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; accretionary lapilli

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 author. Phone: + 49-7144-36596; Fax: +49-

711-9765013

the Galatean region (Fig. 1). The CVP, terminated

and partly transected by major fault zones, extends for about 300 km southwest-northeast and is sur-

rounded 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-

0377-0273/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.

PZZ SO377-0273(96)00106-O

140 R. Schumacher. U. Mues-Schumacher/ Journal of Volcanology and Geothermal Research 78 11997) 139-153

0 C Valibaba-Sofular

Incesu

Kizilkaya

Tdkdy Pumice Fall

Gordeles

Tahar

Karaln Pumice Fall

Cemilktiy

Sarlmaden

AkdaQ- Zelve

Upper Gtireme

Bacehll Surge Depwts

Lower GGreme

Pumice Falls

Surge Deposits Reworked

0

00

'56

a Nevsehir

dzkpOnak 0 0

0

A

Kaymakli l

N

IO km

0 B

0

0

R. Schumacher, U. Mues-Schumacher/JournaE of Valcunology and Geothemurl Research 78 (1997) 139-153 141

Akdag-Zelve Ignimbrite

---I ==-y UPPer -- Surqe Series

Main Pumice Fall Deposit

Secondary volmniclostic

- marginal flats -

Upper surge -

ignimbri te

trUnsitiona1 facies

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.

142 R. Schumacher, U. Mues-Schumacher/ Journal of Volcanology and Geothermal Research 78 (1997) 139-153

‘4 4 0 16 64 mm

2

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, sev- eral pumice falls and surge deposits. Tephra is in- terbedded 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 com-

plexity starting with initial pumice fallout that subse- quently graded through surge deposits into the ign- imbrite-producing phase. The general phreatomag- matic influence during the whole eruption produced unusual volcanological features, and the interpreta- tion of the well exposed accretionary lapilli-bearing pumice fall deposit provides an essential contribution to the understanding of the volcanological evolution of Cappadocia.

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 pyroclas-

tic 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

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 anal- ysed 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)

R. Schumacher, (I. Mues-Schumacher/Journal of Volcanology and Geothermal Research 78 (1997) 139-153 143

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 mi- nor sand-wave bedding) and mutual erosion of indi- vidual layers or sets of layers (Fig. 4c). As distance from source increases, bedding gradually changes into plane-parallel or very low angle cross- stratifica- tion.

Fine-grained ash beds, especially beneath and on top of the intercalated upper pumice beds, enclose abundant accretionary lapilli (rim-type of Schu- macher and Schmincke, 1991). A very thin bedded but widely distributed vesiculated tuff layer from the upper surge series is typically associated with core- type accretionary lapilli. Internal characteristics of the deposit indicate its formation from the coales- cence of accretionary lapilli and other irregular ag- gregates 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.

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 fine- grained 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 BahGeli- Ayvali (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, al- though internal layers of pumice concentration indi- cate a multiple flow deposit. A particular characteris- tic 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., loca- tion 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 sedi- ments in most of the study area. Internal bedding includes subaqueous slumping and convolute bed-

144 R. Schumacher, U. Mues-Schumacher/Journal of Volcanology and Geothermal Research 78 (1997) 139-153

ding resulting from dehydration of individual sedi-

ment layers. The deposits are topped by extremely

fine-grained material which is interpreted as a lacus-

trine stagnant water sediment (Fig. 4a). The thick-

ness 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 interpre- tation 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

&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 interbed-

ded 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 dur-

ing emplacement of the pumice fall. Moreover, alignment of large pumice lumps and the scarcity of

lithic clasts suggesting floating pumice rafts indi- cates in several outcrops in the Urgtip-Goreme-

Avanos area that at least some of the pumice fell into

very shallow water (< 0.5 ml of more or less iso-

lated ponds typical of a marsh or marginal flat environment of a lacustrine basin. The recognition of the depositional environment of a lacustrine sedi-

mentary basin that was progressively filled by pyro-

elastic 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-

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 paleode-

pression fill.

4. Area1 distribution

4. I. Thickness

The eruptive products are well exposed within the

mapped area of about 1200 km2. Outcrops are lim-

ited by sedimentary and younger volcanic cover and

thickness varies by primary depositional factors such

as deposition of the pumice into a subaqueous envi- ronment (north of Gulsehir) where the original thick-

ness 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 (Sigurds- son 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 I’

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6.

lso

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of t

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ain

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and

the

uppe

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mic

e be

ds.

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cura

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of t

he

isop

ach

map

s is

onl

y to

loc

ate

the

sour

ce

with

in

a fe

w

kilo

met

ers

that

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alua

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of

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rnat

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po

sitio

ns

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the

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nter

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a B

ougu

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grav

ity

low

(L

e Pe

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et

al.,

19

94;

Frog

er

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l.,

1994

) so

uth

of N

evse

hir

or n

orth

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thea

st

of K

aym

akli

dedu

ced

from

the

data

pr

esen

ted

here

-

is d

iffi

cult

(see

ch

apte

r ce

nf

posi

tion

for

deta

ils).

(A

) T

he

mai

n pu

mic

e fa

ll de

posi

t ha

s a

seco

nd

thic

knes

s m

axim

um

20-3

0 km

aw

ay

from

th

e

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med

so

urce

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ea.

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ha

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d ar

ea

is w

here

tin

e-gr

aine

d su

rge

depo

sits

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cur

at t

he b

ound

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betw

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the

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sive

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wer

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ied

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r pa

rt

of t

he d

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it.

(B)

The

uppe

r pu

mic

e be

ds

form

a

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ple

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ding

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n.

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4d).

R. Schumacher, U. Mues-Schumacher/ Journal of Volcanology and Geothermal Research 78 (1997) 139-153 147

I s 0 . 1 . a 4

0 10 20 30 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-cm- thick 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 part- ing 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 there-

fore 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

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 crite-

ria such as thickness of pumice fallout, area1 distribu- tion 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). There-

fore, 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 measure- ments 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

o.a /’

n I’ GdSehir’ ,”

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 con-

centration maximum in the Goreme-Zelve area (up- per 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 139-153 149

/’

2.2,1’ ‘. ,,-

3.5 1’ ,’ .

42 I/ /’

. n .3.3 /I

Ii /I

.4.0 ,’ /I

I/ I

I‘

Nevsehir I / . : :

IOkm

0 A

l

A

%g . 2

24816mm

N

IOkm

0 C- r---- wt% Lithics

/I % i

I’ i ,,3~--,;L- t 4816 64mm

. 1' /'

/’ . % I I’ n

% ,' '"IO,2

68 ; t

. :

481664mm ,: /

! ,--._

A N

4816 6Lmm

. 6"

: t 4016 64mm

IOkm

0 B- Wf:% Lithics

150 R. Schumacher, II. Mues-Schumacher/ Journal of Volcanology and Geothermal Research 78 (1997) 139-153

The sensitivity of the isopach maps, flow-direc-

tion 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 loca-

tions 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 prod-

ucts 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, rhy- olitic 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 volcanolog-

ically 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 Yel-

lowstone 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 con-

sider 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 some-

what 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 contem-

poraneous deposition of accretionary lapilli and

pumice lapilli is the Hachinohe Ash (Hayakawa,

1990). This eruption from the Towada caldera evi- dently penetrated the caldera lake which provided

the large amount of external water for formation of accretionary lapilli and armoured lapilli after con-

densation 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 Hachi- noe or Soufribre eruptions. Schumacher and Schmincke (1995) documented experimentally the

R. Schumacher, (I. Mues-Schumacher/ Journal of Volcanology and Geothermal Research 78 (1997) 139-153 151

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 region-

ally modified eruptive style due to local ingress of external water is known from the Laacher See erup-

tion in the East Eifel where Van den Bogaard and Schmincke ( 1984) distinguished between the Mendig

fucies south of the Laacher See dominated by depo-

sition from lateral flow due to locally limited phreatic influence and the Nickenich facies east of the vol-

cano 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 ign- imbrue.

5.3. Column height, deposition and volume

Various methods have been proposed to deduce the eruption column height from the dispersal fan of its deposits. Carey and Sparks (1986) proposed a model based on the areal distribution of lithic frag- ments. Application to the AZ1 eruption is difficult

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 maxi- mum 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 ob-

served 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 erup-

tion 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 loca-

tions. 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 con- tent in the Giireme-Zelve region. Grain-size fre-

quency histograms (Fig. 9C) indicate preferential accumulation of the 2-8 mm fraction and we there-

152 R. Schumacher, U. Mues-Schumacher/Journal of Volcanology and Geothermal Research 78 (1997) 139-1.53

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(thick-

ness)-area’/’ plots and the thickness - half dis-

tance obtained from that plot provides a better evalu-

ation 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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