), but in some areas the deposits contained unburned organic matter, twigs, roots, and chunks of turf (Fig. 16B) indicating that local emplacement temperatures were relatively cool. The emplacement of low-temperature pyroclastic flows may have been the result of mixing of eruptive material with water that was resident in the Kasatochi crater lake (Fig. 15B), all of which was likely expelled during the 21-hour-long eruption. Where the 2008 deposits
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ACCEPTED MANUSCRIPT were thin and emplaced cool, pockets of the underlying vegetation survived and are now serving as nuclei for regrowth of vegetation (Walker et al., 2013). These areas are very
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restricted around the island and the amount of new vegetation emerging in these areas is limited.
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Because of the amount of pyroclastic-flow material and ash emplaced by the
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eruption, all preexisting stream channels, valleys, talus, beaches, and other geomorphic
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surfaces were buried. Following the eruption, surface erosion processes were free to operate on a landscape devoid of vegetation and characterized by thick accumulations of
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loose, easily erodible volcaniclastic deposits (Fig. 16). Gully erosion on the flanks of the volcano began immediately after the eruption ended and led to the generation of
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numerous narrow, several-meter-deep gullies (Fig. 17) on most parts of the island, and
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the drainage density increased by a factor of about 20 in one year (Waythomas et al.,
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2010b). Wave erosion of pyroclastic material along the coastline occurred rapidly, and coastal erosion rates, as defined by the landward retreat of wave-cut cliffs, were as high as 150 my-1 (Fig. 17; Waythomas et al., 2010b). The main impact of surface erosion by gullying and wave erosion of the coastline was the introduction of sediment into the nearshore zone, which suppressed the growth of kelp and other marine biota for several years after the eruption. Continued erosion of the pyroclastic deposits on the island is expected to continue for some time, possibly for decades to centuries, until the gullies progress to a stable configuration and vegetation is able to colonize the surface (Fig. 18). As of 2014, the coastline along the northern half of the island has nearly returned to its pre-eruption configuration, and rates of coastal erosion have declined markedly or ceased in some areas where the coastline has reached bedrock. The exhumation of talus buried by
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ACCEPTED MANUSCRIPT pyroclastic debris is continuing, and seabirds that nest within accumulations of talus, such as Auklets, have not abandoned the island, although it is difficult to know how the loss of
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nesting habitat has affected their population. Formation of new talus at the base of steep bedrock cliffs is occurring locally (Figs. 17C, D) and such areas are being highly utilized
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7. Geomorphic impacts of tephra deposits
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by nesting seabirds (G. Drew, USGS, pers. comm, 2014).
Tephra deposits are common products of explosive eruptions and result from the
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fallout of particles in the eruption cloud through the atmosphere to the ground surface where they typically form a blanketing cover over the landscape. Tephra deposits can
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possess a range of particle sizes from extremely fine ash (<63 μm diameter) to ballistic
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particles up to several meters in diameter (White and Houghton, 2006). The accumulation
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of tephra generated by sedimentation from volcanic eruption clouds typically exhibits roughly exponential changes in thickness and particle size with distance from source (Bonadonna and Costa, 2013), and because of this phenomena the most significant impacts from tephra fallout occur in the proximal to medial areas around the volcano (generally 5–10 km from the vent). The volume of tephra produced by a given eruption is a key parameter in determining eruption magnitude and assigning a VEI value (Newhall and Self, 1982). As discussed previously, about 78 % of the documented historical eruptions in Alaska have been VEI 2 and 3 events. Such eruptions typically produce 106– 107 m3 of tephra and typically more than 50 % of this amount falls in the proximal areas around the volcano (Scott and McGimsey, 1994; McGimsey et al., 2001; Wallace et al., 2013). Thus, the spatial footprint of geomorphically significant tephra fallout for
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ACCEPTED MANUSCRIPT eruptions this size is generally limited to areas within about 10 km of the vent. Larger eruptions, of course, produce more tephra and have a much greater spatial extent of
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fallout. The 1912 Novarupta-Katmai eruption is a good example of the types of the geomorphic impacts associated with a thick and widespread ash fall produced by a large
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historically rare eruption.
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7.1. Tephra-fall impacts from the Novarupta-Katmai eruption of 1912 The 6–8 June 1912 eruption of Novarupta-Katmai produced a cumulative tephra
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fallout volume of roughly 17 km3 that covered an area of about 120,000 m2 with >1-cmthick deposits of light-colored rhyolitic, dacitic, and some andesitic ash (Fig. 19; Fierstein
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and Hildreth, 1992). Locations within about 5 km of the Novarupta vent received more
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than several meters of ash fall and some areas were covered by >12 m of ash as far as 4
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km from the vent (Fierstein and Hildreth, 1992). The early summer eruption occurred when local snow cover was still relatively extensive and the accumulation of warm pumiceous tephra fall on snow and ice led to the generation of hyperconcentrated- and debris-flow lahars in many of the drainages that head on Mt. Katmai (Hildreth and Fierstein, 2012). In addition, many tephra-loaded slopes failed during the eruption, probably because of strong attendant seismicity, which led to a number of blocked drainages where lakes were impounded (Griggs, 1922; Hildreth and Fierstein, 2012). One such lake was the source of a significant dam-break flood in Katmai Canyon (Griggs, 1922) that caused substantial damage at the now abandoned village of Katmai and inundated the Katmai River valley to depths of 3–8 m (Griggs, 1922; Hildreth and Fierstein, 2012).
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ACCEPTED MANUSCRIPT Erosion and incision of the 1912 deposits began during brief pauses in the eruption, and several-meter-thick sequences of pumiceous alluvium within the primary
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eruptive deposits have been recognized in several locations (Hildreth, 1983; Hildreth and Fierstein, 2012). After the eruption ended, erosion of the thick and widespread eruptive
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products commenced rapidly, and in some areas more than 20 m of vertical incision
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occurred by 1917 as documented by Griggs (1922). Systematic documentation of the
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geomorphic impacts and erosion of the 1912 deposits was not a primary focus of the many investigations that followed the initial studies by Griggs (1922). However, it is
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clear from many of the photographs obtained from the Griggs-led expeditions from 1915–1919, and from observations made since then, that rates of erosion were initially
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rapid (Fig. 20), but after about 1917 further changes were less significant (Hildreth and
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Fierstein, 2012). The rapid accumulation and erosion of pyroclastic fall and flow material
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initiated significant aggradation of several drainages that continues today. The accumulation of pumiceous fluvial deposits in the lower Katmai River drainage initially filled in parts of Katmai Bay and caused minor progradation of the coastline (Griggs, 1922), and as reported in Hildreth and Fierstein (2012), tidal mudflats had apparently advanced ~1 km seaward. However, significant geomorphic changes to the lower Katmai River appear to have been minimal since the early 1950s (Hildreth and Fierstein, 2012). In addition to erosion and redistribution of the 1912 tephra deposits by water, the characteristic high winds of the Katmai region have reworked and in many places stripped the landscape of pyroclastic fall deposits. Thick accumulations of pumiceous eolian sand are common in many areas, and intense dust storms occur regularly today. Occasionally such storms are severe enough to loft ash 1–4 km asl and transport it several
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ACCEPTED MANUSCRIPT hundred kilometers from the source area (Hadley et al., 2004) where it interferes with
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local air travel and may pose a public health hazard.
7.2. Tephra-fall impacts associated with other Alaskan eruptions
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Only a few studies have documented the extent, characteristics, and volume of
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tephra fallout associated with historical eruptions in Alaska, but none of these were
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focused specifically on the geomorphic impacts of ash fall. The ash fallout produced during recent eruptions of Redoubt volcano in 1989–1990 and 2009, and during the
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Crater Peak eruption of 1992, has been well enough documented (Scott and McGimsey, 1994; McGimsey et al., 2001; Wallace et al., 2013) to comment on the geomorphic
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impacts of tephra fall associated with these events. Both Redoubt eruptions occurred in
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winter; and by the following summer, rainfall and snowmelt runoff had largely removed
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most of the tephra deposits except for areas within a few kilometers of the vent. Observations made in May 2009 indicated that nearly all of the 19 tephra-fall layers produced by the explosive phase of the 2009 Redoubt eruption had coalesced into a single merged layer of fine ash that lacked any evidence of having originated as multiple fall deposits (Wallace et al., 2013). The amount of tephra removed by erosion following the 2009 Redoubt eruption is not known. Visits to the volcano since the eruption ended revealed that millimeter- to centimeter-thick primary fall deposits are preserved only in areas protected from wind and water erosion. The thickness of proximal ashfall was insufficient to impair vegetation growth, and in many areas around the volcano, plants have grown up through the thin tephra cover.
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ACCEPTED MANUSCRIPT The 1992 eruption of the Crater Peak vent at Mount Spurr volcano consisted of three explosive eruptions on 27 June, 18 August,, and 16–17 September 1992 (Keith,
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1995). The ash clouds from these eruptive events resulted in ash fallout over large parts of south-central Alaska (McGimsey et al., 2001). Most of the distal ash deposits were
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covered by winter snowfall, and then removed the following spring during snowmelt.
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Coarse lapilli tephra deposits from the 1992 eruption formed a blanketing cover in
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proximal areas, and these deposits have been little modified since they accumulated. The geomorphic effect of tephra fallout on distal areas has not been addressed in a
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systematic manner. Some recent studies of distal ash fall from the ca. 1600 yBP eruption of Aniakchak volcano suggest that sites as far as 1100 km from the volcano may have
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experienced a period of retarded vegetation growth following ash deposition possibly
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lasting as long as 100 years (Blackford et al., 2014). Griggs (1920) found that vegetation
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growth on Kodiak Island after the 1912 Katmai eruption increased for several years following the eruption and attributed this to the mechanical effects of the ash fall (Griggs, 1915).
The deposition of volcanic ash in lakes has long been recognized in many parts of Alaska, and most lake core records obtained throughout the state contain recognizable ash layers (Begét et al., 1994; Schiff et al., 2008; Kaufman et al., 2012; Krawiec et al., 2013). The ash beds are typically amenable to dating with radiocarbon techniques and high-resolution records of ash fall based on age–depth models that have been developed (Schiff et al., 2008). Although much emphasis has been placed on tephra correlation and analysis of the frequency and magnitude of ash fall for hazard assessment and paleoclimatic reconstructions, only a few studies aimed at determining the physical and
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ACCEPTED MANUSCRIPT geochemical changes associated with ash fall in Alaska lakes and how this might affect primary productivity have been completed (Eicher and Rounsefell, 1957; Barsdate and
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Dugdale, 1972). Many lakes in Alaska provide unique rearing habitats for salmon and other species of fish that play an important economic role in subsistence and commercial
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fisheries.
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8. Concluding remarks
Clearly from this review, much work remains on the topic of geomorphic impacts
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of eruptions in Alaska. Continued studies at Kasatochi volcano hold promise for providing insight about how an island volcano responds geomorphically to the effects of
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a large explosive eruption that essentially created a new, pristine landscape. The
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Kasatochi example is unique because the island is somewhat isolated geographically
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making it nearly impossible for vegetation to encroach from unaffected areas. In general, the degree of impact scales with eruption magnitude, and thus further analysis of the effects of significant eruptions, such as the 1912 Novarupta-Katmai eruption, would yield the most insight about how such events geomorphically alter affected environments. The most common historical eruptions in Alaska are VEI 2–3 events, and with few exceptions, these types of eruptions have impacts that are restricted to the area immediately surrounding the volcanic edifice. This does not mean that these impacts are unimportant; they clearly are for the particular sector of the volcano that is disturbed. However, for smaller eruptions, the scope of impact is typically at the drainage basin scale, and the effects can be quite limited and transient.
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ACCEPTED MANUSCRIPT At volcanoes that experience frequent eruptions, such as Pavlof volcano, the most significant geomorphic impacts are related to the introduction of significant volumes of
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loose, easily erodible fragmental sediment primarily generated by tephra fallout and small pyroclastic flows. As a result, affected drainages become choked with pyroclastic silt,
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sand, and gravel; and aggrading, unstable reaches of active channels are common. This
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severely limits the development of riparian vegetation, which further promotes channel
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instability.
The role of glacier ice and the nature of the interaction between eruptive products
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and ice is important because this governs the generation of meltwater that can lead to large voluminous (and hazardous) lahars. The emplacement of large lahars and their
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associated deposits can completely alter the geomorphic regime of the valleys they
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inundate, leading to major changes in sediment flux to the sea, altered drainage patterns,
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development of lahar-dammed lakes, and sometimes highly unstable and rapidly shifting stream channels.
Additional research is needed to document how geomorphic, hydrologic, and biologic systems in the Aleutian arc respond to volcanic events. The hazards associated with explosive eruptions at volcanoes in Alaska are generally understood, especially at those volcanoes that pose the greatest risk to people and infrastructure. What is lacking however, is a more comprehensive understanding of the secondary effects that result from eruptive activity and the time scales over which these effects are significant.
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ACCEPTED MANUSCRIPT Acknowledgements The author wishes to thank and acknowledge the support and encouragement of
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his colleagues at the Alaska Volcano Observatory. The author is grateful for the helpful reviews provided by K. Wallace (USGS-AVO), three anonymous reviewers, and for the
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help and support provided by the journal editor, R. Marston, and the review paper
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coordinator, T. Horscoft.
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ACCEPTED MANUSCRIPT References Bacon, C.R., Neal, C.A., Miller, T.P., McGimsey, R.G., Nye, C.J., 2014. Postglacial
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ACCEPTED MANUSCRIPT Hadley, D., Hufford, G. L., Simpson, J. J., 2004. Resuspension of relic volcanic ash and dust from Katmai: still an aviation hazard. Weather and Forecasting 19(5), 829–
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ACCEPTED MANUSCRIPT Jicha, B. R., Scholl, D. W., Singer, B. S., Yogodzinski, G. M., Kay, S. M., 2006. Revised age of Aleutian Island Arc formation implies high rate of magma production.
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Geology 34(8), 661–664. Jones, S.H., Fahl, C.B., 1994. Magnitude and frequency of floods in Alaska and
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Investigations Report 93‐ 4179. 122 pp.
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conterminous basins of Canada. U.S. Geological Survey Water-Resources
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Keith, T.E.C., (Ed.), 1995. The 1992 eruptions of Crater Peak vent, Mount Spurr volcano, Alaska. U.S. Geological Survey Bulletin 2139. Krawiec, A. C., Kaufman, D. S., Vaillencourt, D. A., 2013. Age models and tephrostratigraphy from two lakes on Adak Island, Alaska. Quaternary Geochronology 18, 41–53. Mangan, M.M., Miller, T.P., Waythomas, C.F., Trusdell, F., Calvert, A.T., Layer, P.W., 2009. Diverse lavas from closely spaced volcanoes drawing from a common parent: Emmons Lake Volcanic Center, Eastern Aleutian Arc. Earth and Planetary Science Letters 287, 363–372.
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ACCEPTED MANUSCRIPT McGimsey, R. G., Neal, C. A., Riley, C. M., 2001. Areal distribution, thickness, mass, volume, and grain size of tephra-fall deposits from the 1992 eruptions of Crater
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Peak Vent, Mt. Spurr volcano, Alaska. U.S. Geological Survey Open-File Report OF 01-0370, 38 p.
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McNutt, S. R., 1987. Eruption characteristics and cycles at Pavlof Volcano, Alaska, and
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Miller, T. P., Smith, R. L., 1977. Spectacular mobility of ash flows around Aniakchak
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Miller, T. P., Smith, R. L., 1987. Late Quaternary caldera-forming eruptions in the eastern Aleutian arc, Alaska. Geology 15 (5), 434–438. Miller, T. P., Richter, D. H., 1994. Quaternary volcanism in the Alaska Peninsula and Wrangell Mountains, Alaska. In: Plafker, G., Berg, H. C. (Eds.), The geology of Alaska, v. G-1. Geological Society of America, Boulder, Colorado, pp. 759–779. Myers, J. D., 1988. Possible petrogenetic relations between low-and high-MgO Aleutian basalts. Geological Society of America Bulletin 100(7), 1040–1053. Newhall, C. G., Self, S., 1982. The Volcanic Explosivity Index (VEI) an estimate of explosive magnitude for historical volcanism. Journal of Geophysical Research: Oceans (1978–2012), 87(C2), 1231–1238.
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ACCEPTED MANUSCRIPT Ninkovich, D., Sparks, R. S. J., Ledbetter, M. T., 1978. The exceptional magnitude and intensity of the Toba eruption, Sumatra: an example of the use of deep-sea tephra
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layers as a geological tool. Bulletin Volcanologique 41(3), 286–298. Pearce, N. J., Westgate, J. A., Preece, S. J., Eastwood, W. J., Perkins, W. T., 2004.
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1645 BC date for Minoan eruption of Santorini. Geochemistry, Geophysics,
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Geosystems 5(3) Q03005, doi:10.1029/2003GC000672. Pierson, T. C., 1998. An empirical method for estimating travel times for wet volcanic
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mass flows. Bulletin of Volcanology 60(2), 98-109. Pierson, T. C., 2005. Hyperconcentrated flow—transitional process between water flow
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and debris flow. In: Jakob, M., Hungr, O. (Eds.), Debris-flow hazards and related
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Preece, S. J., Westgate, J. A., Stemper, B. A., Péwé, T. L., 1999. Tephrochronology of late Cenozoic loess at Fairbanks, central Alaska. Geological Society of America Bulletin, 111(1), 71-90. Preece, S. J., Westgate, J. A., Alloway, B. V., Milner, M. W., 2000. Characterization, identity, distribution, and source of late Cenozoic tephra beds in the Klondike district of the Yukon, Canada. Canadian Journal of Earth Sciences, 37(7), 983996. Riehle, J. R., 1985. A reconnaissance of the major Holocene tephra deposits in the upper Cook Inlet region, Alaska. Journal of Volcanology and Geothermal Research 26(1-2), 37–74.
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ACCEPTED MANUSCRIPT Riehle, J. R., Kienle, J., Emmel, K. S., 1981. Lahars in Crescent River valley, lower Cook Inlet, Alaska. Alaska Division of Geological and Geophysical Surveys Geologic
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Report GR 0053, 10 p. Schaefer, J.R. (Ed.), 2012. The 2009 eruption of Redoubt Volcano, Alaska, Alaska
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Division of Geological and Geophysical Surveys Report of Investigation 2011-5,
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Schaefer, J. R., Scott, W.E., Evans, W.C., Jorgenson, J., McGimsey, R.G., Wang, B., 2008. The 2005 catastrophic acid crater lake drainage, lahar, and acidic aerosol
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Geosystems 9(7), 29 p., Q07018, doi:10.1029/2007GC001900.
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Schiff, C. J., Kaufman, D. S., Wallace, K. L., Werner, A., Ku, T. L., Brown, T. A., 2008.
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Modeled tephra ages from lake sediments, base of Redoubt Volcano, Alaska. Quaternary Geochronology, 3(1), 56–67. Scott, W. E., McGimsey, R. G., 1994. Character, mass, distribution, and origin of tephrafall deposits of the 1989-1990 eruption of Redoubt volcano, south-central Alaska. Journal of Volcanology and Geothermal Research 62(1), 251–272. Stelling, P., Gardner, J. E., Beget, J., 2005. Eruptive history of Fisher Caldera, Alaska, USA. Journal of Volcanology and Geothermal Research 139(3-4), 163–183. Sturm, M., Benson, C., MacKeith, P., 1986. Effects of the 1966-68 eruptions of Mount Redoubt on the flow of Drift Glacier, Alaska, U.S.A. Journal of Glaciology 32(112), 355–361.
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ACCEPTED MANUSCRIPT Trabant, D.C., Hawkins, D. B., 1997. Glacier ice-volume modeling and glacier volumes on Redoubt Volcano, Alaska. U.S. Geological Survey Water-resources
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investigations report WRI 97-4187, 29 p. Trabant, D. C., Waitt, R. B., Major, J. J., 1994. Disruption of Drift glacier and origin of
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floods during the 1989-1990 eruptions of Redoubt Volcano, Alaska. Journal of
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Vallier, T. L., Scholl, D.W., Fisher, M.A., Bruns, T.R., Wilson, F.H., von Huene, R., and Stevenson, A.J., 1994. Geologic framework of the Aleutian arc, Alaska. In:
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Plafker, G., Berg, H. C. (Eds.), The geology of Alaska, v. G-1. Geological Society of America, Boulder, Colorado, pp. 367–388.
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Vanderhoek, R., Nelson, R. E., 2007. Ecological roadblocks on a constrained landscape:
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The cultural effects of catastrophic Holocene volcanism on the Alaska Peninsula,
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southwest Alaska. In: Grattan, J., Torrence, R. (Eds.), Living under the shadow: Cultural impacts of volcanic eruptions. Left Coast Press, Walnut Creek California, pp. 133–152. Walker, L.R., Sikes, D.S., Degange, A.R., Jewett, S.C., Michaelson, G., Talbot, S.L., Talbot, S.S., Wang, B., Williams, J.C., 2013. Biological legacies: direct early ecosystem recovery and food web reorganization after a volcanic eruption in Alaska. Ecoscience 20(3), 240–251. Wallace, K.L., Schaefer, J.R., Coombs, M.L., 2013. Character, mass, distribution, and origin of tephra-fall deposits from the 2009 eruption of Redoubt Volcano, Alaska - Highlighting the significance of particle aggregation. Journal of Volcanology and Geothermal Research doi: 10.1016/j.jvolgeores.2012.09.015.
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ACCEPTED MANUSCRIPT Waythomas, C. F., 1999. Stratigraphic framework of Holocene volcaniclastic deposits, Akutan Volcano, east-central Aleutian Islands, Alaska. Bulletin of Volcanology
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61(3), 141–161. Waythomas, C. F., 2001. Formation and failure of volcanic debris dams in the
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Chakachatna River valley associated with eruptions of the Spurr volcanic
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complex, Alaska. Geomorphology 39(3–4), 111-129.
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Waythomas, C.F., 2013. Volcano-ice interactions during recent eruptions of Aleutian Arc volcanoes and implications for melt water generation. Abstract V34C-03
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presented at 2013 Fall Meeting, AGU, San Francisco, Calif., 9-13 Dec.
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Waythomas, C.F., 2007, Mid-Holocene sector collapse at Mount Spurr volcano, south-
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central Alaska, in, Haeussler, P.J., and Galloway, J.P., eds., Studies by the U.S. Geological Survey in Alaska, 2006. U.S. Geological Survey Professional Paper
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Waythomas, C.F., Prejean, S.G., McNutt, S.R., 2008. Alaska's Pavlof Volcano ends 11year repose. Eos 89, 209–211. Waythomas, C. F., Walder, J. S., McGimsey, R. G., Neal, C. A., 1996. A catastrophic flood caused by drainage of a caldera lake at Aniakchak Volcano, Alaska, and implications for volcanic hazards assessment. Geological Society of America Bulletin 108(7), 861–871. Waythomas, C.F., Scott, W.E., Prejean, S.G., Schneider, D.J., Izbekov, Pavel, Nye, C.J., 2010a. The 7-8 August 2008 eruption of Kasatochi Volcano, central Aleutian Islands, Alaska. Journal of Geophysical Research 115(B00B06) 23 p., doi:10.1029/2010JB007437 .
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ACCEPTED MANUSCRIPT Waythomas, C.F., Scott, W.E., Nye, C.J., 2010b. The geomorphology of an Aleutian volcano following a major eruption: the 7-8 August 2008 eruption of Kasatochi
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Volcano, Alaska, and its aftermath: Arctic, Antarctic, and Alpine Research 42(3), 260–275, doi:10.1657/1938-4246-42.3.260.
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Waythomas, C.F., Pierson, T.C., Major, J.J., Scott, W.E., 2013. Voluminous ice-rich and
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water-rich lahars generated during the 2009 eruption of Redoubt Volcano, Alaska.
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Journal of Volcanology and Geothermal Research 259, 389–413, doi:10.1016/j.jvolgeores.2012.05.012.
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Waythomas, C.F., Haney, M.M., Fee, D., Schneider, D.J., Wech, A., 2014. The 2013 eruption of Pavlof Volcano: A spatter eruption at an ice and snow clad volcano.
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Bulletin of Volcanology 76(862) 12 p., doi: 10.1007/s00445-014-0862-2.
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Welch, B.C., Dwyer, K., Helgen, M., Waythomas, C.F., Jacobel, R.W., 2007.
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Geophysical survey of the intra-caldera icefield of Mt Veniaminof, Alaska. Annals of Glaciology 45, 58–65. Westgate, J. A., Preece, S. J., Kotler, E., Hall, S., 2000. Dawson tephra: a prominent stratigraphic marker of Late Wisconsinan age in west-central Yukon, Canada. Canadian Journal of Earth Sciences, 37(4), 621-627. White, J. D. L., Houghton, B. F., 2006. Primary volcaniclastic rocks. Geology 34(8), 677–680. Wilson, C. J. N., Walker, G. P. L., 1985. The Taupo eruption, New Zealand I. General aspects. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, pp. 199–228.
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ACCEPTED MANUSCRIPT Wolfe, B. A., 2001. Paleohydrology of a catastrophic flood release from Okmok caldera and post-flood eruption history at Okmok Volcano, Umnak Island, Alaska:
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University of Alaska Fairbanks unpublished M.S. thesis, 100 p. Figure Captions
Major volcanoes and volcanic fields of the Aleutian arc. The red triangles
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Fig. 1.
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indicate volcanoes that have experienced eruptive activity since A.D. 1700
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and are considered historically active. The yellow triangles indicate volcanoes that have had confirmed eruptive activity within the past 10,000
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years. Also shown are the location of the Aleutian trench and the rates of plate motion for the Pacific Plate (from Freymueller et al., 2008).
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Size of calderas of Holocene age in the Aleutian arc. (A)Plot of caldera
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length versus width. (B)Bar graph of caldera area from west (Semisopochnoi) to east (Hayes).
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Fig. 2.
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ACCEPTED MANUSCRIPT Fig. 3.
Valley-filling pyroclastic-flow deposits associated with the most recent caldera-forming eruption at Veniaminof volcano. (A)View down
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Blueberry Creek valley (the northeast flank of the volcano) showing loose (pf) and welded (w) pyroclastic-flow deposits incised by Blueberry Creek.
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The pyroclastic deposits are nearly 100 m thick in this location and formed
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a continuous, surface-mantling cover that obscured the underlying surface
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morphology. (B)Valley-filling pyroclastic-flow deposits on the southeast flank of Veniaminof volcano. In this area, valleys that were glaciated
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during the late Pleistocene have been partially filled with pyroclastic-flow deposits erupted during the last caldera-forming eruption about 3700 yBP.
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(C)Sketch of valley-filling pyroclastic-flow deposits at Veniaminof
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volcano. Not drawn to scale, but pyroclastic-flow deposits in such areas
Fig. 4.
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are on the order of 25–100 m thick. Generalized surficial geology of the north flank of Pavlof volcano. (A)Shaded relief perspective view of the north flank of Pavlof volcano showing the extent of debris-covered glacier ice, lahar deposits, and the general extent of fountain-fed lava flows and spatter accumulations. View is toward the southwest. (B)Generalized cross section of the north flank of Pavlof volcano showing the relationship between spatter accumulations, lava flows, lahar deposits, and glacier ice.
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ACCEPTED MANUSCRIPT Fig. 5.
Satellite image of Pavlof volcano and vicinity showing areas around the volcano that have been inundated by lahars during historical eruptions,
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and generalized extent of proximal pyroclastic fall and flow deposits. Fountain-fed lava flows on the upper flanks of the cone also shown. Photograph and schematic cross section of lahar deposits along the
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Fig. 6.
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Leontovich River, emplaced by the 1996 eruption of Pavlof volcano. The
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helicopter in the photograph is parked on the surface of a 1–2 m thick lahar deposit emplaced during the 1996 eruption.
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Examples of large lahars blocking drainages and leading to the formation of lahar-dammed lakes. (A)Northwest flank of Veniaminof volcano and
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lahar inundation along the Muddy and Sandy river drainages by lahars
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initiated during the most recent caldera-forming eruption 4100–4400 years ago. (B)South flank of Redoubt volcano and lahar deposits filling the
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Fig. 7.
Crescent River drainage. The Crescent River lahar was emplaced during eruptive activity about 3700 yBP and resulted in the formation of Crescent Lake as the Lake Fork of Crescent River was blocked by the lahar.
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ACCEPTED MANUSCRIPT Fig. 8.
Plot of volcano summit altitude versus distance along the axis of the Aleutian arc from west (Buldir Island) to east (Hayes volcano) for all
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volcanoes with confirmed Holocene activity. The 71 points are plotted with respect to the presence or absence of modern glacier ice on the
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edifice. The dashed line indicates the approximate glaciation threshold and
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shows that volcanoes in the eastern part of the Aleutian arc can support
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glacier ice at lower altitudes than volcanoes in the western part of the arc. All volcanoes indicted are seasonally snow covered, although the amounts
Fig. 9.
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are not known with certainty and vary significantly by location. Shaded-relief map of Redoubt volcano and vicinity showing Drift glacier,
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the Drift River valley, and the Drift River Marine Terminal. Also
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indicated are the Drift River fan and its distributary channels, Montana
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Bill Creek (M), Rust Slough (R), and Cannery Creek (C). The main channel of the Drift River (D) on the fan is also shown.
Fig. 10.
The summit crater of Redoubt volcano, final lava dome of the 2009 eruption (D) and Drift glacier gorge downstream from the lava dome. The summit crater is about 2 km wide, and the volume of the lava dome is about 70 million m3..
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ACCEPTED MANUSCRIPT Fig. 11.
Maps of the lower Drift River valley showing the extent of the active channel of the Drift River during eruptive and noneruptive periods.
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(A)Lower Drift River channel, 13 August 1988, 1 year and four months prior to the start of the 1989–1990 eruption. (B)Lower Drift River channel,
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4 June 1990, showing maximum extent of inundation by lahars during the
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1989–1990 eruption. (C)Lower Drift River channels, 16 October 2005,
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about 15 years after the 1989–1990 eruption. (D)Lower Drift River, 4 April 2009, showing extent of largest lahar of the 2009 eruption.
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(E)Lower Drift River, 30 July 2014, about 5 years after the 2009 eruption, showing abandoned main channel of the Drift River.
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Photographs of the 22–23 March and 4 April 2009 lahar deposits.
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(A)Outcrop of frozen March 23 lahar deposit showing framework assemblage of tabular ice clasts. Scale in photograph is 2 m. (B)March 22–
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Fig. 12.
23 lahar deposits along the main channel of the Drift River and on the upstream part of the protective levees surrounding the DRMT tank farm. In this area the lahar deposits contain numerous rounded to subrounded cobble and smaller-sized ice clasts in a granular ice matrix and many tabular ice blocks several meters in length. The approximate width of the Drift River channel choked with ice-rich lahar debris is about 450 m. (C)Eight-centimeter-thick bed of silt and fine sand that is the deposit resulting from complete melting of the ice within the March 23 lahar deposit. This deposit is overlain by sandy hyperconcentrated-flow deposits
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ACCEPTED MANUSCRIPT emplaced by the April 4 lahar and underlain by a thin organic soil layer developed on 1989–1990 lahar deposits. NASA ALI satellite image of the Drift River valley, 4 April 2009 showing
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Fig. 13.
the extent of lahar inundation associated with eruptive activity on April 4. Cut bank exposure of March 23 and April 4 lahar deposits in the middle
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Fig. 14.
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Drift River valley, May 2009. Combined thickness about 3 m. Note
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contrast in thickness of lahar deposits attributable to the significant ice content of the March 23 deposit. Arrows indicate the position of high-
Fig. 15.
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water mark mud lines on trees in the area. Photographs of Kasatochi Island in 2005 before the 2008 eruption (A, B)
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and (C) on 23 October 2008, 76 days after the 7-8 August 2008 eruption.
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Photos (A) and (B) show pyroclastic deposits associated with young pre-
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historical eruptions of Kasatochi. Photo (C) shows Kasatochi Island covered with pyroclastic-flow and -fall deposits generated by the 2008 eruption.
Fig. 16.
(A)East flank of Kasatochi Island, 22 August 2008, two weeks after the 2008 eruption. The red arrows indicate still-steaming fumaroles, and the areas labeled (p) show zones of sulfurous precipitate on the surface. The distance from the photographer to the fumaroles is about 50 m. (B)Coastal exposure of pyroclastic-flow deposits containing juvenile pumice, lithic material, and unburned vegetal matter (circled). Divisions on scale are in centimeters.
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ACCEPTED MANUSCRIPT Fig. 17.
Photographs of the flanks of Kasatochi volcano taken on 22 August 2008, 14 days after the 2008 eruption ended, and on 12 June 2009, 308 days
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after the eruption ended. (A)Northeast flank of Kasatochi volcano, 22 August 2008. Very little post-emplacement erosion of pyroclastic deposits
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evident in this photograph. Photographer is standing on recently formed
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beach at coastline. Note location of large block labeled (R) compared to its
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location on Fig. 13B. (B)Approximately the same view as Fig. 13A; note gully development and coastline erosion relative to Fig. 13A <1 year after
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the 2008 eruption. Feature labeled (R) is the same in both photos. Wavecut cliff in foreground about 15 m high. (C)Northwest flank of Kasatochi
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volcano on 22 August 2008 showing smooth cover of pyroclastic debris
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and ash. Note accumulation of rock fall talus (T) at the base of steep
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bedrock cliff. (D)Northwest flank of Kasatochi volcano on 12 June 2009 showing gullies developed in pyroclastic fan. After the eruption ended, these pyroclastic deposits were graded to sea level (Fig. 13C) and within <1 year were eroded by waves to form cliffs about 15 m high. Circled area shows an accumulation of rockfall talus that formed within two weeks after the 2008 eruption ended. This talus accumulation has been utilized by crevice-nesting seabirds (least Auklets) that used Kasatochi as prime nesting habitat all of which was destroyed by the 2008 eruption. Fig. 18.
Generalized time scale of geomorphic processes operating on the posteruption landscape of Kasatochi Island.
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ACCEPTED MANUSCRIPT Fig. 19.
(A)Cumulative tephra fallout over the lower Cook Inlet–Shelikof Strait area from the three plinian eruptions at Novarupta, June 1912 (from
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Fierstien and Hildreth, 1992). Contours in centimeters. (B)Extent of proximal tephra fallout and areas containing significant accumulations of
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reworked ash and pyroclastic debris (from Fierstien and Hildreth, 1992).
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Example of fluvial erosion within the proximal tephra fallout zone of the 1912 Novarupta-Katmai eruption. Top of tree to right of men was at the
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ground surface in 1917. By 1919, the channel bed in the foreground had been eroded vertically by about 3 m. Photo by W.L. Henning, National
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Geographic Society Expedition, 1919.
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Fig. 20.
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KV, Katmai Village site.
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Width (km) Semisopochnoi
Unknown
Unknown
6.5
Yunaska
Unknown
Unknown
3.8
Okmok
2140 – 1900
25 – 50
Makushin
9091 – 9284
4–5
Akutan
1554 – 1711
Unknown
Fisher
10,264 – 11,067
10 – 100 (DRE)
Veniaminof
4103 – 4405
>50
Length (km) 9
Area (km2)
3.5
12.4
45
9.4
97
Caldera-forming eruption likely Holocene age. Only proximal eruptive products preserved on Semisopochnoi Island, all other products entered the ocean. Age of caldera-forming eruption not known, but likely Holocene. Only proximal eruptive products preserved on Yunaska Island, all other products entered the ocean. Complete burial of nearly all of eastern Umnak Island by pyroclasticflow deposits meters to tens of meters thick. (2, 3, 6)
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10.3
Geomorphic impacts (references)
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Caldera dimensions
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Bulk eruptive volume (km3)
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Eruption age (Cal. 2 σ age range in yBP)
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Volcano
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Table 1 Holocene calderas and caldera-forming eruptions of the Aleutian arc; eruption ages pertain to the most recent caldera-forming eruption only
2.7
5.5
Extensive pyroclastic flows, but largely confined to glacial valleys on the volcano. Some pyroclastic flows may have crossed water and reached small islands east of the volcano. (1)
2.7
2.6
4.8
Extensive pyroclastic flows, but largely confined to glacial valleys on the volcano. Thick ash fall over most of Akutan Island and the surrounding ocean. (12)
17
137
Extensive pyroclastic flow sheets on western Unimak Island. Flows were highly mobile and rode up and over high topography in the vicinity of Fisher caldera. (10)
10.4
65.5
Extensive, valley-filling pyroclastic flows on all flanks of the volcano extending 40–60 km beyond the caldera. (7, 13)
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2.3
11
8.5
83
7.3
3721 – 3957
>50
10.4
10.4
78.7
Katmai/ Novarupta
1912 AD
10 – 50
2.5
4.2
9
Kaguyak
6311 – 6632
3.5a
2.5
3.4
Hayes
4259 – 4824b
Unknown
3
4.3
Thick pyroclastic deposits in the vicinity of the volcano and extensive ash fallout that formed a prominent regional tephra layer. (7, 13)
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3.4
Extensive, valley-filling pyroclastic flows and sheets on all flanks of the volcano extending 40–60 km beyond the caldera. Extensive ash fallout that formed a prominent regional tephra layer. (7, 11)
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Aniakchak
a
3
Pyroclastic flows extending 10–20 km beyond vent. Extensive and thick ash fallout on areas east and southeast of the vent. Resuspension of ash occurs today during times of high winds and dry conditions. (5)
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10 – 50
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4444 – 4784
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Black Peak
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6.5
Pyroclastic flows and tephra fallout in the vicinity of the caldera. (4)
10.7
Pyroclastic flows and tephra fallout in the vicinity of the caldera and extensive ash fallout over parts of south-central Alaska. (9)
Tephra fallout bulk volume. Modeled age from lake core data. Multiple tephra deposits from Hayes volcano spanning an age range of 3500 to 3800 C-14 yBP are known in south-central Alaska.
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References (1) Bean, K. W., 1999, The Holocene eruptive history of Makushin Volcano, Alaska: University of Alaska Fairbanks unpublished M.S. thesis, Fairbanks, AK, 130 p. (2) Beget, J.E., Larsen, J.F., Neal, C.A., Nye, C.J., and Schaefer, J.R., 2005, Preliminary volcano-hazard assessment for Okmok Volcano, Umnak Island, Alaska: Alaska Division of Geological & Geophysical Surveys Report of Investigation 2004-3, 32 p. (3) Burgisser, A., 2005, Physical volcanology of the 2,050 bp caldera-forming eruption of Okmok volcano, Alaska: Bulletin of Volcanology, v. 67, n. 6, p. 497-525. (4) Fierstein, J., 2007. Explosive eruptive record in the Katmai Region, Alaska Peninsula: an overview. Bull. Volcanol. 69, 469–509. (5) Fierstein, Judy, and Hildreth, Wes, 2008, Kaguyak dome field and its Holocene caldera, Alaska Peninsula: Journal of Volcanology and Geothermal Research, v. 177, n. 2, p. 340-366
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(6) Larsen, J. F., Neal, Christina, Schaefer, Janet, Beget, Jim, and Nye, Chris, 2007, Late Pleistocene and Holocene caldera-forming eruptions of Okmok Caldera, Aleutian Islands, Alaska, in Eichelberger, John, Gordeev, Evgenii, Izbekov, Pavel, Kasahara, Minoru, and Lees, Jonathan, eds., Volcanism and Subduction: The Kamchatka Region: Geophysical Monograph 172, American Geophysical Union, p. 343-364.
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(7) Miller, T. P., and Smith, R. L., 1987, Late Quaternary caldera-forming eruptions in the eastern Aleutian arc, Alaska: Geology, v. 15, n. 5, p. 434-438.
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(8) Miller, T. P., 2004, Geology of the Ugashik-Mount Peulik volcanic center, Alaska: U.S. Geological Survey Open-File Report OF 2004-1009, 19 p.
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(9) Schiff, C.J., Kaufman, D.S., Wallace, K.L., Werner, A., Ku, T.L., and Brown, T.A., 2008, Modeled tephra ages from lake sediments, base of Redoubt Volcano, Alaska: Quaternary Geochronology, v. 3, p. 56-67.
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(10) Stelling, P., Gardner, J. E., and Beget, J., 2005, Eruptive history of Fisher Caldera, Alaska, USA: Journal of Volcanology and Geothermal Research, v. 139, no. 3-4, p. 163-183. (11) Waythomas, C. F., and Neal, C. A., 1998, Tsunami generation by pyroclastic flow during the 3500-year B.P. caldera-forming eruption of Aniakchak Volcano, Alaska: Bulletin of Volcanology, v. 60, n. 2, p. 110-124.
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(13) Waythomas, C.F., unpublished data.
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(12) Waythomas, C. F., 1999, Stratigraphic framework of Holocene volcaniclastic deposits, Akutan Volcano, east-central Aleutian Islands, Alaska: Bulletin of Volcanology, v. 61, n. 3, p. 141-161.
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Table 2 Principal geomorphic effects of caldera-forming eruptions in Alaska Effects on glaciers
Effects on edifice morphology
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Tsunamis generated where thick, voluminous pyroclastic flows entered the ocean New water bodies generated where drainages are blocked Temporary dams form that may fail catastrophically
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Upper edifice truncated by circular caldera structure Large lakes or glaciers develop in caldera
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Glacier ice destroyed Upper parts of glaciers truncated by eruptive activity Glacier ice heavily mantled by pyroclastic debris
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Valleys filled with loose pyroclastic debris; drainage networks altered, riparian habitat destroyed Post-eruption incision and erosion contributes substantial amounts of sediment to newly developing stream channels. Sediment flux to the ocean increased significantly
Effects on water bodies
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Effects on stream channels and drainages
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Several Alaska eruptions are the focus of this article, including the 1912 Novarupta-Katmai eruption, the 2008 eruption of Kasatochi volcano in the Aleutian Islands, historical eruptions of Pavlof Volcano, and the 2009 eruption of Redoubt Volcano. These eruptions and their products are reviewed from the perspective of their geomorphic impacts to surrounding environments, although the geomorphic impacts of these events have been addressed in varying degrees of detail. Some eruptions, such as the 1912 Novarupta-Katmai eruption and the Redoubt eruptions of 1989–90 and 2009, have been evaluated comprehensively, whereas others, such as the recent eruptions of Pavlof Volcano have received more limited study due to remote location and logistical problems accessing field sites. Given the constraints imposed by location, weather, and cost of fieldwork, it is not possible to provide the same level of analysis for all of the eruptions described in this article. With these considerations in mind, the objectives of this review are: (1) to address the geomorphic impacts associated with large infrequent eruptions, (2) to address the geomorphic impacts of small, frequent eruptions, (3) to address the geomorphic impacts of large lahars on streams and channel networks, and (4) to briefly describe the geomorphic impacts of tephra fallout. The events discussed illustrate how eruptions of various sizes and characteristics can have significant impacts on the landscape that may persist for decades to millennia.
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