Massive edifice failure at Aleutian arc volcanoes

Earth and Planetary Science Letters 256 (2007) 403 – 418 www.elsevier.com/locate/epsl

Massive edifice failure at Aleutian arc volcanoes Michelle L. Coombs a,⁎, Scott M. White b , David W. Scholl c a

Alaska Volcano Observatory, Alaska Science Center, U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508, USA b Department of Geological Sciences, 701 Sumter Street, University of South Carolina, Columbia, SC, 29208, USA c Moss Landing Marine Laboratories, 8272 Moss Landing Road, Moss Landing, CA 95039, USA Received 4 October 2006; received in revised form 19 January 2007; accepted 25 January 2007 Available online 2 February 2007 Editor: C.P. Jaupart

Abstract Along the 450-km-long stretch of the Aleutian volcanic arc from Great Sitkin to Kiska Islands, edifice failure and submarine debris-avalanche deposition have occurred at seven of ten Quaternary volcanic centers. Reconnaissance geologic studies have identified subaerial evidence for large-scale prehistoric collapse events at five of the centers (Great Sitkin, Kanaga, Tanaga, Gareloi, and Segula). Side-scan sonar data collected in the 1980s by GLORIA surveys reveal a hummocky seafloor fabric north of several islands, notably Great Sitkin, Kanaga, Bobrof, Gareloi, Segula, and Kiska, suggestive of landslide debris. Simrad EM300 multibeam sonar data, acquired in 2005, show that these areas consist of discrete large blocks strewn across the seafloor, supporting the landslide interpretation from the GLORIA data. A debris-avalanche deposit north of Kiska Island (177.6° E, 52.1° N) was fully mapped by EM300 multibeam revealing a hummocky surface that extends 40 km from the north flank of the volcano and covers an area of ∼ 380 km2. A 24-channel seismic reflection profile across the longitudinal axis of the deposit reveals a several hundredmeter-thick chaotic unit that appears to have incised into well-bedded sediment, with only a few tens of meters of surface relief. Edifice failures include thin-skinned, narrow, Stromboli-style collapse as well as Bezymianny-style collapse accompanied by an explosive eruption, but many of the events appear to have been deep-seated, removing much of an edifice and depositing huge amounts of debris on the sea floor. Based on the absence of large pyroclastic sheets on the islands, this latter type of collapse was not accompanied by large eruptions, and may have been driven by gravity failure instead of magmatic injection. Young volcanoes in the central and western portions of the arc (177° E to 175° W) are located atop the northern edge of the ∼4000-m-high Aleutian ridge. The position of the Quaternary stratocones relative to the edge of the Aleutian ridge appears to strongly control their likelihood for, and direction of, past collapse. The ridge's steep drop to the north greatly increases potential runout length for slides that originate at the island chain. © 2007 Elsevier B.V. All rights reserved. Keywords: landslide; volcano; debris avalanche; Aleutian arc; submarine; edifice collapse

1. Introduction Owing to their rapid growth, interlayered weak rocks, and destabilizing volcanic activity, volcanoes are frequent sites of catastrophic mass wasting. Debris ⁎ Corresponding author. Tel.: +1 907 786 7403; fax: +1 907 786 7425. E-mail address: [email protected] (M.L. Coombs). 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.01.030

avalanches from the flanks of ocean island volcanoes in particular are an increasingly recognized tsunamogenic hazard [1–4]. Understanding the frequency, distribution, and size of these events preserved in the geologic record can enable the assessment of future collapse sites and the resulting tsunami potential. Much work on the submarine landslide deposits from volcanic islands has focused on large, hotspot-generated ocean island volcanoes, for example the Hawaiian

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Islands [e.g., [3,5,6]], but recent investigations have identified the collapse and subsequent submarine emplacement of debris avalanches at island arcs. Massive submarine avalanches have been recognized in the Lesser Antilles arc [7]; Bismarck arc, Papua New Guinea [8]; and the Tonga-Kermadec arc [9]. In this paper we focus on evidence for edifice collapse in the Aleutian arc, Alaska. Limited field work has been conducted in the remote western sector of the arc, but even reconnaissance geologic studies have revealed subaerial evidence for large collapse events at many of the volcanically active islands [10–13]. GLORIA side-scan sonar data collected in the 1980s [14,15] reveal hummocky debris on the seafloor north of many of the islands. We summarize previous observations, and present new, high-resolution multibeam bathymetry data collected over submarine deposits, as well as multi-channel seismic data for one of the deposits. Our synthesis reveals that edifice collapse, at a range of scales, has been common in the eruptive histories of Aleutian arc volcanoes. Additionally, many of these volcanoes are located along the north edge of the Aleutian Ridge, allowing collapses to have significant submarine runouts; moreover, the position along the edge may partially contribute to the propensity for collapse. Finally, magmatic triggers may have played a role in causing collapses, but most of these catastrophic events were evidently not accompanied by large-scale eruptions, suggesting non-magmatic factors may have triggered some collapses. Although much of the information presented here is reconnaissance in nature, our intent is to draw attention to the value of more detailed surveying efforts of specific volcanoes and their collapse deposits. Such work could contribute to a greatly improved understanding of arc volcanic slope failures and the hazards they pose. 2. The Aleutian arc The Aleutian arc is a classic subduction-related island arc, stretching as a chain of active volcanic centers from the Alaska Peninsula in the east to the westernmost active volcano in the Aleutian island chain, Buldir, at ∼ 175° E (Fig. 1A). To the east, the Alaska Peninsula sector of the arc sits atop continental crust. The Aleutian–Alaska arc has formed by subduction, increasingly oblique to the west, of the Pacific plate beneath the North American plate. Many of the larger islands in the Aleutian chain share a common makeup: mildly metamorphosed to virtually unaltered late Tertiary volcanic units compose their topographically subdued southern extents, and Quaternary volcanic

cones sit atop their northern sectors. Some Quaternary volcanoes (e.g., Okmok, Semisopochnoi) have circular collapse calderas, but most do not. The Quaternary volcanoes sit atop the ∼4000-mhigh Aleutian Ridge, which has been constructed by arc volcanism starting at ∼ 46 Ma [16]. The arc has essentially been in its current geographic position and massif configuration since the middle Eocene, although it has been much disrupted by arc-parallel lateral extension and strike-slip faulting and the focus of active volcanism has generally moved north with time [17]. This northward migration has perched many of the Quaternary volcanoes on the precipitous north edge of the ridge. This configuration is especially true in the arc segment discussed here (Fig. 1B). Most of the islands rise above the ridge's summit platform, ∼ 150 m deep at its crest and tilted down to the north and south, the result of both wave-cut erosion and subsidence of the ridge and thought to be formed by the end of the Miocene [17]. This configuration implies that Pliocene through Quaternary volcanoes on the northern edge of the ridge are built over thickening piles of their own debris and above a stronger framework of this late Miocene platform. Because detailed bathymetric data are not available for much of the Aleutian Ridge, little is known about its submarine landslide history. Perhaps the best known suspected landslide to have occurred along the arc, although not of a volcanic edifice, is the enigmatic 1946 Scotch Cap slide, located off the western end of the Alaska Peninsula, and thought to have triggered a tsunami that reached Hawaii and beyond [18]. Reconnaissance studies of several central Aleutian volcanoes have identified subaerial evidence for large-scale collapse [10,12]. Perhaps the best-understood example of an Aleutian–Alaska arc volcano that has undergone multiple episodes of collapse is Augustine volcano, far to the east in Cook Inlet [19,20] (Fig. 1A). 3. Data sources and methodology 3.1. Subaerial edifice geology Understanding of the geology of the islands discussed here derives from two main sources. Preliminary studies were conducted by USGS scientists in the 1940s and published as a series of geologic maps and reports [21–24]. Renewed work, beginning in the 1990s, was conducted to better understand the volcanic hazards and recent eruptive histories of central and western Aleutian volcanoes. Work at Kanaga and Great Sitkin has been previously published [10–12,25];

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Fig. 1. (A) Index map of North Pacific region. Bathymetry from [58]. (B) Bathymetric map of the central and western Aleutian Islands, showing location of identified submarine debris deposits in white. Contour interval is 400 m, with the 200 m contour also shown. Centers of Quaternary volcanism are shown as black triangles. Bathymetry from [59]; digital version available at www.absc.usgs.gov/research/walrus/bering/bathy/index. htm. Note steep drop to the north from the arc massif into the Bering Sea.

newly acquired information pertaining to Gareloi and Tanaga is presented here for the first time. 3.2. Marine data During the 1980s, a side-scan survey of the U.S. exclusive economic zone (EEZ) was conducted that covered the seafloor 200 nautical miles (370 km) around U.S. territories. This survey used the GLORIA side-scan sonar, which provided systematic coverage capable of resolving seafloor features as small as several hundred meters [14]. GLORIA data have been used effectively to identify and map the astonishing

seaward extent of gigantic slides off the flanks of the Hawaiian Islands [3,26] and others along the continental margins of the U.S. [27,28]. Surveys of the Bering Sea and North Pacific seafloor around the westcentral Aleutians were conducted in 1986 and 1987 [29,30]. GLORIA data were used to identify a seafloor slide deposit north of Kanaga [12], but the Aleutian dataset has not been systematically examined for possible debris-avalanche deposits until this study. For some of the larger Aleutian slide deposits, avalanche debris can also be inferred from 1:400,000scale regional bathymetry [31]. For many of the interpreted submarine slide deposits, it is likely that

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thinner, unrecognized facies extend beyond the limits of acoustic recognition; the regions indicated in this study are therefore likely minimum area estimates. Where possible, we augmented the older GLORIA data with higher resolution swath data acquired during a July–August 2005 cruise of the R/V Thompson operated by the University of Washington. While the primary goal of this cruise was to locate and dredge young volcanic cones, some multibeam swaths crossed debrisavalanche deposits, and in the case of Kiska and Gareloi, we conducted dedicated surveys over GLORIA-identified slide deposits. Multibeam data were collected using the Thompson's Simrad EM300 30 kHz multibeam echosounder with navigational control by globally corrected differential GPS. The bathymetric soundings were corrected using daily sound velocity profiles, cleaned, and gridded to produce bathymetric maps with 50 m horizontal and b10 m vertical resolution. For regions of interest, EM300 multibeam data were merged with digitized regional bathymetry to form unified grids. We merged these data with subaerial topography collected during the Shuttle Radar Topography Mission (SRTM) [32] to produce complete images of collapse sources and deposits. Multi-channel seismic data of the Kiska landslide were also acquired during the R/V Thompson cruise. The seismic source was a single airgun consisting of a 45-cubic-inch firing chamber married to a 105-cubicinch injector chamber. A 300-m-long 24-channel hydrophone streamer received reflection returns. Record length was 5.6 s and the shot interval was 8 s. Shot gathers were normal move-out (NMO) corrected and bandpass, automatic gain control (AGC), and F-X deconvolution filters were applied. The seismic section was migrated using a single velocity-depth table for every ∼100 shots with constant velocities of 1500 m/s above seafloor, 2000 m/s from seafloor to basement, and 5000 m/s below basement. A brute stack was found to show the contrast across unit boundaries more clearly than the migrated data in spite of some minor diffractions visible near the edges of some units (Fig. 3). 4. Subaerial edifice failure and submarine debris-avalanche deposits Along the 450-km stretch of the Aleutian arc from Great Sitkin (176.1° W) to Kiska (177.6° E), we recognized evidence for edifice collapse at seven of ten Quaternary volcanic centers (Table 1). Below, we describe these seven Aleutian volcanic centers from west to east, and the available evidence implying edifice failure at each.

Large landslide deposits thus far identified along the Aleutian Ridge are all of the debris-avalanche type, rather than slumps, using the general classification scheme of Varnes [33]. These deposits are typically longer than they are wide, and are characterized by wellformed amphitheatres at their heads, midsections containing large blocks, and distal hummocky terrain. For many of the slides described below, the upper headwall scarp has been filled in by younger volcanic flows and fragmental debris. For some, a subaerial collapse amphitheatre is clearly evident. Earlier work on volcanic edifice failures has shown that failure geometry can vary from deep-seated collapses that remove much of an edifice including its summit, to thinner-skinned sector collapses that remove a narrow sector but leave the majority of the edifice intact [e.g., [34]]. As described below, past collapses of Aleutian arc volcanoes have spanned this spectrum of failure geometry. 4.1. Kiska Kiska Volcano (177.4° E) is a Holocene stratocone that occupies the northern tip of Kiska Island (Figs. 1 and 2). The volcano has erupted seven times historically, most recently in 1990 [35]. The southern half of the island consists of deformed and faulted submarine volcanic and volcaniclastic units of the Vega Formation, thought to be middle Tertiary in age [23]. This unit is unconformably overlain by the late Tertiary to early Pleistocene Kiska Harbor Formation, the remnants of a composite stratovolcano thought to have occupied nearly the same site as present-day Kiska Volcano [23,36]. A single K–Ar age is 5.5 +/ − 0.7 m.y. for an andesitic lava flow in this older volcano [37]. The only comprehensive study of the island's stratigraphic sequence does not describe the contact between the Kiska Harbor formation and Kiska Volcano, nor does it present evidence for edifice failure of the Kiska Harbor stratovolcano or the younger Kiska volcano [23]. Despite the absence of subaerial evidence for edifice failure, GLORIA data show a region of hummocky seafloor terrain extending ∼30 km NNW of the volcano. North of Kiska, the Aleutian Ridge drops down in two submarine steps — one immediately offshore, to a depth of 1000 mbsl, and another 30 km to the north drops down from about 1800 mbsl to the abyssal sea floor of the Bering Sea at about 3800 mbsl (Fig. 2). The hummocky region descends to the base of the first of these two steps and appears to just reach the top of the second descent.

Island

Collapse type

Timing of collapse

Current Subaerial Subaerial Volume lost edifice evidence scar area during elevation for collapse? (km2) collapse (m) (km3)

Great Sitkin Kanaga Tanaga Sajaka Gareloi East Gareloi North Gareloi NW Segula Kiska

Deep-seated Deep-seated Deep-seated Bezymianny-type Thin-skinned Unknown Thin-skinned Unknown Deep-seated

Pleistocene c Pleistocene d 240–120 ka f Late Holocene f Holocene f Unknown Holocene Holocene? Pleistocene?

1740 1307 1806 1354 1573 1573 1573 1153 1220

a b c d e f g

Yes c Yes e Yes Yes Yes Yes Yes Yes g No

25 27 n/a 10 5.78 n/a 18 n/a n/a

MB, multibeam; MCS, multi-channel seismic. Height/length; H estimated using current edifice height plus depth to distal toe. [10]. [34]. [12]. A. Calvert, unpublished Ar–Ar dating. [43].

25 e n/a 2.9 N0.6 n/a 9 n/a n/a

Submarine deposit identified using a

Area of Estimated Deposit Depth to Maximum submarine volume length distal toe block size deposit (km3) (km) (mbsl) (m across) (km2)

H/L b

GLORIA GLORIA n/a MB GLORIA, MB GLORIA, MB GLORIA, MB GLORIA, MB GLORIA, MB, MCS

890 230 n/a n/a 95 300 185 205 196

N1000 750 × 1000 n/a n/a 800 × 500 n/a N1000 1300 400

0.127 0.152 n/a n/a 0.193 0.117 0.183 0.128 0.122

25 e

59

42 29 n/a n/a 19 36 19 18 40

3600 3100 n/a n/a 2100 2650 1910 1143 3660

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Table 1 Characteristics of Aleutian arc volcano collapses and associated submarine deposits

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Fig. 2. Topography and bathymetry (A) and Simrad side-scan (B) of Kiska volcano and its northern submarine slopes. For (A), topography is from SRTM [32] and bathymetric grid has been merged from [31] and EM300 multibeam data from this study. Contour interval is 100 m for bathymetry, 200 m for topography. Dashed line shows location of seismic line of Fig. 3.

A dedicated EM300 multibeam survey of the region north of Kiska was conducted over the area of hummocky topography evident in GLORIA imagery (Fig. 2). The bathymetry and multibeam backscatter data reveal two regions with contrasting textures forming the inferred slide deposit. The first is a block-rich zone extending 13 km offshore. The zone is about 7 km wide, and contains abundant, chaotically distributed blocks as large as 600 by 500 m in map view and as tall as 70 m. The second, more distal zone contains fewer blocks and extends about 28 km from the island, almost to the edge of the steep slope into the Bering Sea Basin. Based on available data, it is unclear if the debris-avalanche deposit reached this deeper steep slope. The entire debris-avalanche deposit is located within a subtle bathymetric low flanked on either side by smooth sedimented slopes. A zone of contour-parallel steps, separated by troughs 20 to 60 m deep and 200 to 300 m across, is present east of

the deposit at a distance of 15 to 28 km offshore. Similar bedforms have been observed offshore of Maui Island and were interpreted as coarse-grained sediment waves typical of gravel-rich turbidite systems [38]. East of these bedforms, a submarine canyon cuts into the slope sediments (Fig. 2). To image the internal structure of the Kiska debrisavalanche deposit, we collected a transverse seismic line across its east–west width 17 km north of the island (Fig. 3). The profile reveals three acoustically contrasting units, two of which were previously defined [17]: the lower or middle series, which forms a basement unit characterized by irregular, laterally discontinuous reflectors, and the upper series exhibiting laterally coherent reflection horizons. According to [37], the lower series corresponds to subaerial exposures of deformed and faulted greenstones (i.e., the Vega Formation on Kiska, and the Finger Bay volcanics of Great Sitkin, Kanaga, and elsewhere). Potentially less

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Fig. 3. Uninterpreted (A) and interpreted (B) seismic line perpendicular to the Kiska debris-avalanche deposit. Interpretation is based on migrated data, though version shown here is a brute stack, unmigrated but NMO corrected. Line location shown in Fig. 2. Upper, middle, and lower series sediments are discussed in text. 5× vertical exaggeration.

deformed Oligocene and Miocene beds form the basement recorded along the seismic profile. The upper series corresponds to late Tertiary and younger volcaniclastic and diatom-rich marine slope and basinal deposits. In our seismic profile, the upper surface of the lower or middle series is highly irregular where it is overlain unconformably by the upper series. In the area of our seismic section, the upper series beds are mostly horizontal and total thickness ranges from 350 m to 900 m thick. Where the profile crosses the debris-avalanche deposit as defined in map view, a lens of chaotically bedded material interrupts the otherwise horizontal beds of the upper series. This lens of disrupted continuity is thickest (∼ 700 m) in the center, and tapers off to the west and east across the width of the inferred slide deposit. The lens is overlain by thin, discontinuous horizontal beds of sediment; in places the underlying chaotic unit reaches the seafloor. A similar relation exists between debris material and slope sediment for a debris-avalanche deposit emplaced off the west flank of St. Lucia, Lesser Antilles Arc [7]. There a seismic line near the distal end of the avalanche deposit displays chaotic debris cut into surrounding sediment to a depth of ∼ 200 m, onlapping at the edges. It was inferred that the debris avalanche incorporated sediment during transport, and that where the avalanche came to rest a large fraction of the deposit was slope sediment, not volcaniclastic material [7]. We envision a similar mechanism for the Kiska slide.

4.2. Gareloi Gareloi (178.8° W) is an island stratovolcano, 10 km by 8 km at its subaerial base, with evidence for multiple, relatively small collapse events (Fig. 4). Gareloi's two summit craters have both been historically active, with 16 described eruptions, the most recent in 1996 [35]. The oldest rocks on the island are Pleistocene, and crop out as deeply dissected stacks of lavas and pyroclastics that form wedge-shaped sectors on the southwest, southeast, and northeast flanks [21]. Holocene lavas fill the gaps in between these older sectors. The northwestern and eastern gaps appear to have formed by sector collapse. The eastern scar is about 2 km across, reaches from summit to coastline, and is now 100 m deep. The northwest sector scar is broader, 5 km wide at its base, and currently up to 120 m deep. Both were likely deeper prior to infilling by younger lavas. The northern crater is enclosed, although the intra-crater eruptive stratigraphy is abruptly interrupted by near-vertical local unconformities on the northwest wall, representing a possible collapse scar that has since been filled [39]. Unlike most other Aleutian volcanoes, Gareloi is located slightly north of the summit plateau of the Aleutian ridge, and sits upon its own constructional pedestal that rises from 2600 mbsl on the north and east (Fig. 4B). Submarine debris fields are present east and north–northwest of the island. EM300 multibeam coverage offshore of Gareloi is more extensive than for Kanaga and Tanaga, and covers most of Gareloi's steep submarine flanks in an arc across the northern half

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Fig. 4. GLORIA side-scan image (A) and topography and bathymetry (B) of Gareloi volcano. Topography and bathymetry data sources as in Fig. 2. Contour interval is 200 m. N, North Gareloi debris avalanche; NW, Northwest Gareloi debris avalanche; E, East Gareloi debris avalanche. Yellow circle shows location of two 2005 dredges on satellite cone.

of the pedestal (Fig. 4B). The submarine flank to the northeast, which lies immediately off an inactive sector of the subaerial volcano, is characterized by closely

spaced canyons separated by smooth, uniform ramparts. Apparent in GLORIA imagery, debris-avalanche deposits form two long lobes north–northwest of the island;

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this area is headed in a 2.5-km-wide chute that sweeps down from the island. To the northwest, a stubbier, rougher region appears to partially overlap this chute. This smaller region, visible in multibeam and GLORIA imagery, has higher relief than most other debrisavalanche deposits discussed here, and includes a distinct, ∼ 7-km-long ridge that rises as much as 300 m off the surrounding seafloor. Without direct observation it is unclear if this feature is constructional in origin or is composed of landslide debris. We tentatively consider the high-reflectivity region (marked “NW” on Fig. 4B) to be a landslide deposit but further work is necessary to confirm this. The debris-avalanche deposit to the east of Gareloi is smaller than those to the north and northwest, widening seaward from a narrow zone near the shoreline to about 13 km across at its distal end, about 13 km from shoreline. The area is just downslope of the east scar in the subaerial edifice. This region contains a dense array of individual blocks on relatively smooth seafloor in both GLORIA and multibeam data sets. The largest block is 750 by 500 m in map view, and ∼80 m high. The deposit covers an area of ∼ 95 km2. Since the

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volume lost from the subaerial edifice is probably on the order of 1 km3 (Table 1), this suggests that either the submarine deposit is, on average, only ∼ 10 m thick or else has bulked up by incorporation of submarine substrate during flow. The age of this deposit is unknown, but is likely middle to late Holocene based on the lack of sediment cover and the sharp morphology of the subaerial collapse scar. 4.3. Tanaga The northern half of Tanaga Island (178° W) comprises three discrete Quaternary volcanic centers, from west to east: Sajaka, Tanaga, and Takawangha (Fig. 5). The two western centers are steep-sided cones of Holocene age that rise from shoreline to an elevation of about 1800 m. East of Sajaka and Tanaga, a N 300-mthick sequence of Pleistocene and older volcanic and volcaniclastic rocks underlies the broad edifice of Takawangha volcano, which has been active throughout the Pleistocene and Holocene [13]. Early Pleistocene and Tertiary volcanic and volcaniclastic rocks are exposed on the southern half of Tanaga Island [40].

Fig. 5. Topography and bathymetry of northern Tanaga Island. Topography and bathymetry data sources as in Fig. 2. Contour interval is 200 m. Holocene subaerial vents marked by triangles. Location of submarine cones (marked with “C”) from [42]. Circles indicate dredge locations from 2005 R/V Thompson cruise.

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A broadly crescent-shaped scarp, concave to the west, separates the Takawangha and Tanaga edifices. This ridge appears to be the eroded rim of an amphitheatre that formed as the northwest portion of Tanaga Island collapsed into the sea. Lavas from the preTanaga volcano that form the amphitheatre wall have been dated using 40Ar/39Ar at 240 ka; the oldest lavas west of this scarp underlie the current Tanaga cone and have been dated at 140 ka, thus constraining amphitheatre formation to 240–140 ka (A. Calvert, unpublished data). This age is consistent with the eroded, scalloped morphology of the collapse scarp, which appears modified by glaciation. No extensive pyroclastic sheet or tephra was found that could correlate to caldera formation; however, glaciation may have destroyed evidence of such a deposit. A smaller edifice collapse also affected the west side of Sajaka volcano (Fig. 5). The earlier edifice, Sajaka I, was truncated nearly in half, with a new, younger cone (Sajaka II) growing in the scar. The resulting debrisavalanche deposit is now below sea level, but limited subaerial exposures show a pumiceous pyroclastic flow deposited on Sajaka I lavas. The presence of the pyroclastic-flow deposit suggests that this sector collapse may have been accompanied by an explosive eruption. The lower part of the Sajaka amphitheatre is obscured by the new cone, but the upper reach of the Sajaka I edifice is truncated by a 1.5-km-diameter crater that resembles those at Bezymianny and Mount St. Helens, where landslides were accompanied by eruptions as pressure was released on shallow magmatic intrusions [41]. The age of the Sajaka collapse is late Holocene, because lavas from beneath the pyroclastic-flow deposit and from the Sajaka II cone yield 40Ar/39Ar ages of 5–0 ka (A. Calvert, unpublished data). The north half of Tanaga Island rests on a submarine pedestal that rises steeply from a depth near 2600 mbsl above a more gradual slope to the Bering Sea floor (Fig. 1). High-resolution multibeam data closer to the island reveal a debris field west of Sajaka that contains several large blocks (Fig. 5). The larger of the two subaerial collapse scarps appears to continue underwater to the north and south. On the north, the submarine flank of the island is deeply eroded east of the larger, older scarp, exhibiting morphology like that of Gareloi's inactive northeast flank. West of it, the flanks are smoother, likely from Holocene volcaniclastic deposits that accumulate as pyroclastic and lava flows enter the sea from Tanaga and Sajaka volcanoes. South of the volcanoes, the edge of the main bedrock platform underlying the island lines up with the subaerial scarp, with the cones of Tanaga and Sajaka jutting out beyond it to the west, suggesting that

they are relatively new constructions. With the exception of the blocks offshore of Sajaka, neither GLORIA nor multibeam data clearly show landslide debris on the seafloor north and northwest of Tanaga Island. This may be because of the greater age of the largest Tanaga collapse (> 140 ka), and thus any debris that was deposited may be obscured by younger sediment. Small cones on the submarine west flank of Sajaka have been identified by submersible diving as volcanic edifices, not landslide debris [42]. Dredges in 2005 on two cones south of Tanaga and Sajaka recovered monogenetic lava (G. Yogodzinski, unpublished data). 4.4. Kanaga Kanaga Volcano (177.2° W) is a 1300-m-high, Holocene stratovolcano on the north end of Kanaga Island (Fig. 6). Kanaga has erupted numerous times in the past 10,000 yr, including at least ten Strombolian to subPlinian eruptions in the past 250 yr; it erupted most recently in 1993–95 [12,35]. Concentric to Kanaga Volcano to the southeast is the arcuate Kanaton Ridge (Fig. 6), inferred to be the remnant of Mount Kanaton, an ancestral volcano destroyed by structural collapse [22,43]. The lava flows exposed in the rim of this amphitheatre dip gently south, suggesting that the ancestral edifice was in about the same position as the modern cone of Kanaga Volcano. Lavas at the base and near the top of Kanaton Ridge have been dated by 40 Ar/39Ar at 383–352 ka and 199.1 +/− 2.5 ka, respectively [44], indicating that collapse occurred after 199 ka. The scarp was originally presumed to have originated during caldera-forming eruption [22], but recent study shows that no extensive pyroclastic sheet is present on the island south of the ridge. This indicates either complete erosion by Pleistocene glaciers, or that the collapse was not accompanied by an ignimbrite-forming eruption [12]. Reconstruction of ancestral Mount Kanaton suggests a volcano with a basal diameter of about 13 km, an estimated elevation of about 2.3 km above sea level, and a volume of at least 75 km3, roughly 25 km3 of which was removed by gravity-driven structural collapse [12]. GLORIA imagery of the southern Bering Sea floor, north of Kanaga Island, shows a region of irregular reflectivity extending northwestward from Kanaga Island, recognized as probable landslide debris [12] (Fig. 6A). As mapped in the current study, the deposit is 30 km long, 10 km across, and covers roughly 230 km2. The proximal part of the deposit is obscured by deposition of Holocene volcaniclastic deposits from the Kanaga cone that are uniformly reflective on GLORIA side-scan images. Discrete blocks closest to

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Fig. 6. GLORIA side-scan seafloor imagery (A) and topography and bathymetry (B) for Kanaga volcano and its north submarine flanks. Topography and bathymetry data sources as in Fig. 2. Contour interval is 200 m. Position of amphitheatre walls from [22] and [12].

the island are ∼ 6 km from the current shoreline. The middle section of the debris-avalanche deposit reaches to a distance of 18 km from the island and is marked by hummocks that show up in GLORIA data as blocky sonar reflectors. The distal reach of the deposit has fewer blocks, and its toe is poorly defined. Bathymetry in the area of the Kanaga debrisavalanche deposit is primarily at 1:400,000 scale [31], except for a single, 5-km-wide swath of EM300 multibeam data acquired in 2005 near the island (Fig. 6B). This swath shows several blocks that appear as blocky reflectors in the GLORIA image. The largest one, at 1600 mbsl, is 750 by 1000 m (Fig. 6B). Just discernable in the bathymetry is the suggestion of a thicker debris lobe immediately northeast of this block. The Kanaga debris-avalanche deposit may be partially covered by a younger one shed from Bobrof Island, directly to the west. See Section 4.6 for further discussion of Bobrof. 4.5. Great Sitkin Great Sitkin Island (176.1° W; Fig. 1) is dominated by the Quaternary Great Sitkin volcano, which occupies

its northern sector (Fig. 7). Historical eruptive activity at Great Sitkin has been relatively minor — the most notable event occurred in 1974 with ash fall and dome extrusion [10]. Two older units of volcanic rocks underlie the modern edifice: greenstone of the Finger Bay volcanics, known to be of middle Eocene age on nearby Adak island, that are exposed on the southern part of Great Sitkin, and the unconformably overlying and undeformed Sand Bay volcanics, likely late Tertiary in age [24]. The Sand Bay unit is the remnant of a shield volcano whose summit was located near that of the modern cone [24]. The Quaternary edifice of Great Sitkin is truncated by a horseshoe-shaped amphitheatre open to the northwest toward the Bering Sea, formed during edifice collapse (Fig. 7) [10,11]. A Holocene cone has grown within the scarp, indicating that the collapse likely occurred in the late Pleistocene or early Holocene. No widespread pyroclastic deposit has been found to correlate with edifice failure, suggesting it formed by mass wasting rather than from magma withdrawal [10]. Multibeam bathymetry is not available for the area north of Great Sitkin Island. Regional bathymetry drops steeply to the north, reaching a depth of ∼ 2750 mbsl at a

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Fig. 7. GLORIA side-scan imagery (A) and topography and bathymetry (B) for Great Sitkin volcano and its north submarine flanks. Topography from SRTM [32], bathymetric contours (in fathoms) from [31], position of amphitheatre walls from [11].

distance of 13.5 km from the island. North of this, the slope flattens, reaching 3700 m at a distance of 40 km from the island. To the east, west, and south, the flanks of the volcano rise above the Aleutian Ridge's shallow summit platform. GLORIA side-scan data show an area of hummocky terrain north of Great Sitkin, interpreted as a debrisavalanche deposit [10]. The hummocky region appears confined to the flatter area between 13 and 42 km offshore, but high backscatter values associated with steep slope obscure the side-scan image. Within the hummocky region, many individual blocks can be identified, some exceeding 1 km in length. Assuming that the deposit fans out to its distal extent from a narrow sector of the island that corresponds to the collapse scar, the slide-disturbed area covers roughly 900 km2 (Table 1; Fig. 7). 4.6. Other volcanoes with evidence for landslides Segula (178.1° E) and Bobrof (177.4° W) Islands both show evidence of submarine landslides, but much less is known about the geology of these islands. Most of the landslide evidence is interpreted from GLORIA

data, and we obtained sparse multibeam coverage in their vicinity to corroborate the side-scan interpretation. Segula Island is a single stratocone approximately 6 km in basal diameter (Fig. 1). Most of the exposed volcanic material on the island is a relatively young pyroclastic sequence, except for lava flow fields on the northeast, and to a lesser extent, southeast, flank [45]. No obvious subaerial amphitheatre or collapse scar has been described, but a 500-foot-thick sequence of “unconsolidated crudely sorted moderately well-bedded volcanic detritus”, exposed along a cove on the north flank of the island, comprises fragments ranging in grain size from fine sand to blocks several m across [45, p. 264]. This accumulation may be a debris-avalanche deposit that corresponds to an area of hummocky topography 6 to 18 km offshore to the north, as seen in GLORIA images. Bobrof is a small island, 3.5 by 2.8 km in diameter (Fig. 6). Little is known about its composition; a single visit identified pyroclastic and lava flow deposits [46], but the island's submarine flanks reveal evidence of mass wasting. Bobrof volcano sits atop a wave-planed pedestal cut across Tertiary rocks in an indentation in the northern side of the summit plateau of the Aleutian

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Ridge, with submarine slopes that drop steeply on all sides (Fig. 1). Particularly on the east flank (Fig. 6), steep chutes lead from the platform at 40 mbsl down to depths greater than 2000 mbsl. A swath of multibeam data near the base of the east chutes reveals apparent avalanche debris (Fig. 6B). 5. Nature of Aleutian debris avalanches The Aleutian debris avalanches described here all share several traits. The deposits are longer than they are wide, and typically contain blocks that become less abundant and smaller with distance from the volcano. All appear to have originated within the subaerial edifice, and for most, substantial sections of the edifice were removed. Most deposits show little bathymetric relief beyond individual blocks. These traits are all characteristic of debris avalanches commonly found at island arc volcanoes. This uniform style of landsliding is in contrast to the larger failures found at hotspot-related, ocean island volcanoes, where both slow-moving, more coherent slumps and rapidly emplaced debris avalanches have been observed [e.g., [3]]. Based on analogy with observed subaerial debris avalanches, such as Mount St. Helens [47], edifice collapse probably initially produces large, intact blocks that move downslope, disintegrating into a debris avalanche consisting of blocks of varying sizes. As the avalanche continues downslope, further disintegration leads to smaller blocks deposited farther from the volcano. Debris avalanches are thought to move at a high velocity (up to ∼ 100 m/s) for a short duration, as observed at Mount St. Helens in 1980 [47]. The 1888 submarine Ritter landslide in Papua New Guinea traveled at more than 40 m/s, based on observations of the resulting tsunami coupled with numerical modeling, [48]. Most of the heights descended by the Aleutian landslides described here exceed the 800-m vertical drop of the Ritter event, suggesting that travel speeds could have been greater. The thicknesses of most of the deposits near the Aleutian arc volcanoes are unknown, but for those where high-resolution bathymetry is available, surface relief is not great — generally less than 100 m even in the proximal sections. Subsurface information for the Kiska debris-avalanche deposit suggests, however, that surface expression may be a poor indicator of true deposit thickness. Other Aleutian landslides may also have eroded into, and incorporated material from, submarine surface sediments in a manner similar to the Kiska debris avalanche. The emplacement of the Kiska slide implies that in areas of easily eroded marine sediments, the

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volume of material ultimately set in motion from a potential subaerial scar exceeds the initial subaerial volume source. Instead of only the subaerial mass, three possible moving sediment masses or bodies are involved: 1) material initially shed from the subaerial edifice, 2) marine surface sediment eroded and incorporated into the avalanching subaerial debris, and 3) the sympathetic disruption and downslope movement of the underlying, weakly lithified sedimentary section. While the deposit volumes are difficult to determine without subsurface imaging to determine thickness, the distances traveled by the Aleutian landslides are at the upper range shown by other Quaternary subaerial debris avalanches from subduction volcanoes (Fig. 8). The sizes of the Aleutian deposits are much smaller than Hawaiian or other oceanic island landslides, but are similar to deposits seen for other arc volcanoes of similar size to those of the Aleutian Ridge. Despite general similarities in the character of the observed submarine deposits, the Aleutian subaerial collapse scars, where known, vary greatly in magnitude and geometry. Below we look at the various types of edifice failure that led to Aleutian debris avalanches, and discuss possible triggering mechanisms of the different failures. 5.1. Deep-seated failures At Great Sitkin, Kanaga, and Tanaga, large, arcuate amphitheatres presumably mark the site of catastrophic edifice collapse, where a large part of the volcanic edifice was lost into the sea. No amphitheatre has been

Fig. 8. Runout distance (L) versus vertical drop (H) for Aleutian debris avalanches. Other Quaternary debris avalanches shown here are almost all subaerial; data from [56]. Submarine Hawaiian debris-avalanche data from [3]. Solid lines indicate representative H/L ratios.

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identified at Kiska, but the offshore debris-avalanche deposit suggests large-scale collapse occurred there as well. The scarps, where present, are all open to the north or northwest, and all of these volcanoes are currently perched at the edge of the 1000- to 3600-m-high slope that marks the north edge of the Aleutian Ridge. To the east, where Quaternary volcanic centers are set back from the north edge, large edifice collapses have not been identified. What factors led to the preferentially oriented collapse at these volcanoes? As volcanism migrates north within the arc, the volcanic edifices are built atop a platform constructed of volcaniclastic units previously shed north over the slope. At greater depths, water-rich hemipelagic deposits of diatom debris also accumulate. As the edifices grow, loading of the potentially unstable substrate could lead to gravitational instability. The Aleutian Ridge provides a substantial buttress to the south, favoring northward collapse. This tendency may be further exacerbated by the northward tilt of the underlying and possibly more competent Miocene summit platform [17]. This preferential orientation is similar to that observed in the Lesser Antilles arc, where active volcanoes are all built along the western edge of the older volcanic arc, and all large-scale flank collapse events have been directed to the west [7]. The western flanks of these volcanoes face the Caribbean Sea, are less buttressed, and have the steepest slopes. It has been suggested then that the orientation of large Antilles collapse events is mainly controlled by the large-scale arc structure [7]. Other factors make volcanoes particularly susceptible to gravitational failure and may play a role in these largescale collapses. Weakening through hydrothermal alteration [49,50] can make volcanic edifices more prone to collapse. Pore fluid pressurization resulting from magmatic intrusion may affect the geometry of collapse, leading to deeper failure than if no pressurization was present [51]. Tectonics may also play a role, as significant basement faulting, both normal and strike-slip, likely occurs near or beneath Aleutian volcanic edifices, in particular west of Atka. Some of the major volcanic centers are located at the axis of counter-clockwise rotating fault blocks [52]. Finally, shallow magmatic processes and regional seismicity likely often act as collapse triggers in systems that are prone to failure. 5.2. Thin-skinned failures In contrast to the large events discussed above, the collapse scarps on the north and east flanks of Gareloi are narrower, smaller in area, and did not excavate

deeply into the edifice. Gareloi's setting is different from the other volcanoes discussed here — it sits on a pedestal composed of its own debris that rises above the highly fractured basement fabric of Amchitka Pass. This suggests that Gareloi did not grow on a pre-existing subaerial platform or topographic high and has instead reached sea level through its own constructional vigor. The type of sector collapse seen at Gareloi is similar to collapse at other exceptionally active island stratovolcanoes, such as Stromboli. Stromboli has experienced multiple sector collapses of 0.7–2.0 km3 volumes in the past 13,000 k.y. [53], each of which have affected a small sector of the volcano. These events possibly result from surface loading by eruptive products [54] and may be more prevalent at volcanoes like Gareloi (and Stromboli) where thin mafic lava flows and pyroclastic accumulations predominate over thick lava coulees and domes. 5.3. Failure accompanied by Bezymianny-style eruption Finally, Sajaka on Tanaga Island provides a probable example of a sector collapse accompanied by magmatic eruption, similar to that first described at Bezymianny Volcano in Kamchatka [55]. During the event, the western half of Sajaka I was destroyed. This destruction appears to have been accompanied by a directed lateral blast rather than a vertical eruption column, as evidenced by the resulting pyroclastic-flow deposits distributed only on the western flank of the old Sajaka edifice. Lateral blasts occur as a landslide unroofs a hydrothermal–magmatic system within a volcanic edifice, leading to depressurization and explosive eruption [56]. The direction of landsliding controls the blast direction as the intact portion of the edifice deflects the explosion in the direction of the collapse. Cryptodome intrusion may also play a role in Bezymianny-type events [47,57]. 6. Concluding remarks Many volcanoes in the Aleutian arc from 176° W to 177° E show evidence of edifice failure. These collapses span an array of styles and triggers, but the majority appears to have been driven primarily by gravitational collapse directed toward the north. If Aleutian arc volcanism continues to migrate northward, which has been occurring during the past 46 Myr [16], the gravitational instability that led to large-scale edifice failure will likely continue, and future similar events are possible. Events like those in the Pleistocene could occur again as post-collapse edifices regrow. Similarly,

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large edifices currently located on the north edge of the ridge could be sites of future collapse. These edifices include those on Atka and Adak Islands. The large events described here may have resulted in tsunamis, though no deposits are known. Because of the northern direction of most of the deposits, tsunamis would have traveled into the Bering Sea and hit the coasts of western Alaska, northern Kamchatka, and Siberia, making these sites possibilities for finding evidence of tsunami inundation. Future volcanic collapses in this section of the arc would likely also fail to the north, possibly affecting these coastal areas as well as north Pacific shipping routes between Asia and North America, and communities within the Aleutian Islands. The recurrence of past collapse events, coupled with a likelihood of future events, makes further study of the Aleutian edifices of significant scientific and societal importance. Improved bathymetric coverage could reveal currently unknown deposits, as well as provide greater understanding of the volume and fabric of the many collapse deposits described here, and dives could provide direct observations that would further distinguish landslide blocks from constructional volcanic cones. Seismic profiling coupled with direct sampling would provide information critically needed to reconstruct emplacement mechanisms and guide tsunami modeling. Acknowledgements We thank Capt. McClengnahan and the crew of the R/V Thompson for their outstanding shiphandling. We offer a special thanks to Gene Yogodzinski, chief scientist on TN182, for his support. John Diebold provided the seismic instruments and assisted in the processing. Julia Morgan generously provided insight into the Kiska seismic profile. Evan Thoms digitized sections of regional bathymetric charts, merged results with multibeam data, and helped with figure preparation. Several swaths of multibeam data, collected by the R/V Roger Revelle in 2004, were kindly provided by Jennifer Reynolds. Reviews by Peter Lipman, Mark Reid, Charlie Bacon, Wendell Duffield, and an anonymous reviewer improved the manuscript. This work was supported by the Volcano Hazards Program of the U.S. Geological Survey and NSF Grant OCE-0242585 to Gene Yogodzinski. References [1] R.T. Holcomb, R.C. Searle, Large landslides from oceanic volcanoes, Mar. Geotechnol. 10 (1991) 19–32.

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