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The pre-ignimbrite (phreato) plinian and phreatomagmatic phases of the Akdag-Zelve ignimbrite eruption in Central Anatolia, Turkey
Journal of Volcanology
and Geothermal
Research 78 (1997) 139- 153
The pre-ignimbrite ( phreato) plinian and phreatomagmatic phases of the Akdag-Zelve ignimbrite eruption in Central Anatolia, Turkey Rolf Schumacher
*,
Ulrike Mues-Schumacher
HauptstraJe 3, 71672 Marbach Germany
Received 26 April 1996; accepted 13 December 1996
Abstract The eruptive sequence of the 7.6-Ma-old Akdag-Zelve ignimbrite (AZ11 eruption comprises five units totalling up to > 50 m on Akdag mountain. They are, in stratigraphic order: (a) the main pumice full deposit (up to 10 ml; (b) the lower surge series (up to 4 m); (cl the upper pumice beds (up to 0.8 ml; (d) the upper surge series (up to 5 ml; and (e) the Akdug-Z&e ignimbrite (up to 40 m). The main pumice fall deposit shows a second distal thickness maximum about 20 km from the source, whereas the upper pumice beds form a simple NNE-extending fan. The source area, about 6 km north-northeast of Kaymakli, is inferred from fallout thickness, size and amount of lithic clasts, depositional facies and flow directions of the surge deposits. The main pumice fall deposit encloses accretionary lapilli up to 6 cm in size indicating a steady phreatomagmatic influence on the eruptive style from the very beginning. The major surge sequence beneath the ignimbrite is similar to that of the 1400 yr B.C. Minoan eruption on Santorini, Greece, and resembles the Minoan B stratigraphic unit attributed to phreatomagmatic eruptions due to ingression of seawater into the conduit. Similarly, the depositional environment of marshes and marginal flats of a lacustrine basin in Cappadocia accounts for the external water supply and the generation of surges during the AZ1 eruption. Estimates of eruption column height of 2 45 km indicate that the AZ1 eruption ranks in the order of the 1956 Bezymianny, 1815 Tambora and the Waimihia Plinian pumice eruption of
New Zealand. Volume estimates of the erupted tephra are on the order of 41.5 km3 for the main pumice and 0.35 km3 of the upper pumice. Keywords:
Central Anatolia; phreatoplinian
eruptions;
eruptive successions;
1. Introduction The Central or Cappadocian Volcanic Province (CVP) is one of the four major volcanic provinces in Turkey: Eastern, Central and Western Anatolia, and
* Corresponding 711-9765013
author.
Phone:
+ 49-7144-36596;
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0377-0273/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZZSO377-0273(96)00106-O
accretionary
lapilli
the Galatean region (Fig. 1). The CVP, terminated and partly transected by major fault zones, extends for about 300 km southwest-northeast and is surrounded by tectonic basins and massifs of the central part of the Anatolian block (Toprak et al., 1994, and references therein). The 7.6-Ma-old Akdag-Zelve ignimbrite (AZI; K-Ar age, Mues-Schumacher and Schumacher, 1997) is part of the Cappadocian tephra series that com-
140
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Journal
ofVolcanology and Geothermal Research 78 11997) 139-153
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of Valcunology and Geothemurl Research 78 (1997) 139-153
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Akdag-Zelve
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Ignimbrite
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Secondary volmniclostic -
marginal
flats
-
Fig. 2. Eruptive sequence of the Akdag-Delve ignimbrite eruption documented from proximal location 22 and distal location 27. Five major units can be identified in near-vent locations. Farther away from the source, definition of the upper surge series is hampered due to a fine-grained distal pyroclastic flow facies; therefore, the surge-ignimbrite transitionalfacies was defined.
Fig. 1. Location of study area. (A) The geological sketch map of the Cappadocian Volcanic Province (CVP) in central Turkey illustrating the main geological and volcanological features. The CVP is composed of a complex succession of volcaniclastic deposits and several volcanic complexes - the most important are Erciyes Dag (I), Melendiz Dag (2) and Hasan Dag (3) stratovolcanoes. Major geotectonic fault zones transect the province. (B) The location map of the study area shows sample locations of the AZI eruptive products. Figures denote the locations mentioned in the text; circled figures represent the locations of the stratigraphic sections shown in Fig. 2. The triangle denotes Akdag mountain. (C) The stratigraphic section illustrates the ignimbrite stratigraphy of Cappadocia that comprises at least 10 ignimbrites intercalated with pumice fall, surge deposits and extensive reworked material.
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2. Eruptive
sequence
The deposits of the initial, pre-ignimbrite eruptive phase consist of rhyolitic pumice fall and fine-grained ash deposits, the latter chiefly coming from pyroclastic surges. The eruption products can be subdivided into five units that are in stratigraphic order (Fig. 2): (5) the Akdag-Zelve ignimbrite; (4) the upper surge series; (3) the upper pumice beds; (2) the lower surge series; (1) the main pumice fall deposit. Minor ash-and-pumice deposits of pyroclastic flow origin locally occur at the base of the lower surge series in proximal locations. The thickness of the entire sequence is up to 50 m on Akdag mountain. 2.1. Main pumice fall deposit
‘4
4 0 16 64 2
mm
4 8 16 64mm Fig. 3. Variation of lithic content through the main pumice fall deposit as analysed in proximal location 22. There is a general decrease in size and amount with height in stratigraphy except for two lithic concentration layers. The grain-size distribution of the ash layer is shown in Fig. 5.
prizes eight major and two minor ignimbrites, several pumice falls and surge deposits. Tephra is interbedded with extensive secondary volcaniclastic sediments and fossiliferous lacustrine limestones (Schumacher et al., 1990; Toprak et al., 1994). We have studied the AZ1 eruption because of its complexity starting with initial pumice fallout that subsequently graded through surge deposits into the ignimbrite-producing phase. The general phreatomagmatic influence during the whole eruption produced unusual volcanological features, and the interpretation of the well exposed accretionary lapilli-bearing pumice fall deposit provides an essential contribution to the understanding of the volcanological evolution of Cappadocia.
The main pumice fall deposit consists of rhyolite pumice and is coarser than other similar deposits of the Cappadocian tephra series. It is conspicuous for its intense induration (up to building stone quality with a bulk rock density of 1050-l 100 kg/m? that frequently causes its faint internal bedding to be obscured. The deposit consists of two well-defined parts with contrasting internal characteristics. The lower massive part (Fig. 2) is nearly homogeneous throughout its thickness, but locally shows a faint internal stratification due to fluctuations in grain size and lithic content. The lithic concentration, as analysed in location 22, is as high as 9 wt.% and generally decreases upward (Fig. 3). The upper strut@ied part is an internally layered pumice deposit. In proximal locations (e.g., location 22) in the Bahgeli-Ayvali region, strongly undulating and anastomosing ash beds, 5-20 cm thick, occur in this unit (Fig. 4b). Individual ash layers are normally graded as is typical of laminated surge deposits higher up in the stratigraphy and is also known from other phreatomagmatic eruptions - e.g., the final stage of the Laacher See (Van den Bogaard and Schmincke, 1984) and the Rieden volcano, East Eifel (Viereck, 1984). Fine-ash beds intercalated with pumice falls are also reported from the deposits of proximal Fogo A (Walker and Croasdale, 1970) and the Waimihia plinian pumice deposit (Walker, 1981a). Grain-size analyses of one layer show (a)
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of Volcanology and Geothermal Research 78 (1997) 139-153
fairly well sorted unimodal distribution within each sublayer, and (b> unimodal grain-size distribution of the entire bedding set (Fig. 5). Accretionary lapilli occur in the lower part of the main pumice fall deposit as ellipsoidal, more or less concentric mud b&s up to 6 cm in diameter (Fig. 4a). They are generally rare components of the pumice fall throughout the whole study area but are abundant in location 17 north of Ayvali. Apparently, individual showers of aggregates accompanied the steady pumice fall. Accretionary lapilli associated with pumice fall deposits are documented from the Hachinohe Ash Towada Caldera, Japan (Hayakawa, 19901 - where they frequently enclose pumice and other coarse material; similarly they are found at the So&i&e of St. Vincent (Brazier et al., 19821. Compared to these examples, the accretionaty lapilli of the AZ1 main pumice are very fine-grained and no coarse material was found in the center. 2.2. Surge series deposits Both surge series consist of alternating, internally bedded,particulate ash-and-pumice and fine-ash beds. Proximal locations, e.g., 19, 22 and 30 south of Bahseli, show distinct cross-stratification (with minor sand-wave bedding) and mutual erosion of individual layers or sets of layers (Fig. 4c). As distance from source increases, bedding gradually changes into plane-parallel or very low angle cross- stratification. Fine-grained ash beds, especially beneath and on top of the intercalated upper pumice beds, enclose abundant accretionary lapilli (rim-type of Schumacher and Schmincke, 1991). A very thin bedded but widely distributed vesiculated tuff layer from the upper surge series is typically associated with coretype accretionary lapilli. Internal characteristics of the deposit indicate its formation from the coalescence of accretionary lapilli and other irregular aggregates and ash clots as described by Rosi (1992) and Schumacher and Schmincke (1995). A distinctly fines-depleted particulate facies of the surge series, exposed on Sarimaden Tepe and an adjacent hill (locations 8 and 131, is attributed to locally increased turbulence that winnowed out the fines.
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2.3. Upper pumice beds The upper pumice beds represent a minor plinian pumice fall intercalated between the lower and the upper surge series (Fig. 4d). The deposit is readily identified by its tripartite subdivision into a finegrained basal layer, a medium-grained central layer, and a coarser-grained upper layer. The uppermost layer has a distinct basal parting throughout most of the study area that indicates a short pause in fallout deposition. Proximal locations south of BahGeliAyvali (19, 22, 30 and 58) expose a corresponding ash layer, lo- 15 cm thick, that shows internal (cross)bedding structures, accretionary and armoured lapilli. The combination of these features indicates deposition from a small and brief lateral flow that shortly interrupted the pumice fall.
2.4. The Akdag-Z&e
ignimbrite
The ignimbrite proper is generally massive, although internal layers of pumice concentration indicate a multiple flow deposit. A particular characteristic of the basal part is the presence of at least three intermittent surge layers, each of which is up to 1 m thick (Fig. 4e). They extend throughout the study area and occur as far as &konak, at least 30 km from the assumed vent. The deposits are finer than the ignimbrite and have internal cross-bedding and rim-type accretionary lapilli. Parting lineations on bedding planes indicate deposition in the upper flow regime. Distinction of this basal ignimbrite from the upper surge series is difficult in distant places, e.g., location 27 Hasan Kilesi, Avanos (Fig. 21, where both deposits have planar beds and the pyroclastic flow deposits are relatively thin and also enclose abundant accretionary lapilli. These deposits are therefore termed transitional facies between the upper surges and the massive ignimbrite.
3. Depositional environment The main pumice fall deposit overlies secondary volcaniclastic, reworked shallow lacustrine sediments in most of the study area. Internal bedding includes subaqueous slumping and convolute bed-
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ding resulting from dehydration of individual sediment layers. The deposits are topped by extremely fine-grained material which is interpreted as a lacustrine stagnant water sediment (Fig. 4a). The thickness is between 3 and 6 cm and no systematic area1
thickness variation was detected (e.g., thickening either toward the assumed source area or within topographic depressions). This rules out the interpretation that this fine-grained ash is an integral part of the eruption.
R. Schumacher, U. Mues-Schumacher/
Journal of Volcanology and Geothermal Research 78 (1997) 139-153
145
poraneous reworking. The best examples are exposed south of Ulasli, in Sarihidir and northeast of Gulsehir. A striking example of subaerial paleotopography is exposed at location 54, Urglip. The lower surge series was emplaced on a steep slope and it thickens as the dip decreases in the valley bottom. This transition of the marginal facies of a ponded deposit into the overbank veneer facies indicates a paleodepression fill.
&J 0
530 20 IO .063.125.25 .5 I. 2. 4. 8.mm Grainsize Fig. 5. Grain-size analyses of ash from a surge deposit interbedded within the proximal main pumice fall deposit. Dotted part: coarse basal sublayer; hatched part: fine-grained upper sublayer-both sublayers are unimodal in grain-size distribution. The cross-hatched insert shows the unimodal size distribution of the entire bedding set.
Soft sediment deformation, especially load casting of pumice, indicates that the sediment was wet during emplacement of the pumice fall. Moreover, alignment of large pumice lumps and the scarcity of lithic clasts suggesting floating pumice rafts indicates in several outcrops in the Urgtip-GoremeAvanos area that at least some of the pumice fell into very shallow water (< 0.5 ml of more or less isolated ponds typical of a marsh or marginal flat environment of a lacustrine basin. The recognition of the depositional environment of a lacustrine sedimentary basin that was progressively filled by pyroelastic deposits is essential to understand the mode of eruption and emplacement of AZ1 tephra. Some other locations, chiefly in the northern and northwestern part of the study area indicate deeper water facies of subaqueous deposition and contem-
4. Area1 distribution 4. I. Thickness The eruptive products are well exposed within the mapped area of about 1200 km2. Outcrops are limited by sedimentary and younger volcanic cover and thickness varies by primary depositional factors such as deposition of the pumice into a subaqueous environment (north of Gulsehir) where the original thickness was syn-depositionally modified by floating and reworking of pumice. The main pumice fall shows a distinct distal thickness maximum of > 8.5 m in the Zelve-Avanos area (Fig. 6A), nearly as high as the measured near-vent maximum of about 10 m (11 m extrapolated maximum thickness To according to Pyle, 1989). Similar distal thickness maxima in pumice fall deposits are reported, for example, from the 79 A.D. eruption of Somma-Vesuvius (Sigurdsson et al., 1985), from the MLST-Cl deposit of the Laacher See (Van den Bogaard and Schmincke, 1984), and from the 1980 eruptions of Mount St. Helens (Fig. 7). An unexpected occurrence of about 2 m of main pumice fall was detected in Akkoy, about 30 km away from the vent in a cross-wind direction perpendicular to the main fan axis.
Fig. 4. Photographs illustrating depositional characteristics of the AZ1 eruptive products. (a) Basal zone of the main pumice fall deposit overlying the very fine-grained lacustrine stagnant water sediment (location 17 north of Ayvali). The pumice deposit is characterized by abundant accretionary lapilli (some are indicated by arrows) which deposit fine ash together with coarse pumice lapilli. Maximum size of aggregates is up to 6 cm. (b) Undulating ash layers within the main pumice fall deposit (location 22) that are interpreted as phreatomagmatic surge deposits at the boundary between the frequently massive lower part and the generally stratified upper part of the pumice deposit, (c) Tbe lower surge series in proximal location 30 is composed of internally bedded ash layers showing cross-stratification and mutual erosion of individual sets of layers. (d) The upper pumice beds (location 22) are subdivided into 3 sublayers, the uppermost of which is coarsest, Note the tine-grained ash (arrow) beneath the upper pumice sublayer, which was probably deposited from lateral flow indicated by internal bedding structures. (e) Entire eruptive sequence of AZ1 as exposed in proximal location 58. MF = main pumice fall deposit; LS = lower surge series; UP = upper pumice beds; US = upper surge series; AZ1 = Akdag-Zelve ignimbrite proper; IS = intermittent surge deposits which are intercalated between basal flow units of the proximal ignimbtite.
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Journal of Volcanology and Geothermal Research 78 (1997) 139-153
4
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50km Distance from source
Fig. 7. Thickness variation of fall deposits with distance from source. The main pumice is compared with similar deposits that have a distal thickness maximum. After Sigurdsson et al. (1985); data for the Laacher See MIST-CI pumice from Van den Bogaard and Schmincke (1984).
The upper pumice beds representing the second, minor plinian phase of the eruption form a simple depositional NNE-elongated fan (Fig. 6B). The thickness of the whole deposit consistently decreases from 83 cm at location 30 south of Bahqeli to 6 cm in location 28 near Avanos, about 25 km from the source. Farther north, around Gzkonak, a l-1.5-cmthick particulate layer overlying fine-grained ash with accretionary lapilli is interpreted as the upper pumice beds. The lower surge series was identified throughout the study area and is thickest in the Bahqeli-Ayvali region where the isopach map indicates individual lobes (Fig. 8). Flow-direction measurements of parting lineations on bedding planes indicate a more or less radial dispersal. 4.2. Lithic content Bulk tephra grain-size data are not available due to the intensive induration of the deposit. We therefore determined from crushed samples of at least 3 kg the amount of lithic fragments > 4 mm in the basal zone of the main pumice fall and the lithic content > 2 mm from 1.5 kg samples of the top layer of the upper pumice. Little systematic variation is visible within the main pumice fall deposit in the diameter of the five largest lithics (ML5 value; Fig. 9A), or the median (Md) > 4 mm lithic content. Nevertheless, proximal
147
locations south of BahGeli-Ayvali are lithic rich (Fig. 9B) and show high ML5 values as well as coarse Md values. A continuous outward decrease of lithic size is observed only along the N-S axis extending from locations 8 and 13 northward to Goreme-Avanos-ozkonak that roughly coincides with the dispersial axis of the bulk tephra obtained from the isopach map (Fig. 8b). Unexpected high amounts of quite large lithics (ML5 up to 6.3 cm in location 56) occur around Gulsehir, about 15 km off the dispersial axis. In the coarse-grained top layer of the upper pumice beds, distribution histograms show a more or less continous decrease in grain size with distance from vent (Fig. 9C). A second distal maximum in lithic content developed between Giireme and Zelve, about 15 to 20 km from the assumed vent area. The distribution histograms show a preferential increase in the amount of the 2-8 mm fractions which is responsible for the distal maximum. 4.3. Vent position The vent area was located by volcanological criteria such as thickness of pumice fallout, area1 distribution of accidental lithic clasts, depositional facies of surge deposits, and flow direction measurements of parting lineations. The most useful criterion is the increasing thickness of pumice fall deposits toward the vent. This is invalid, however, for those deposits which have their maximum thickness away from the vent. The combination of both isopach maps for the main pumice fall and the upper pumice, however, provides evidence to locate the vent area about 6 km north-northeast of Kaymakli (Fig. 6). Isopach maps are less valuable for flow deposits whereas the change in depositional facies from distal plane-parallel to proximal cross-stratified deposits is more reliable (Wohletz and Sheridan, 1979). Therefore, we interprete the cross-stratified deposits of the lower surge series south of Bahqeli to be deposited within a few km of the vent. Flow direction measurements of parting lineations on bedding planes are consistent with the other criteria and again locate the source area of the AZ1 products north-northeast of Kaymakli (Fig. 8). The increasing grain size towards the vent, either measured as the median of bulk tephra samples or by the maximum lithic and pumice size is also widely
148
R. Schumacher, U. blues-Schumacher/
Journal of Vokanology and Geothermal Research 78 (19971139-153
ijzkonak
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-- Nevsehir
Fig. 8. Area1 distribution, thickness, and flow directions measured from parting lineations of the lower surge series. Flow directions, especially in the Bahgeli-Ayvali region, are consistent with the vent location north-northeast of Kaymakli. A deviation in flow direction of + 22.5“ is attributed to individual lineation measurements.
used. The data for the distribution of lithic clasts (Fig. 9) show consistent high values of both size and amount southwest of Bahseli-Ayvali. Despite the complications owing to the distal clast-size maxi-
mum around Gulsehir (main pumice) and the concentration maximum in the Goreme-Zelve area (upper pumice), the clast data sufficiently support the vent position indicated by the other criteria.
Fig. 9. Regional variations of the amount and size of accidental lithic clasts in the main pumice fall deposit and the uppermost sublayer of the upper pumice beds. (A, B) The mean of the 5 hugest lithics (MU) and weight percentage of lithics > 4 mm for the main pumice fall deposit indicate relatively high amounts of large clasts in the Bahceli-Ayvali region and northwest of Nevsehir. Thin stippled lines indicate isopachs from Fig. 6 for comparative orientation. (Cl The weight percentage of lithics > 2 mm for the upper pumice beds indicates a second distal high in the Giireme-Zelve region due to preferential accumulation of particles 2-8 mm in size. Thin stipples lines indicate 1 cm and 30 cm isopachs from Fig. 6 for comparative orientation.
R. Schumacher, U. Mues-Schumacher/
Journal of Volcanology and Geothermal Research 78 (19971 139-153
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R. Schumacher, II. Mues-Schumacher/
Journal of Volcanology and Geothermal Research 78 (1997) 139-153
The sensitivity of the isopach maps, flow-direction measurements and clast-size distribution suffices to locate the vent only within a few kilometers. Moreover, experience shows that plinian deposits containing lithic fragments always include ballistic clasts within 3 to 5 km of their vents (Walker, 1981a). Maximum clast size of + 10 cm at location 30 southwest of BahGeli is barely large enough to be interpreted as ballistic. We therefore conclude that the vent is beyond that critical distance, supported by the depositional facies of the surge deposits in locations 19, 22 and 30 which lack antidunes that are expected closer than about 4 km to the vent. A rival vent position for the AZ1 eruption products was previously inferred from a combination of volcanological (isopach maps) and geophysical data (Bouguer gravity) to lie about 8 km due south of Nevsehir (Le Pennec et al., 1994). In general, rhyolitic vent areas frequently have gravity anomalies on the order of - 10 to -30 mgal, owing to a large mass deficit (Williams and McBirney, 1979). The -95 mgal anomaly south of Nevsehir (Ekingen and Guven, 1978; Froger et al., 1994) is therefore a tempting site to locate a vent. So, we are left with the alternative of a vent position in the center of the gravity anomaly that would be supported volcanologically by the high total amount of lithics in locations 8 and 13 and, to some extent, also by the large lithics around Gulsehir (Fig. 9A). However, some vents show little or no gravity anomaly. For example, there is little evidence from the gravity map of the Yellowstone area to locate the eruptive center of the 250 km3 Mesa Falls Tuff (Christiansen, 19821, and Crater Lake Caldera, Oregon, exhibits only -5 mgal in gravity (Williams and McBimey, 1979). We consider that the volcanological data presented here, obtained from 58 locations - 30 of which hold for the upper pumice better fit to a vent location north-northeast of Kaymakli and that the center of the gravity anomaly is related to different eruptions.
5. Interpretation
and discussion
5.1. Eruptive succession The stratigraphic succession of the AZ1 eruption, a major surge series between the initial plinian
pumice fall and the final pyroclastic flows, is somewhat unusual compared with many other plinian style eruptions, e.g., the 79 A.D. eruption of Somma-Vesuvius (Sigurdsson et al., 1985). At Vesuvius, surge deposits mark the final stages of the eruption when the mass eruption rate decreased and external water probably gained access to the conduit. Such a mechanism is also inferred by Van den Bogaard and Schmincke (1984) to explain the final surge deposits of the Laacher See eruption in the East Eifel. A close analogy to the AZ1 eruptive cycle, however, is provided by the 1400 yr B.C. Minoan eruption of the island of Santorini (Bond and Sparks, 1976; Druitt et al., 1989). The surge deposits of the Minoan B stratigraphic unit are analogous to the lower and upper surge series of the AZ1 eruption. The formation of Minoan B is attributed to the influx of seawater into the conduit and/or the upper part of the magma chamber (Druitt et al., 1989). A similar process must be assumed for the AZ1 eruption and can be best explained by water from the marshes and marginal flats of the northern lacustrine sedimentary basin mentioned above. 5.2. Eruptive style Close examination shows that not only the lower and upper surge series were phreatomagmatic but the ubiquitous accretionary lapilli indicate the main pumice fall was also influenced by external water from the very beginning. A close analogy of contemporaneous deposition of accretionary lapilli and pumice lapilli is the Hachinohe Ash (Hayakawa, 1990). This eruption from the Towada caldera evidently penetrated the caldera lake which provided the large amount of external water for formation of accretionary lapilli and armoured lapilli after condensation in the eruption plume. Similarly, the 1979 eruption of Soufriere of St. Vincent burst through the crater lake and the fallout of accretionary lapilli was directly observed (Brazier et al., 1982). We assume only a minimum phreatomagmatic influence on the eruptive style of the initial plinian phase of the AZ1 eruption. The lack of coarse-grained particles in the accretionary lapilli indicates minor amounts of external water compared with the Hachinoe or Soufribre eruptions. Schumacher and Schmincke (1995) documented experimentally the
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amount of water necessary for the formation of aggregates increases with grain size, and, in turn, the grain size of ash found in aggregates may indicate the amount of water during their formation. We therefore assume that the accretionary lapilli of the AZ1 main pumice formed at or close to the minimum conditions of aggregate formation whereas those of the Hachinohe Ash formed under more favourable conditions of higher water content. Phreatomagmatic activity was probably greater in the sector of Bahgeli-Ayvali due to a local higher influx of external water which could account for: (a) the high amounts of coarse lithic lapilli; (b) the higher amount of accretionary lapilli in location 17 than elsewhere; (c) the ash layers of surge origin in the upper part of the main pumice fall deposit; and (d) the fine-grained ash beds coming from lateral flow within the upper pumice beds. A similar regionally modified eruptive style due to local ingress of external water is known from the Laacher See eruption in the East Eifel where Van den Bogaard and Schmincke ( 1984) distinguished between the Mendig fucies south of the Laacher See dominated by deposition from lateral flow due to locally limited phreatic influence and the Nickenich facies east of the volcano dominated by contemporaneous pumice fall deposits out of a stable convecting eruption plume. The influx of external water gradually increased during the eruption of the main pumice, as suggested by the layered upper part of the deposit. To account for the layering, we assume distinct fluctuations in the continuity of eruption accompanied by a change from a sustained eruption plume to an intermittent one. In general, due to accidental phreatomagmatic explosions, the phreatic influence on the eruptive style persisted into the ignimbrite producing phase, at least up to the end of deposition of the intermittent surges which are intercalated with pyroclastic flow deposits in the basal zone of the Akdag-Zelve ignimbrue. 5.3. Column height, deposition Various methods the eruption column its deposits. Carey model based on the ments. Application
and volume
have been proposed to deduce height from the dispersal fan of and Sparks (1986) proposed a areal distribution of lithic fragto the AZ1 eruption is difficult
151
because only parts of the dispersal fan are exposed, and, moreover, a deviation between the assumed lithic fan and the bulk tephra fan is indicated in both fall deposits that affects the measure of the maximum cross-wind range. Best estimates for the main pumice fall deposit imply column heights > 45 km and wind velocities of about lo-20 m/s. The observed 2 m of fallout in Akkijy, 30 km southeast of the vent, indicate a considerable upwind umbrella region of the eruption column. Estimates for the culminating phase of the upper pumice beds appear to be more precise and a column height of about 25 km and a wind velocity of I 30 m/s is obtained. Mass eruption rates on the order of roughly (3-)5 X lo8 kg/s for the main pumice fall and (3-5) X 10’ kg/s for the culmination of the upper pumice eruption are given by the models of Sparks et al. (1978) and Sparks (1986). The main plinian phase thus ranks in the same order of magnitude with the 1956 eruption of Bezymianny, the Waimihia plinian pumice eruption, New Zealand (Walker, 1981a) or the 1815 Tambora eruption (Sigurdsson and Carey, 1989) which are slightly less than the Taupo (ultra)plinian phase (Walker, 1980). The culmination of the second plinian phase may have resembled that of the 1947 eruption of Hekla and was slightly less than the eruptive phase of the white pumice of the 79 A.D. eruption of Somma-Vesuvius. Deposition of tephra from these eruption clouds produced a distinct second thickness maximum 20 km from vent in the main pumice fall, which is quite far downwind compared with similar maxima of other eruptions, e.g., 79 A.D. Vesuvius or Laacher See MLST-Cl pumice (Fig. 7). These thickness maxima in pumice fall deposits can be related to eruption column height, wind velocity, and primary grain-size distribution in the plume. This relationship implies a local increase in the frequency of a certain grain-size fraction within the thickness maximum relative to neighbouring upwind and downwind locations. However, bulk tephra grain-size analyses could not be obtained and a substitute, the lithic fraction, does not show this relationship (Fig, 9A, B). In contrast, the upper pumice beds forming a simple, elongated fallout fan show an increase in lithic content in the Giireme-Zelve region. Grain-size frequency histograms (Fig. 9C) indicate preferential accumulation of the 2-8 mm fraction and we there-
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fore assume that the second distal high in lithic content is due to wind sorting processes. Reliable volume estimates are also difficult to obtain, because the dispersal fans are incomplete. A conventional estimate can be based on extrapolations of area-thickness plots (Walker, 1981b) and on the isopach maps of Fig. 6 assuming a distinct upwind umbrella region of the eruption plume. The method proposed by Pyle (1989) on the basis of In(thickness)-area’/’ plots and the thickness - half distance obtained from that plot provides a better evaluation of the far downwind mass according to the equation V = 13.08 X To b: (V = volume; T, = extrapolated maximum thickness; b, = the thickness - half distance). Corresponding tephra volumes are about 41.5 km3 for the main pumice and 0.35 km3 for the upper pumice, respectively. Adding the bulk tephra volume of the ignimbrite of 120 km3 (Le Pennec et al., 1994) and the other deposits coming from lateral flow, a total of 180 km” of tephra appears to be a valid approximation for this unusual Cappadocian eruption.
Acknowledgements We thank our Turkish collegues Yilmaz Savascin (Izmir), Vedat Toprak (Ankara) and Orhan Tartar (Sivas) who helped us exporting samples. George Walker and Don Swanson reviewed the manuscript; we gratefully acknowledge their helpful comments and their patience to smooth the language. Hilary Downes again performed a final language editing.
References Bond, A. and Sparks, R.S.J., 1976. The Minoan eruption of Santorini, Greece. J. Geol. Sot. London, 132: l-16. Brazier, S., Davis, A.N., Sigurdsson, H. and Sparks, R.S.J., 1982. Fallout and deposition of volcanic ash during the 1979 explosive eruption of the Soufri&re of St. Vincent. J. Volcanol. Geotherm. Res., 14: 335-359. Carey, S. and Sparks, R.S.J., 1986. Quantitative models of the fallout of pyroclasts from explosive eruption columns. Bull. Volcanol., 48: 109-125. Christiansen, R.L., 1982. Late Cenozoic volcanism of the Island Park area, eastern Idaho. In: B. Bonichsen and R.M. Breckenridge (Editors), Cenozoic Geology of Idaho. Idaho Bur. Mines Geol. Bull., 26: 345-368.
Druitt, T.H., Mellors, R.A., Pyle, D.M. and Sparks, R.S.J., 1989. Explosive volcanism on Santorini, Greece. Geol. Mag., 126: 95-126. Ekingen, A. and Gllven, C., 1978. Jeotermik enerji aramalari AcigGl (Nevsehir) bolgesi gravite etlidl. Int. Rep. Miner. Res. Explor. Inst. Turk. 6289. Froger, J.L., Le Pennec, J.-L., Lenat, J.F., Bourdier, J.L. and Kose, 0.. 1994. The missing calderas of Cappadocia. IAVCEI Congr. Ankara, Abstract Vol. Hayakawa, Y., 1990. Mode of eruption and deposition of the Hachinohe phreatoplinian ash from the Towada Volcano, Japan. Geogr. Rep. Tokyo Metropolitan Univ., 25: 167-182. Le Pennec, J.-L., Bourdier. J.-L., Froger, A.. Temel, A., Camus, G. and Gourgaud, A., 1994. Neogene ignimbrites of the Nevsehir plateau (central Turkey): stratigraphy, distribution and source constraints. J. Volcanol. Geotherm. Res., 63: 59-87. Mues-Schumacher, U. and Schumacher, R., 1997. Problems of stratigraphical correlation and new K-Ar data of ignimbtites in Cappadocia, Central Turkey. Intemat. Geol. Rev., Spec. Iss. Proc. 2nd Int. Workshop Geol. Turkey, Sivas (in press). Pyle, D.M., 1989. The thickness, volume and grainsize of tephra fall deposits. Bull. Volcanol., 51: l-15. Rosi, M., 1992. A model for the formation of vesiculated tuff by the coalescence od accretionaty lapilli. Bull. Volcanol., 54: 4299434. Schumacher, R. and Schmincke, H.-U., 1991. Internal structure and occurrence of accretionary lapilli a case study at Laacher See volcano. Bull. Volcanol., 53: 612-634. Schumacher, R. and Schmincke, H.-U., 1995. Models for the roigin of accretionary lapilli. Bull. Volcanol., 56: 626-639. Schumacher, R., Keller, J. and Bayhan, H.. 1990. Depositional characteristics of ignimbrites in Cappadocia, Central Anatolia, Turkey. In: M.Y. Savascin and A.H. Eronat (Editors), Proc. Int. Earth Sci. Congr. on Aegean Regions (IESCA 1990). 2: 435-449. Sigurdsson, H. and Carey, S.. 1989. Plinian and co-ignimbrite tephra fall from the 18 15 eruption of Tambora volcano. Bull. Volcanol., 51: 243-270. Sigurdsson, H., Carey, S., Cornell, W. and Pescatore, T., 1985. The eruption of Vesuvius in A.D. 79. Natl. Geogr. Res., 1: 332-387. Sparks, R.S.J., 1986. The dimensions and dynamics of volcanic eruption columns. Bull. Volcanol., 48: 3-15. Sparks, R.S.J.. Wilson, L. and Hulme, G., 1978. Theoretical modeling of the generation, movement, and emplacement of pyroclastic flows by column collapse. J. Geophys. Res.. 83: 1727-1739. Top&, V., Keller, J. and Schumacher, R.. 1994. Volcano-tectonic features of the Cappadocian volcanic province. lAVCE1 Int. Volcanol. Congr., Ankar?, Excursion Guide, pp. l-58. Van den Bogaard, P. and Schmincke, H.-U., 1984. The eruptive center of the late Quatemary Laacher See Tephra. Geol. Rundsch., 73: 933-980. Viereck, L.G., 1984. Geologische und petrologische Entwicklung des pleistozlnen Vulkankomplexes Rieden, Osteifel. Bochumer Geol. Geotech. Arb., 17: l-337. Walker, G.P.L., 1980. The Taupo pumice: product of the most
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powerful known (ultraplinian) eruption. J. Volcanol. Geotherm. Res., 8: 69-94. Walker, G.P.L., 1981a. The Waimihia and Hatepe plinian deposits from the rhyolitic Taupo volcanic center. New Zealand. J. Geol. Geophys., 24: 305-324. Walker, G.P.L., 1981b. Plinian eruptions and their products. Bull. Volcanol., 44: 223-240.
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Walker, G.P.L. and Croasdale, R., 1970. Two plinian-type eruptions in the Azores. J. Cieol. Sot., 127: 17-55. Williams, H. and McBimey, A.R., 1979. Volcanology. Freeman, San Francisco, CA, 397 pp. Wohletz, K.H. and Sheridan, M.T., 1979. A model of pyroclastic surge. Geol. Sot. Am. Spec. Pap., 180: 177-194.
and Geothermal
Research 78 (1997) 139- 153
The pre-ignimbrite ( phreato) plinian and phreatomagmatic phases of the Akdag-Zelve ignimbrite eruption in Central Anatolia, Turkey Rolf Schumacher
*,
Ulrike Mues-Schumacher
HauptstraJe 3, 71672 Marbach Germany
Received 26 April 1996; accepted 13 December 1996
Abstract The eruptive sequence of the 7.6-Ma-old Akdag-Zelve ignimbrite (AZ11 eruption comprises five units totalling up to > 50 m on Akdag mountain. They are, in stratigraphic order: (a) the main pumice full deposit (up to 10 ml; (b) the lower surge series (up to 4 m); (cl the upper pumice beds (up to 0.8 ml; (d) the upper surge series (up to 5 ml; and (e) the Akdug-Z&e ignimbrite (up to 40 m). The main pumice fall deposit shows a second distal thickness maximum about 20 km from the source, whereas the upper pumice beds form a simple NNE-extending fan. The source area, about 6 km north-northeast of Kaymakli, is inferred from fallout thickness, size and amount of lithic clasts, depositional facies and flow directions of the surge deposits. The main pumice fall deposit encloses accretionary lapilli up to 6 cm in size indicating a steady phreatomagmatic influence on the eruptive style from the very beginning. The major surge sequence beneath the ignimbrite is similar to that of the 1400 yr B.C. Minoan eruption on Santorini, Greece, and resembles the Minoan B stratigraphic unit attributed to phreatomagmatic eruptions due to ingression of seawater into the conduit. Similarly, the depositional environment of marshes and marginal flats of a lacustrine basin in Cappadocia accounts for the external water supply and the generation of surges during the AZ1 eruption. Estimates of eruption column height of 2 45 km indicate that the AZ1 eruption ranks in the order of the 1956 Bezymianny, 1815 Tambora and the Waimihia Plinian pumice eruption of
New Zealand. Volume estimates of the erupted tephra are on the order of 41.5 km3 for the main pumice and 0.35 km3 of the upper pumice. Keywords:
Central Anatolia; phreatoplinian
eruptions;
eruptive successions;
1. Introduction The Central or Cappadocian Volcanic Province (CVP) is one of the four major volcanic provinces in Turkey: Eastern, Central and Western Anatolia, and
* Corresponding 711-9765013
author.
Phone:
+ 49-7144-36596;
Fax:
+49-
0377-0273/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZZSO377-0273(96)00106-O
accretionary
lapilli
the Galatean region (Fig. 1). The CVP, terminated and partly transected by major fault zones, extends for about 300 km southwest-northeast and is surrounded by tectonic basins and massifs of the central part of the Anatolian block (Toprak et al., 1994, and references therein). The 7.6-Ma-old Akdag-Zelve ignimbrite (AZI; K-Ar age, Mues-Schumacher and Schumacher, 1997) is part of the Cappadocian tephra series that com-
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0 C
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Akdag-Zelve
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141
Ignimbrite
Series Upper ignimbri trUnsitiona1
surge
-
te facies
Main Pumice Fall Deposit
Secondary volmniclostic -
marginal
flats
-
Fig. 2. Eruptive sequence of the Akdag-Delve ignimbrite eruption documented from proximal location 22 and distal location 27. Five major units can be identified in near-vent locations. Farther away from the source, definition of the upper surge series is hampered due to a fine-grained distal pyroclastic flow facies; therefore, the surge-ignimbrite transitionalfacies was defined.
Fig. 1. Location of study area. (A) The geological sketch map of the Cappadocian Volcanic Province (CVP) in central Turkey illustrating the main geological and volcanological features. The CVP is composed of a complex succession of volcaniclastic deposits and several volcanic complexes - the most important are Erciyes Dag (I), Melendiz Dag (2) and Hasan Dag (3) stratovolcanoes. Major geotectonic fault zones transect the province. (B) The location map of the study area shows sample locations of the AZI eruptive products. Figures denote the locations mentioned in the text; circled figures represent the locations of the stratigraphic sections shown in Fig. 2. The triangle denotes Akdag mountain. (C) The stratigraphic section illustrates the ignimbrite stratigraphy of Cappadocia that comprises at least 10 ignimbrites intercalated with pumice fall, surge deposits and extensive reworked material.
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2. Eruptive
sequence
The deposits of the initial, pre-ignimbrite eruptive phase consist of rhyolitic pumice fall and fine-grained ash deposits, the latter chiefly coming from pyroclastic surges. The eruption products can be subdivided into five units that are in stratigraphic order (Fig. 2): (5) the Akdag-Zelve ignimbrite; (4) the upper surge series; (3) the upper pumice beds; (2) the lower surge series; (1) the main pumice fall deposit. Minor ash-and-pumice deposits of pyroclastic flow origin locally occur at the base of the lower surge series in proximal locations. The thickness of the entire sequence is up to 50 m on Akdag mountain. 2.1. Main pumice fall deposit
‘4
4 0 16 64 2
mm
4 8 16 64mm Fig. 3. Variation of lithic content through the main pumice fall deposit as analysed in proximal location 22. There is a general decrease in size and amount with height in stratigraphy except for two lithic concentration layers. The grain-size distribution of the ash layer is shown in Fig. 5.
prizes eight major and two minor ignimbrites, several pumice falls and surge deposits. Tephra is interbedded with extensive secondary volcaniclastic sediments and fossiliferous lacustrine limestones (Schumacher et al., 1990; Toprak et al., 1994). We have studied the AZ1 eruption because of its complexity starting with initial pumice fallout that subsequently graded through surge deposits into the ignimbrite-producing phase. The general phreatomagmatic influence during the whole eruption produced unusual volcanological features, and the interpretation of the well exposed accretionary lapilli-bearing pumice fall deposit provides an essential contribution to the understanding of the volcanological evolution of Cappadocia.
The main pumice fall deposit consists of rhyolite pumice and is coarser than other similar deposits of the Cappadocian tephra series. It is conspicuous for its intense induration (up to building stone quality with a bulk rock density of 1050-l 100 kg/m? that frequently causes its faint internal bedding to be obscured. The deposit consists of two well-defined parts with contrasting internal characteristics. The lower massive part (Fig. 2) is nearly homogeneous throughout its thickness, but locally shows a faint internal stratification due to fluctuations in grain size and lithic content. The lithic concentration, as analysed in location 22, is as high as 9 wt.% and generally decreases upward (Fig. 3). The upper strut@ied part is an internally layered pumice deposit. In proximal locations (e.g., location 22) in the Bahgeli-Ayvali region, strongly undulating and anastomosing ash beds, 5-20 cm thick, occur in this unit (Fig. 4b). Individual ash layers are normally graded as is typical of laminated surge deposits higher up in the stratigraphy and is also known from other phreatomagmatic eruptions - e.g., the final stage of the Laacher See (Van den Bogaard and Schmincke, 1984) and the Rieden volcano, East Eifel (Viereck, 1984). Fine-ash beds intercalated with pumice falls are also reported from the deposits of proximal Fogo A (Walker and Croasdale, 1970) and the Waimihia plinian pumice deposit (Walker, 1981a). Grain-size analyses of one layer show (a)
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fairly well sorted unimodal distribution within each sublayer, and (b> unimodal grain-size distribution of the entire bedding set (Fig. 5). Accretionary lapilli occur in the lower part of the main pumice fall deposit as ellipsoidal, more or less concentric mud b&s up to 6 cm in diameter (Fig. 4a). They are generally rare components of the pumice fall throughout the whole study area but are abundant in location 17 north of Ayvali. Apparently, individual showers of aggregates accompanied the steady pumice fall. Accretionary lapilli associated with pumice fall deposits are documented from the Hachinohe Ash Towada Caldera, Japan (Hayakawa, 19901 - where they frequently enclose pumice and other coarse material; similarly they are found at the So&i&e of St. Vincent (Brazier et al., 19821. Compared to these examples, the accretionaty lapilli of the AZ1 main pumice are very fine-grained and no coarse material was found in the center. 2.2. Surge series deposits Both surge series consist of alternating, internally bedded,particulate ash-and-pumice and fine-ash beds. Proximal locations, e.g., 19, 22 and 30 south of Bahseli, show distinct cross-stratification (with minor sand-wave bedding) and mutual erosion of individual layers or sets of layers (Fig. 4c). As distance from source increases, bedding gradually changes into plane-parallel or very low angle cross- stratification. Fine-grained ash beds, especially beneath and on top of the intercalated upper pumice beds, enclose abundant accretionary lapilli (rim-type of Schumacher and Schmincke, 1991). A very thin bedded but widely distributed vesiculated tuff layer from the upper surge series is typically associated with coretype accretionary lapilli. Internal characteristics of the deposit indicate its formation from the coalescence of accretionary lapilli and other irregular aggregates and ash clots as described by Rosi (1992) and Schumacher and Schmincke (1995). A distinctly fines-depleted particulate facies of the surge series, exposed on Sarimaden Tepe and an adjacent hill (locations 8 and 131, is attributed to locally increased turbulence that winnowed out the fines.
143
2.3. Upper pumice beds The upper pumice beds represent a minor plinian pumice fall intercalated between the lower and the upper surge series (Fig. 4d). The deposit is readily identified by its tripartite subdivision into a finegrained basal layer, a medium-grained central layer, and a coarser-grained upper layer. The uppermost layer has a distinct basal parting throughout most of the study area that indicates a short pause in fallout deposition. Proximal locations south of BahGeliAyvali (19, 22, 30 and 58) expose a corresponding ash layer, lo- 15 cm thick, that shows internal (cross)bedding structures, accretionary and armoured lapilli. The combination of these features indicates deposition from a small and brief lateral flow that shortly interrupted the pumice fall.
2.4. The Akdag-Z&e
ignimbrite
The ignimbrite proper is generally massive, although internal layers of pumice concentration indicate a multiple flow deposit. A particular characteristic of the basal part is the presence of at least three intermittent surge layers, each of which is up to 1 m thick (Fig. 4e). They extend throughout the study area and occur as far as &konak, at least 30 km from the assumed vent. The deposits are finer than the ignimbrite and have internal cross-bedding and rim-type accretionary lapilli. Parting lineations on bedding planes indicate deposition in the upper flow regime. Distinction of this basal ignimbrite from the upper surge series is difficult in distant places, e.g., location 27 Hasan Kilesi, Avanos (Fig. 21, where both deposits have planar beds and the pyroclastic flow deposits are relatively thin and also enclose abundant accretionary lapilli. These deposits are therefore termed transitional facies between the upper surges and the massive ignimbrite.
3. Depositional environment The main pumice fall deposit overlies secondary volcaniclastic, reworked shallow lacustrine sediments in most of the study area. Internal bedding includes subaqueous slumping and convolute bed-
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ding resulting from dehydration of individual sediment layers. The deposits are topped by extremely fine-grained material which is interpreted as a lacustrine stagnant water sediment (Fig. 4a). The thickness is between 3 and 6 cm and no systematic area1
thickness variation was detected (e.g., thickening either toward the assumed source area or within topographic depressions). This rules out the interpretation that this fine-grained ash is an integral part of the eruption.
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145
poraneous reworking. The best examples are exposed south of Ulasli, in Sarihidir and northeast of Gulsehir. A striking example of subaerial paleotopography is exposed at location 54, Urglip. The lower surge series was emplaced on a steep slope and it thickens as the dip decreases in the valley bottom. This transition of the marginal facies of a ponded deposit into the overbank veneer facies indicates a paleodepression fill.
&J 0
530 20 IO .063.125.25 .5 I. 2. 4. 8.mm Grainsize Fig. 5. Grain-size analyses of ash from a surge deposit interbedded within the proximal main pumice fall deposit. Dotted part: coarse basal sublayer; hatched part: fine-grained upper sublayer-both sublayers are unimodal in grain-size distribution. The cross-hatched insert shows the unimodal size distribution of the entire bedding set.
Soft sediment deformation, especially load casting of pumice, indicates that the sediment was wet during emplacement of the pumice fall. Moreover, alignment of large pumice lumps and the scarcity of lithic clasts suggesting floating pumice rafts indicates in several outcrops in the Urgtip-GoremeAvanos area that at least some of the pumice fell into very shallow water (< 0.5 ml of more or less isolated ponds typical of a marsh or marginal flat environment of a lacustrine basin. The recognition of the depositional environment of a lacustrine sedimentary basin that was progressively filled by pyroelastic deposits is essential to understand the mode of eruption and emplacement of AZ1 tephra. Some other locations, chiefly in the northern and northwestern part of the study area indicate deeper water facies of subaqueous deposition and contem-
4. Area1 distribution 4. I. Thickness The eruptive products are well exposed within the mapped area of about 1200 km2. Outcrops are limited by sedimentary and younger volcanic cover and thickness varies by primary depositional factors such as deposition of the pumice into a subaqueous environment (north of Gulsehir) where the original thickness was syn-depositionally modified by floating and reworking of pumice. The main pumice fall shows a distinct distal thickness maximum of > 8.5 m in the Zelve-Avanos area (Fig. 6A), nearly as high as the measured near-vent maximum of about 10 m (11 m extrapolated maximum thickness To according to Pyle, 1989). Similar distal thickness maxima in pumice fall deposits are reported, for example, from the 79 A.D. eruption of Somma-Vesuvius (Sigurdsson et al., 1985), from the MLST-Cl deposit of the Laacher See (Van den Bogaard and Schmincke, 1984), and from the 1980 eruptions of Mount St. Helens (Fig. 7). An unexpected occurrence of about 2 m of main pumice fall was detected in Akkoy, about 30 km away from the vent in a cross-wind direction perpendicular to the main fan axis.
Fig. 4. Photographs illustrating depositional characteristics of the AZ1 eruptive products. (a) Basal zone of the main pumice fall deposit overlying the very fine-grained lacustrine stagnant water sediment (location 17 north of Ayvali). The pumice deposit is characterized by abundant accretionary lapilli (some are indicated by arrows) which deposit fine ash together with coarse pumice lapilli. Maximum size of aggregates is up to 6 cm. (b) Undulating ash layers within the main pumice fall deposit (location 22) that are interpreted as phreatomagmatic surge deposits at the boundary between the frequently massive lower part and the generally stratified upper part of the pumice deposit, (c) Tbe lower surge series in proximal location 30 is composed of internally bedded ash layers showing cross-stratification and mutual erosion of individual sets of layers. (d) The upper pumice beds (location 22) are subdivided into 3 sublayers, the uppermost of which is coarsest, Note the tine-grained ash (arrow) beneath the upper pumice sublayer, which was probably deposited from lateral flow indicated by internal bedding structures. (e) Entire eruptive sequence of AZ1 as exposed in proximal location 58. MF = main pumice fall deposit; LS = lower surge series; UP = upper pumice beds; US = upper surge series; AZ1 = Akdag-Zelve ignimbrite proper; IS = intermittent surge deposits which are intercalated between basal flow units of the proximal ignimbtite.
I
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.
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Fig. 6. lsopach maps of the main pumice fall and the upper pumice beds. The accuracy of the isopach maps is only to locate the source within a few kilometers that evaluation of alternative vent positions - at the center of a Bouguer gravity low (Le Pennec et al., 1994; Froger et al., 1994) south of Nevsehir or north-northeast of Kaymakli deduced from the data presented here - is difficult (see chapter cenf position for details). (A) The main pumice fall deposit has a second thickness maximum 20-30 km away from the assumed source area. The hatched area is where tine-grained surge deposits occur at the boundary between the massive lower and the stratified upper part of the deposit. (B) The upper pumice beds form a simple NNE-extending fan. The hatched area is where a fine-grained ash layer was found beneath the top pumice sublayer (see also Fig. 4d).
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4
40
50km Distance from source
Fig. 7. Thickness variation of fall deposits with distance from source. The main pumice is compared with similar deposits that have a distal thickness maximum. After Sigurdsson et al. (1985); data for the Laacher See MIST-CI pumice from Van den Bogaard and Schmincke (1984).
The upper pumice beds representing the second, minor plinian phase of the eruption form a simple depositional NNE-elongated fan (Fig. 6B). The thickness of the whole deposit consistently decreases from 83 cm at location 30 south of Bahqeli to 6 cm in location 28 near Avanos, about 25 km from the source. Farther north, around Gzkonak, a l-1.5-cmthick particulate layer overlying fine-grained ash with accretionary lapilli is interpreted as the upper pumice beds. The lower surge series was identified throughout the study area and is thickest in the Bahqeli-Ayvali region where the isopach map indicates individual lobes (Fig. 8). Flow-direction measurements of parting lineations on bedding planes indicate a more or less radial dispersal. 4.2. Lithic content Bulk tephra grain-size data are not available due to the intensive induration of the deposit. We therefore determined from crushed samples of at least 3 kg the amount of lithic fragments > 4 mm in the basal zone of the main pumice fall and the lithic content > 2 mm from 1.5 kg samples of the top layer of the upper pumice. Little systematic variation is visible within the main pumice fall deposit in the diameter of the five largest lithics (ML5 value; Fig. 9A), or the median (Md) > 4 mm lithic content. Nevertheless, proximal
147
locations south of BahGeli-Ayvali are lithic rich (Fig. 9B) and show high ML5 values as well as coarse Md values. A continuous outward decrease of lithic size is observed only along the N-S axis extending from locations 8 and 13 northward to Goreme-Avanos-ozkonak that roughly coincides with the dispersial axis of the bulk tephra obtained from the isopach map (Fig. 8b). Unexpected high amounts of quite large lithics (ML5 up to 6.3 cm in location 56) occur around Gulsehir, about 15 km off the dispersial axis. In the coarse-grained top layer of the upper pumice beds, distribution histograms show a more or less continous decrease in grain size with distance from vent (Fig. 9C). A second distal maximum in lithic content developed between Giireme and Zelve, about 15 to 20 km from the assumed vent area. The distribution histograms show a preferential increase in the amount of the 2-8 mm fractions which is responsible for the distal maximum. 4.3. Vent position The vent area was located by volcanological criteria such as thickness of pumice fallout, area1 distribution of accidental lithic clasts, depositional facies of surge deposits, and flow direction measurements of parting lineations. The most useful criterion is the increasing thickness of pumice fall deposits toward the vent. This is invalid, however, for those deposits which have their maximum thickness away from the vent. The combination of both isopach maps for the main pumice fall and the upper pumice, however, provides evidence to locate the vent area about 6 km north-northeast of Kaymakli (Fig. 6). Isopach maps are less valuable for flow deposits whereas the change in depositional facies from distal plane-parallel to proximal cross-stratified deposits is more reliable (Wohletz and Sheridan, 1979). Therefore, we interprete the cross-stratified deposits of the lower surge series south of Bahqeli to be deposited within a few km of the vent. Flow direction measurements of parting lineations on bedding planes are consistent with the other criteria and again locate the source area of the AZ1 products north-northeast of Kaymakli (Fig. 8). The increasing grain size towards the vent, either measured as the median of bulk tephra samples or by the maximum lithic and pumice size is also widely
148
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Journal of Vokanology and Geothermal Research 78 (19971139-153
ijzkonak
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used. The data for the distribution of lithic clasts (Fig. 9) show consistent high values of both size and amount southwest of Bahseli-Ayvali. Despite the complications owing to the distal clast-size maxi-
mum around Gulsehir (main pumice) and the concentration maximum in the Goreme-Zelve area (upper pumice), the clast data sufficiently support the vent position indicated by the other criteria.
Fig. 9. Regional variations of the amount and size of accidental lithic clasts in the main pumice fall deposit and the uppermost sublayer of the upper pumice beds. (A, B) The mean of the 5 hugest lithics (MU) and weight percentage of lithics > 4 mm for the main pumice fall deposit indicate relatively high amounts of large clasts in the Bahceli-Ayvali region and northwest of Nevsehir. Thin stippled lines indicate isopachs from Fig. 6 for comparative orientation. (Cl The weight percentage of lithics > 2 mm for the upper pumice beds indicates a second distal high in the Giireme-Zelve region due to preferential accumulation of particles 2-8 mm in size. Thin stipples lines indicate 1 cm and 30 cm isopachs from Fig. 6 for comparative orientation.
R. Schumacher, U. Mues-Schumacher/
Journal of Volcanology and Geothermal Research 78 (19971 139-153
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R. Schumacher, II. Mues-Schumacher/
Journal of Volcanology and Geothermal Research 78 (1997) 139-153
The sensitivity of the isopach maps, flow-direction measurements and clast-size distribution suffices to locate the vent only within a few kilometers. Moreover, experience shows that plinian deposits containing lithic fragments always include ballistic clasts within 3 to 5 km of their vents (Walker, 1981a). Maximum clast size of + 10 cm at location 30 southwest of BahGeli is barely large enough to be interpreted as ballistic. We therefore conclude that the vent is beyond that critical distance, supported by the depositional facies of the surge deposits in locations 19, 22 and 30 which lack antidunes that are expected closer than about 4 km to the vent. A rival vent position for the AZ1 eruption products was previously inferred from a combination of volcanological (isopach maps) and geophysical data (Bouguer gravity) to lie about 8 km due south of Nevsehir (Le Pennec et al., 1994). In general, rhyolitic vent areas frequently have gravity anomalies on the order of - 10 to -30 mgal, owing to a large mass deficit (Williams and McBirney, 1979). The -95 mgal anomaly south of Nevsehir (Ekingen and Guven, 1978; Froger et al., 1994) is therefore a tempting site to locate a vent. So, we are left with the alternative of a vent position in the center of the gravity anomaly that would be supported volcanologically by the high total amount of lithics in locations 8 and 13 and, to some extent, also by the large lithics around Gulsehir (Fig. 9A). However, some vents show little or no gravity anomaly. For example, there is little evidence from the gravity map of the Yellowstone area to locate the eruptive center of the 250 km3 Mesa Falls Tuff (Christiansen, 19821, and Crater Lake Caldera, Oregon, exhibits only -5 mgal in gravity (Williams and McBimey, 1979). We consider that the volcanological data presented here, obtained from 58 locations - 30 of which hold for the upper pumice better fit to a vent location north-northeast of Kaymakli and that the center of the gravity anomaly is related to different eruptions.
5. Interpretation
and discussion
5.1. Eruptive succession The stratigraphic succession of the AZ1 eruption, a major surge series between the initial plinian
pumice fall and the final pyroclastic flows, is somewhat unusual compared with many other plinian style eruptions, e.g., the 79 A.D. eruption of Somma-Vesuvius (Sigurdsson et al., 1985). At Vesuvius, surge deposits mark the final stages of the eruption when the mass eruption rate decreased and external water probably gained access to the conduit. Such a mechanism is also inferred by Van den Bogaard and Schmincke (1984) to explain the final surge deposits of the Laacher See eruption in the East Eifel. A close analogy to the AZ1 eruptive cycle, however, is provided by the 1400 yr B.C. Minoan eruption of the island of Santorini (Bond and Sparks, 1976; Druitt et al., 1989). The surge deposits of the Minoan B stratigraphic unit are analogous to the lower and upper surge series of the AZ1 eruption. The formation of Minoan B is attributed to the influx of seawater into the conduit and/or the upper part of the magma chamber (Druitt et al., 1989). A similar process must be assumed for the AZ1 eruption and can be best explained by water from the marshes and marginal flats of the northern lacustrine sedimentary basin mentioned above. 5.2. Eruptive style Close examination shows that not only the lower and upper surge series were phreatomagmatic but the ubiquitous accretionary lapilli indicate the main pumice fall was also influenced by external water from the very beginning. A close analogy of contemporaneous deposition of accretionary lapilli and pumice lapilli is the Hachinohe Ash (Hayakawa, 1990). This eruption from the Towada caldera evidently penetrated the caldera lake which provided the large amount of external water for formation of accretionary lapilli and armoured lapilli after condensation in the eruption plume. Similarly, the 1979 eruption of Soufriere of St. Vincent burst through the crater lake and the fallout of accretionary lapilli was directly observed (Brazier et al., 1982). We assume only a minimum phreatomagmatic influence on the eruptive style of the initial plinian phase of the AZ1 eruption. The lack of coarse-grained particles in the accretionary lapilli indicates minor amounts of external water compared with the Hachinoe or Soufribre eruptions. Schumacher and Schmincke (1995) documented experimentally the
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Journal of Volcanology and Geothermal Research 78 (1997) 139-153
amount of water necessary for the formation of aggregates increases with grain size, and, in turn, the grain size of ash found in aggregates may indicate the amount of water during their formation. We therefore assume that the accretionary lapilli of the AZ1 main pumice formed at or close to the minimum conditions of aggregate formation whereas those of the Hachinohe Ash formed under more favourable conditions of higher water content. Phreatomagmatic activity was probably greater in the sector of Bahgeli-Ayvali due to a local higher influx of external water which could account for: (a) the high amounts of coarse lithic lapilli; (b) the higher amount of accretionary lapilli in location 17 than elsewhere; (c) the ash layers of surge origin in the upper part of the main pumice fall deposit; and (d) the fine-grained ash beds coming from lateral flow within the upper pumice beds. A similar regionally modified eruptive style due to local ingress of external water is known from the Laacher See eruption in the East Eifel where Van den Bogaard and Schmincke ( 1984) distinguished between the Mendig fucies south of the Laacher See dominated by deposition from lateral flow due to locally limited phreatic influence and the Nickenich facies east of the volcano dominated by contemporaneous pumice fall deposits out of a stable convecting eruption plume. The influx of external water gradually increased during the eruption of the main pumice, as suggested by the layered upper part of the deposit. To account for the layering, we assume distinct fluctuations in the continuity of eruption accompanied by a change from a sustained eruption plume to an intermittent one. In general, due to accidental phreatomagmatic explosions, the phreatic influence on the eruptive style persisted into the ignimbrite producing phase, at least up to the end of deposition of the intermittent surges which are intercalated with pyroclastic flow deposits in the basal zone of the Akdag-Zelve ignimbrue. 5.3. Column height, deposition Various methods the eruption column its deposits. Carey model based on the ments. Application
and volume
have been proposed to deduce height from the dispersal fan of and Sparks (1986) proposed a areal distribution of lithic fragto the AZ1 eruption is difficult
151
because only parts of the dispersal fan are exposed, and, moreover, a deviation between the assumed lithic fan and the bulk tephra fan is indicated in both fall deposits that affects the measure of the maximum cross-wind range. Best estimates for the main pumice fall deposit imply column heights > 45 km and wind velocities of about lo-20 m/s. The observed 2 m of fallout in Akkijy, 30 km southeast of the vent, indicate a considerable upwind umbrella region of the eruption column. Estimates for the culminating phase of the upper pumice beds appear to be more precise and a column height of about 25 km and a wind velocity of I 30 m/s is obtained. Mass eruption rates on the order of roughly (3-)5 X lo8 kg/s for the main pumice fall and (3-5) X 10’ kg/s for the culmination of the upper pumice eruption are given by the models of Sparks et al. (1978) and Sparks (1986). The main plinian phase thus ranks in the same order of magnitude with the 1956 eruption of Bezymianny, the Waimihia plinian pumice eruption, New Zealand (Walker, 1981a) or the 1815 Tambora eruption (Sigurdsson and Carey, 1989) which are slightly less than the Taupo (ultra)plinian phase (Walker, 1980). The culmination of the second plinian phase may have resembled that of the 1947 eruption of Hekla and was slightly less than the eruptive phase of the white pumice of the 79 A.D. eruption of Somma-Vesuvius. Deposition of tephra from these eruption clouds produced a distinct second thickness maximum 20 km from vent in the main pumice fall, which is quite far downwind compared with similar maxima of other eruptions, e.g., 79 A.D. Vesuvius or Laacher See MLST-Cl pumice (Fig. 7). These thickness maxima in pumice fall deposits can be related to eruption column height, wind velocity, and primary grain-size distribution in the plume. This relationship implies a local increase in the frequency of a certain grain-size fraction within the thickness maximum relative to neighbouring upwind and downwind locations. However, bulk tephra grain-size analyses could not be obtained and a substitute, the lithic fraction, does not show this relationship (Fig, 9A, B). In contrast, the upper pumice beds forming a simple, elongated fallout fan show an increase in lithic content in the Giireme-Zelve region. Grain-size frequency histograms (Fig. 9C) indicate preferential accumulation of the 2-8 mm fraction and we there-
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fore assume that the second distal high in lithic content is due to wind sorting processes. Reliable volume estimates are also difficult to obtain, because the dispersal fans are incomplete. A conventional estimate can be based on extrapolations of area-thickness plots (Walker, 1981b) and on the isopach maps of Fig. 6 assuming a distinct upwind umbrella region of the eruption plume. The method proposed by Pyle (1989) on the basis of In(thickness)-area’/’ plots and the thickness - half distance obtained from that plot provides a better evaluation of the far downwind mass according to the equation V = 13.08 X To b: (V = volume; T, = extrapolated maximum thickness; b, = the thickness - half distance). Corresponding tephra volumes are about 41.5 km3 for the main pumice and 0.35 km3 for the upper pumice, respectively. Adding the bulk tephra volume of the ignimbrite of 120 km3 (Le Pennec et al., 1994) and the other deposits coming from lateral flow, a total of 180 km” of tephra appears to be a valid approximation for this unusual Cappadocian eruption.
Acknowledgements We thank our Turkish collegues Yilmaz Savascin (Izmir), Vedat Toprak (Ankara) and Orhan Tartar (Sivas) who helped us exporting samples. George Walker and Don Swanson reviewed the manuscript; we gratefully acknowledge their helpful comments and their patience to smooth the language. Hilary Downes again performed a final language editing.
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Walker, G.P.L. and Croasdale, R., 1970. Two plinian-type eruptions in the Azores. J. Cieol. Sot., 127: 17-55. Williams, H. and McBimey, A.R., 1979. Volcanology. Freeman, San Francisco, CA, 397 pp. Wohletz, K.H. and Sheridan, M.T., 1979. A model of pyroclastic surge. Geol. Sot. Am. Spec. Pap., 180: 177-194.