caldera development during the minoan eruption, thira, cyclades, greece

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B10, PAGES 8441-8462, SEPTEMBER 30, 1984

Caldera Development During the Minoan Eruption, Thira, Cyclades, Greece

ORANT HEIKEN

Los Alamos National Laboratory, New Mexico

FLOYD McCoY, JR. ß

Lamont-Doherty Geolo•?ical Observatory of Columbia University

The well-known caldera of Thira (Santorini), Greece, was not formed during a single eruption but is composed of two overlapping calderas superimposed upon a complex volcanic field that developed along a NE trending line of vents. Before the Minoan eruption Of 1400 B.C., Thira consisted of three lava shields in the northern half of the island and a flooded depression surrounded by tuff deposits in the southern half. Andesitic lavas formed the overlapping shields of the north and were contemporaneous with and, in many places, interbedded with the southern tuff deposits. Although there appears to be little difference between the composition of magrnas erupted, differences in eruption style indicate that most of the activity in the northern half of the volcanic field was subaerial, producing lava flows, whereas in the south, eruptions within a flooded depression produced a sequence of mostly. phreatomagrnatic tuffs. Many of these tuffs are plastered onto the walls of what appears to have been an older caldera, most probably associated with an eruption of rhyodacitic tephra 100,000 years ago. The Minoan eruption of about 1400 B.C. had four distinct phases, each reflecting a different vent geometry and eruption mechanism. The Minoan activity was preceded by minor eruptions of fine ash. (1) The eruption began with a Plinian phase, from subaerial vent(s) located on the easternmost of the lava shields. (2) Vent(s) grew toward the SW into the flooded depression. Subsequent activity deposited large-scale base surge deposits during vent widening by phreatomagmatic activity. (3) The third eruptive phase was also phreatomagmar tic and produced 60% of the volume of the Minoan Tuff. This activity was nearly continuous and formed a large featureless tuff ring with poorly defined bedding. This deposit contains 540% lithic fragments that are typical of the westernmost lava shield and appears to have been erupted when caldera collapse began. (4) The last phase consisted of eruption of ignimbrites from vent(s) on the eastern shield, not yet involved in collapse. Collapse continued after eruption of the ignimbrites with foundering of the eastern half of the caldera. Total volume of the collapse was about 19 km 3, overlapping the older caldera to form the caldera complex visible today. Intracaldera eruptions have formed the Kameni Islands along linear vents concomitant with vents ihat may have been sources for the Minoan Tuff.

INTRODUCTION

The Thira volcanic field is one of several small volcanic

fields of the Hellenic arc (Figure 1). Eruption history of Thira has been of interest to volcanologists and archeologists because of the large eruption of about 1400 B.C. that buried and preserved at least one and perhaps several Minoan villages.

Most previous investigators who have worked in the Thira volcanic field assumed that the Minoan eruption was solely responsible for the present caldera and that before the eruption, the island of Thira was composed of several large stratovolcanoes [Pichler and Friedrich, 1980]: Pichler and Friedrich proposed a single island, which they called "Stronghyle," with an elevation of 500-600 m. As calderas are formed by collapse, the volume of material erupted should be approximately equal to that of the caldera; the volume of magma erupted during the Minoan eruption should equal the volume difference between "Stronghyle" and the caldera.

Volume of tephra from the Minoan eruption was estimated by Watkins et al. [1978] to be between 13 and 18 km 3 (dense rock equivalent). They used best available measurements from the island and from piston-core samples of Minoan ash in deep-sea sediments of the eastern Mediterranean. If one uses

This paper is not subject to U.S. copyright. Published in 1984 by the American Geophysical Union.

Paper number 4B0328.

the paleogeologic reconstruction of Minoan Thira by Pichler and Friedrich [1980], the caldera volume is nearly 60 km 3. This ratheLlarge discrepancy between caldera and tephra vol- umes could be explained if a caldera or calderas had been present in the volcanic field before the Minoan eruption. On first glance, it is evident that the Thira volcanic field was composed of a large variety of volcanic landforms with diverse compositions and not several stratovolcanoes; this observa- tion was noted by Fouqu• [1879].

To determine if a caldera were present on Thira before the Minoan eruption, the following field approach was used.

1. The structural framework of pre-Minoan Thira was studied in order to evaluate structural controls for devel-

opment of the volcanic field. 2. DiStribution, stratigraphy, and petrogenesis of pyroclas-

tic deposits and lavas that erupted between 100,000 years B.P. and 1400 B.C. were studied and used for a paleogeologic and paleotopographic reconstruction of Minoan Thira. This straii- graphic interval was chosen, as it represents a Well-dated period between two major volcanic events, the "unter bimstein" (lower pumice), dated at 100,000 years B.P. [Seward et al., 1980], and the "ober bimstein" (upper pumice, the Minoan Tephra), dated at about 1400 B.C. Within this time frame there are two distinct correlative facies within the eruption

,

sequence. In the southern part of the volcanic field there are only pyroclastic deposits, and in the northern section, mostly overlapping lava shields with tuffs interbedded between flows and filling some palcovalleys.

8441

8442 HEIKEN AND McCoY: CALDERA DEVELOPMENT DURING THE MINOAN ERUPTION

38 •

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Fig. 1. Location map, showing Thira and the Cyclades volcanic chain (dotted line), which includes the young volcanoes Milos and Nisiros. The dashed line with triangles marks the approximate boundary of the Aegean plate.

STRUCTURAL FRAMEWORK OF THE THIRA VOLCANIC FIELD

Prevolcanic Thira

Prevolcanic Thira consists of ridges located in the southwestern part of the island; they are mostly covered by younger volcanic rocks but are well exposed on the highest ridges, including Mount Profitis Ilias (elevation 564 m), Mesa Vouno (elevation 369 m), and Gavrilos Ridge (elevation 180 m). Li- thologies exposed in the ridges are similar to those present in much of southern Greece: massive gray marble, phyllites, me- taconglomerate, and metatuffs. The highest hills, exposures along caldera walls, and a few isolated outcrops near the shoreline indicate the extent of the prevolcanic island to have been about 9 x 6 km. Interpretations of bathymetric and seismic reflection profiles [Hoskins and Edgerton, 1971] indicate that a submarine ridge, trending SSW from Thira is an extension of the prevolcanic island.

The major structural feature visible on prevolcanic Thira is a thrust fault, with an average strike of N20øE, dipping 30øE that underlies Mesa Vouno. The fault plane begins at Perissa Beach, passes through the saddle between Mesa Vouno and Mount Profitis Ilias, then intersects the sea at Kamari Beach on the north side of the mesa (Figure 2).

Blake et al. [1981] have summarized structural and lithologic data that suggest the presence of a subduction zone in the Aegean region during late Mesozoic-early Cenozoic time. The southern segment of their Cycladic blueschist belt includes Milos and Thira; the inferred direction of subduction for this plate segment is from the ESE, consistent with the northeast striking thrust faults found on Thira. Auboin et al. [1963] propose that most of the normal faulting within the

Aegean plate (as defined by McKenzie [1972]) began at about 10 Ma. Extension and normal faulting within the Aegean plate have resulted in subsidence to form the present sea. Tectonic analyses by Angelier et al. [1982] in the central-southern Aegean Sea indicate directions of extension in the area of Thira, during Recent and Quaternary time, as NW-SE, consistent with northeast trending normal faults and vents within the volcanic field.

Pre-1 Ma Volcanic Sequence

Vents within volcanic rocks older than 1.0 _+ 0.08 Ma (fission track date of white vitric tuffs overlying Akrotiri rocks, by Seward et al. [1980]) are exposed along a 2- to 5-km-wide zone on the Akrotiri Peninsula. Due to their age and degree of erosion, it is difficult to determine vent orientations within these older andesitic cinder cones and dacitic domes. How-

ever, cinder cones located at Cape Mavrorachidi, Point Mavros, and just north of the village of Akrotiri form re- sistent, NE trending spatter ridges; at Cape Mavrorachidi, attitudes of cinder deposits indicate that this cone had a NE trending vent.

Dike Systems in Pre-Minoan Lava Shields

Dikes are exposed in the caldera walls and near summits of the lava shields of Mikro Profitis Ilias, Megalo Vouno, and Thirasia volcanoes. Most of the dikes are vertical or near

vertical and 114 m wide, cutting older lavas and tuffs of the shield complexes. Mean orientation of 33 dikes is N28øE (standard deviation of 14ø). Four dikes within Megalo Vouno volcano, including two that flank a 25-m-wide vent filled with tuff-breccia, trend NW. Most of these dikes are feeders for the

HEIKEN AND McCoY.' CALDERA DEVELOPMENT DURING THE MINOAN ERUPTION 8443

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COLOMBO BANK

(Colombo Volcano)

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EXPLANATION

Dacite Dome ....... Approximate Outline of

Cinder Cone Pre-volcanic Thera • Coastline Submarine Volcano?

Dikes • Phyllites, Metasand- stones, Limestone

Crater ..".'i:!• Crystalline Limestone Normal Fault

•'200 j Depth in Meters Thrust Fault

P83 Soundings in Meters Major Submarine Scarp

EI•E] Seismic Line Structural Trend or

Line of Vents

Fig. 2. Structural geologic map of the Thira volcanic field. Included is a simplified structural map of the prevolcanic island (Triassic and early Tertiary metasedimentary and metavolcanic rocks), vent orientations, bathymetry (in meters), and lines for seismic profiles (Figure 3).

8444 HEIKEN AND McCoY' CALDERA DEVELOPMENT DURING TI-IE MINOAN ERUPTION

A B D C

0 I 2 3 4 5km i i i ! ß ß 10x vertical exaggeration

D E E • F F G

Fig. 3. Seismic profiles in and around the Tliira volcanic field, Aegean Sea. Profile locations are shown in Figure 2. Note the 10x vertical exaggeration. Based upon data published by Hoskins and Edgerton [1971] and the Woods Hole Oceanographic Institute [1967].

Megalo Vouno and Mikro Profitis Ilias volcanoes and are located within a 2-km-wide zone. The youngest vents located on the lava shields are the cinder cones, Megalo Vouno, and Kokkino Vouno. Cape Colombo, along Thira's northern shore, is a well-preserved maar volcano. The alignment of cinder cones and maar volcano is N55øE. Along this same trend, 7 km NE of Thira, is the Colombo Bank, a large submarine volcano that erupted in 1650 A.D. (Figure 2).

Seismic Profiles

Interpretation of the structural flamework of Thira has been helped by seismic profiles and sonar profiles collected by the Woods Hole Oceanographic Institute [1967; Haskins and Edgerton, 1971; Mavar, 1969]. Tracks for these seismic lines are shown in Figure 2; track locations were based on navi- gational notations on margins of the profiles and by comparison with bathymetric maps published by the British Admiralty (t•o. 2043, 1963).

Figure 3 is an interpretation of the Woods Hole seismic data. A to B is parallel to the channel between Thirasia and Thira, a 1.5-km-wide, 8-km-long graben. Visible along the axis of the graben are a series of tilted fault blocks. They appear to have been formed during slumping into the Minoan caldera. The headwall of this slump extends 6 km beyond the island. Within the caldera the slump is overlain by caldera fill. The flat caldera floor is visible in the profile, at a depth of 380 m. The slopes of Nea Kameni rise from the caldera floor to a ridge that has been interpreted by Haskins and Edgerton [1971] as "ridge-vents." This seems likely, as the ridge vents continued along the trend of fissures on the Kameni Islands.

From C to D there is flat caldera floor, then a slump similar to that forming the NW channel. At the caldera rim there are volcanic rocks(?) dipping west, parallel to those visible in the adjacent scarp on Thirasia. From D to E, at depths of about 200 m, the profile of the western flank of Thirasia shows little if any faulting or folding.

From E to F the northwest channel between Thira and

Thirasia is crossed at right angles. In cross section it is a narrow (2 km) and deep (200-280 m) graben. Beyond it, immediately north of Thira, a NE trending(?) fault scarp is visible; this may be a scarp flanking a NE trending, 8- to 10-km- wide graben (Figures 2 and 3). Near F and in the profile F-G are the Colombo Banks, a 250-m-high, historically active submarine volcano located near the center of the NE trending

graben. The SE side of the graben, north of Thira, is poorly defined but many coincide with the fault visible near G.

From G to H (Figure 4), along the eastern flank of the island, deeper folds visible on the seismic profile are most likely within metamorphosed marble and ciastic rock units of prevolcanic Thira. From H to I a structural and bathymetric high; located south of Thira, is crossed. It has been interpreted as the SW extension of prevOlcanic Thira [Haskins and Edgerton, 1971]. The steep western slope of this ridge is visible in profile I-J. It may coincide with the southeastern scarp of the NE-SW trending graben described in profiles E, F, and G. Many steep, pointed ridges are visible in profile Lj and may be submarine volcanoes; they are along the trend of the dacitic and andesitic volcanoes of the Akrotiri peninsula and are

located only 3 km from the shore. Profile J-K cuts across the SW trending graben once again, but is passing downslope into the Cret•in Trough. The two ridges crossed near K may be horsts and/or submarine volcanoes.

D•scussian of the Structural Framework The shape and extent of most volcanic fields are controlled

by the prevolcanic structural framework, and Thira is no ex- ception. Throughout Thira's history, multiple eruptions of a variety of magma types, from fissures, have been restricted to what is interpreted here as a 3- to 4-km-wide, northeast trending graben. Alignments of cones, domes, and dikes are parallei or subparallel to the graben. Located within the graben are also what appear to be submarine volcanoes, located southwest of Thira, and the submarine volcano of Colombo Bank, located northeast of the island. This graben is parallel to the major faults of prevolcanic Thira. The shapes of calderas, formed late in the history of the volcanic field, were also controlled by this northeast trending fabric. Neumann Van Padang and Reck [in Reck, 1936] noted numerous northeast- trending lineaments and linked them to a line between the Christiana Islands, Thira, and the Colombo Bank (striking N45øE).

MIDDLE TUFF SEQUENCE

Distribution

The Middle Tuff Sequence crops out in cliffs along the entire southern half of the caldera complex, from Thira to the western tip of the Akrotiri Peninsula and on the island of


G H H I

200: f 4OOml'

0 1 2:3 4 5km ß ß ß . i 10X vertical exaggeration

Fig. 4. Seismic profiles in and around the Thira volcanic field, Aegean Sea. Profile locations are shown in Figure 2. Note the 10x vertical exaggeration. Based upon data published by Hoskins and Edgerton [1971] and the Woods Hole Oceanographic Institute [1967].

Aspronisi. Along these cliffs the tuffs range in thickness from 20 to 40 m.

In the northern half of the field, most tuff outcrops are quite thin (a few tens of centimeters) and are interbedded with lavas of the shields. Only in a valley on the western slopes of Megalo Vouno volcano (below the town of Oia) and along a narrow zone below the lavas of Thirasia does the Middle Tuff

Sequence reach any appreciable thickness (greater than a few meters). Part of this thickening is due to the presence of a massive scoria unit of limited extent (Figure 7a). Along the flanks of Thira, many outcrops of the Middle Tuff Sequence are exposed in ravines. Thicknesses vary laterally as many of the deposits fill paleovalleys.

Age

The age of the Middle Tuff Sequence, as mentioned earlier, is bracketed by the underlying Lower Pumice and the Minoan Tuff. Fission track ages of obsidian fiamme from the lowest part of the Lower Pumice, northern Thira, are 106,000 + 21,000 and 97,000 + 20,000 years [Seward et al., 1980]. These data have been discussed by Federman and Carey [1980], who tentatively correlate an ash bed within abyssal sediments, located 200 km west of Cyprus, with the Middle Pumice Series (the Middle Tuff Sequence) and not the Lower Pumice; they have an approximate age of 100,000 years for this ash bed. This correlation is questionable, as it was based upon one ash layer in one core located 500 km east of Thira; Federrnan and Carey [1980] studied 15 cores located between Thira and the core site and did not find this ash layer. A better source for Federman and Carey's "Middle Pumice Series" ash may be either Nisiros or the Kos Plateau.

Pichler and Friedrich [1976] have dated plant-bearing pa- leosoils within the Middle Tuff Sequence at 13,000 and 18,500 •'•C years.

As will be discussed later in this paper, the age of the overlying Minoan Tuff is about 1400 B.C.

Stratigraphy

Between 37% and 70% of these deposits (estimated from thicknesses measured in stratigraphic sections) consist of tuffs

of phreatomagmatic origin. They are characterized by well- developed beds (1-50 cm thick) of brown, tan, or black fine ash to lapilli tuffs (Figure 5). Most are plane beds, with both normal and reverse grading, but many consist of small surge dunes. Some beds contain accretionary lapilli and bedding plane sags. The fine-grained brown tuffs are very well consoli- dated and resistant to erosion. It is within these beds where

base surge current directions are most easily measured. Interbedded with the phreatomagmatic tuffs are one to

three eruption sequences consisting of a Plinian pumice fall and pyroelastic flows (Figure 5). Thicknesses of these tuff units vary greatly from place to place; in paleovalleys and ravines there are well-developed pyroelastic flows and lag breccias; in other locations, especially on former highlands, there are only pumice falls from these eruptions. Druitt and Sparks [1982] have described the near-vent lag deposits and discussed depositional mechanisms for one of these pyroelastic flows.

Phreatomagmatic segments of the Middle Tuff Sequence have depositional dips, indicating that they were deposited on walls of an older caldera on slopes similar to those visible today. Along the north coast of the Akrotiri Peninsula and west slope of the Athinios "highland," depositional dips of the phreatomagmatic tuffs are 5-30 ø, dipping into the older caldera (Figure 6). There are also a number of broad shoals cut into these inward dipping Middle Tuff Sequence deposits. Steep original dips such as those described here, plastered onto crater walls, are typical of phreatomagmatic tuffs. These deposits are wedge shaped, stacked up against walls of the older caldera. Beyond the caldera rim they are concentrated in paleovalleys and arroyos.

Farther north, near the town of Thira, these tuffs have both inward and outward dips and may mark the rim of a large maar volcano; here the deposits do not slope into the caldera but were truncated by Minoan caldera collapse.

Overlapping channels, best seen in the Thira Quarries, are present in the phreatomagmatic tuffs. They are up to 10 m deep and 30 m wide, cut into lower bedded tuffs and are, in turn, filled with thinly bedded brown, black, and tan tuffs. There is no alluvial debris in these channels, and tuff beds within them exhibit both grading and surge dunes. Fisher


STRATIGRAPHIC SECTION -- MIDDLE TUFF SEQUENCE

-- WESTERN AKROTIRI PENINSULA

(Located on the northwest flank of an Akrotiri cinder cone)

UNIT LITHOLOGY

E

MINOAN TUFF SEQUENCE

-Brown pebbly loam in upper 20cm. -Massive, lithic-rich, lapilli-bearing tuff. Tan lapdh (20%) m matrix of tan tuff (50%) w•th 30% hth•c fragments Pottery shards in sod at top.

Interbedded tan, gray and black hne tuff, lapdh-bear•ng tuff and lapilh beds. Some accret•onary lapdh in the lapdh-bearmg

hne tuff beds.

Massive lapdh bed, consisting of 50% tan pumice lapdh and

coarse ash, 35% gray-black pumice and 15% hthlc fragments

Well-bedded hght-tan free tuff m '/2 cm to 80cm thick plane beds

Some beds contain small accret•onary lapdh

Tan, gray and dark gray lapalii-bearing tuffs and hne tuffs

Mostly beds are normally graded, from black scoria lapdh to medium or fine ash

Some of the thinner beds consist of fine brown ash

Massive, pumice-bearing fine ash

Grayish-brown volcamc brecc•a. 60% hthlc fragments in tan lapJill and ash matrix Is lenticular, pinching out upslope Masswe pinkish-tan pumice beds, consisting of pumice lapdh and bombs. 40% hth•c fragments near base, decreasing to 3% near top

Middle pumice of P•chler

Well-bedded dark gray and tan lapilh tuffs, m beds 20 to 50cm thick Both normal and reverse grading

Some thin beds of brown free ash

Lower Pumice Series

Fig. 5. Representative stratigraphic section of the Middle Tuff Se- quence, Akrotiri Peninsula.

[1977] described channels similar to these in a variety of maar volcanoes and provided convincing arguments that they were, all or in part, cut by base surges and were subsequently filled by deposits from later surges.

Surge dunes, ranging in amplitude from a few to a few tens of centimeters, are present in many of the phreatomagmatic tuffs. Flow directions indicate that many of the surges were moving upslope, out of the older caldera. This is not an unus- ual occurrence and has been documented in many maar volcanoes [e.g., Fisher and Waters, 1969]. Current directions indicate that a source or sources for the phreatomagmatic tuffs of the Middle Tuff Sequence were located within the southern part of the present-day caldera complex.

PRE-MINOAN LAVA SHIELDS

Contemporaneous with and, in places, interbedded with the Middle Tuff Sequence are three overlapping lava shields in the northern half of the volcanic field. The upper portions of the Megalo Vouno volcano, the Skaros volcano, and the upper Thirasia volcano overlie the Lower Pumice Sequence and un- derlie the Minoan Tuff.

Megalo Vouno Volcano

The highest point on this northernmost of the Thira volcanoes is 329 m above sea level. The volcano is beautifully exposed in cliffs along the north wall of the caldera complex. On the east it is in depositional and fault contact with the older composite cone of Mikro Profitis Ilias. On the west, its lavas are interbedded with tuffs of the Middle Tuff Sequence and lavas of the Thirasia volcano (Figures 7a and 8b). Dikes within Megalo Vouno are exposed in caldera walls; most trend NE, as was discussed earlier. As it is exposed now, this volcano appears to have been about 5 km wide at the base and trended NE-SW. The youngest flows in Megalo Vouno are thinly bedded andesitcs that overlie the Lower Pumice series. They are separated by thin cinder and flow-breccia deposits. The youngest volcanoes on this shield are the two cinder cones, Megalo Youno and Kokkino ¾uono, and a maar volcano (Colombo Maar). Both cinder cones consist of bedded red and black coarse ash and lapilli typical of Strom- bolian eruptions. Colombo Maar is exposed at sea level and is located along a SW-NE trending line with the two cinder cones. It is mostly buried by Minoan Tuff and consists of 2- to 8-cm-thick, reversely graded beds of greenish gray, lapilli- bearing, medium- to fine-grained tuff beds that contain accre- tionay lapilli and bedding plane sags. The tuff ring is about 60 m thick and 1 km in diameter.

The southern slopes (caldera side) of Megalo Vouno were only partly truncated by the Minoan caldera collapse. There is a distinct change in slope at an elevation of 200 m, above which slopes are about 27 ø, have a poorly developed soil, and weathered talus; below the break in slope the caldera wall is much steeper (45 ø) and outcrops are less weathered.

Skaros Volcano

Easternmost of the lava shields is the Skaros Volcano, upon which is located the village of Merovigli. This shield extends from Mount Mikro Profitis Ilias to the town of Thira. It is

bisected by the Minoan caldera wall and has a maximum elevation of 270 m. No dikes (vents) for these lavas are exposed; lavas of this shield are flat lying at the caldera wall and dip steeply to the east (original attitudes) on the well- preserved eastern slope, implying that the vents for these flows were near the caldera wall. Lava flows range in thickness from 2 m to somewhat over 10 m, but most are 3-4 m thick. Eigh- teen lava flows fill a 3-km-wide valley between the eroded slopes of the Middle Tuff Sequence on the south and the slopes of the composite cone, Mount Profitis Ilias, on the north (Figure 7b). Between some of the lava flows are thin layers of brown phreatomagmatic tuffs, typical of the Middle Tuff Sequence. Most of the flows are andesitc or basaltic andesitc (J. Huijsmans and M. Barton, personal communication, 1983).

The Skaros lava shield is overlain by a 2- to 60-m-thick sequence of pyroelastic rocks of the Middle Tuff Sequence, including a 20- to 30-m-thick, cliff-forming densely welded ignimbrite (Cape Riva ignimbrite of Druitt and Sparks [1982]), and a sequence of well-bedded phreatomagmatic tuffs. These tuffs are partly buried by a lava flow consisting of black, glassy porphyritic dacite; this flow, which is up to 50 m thick, is visible in the cliffs between Merovigli and the town of Thira.

Thirasia Volcano

The island of Thirasia, 6 km long and 2.5 km wide, is the largest remnant of the Thirasia volcano. Some flows from this

HEIKEN AND McCoY: CALDERA DEVELOPMENT DURING THE MINOAN ERUPTION 8447

0 2 4 km

A A'

South North

B B'

100 m •......•. S.L.

lOO m

S.L.

-100

-200

0 500 m i i

2x Vertical Exaggeration

E E' 200 m

100

S.L.

-100 / -200

200 m

100

S.L.

-100

-200

C C'

,oo -lOO

LEGEND

Cross Sections AA' to EE'

•!'""•• ..... Minoan Tuff Well-bedded phreatomagmatic and Strombolian tuffs (some ignimbrite)

'• Lower pumice series • Pre-100,000 year-old tuffs

• Pre-100,000 year-old lavas • Older dacite flows, dacitic tuffs and andesitic cinder cones of the

Akrotiri Penninsula (pre I Myr)

• Metamorphic 'basement"- phyllites, marble, metatuffs (Triassic and early Tertiary)

Fig. 6. Cross sections along the southern and eastern walls of the caldera complex that show depositional relationships of the Middle Tuff Sequence. In cross sections A-D the Middle Tuff Sequence is present as wedge-shaped tuffs deposited on the wall of the 100,000-year-old caldera formed during eruption of the Lower Pumice series. In cross section E the Middle Tuff Sequence has been cut by the Minoan caldera.

volcano are visible below the town of Oia, located on Thira, where they are underlain and overlain by lavas from the Megalo Vouno volcano.

Thirasia volcano's western slopes are overlain by the Minoan Tuff (Figure 9). Much of this volcano was cut by caldera collapse and now forms the western wall of the Minoan caldera. The highest remnants of Thirasia volcano are located in southern Thirasia at a height of 260 m above sea level. From flow directions, flow thicknesses, and distribution of a massive scoria deposit, it appears that the Thirasia shield was erupted along a line of SW-NE trending fissures. Several NE trending dikes are visible in caldera walls in the southern part of the island; other vents must have been located immediately east of the caldera wall.

The base of the volcano consists of interbedded, thin andesite flows and tuffs of the Middle Tuff Sequence. There are brown, tan, and black phreatomagmatic tuffs and thin ignimbrites interbedded with the lavas. Above the lowest tuffs and

lavas is a very distinctive unit consisting of massive red scoria (the scoria of Pichler and Kussmaul [1980]). It is thickest (80 m) below the town of Oia, on Thira, and at Cape Simandiri, on Thirasia. It thins to the east and west, forming an elongate, 3-km-wide, NE-SW trending deposit. It consists of red bombs that range in size from a few centimeters to 2 m long. The deposit also contains 25% cobble to boulder-size, rounded ultramarie xenoliths. The shape of the deposit and large size of bombs and blocks implies that Strombolian eruptions formed a NE trending spatter rampart and cone.

The scoria deposit of Thirasia volcano is capped by massive flows of black, glassy andesite and dacite that are up to 50 m thick in paleovalleys on the volcano's western slopes. Sources for these flows were mostly to the east, somewhere in what is now the Minoan caldera. They are short flows that thin to only a few meters over a distance of 2 km.

On Thira and Thirasia, the massive red scoria unit and the uppermost lava flows are overlain by more tuffs of the Middle Tuff Sequence, including some of phreatomagmatic origin, and by the ignimbrite of the Cape Riva member of Druitt and Sparks [1982]. These deposits are, however, concentrated mostly in paleovalleys.

PALEOGEOLOGY AND PALEOTOPOGRAPHY OF

PRE-MINOAN THIRA

Where the thickness of the Minoan Tuff was known or

could be inferred, it was "stripped" and the elevation and lithology of the underlying rock units noted. Units mapped for the paleogeological reconstruction included the prevolcanic metamorphic units, older volcanic rocks of the Akrotiri Penin- sula, pyroelastic rocks of the Middle Tuff Sequence, and rocks of the lava shields and composite cones of the northern part of the Thira volcanic field.

A key to reconstructing the southern half of the field lay in the distribution and origin of tuffs of the Middle Tuff Se- quence. As was discussed earlier, most of the tuffs are of phreatomagmatic origin and are present as wedges plastered onto steeply sloping walls of the southern part of a depression

8448 HEIKEN AND McCoY' CALDERA DEVELOPMENT DURING THE MINOAN ERUPTION

A Skaros

North I Southern 300m ' .*. '

200m

lOOm

Sea Level

A'

Thira Alonaki bend in South Quarries Quarries section Athinios Plaka 300m

200m

........ , lOOm

""!?/'•?'(2'•"•.?•!:•'•;•';•:• Sea Level 2x Vertical Exaggeration

LEGEND

Cross-section AA' (North-South)

Uppermost lava flow of Skaros Volcano ,• Welded scoria •'• Well bedded

Lavas of Skaros Volcano • Middle Tuff phreatomagmatic, Sequence strombolian tuffs (some ignimbrites )

Ignimbrite of Cape Thira

Lavas, interbedded tuffs of Mikro Profitis Ilias Volcano (dacite dome-stippled)

Lower pumice series and pre-100,000 year-old tuffs (there are a few thin interbedded lava flows)

Metamorphic rocks at 'Athinios Ridge' (phyllite, metasandstone, limestone)

Fig. 7a

B I B East

West

r

•a Level • '•---"'"'""•••'•••'••.•';;"•::._.= 7 Sea Level I { 2x Venial Exaggeration 0 lkm

Upper lavas and cinders of / Megalo Vouno and Kokkino Vouno Volcanos

LEGEND Cross-section B-8 • (West-East)

Interbedded lavas and tuffs of Megalo Vouno below the lower pumice series

Lavas of Thirasia Volcano

Lavas. interbedded tuffs of Mikro Profitis Ilias Volcano

Fig. 7b

Dikes 1[•'•

Ignimbrite and upper bedded tuffs

Middle Welded scoria of Tuff Thirasia Sequence

Thin, brown pumice fall deposits

Lower pumice series "'/'"'""... '. •. ••

Fig. 7. (a) NS and (b) EW pre-Minoan cross sections through the northern Thira volcanic field, showing simplified stratigraphic relationships between the Middle Tuff Sequence and lavas of the Skaros and Megalo Vouno volcanoes. Lines of profiles are shown in Figure 8.

not truncated by collapse of the Minoan caldera (Figures 6-8). Distribution of these deposits implies that there was a circular, 6-km-diameter depression located here before the Minoan eruption. Eruption of phreatomagmatic tuffs of the Middle Tuff Sequence may have been from fissures located within this depression, open to the southwest and flooded by the sea. The presence of phreatomagmatic tuffs implies that this depression was flooded then, as it is now. The size of the depression appears to have been about half of the present "caldera." Within this depression the Lower Pumice series is cut by steep scarps and overlain by Middle Tuff Sequence deposits (Figure 6). The depression may have been, in part, a NE trending

graben, cutting the metamorhic rocks of Athinios. The polygonal, 6-km-diameter depression shown in Figure 8 is here interpreted as a caldera with the shape controlled, in part, by precaldera normal faults. The depression is located south of the Kameni Islands, where water depth is 280 m; in contrast, what is believed to be the Minoan caldera has an average water depth of 380 m.

The older, pre-Minoan caldera could have been formed during eruption of the Lower Pumice series. In relative mag- nitude and eruption history, the Lower Pumice series is similar to the Minoan Tuff and could have been responsible for caldera collapse. Formation of an older caldera due to the

PALEOTOPOGRAPHY - MINOAN THIRA (in m)

Fig. 8a

Paleogeology of Minoan Thira EXPLANATION

,2 km

Thirasia Scoria

Volcano ......

Lavas of Skaros I,,,,,,',.?•,l Volcano

Lavas and Tuffs of Megalo Vouno Volcano

Pre- 1 Myr - old dacitic tuffs and andesitic cinder cones of

The Akrotiri Peninsula

Metamorphic 'basement" phyllites, marble, meta-tuffs (Triassic and early Tertiary)

Middle Tuff Sequence

Fig. 8b

Fig. 8. (a) Paleotopographic and (b) paleogeologic maps of Minoan Thira. The most notable feature is the 5- to 6-km-diameter flooded caldera present in the southern part of the volcanic field.


cc

3Oom

2Oom I loom

S.L.

-lOOm

THIRASIA MEGALO VOUNO SKAROS CC' 300m

200m

100m

-100m

cc

3OOm

2Oom

loom -lOOm

-200m •. -300m

-400m ß

Lavas of

Thorasia Volcano

MIKRO PROFITIS ILlAS

CC' 300m

200m

100m

S.L.

-100m

-200m

-300m

................................... ==============:='-=:=============-==== - 400 m O, ......... 1, km 2x Vertical exa•l•eratlon

LEGEND

Cross-section CC-CC'

Minoan Tuff :":::..?• Upper lavas and cinders of Megalo Vouno Volcano

Welded scoria of Thirasia

Middle tuff sequence-• Well-bedded brown tuffs

Lower pumice series

Lavas and interbedded tuffs of Megalo Vouno Volcano below the lower pumice series

Lavas, interbedded tuffs of Mikro Profitas Ilias Volcano Fig. 9.

Lavas of

Skaros Volcano

Pre- and post-Minoan eruption, reconstructed EW cross sections, showing stratigraphic relationships between the Skaros, Megalo Vouno, and Thirasia volcanoes.

eruption of the Lower Pumice series was first proposed by Van Padang [1936]; he also suggested that the Middle Tuff Sequence filled part of this caldera.

Reconstruction of the lava shields and composite cones was more difficult. As was discussed earlier, they appear to have developed within a NE trending graben. Orientations of palcovalleys, flow directions, and flow and cinder deposit thicknesses support the hypothesis that flows and cinders were erupted from NE trending fissures (now mostly buried by caldera collapse). Megalo Vouno, with its southern slopes partly preserved, appears to have consisted of a line of cinder cones, associated lava flows, and one maar volcano. Vents for the Thirasia lavas appear to have been coincident with fissure vents responsible for a narrow, NE trending deposit of massive red scoria. Slopes are nearly level, where Thirasia flows are thickest, at the caldera edge. Lava flows near the present summit of Skaros volcano (also at the caldera edge) are nearly flat and appear to be near the summit of that volcano. All Skaros vents had to be implied in this reconstruction, as no dikes are visible.

Profiles of reconstructed lava shields were also composed by analogy with the Kameni Islands, a postcaldera shield that consists of lavas very similar to those of the three pre-Minoan shields. The Kamenis have been erupted from SW-NE trending fissures during 11 observed periods of activity over the last 2000 years [Georgalas, 1962]. They are still active, the latest eruption occurring in 1950. The Kameni shield is 4 km wide (at the base) and about 400 m high (above the caldera floor) and has a profile very similar to that of remnants of the Skaros volcano.

This pre-Minoan palcogeologic and palcotopographic reconstruction is very different than that proposed before by Pichler and Kussmaul [1980]. They proposed that Thira was a

large, round island consisting of a pair of 600-m-high composite cones. This does not fit the geologic observations. Pre- Minoan Thira was a complex volcanic field, consisting of a 6-km-diameter flooded depression in the south, here interpeted as a caldera, and a group of overlapping shield volcanoes and composite cones in the north (Figure 8). Although geo- chemical correlations are not yet complete, tuffs in the southern half of the volcanic field appear to be similar in composition to the lavas of the north. The main difference seems to

be mode of eruption. All appear to have been erupted from SW-NE trending fissures; vents in the northern half of the volcanic field were subaerial and erupted cinders and lava flows, whereas those in the south, within the flooded depression, erupted mostly phreatomagmatic tephra. Ignimbrites within the Middle Tuff Sequence appear to have been erupted from subaerial vents within the shields, but much more work is needed before this can be confirmed. Presence of a flooded

depression (here interpreted as a caldera) within pre-Minoan Thira is of great importance in understanding the Minoan eruption.

CALDERA DEVELOPMENT

Introduction

The Minoan Tuff has been studied by dozens of workers over the last 120 years. The most thorough studies to date have been by Fouqu• [1879] and Reck [1936]. Over the last 10 years the most informative articles concerning the eruption were published by Bond and Sparks [1976] and Watkins et al. [1978]; a geologic map was published by Pichler and Kuss- maul [1980].

Each of the four phases of the Minoan eruption sequence was distinct, reflecting a different vent geometry and eruption

HEIKEN AND McCoY' CALDERA DEVELOPMENT DURING THE MINOAN ERUPTION 8451

mechanism (Figure 10). All eruption phases produced mostly glass pyroclasts of rhyodacitic composition, with varying amounts of lithic fragments.

Most recent authors agree that the first eruption phase was a Plinian pumice fall and that the second phase was phreatomagmatic. There was a variety of interpretations of eruption mechanisms for the third and fourth phases. The third eruption phase has been called a mudflow [Bond and Sparks, 1976] and a pyroclastic flow [Pichler and Friedrich, 1980; Gfi'nther and Pichler, 1973]; neither are totally correct, as will be dem- onstrated. The fourth phase was called reworked third-phase pumice by Pichler and Friedrich [1980] and an ignimbrite by Bond and Sparks [1976] and Wright [1978]. Comprehensive stratigraphic data concerning the Minoan tuff will be published elsewhere; only those data necessary to illustrate the mechanism of eruption and caldera collapse will be included in this paper.

Date of the Minoan Eruption

Remnants of wood from the site of ancient Akrotiri range in age from 1100 + 190 to 1710 + 60 years B.C. for long-lived samples, with an average of 1660 B.C. [Michael, 1978]. Fava beans from a jug in the Minoan village of Akrotiri have •'•C dates of 1370 + 20, 1410 + 70, and 1390 + 55 B.C. [Friedrich et al., 1980], which gives an average age of 1390 B.C. Validity of all Akrotiri •'•C ages have been questioned by Michael [1978] and Biddie and Ralph [1980]. The general view is that the eruption occurred sometime between 1400 and 1600 B.C. Another means of dating this eruption lie in the ice cores from Greenland [Hammer et al., 1980], where high acidity of ice layers matches large-scale explosive eruptions. Hammer and co-workers have dated an unusually large event at 1390 + 50 B.C., a date that correlates well with the •'•C dates of Friedrich et al. [1980] and some of those published by Michael [1978].

Eruption Precursors

Pumice falls from the first phase of the Minoan eruption have preserved several Minoan villages, the largest and best known being near Akrotiri. No human skeletons have been discovered in the excavations at Akrotiri [Doumas, 1983]. Data from these excavations indicate that most people had left the island before eruption of the Plinian phase of the Minoan eruption had begun. What prompted them to leave? A careful search was made at the base of the Minoan eruption sequence for any tephra that might have been erupted before, the cata- strophic Plinian phas e.

From 1 to 4 cm of very fine-grained light gray and pale orange ash layers were found at the base of the Minoan Tuff above Porto Athinios, on Akrotiri and near Exomiti, in the southeastern part of Thira. These ash beds are present below the massive pumice fall of the Plinian phase, deposited on a poorly developed s0il of the Middle Tuff Sequence. These ash layers have not been found anywhere else on the islands. It is here interpeted that the ash beds were from a small phreatic eruption that Preceded the Minoan eruption and, coupled with earthquakes, were responsible for evacuation of the island before Akrotiri was buried by pumice of the Plinian phase.

Plinian Pumice Fall (First Phase)

The first phase ranges in thickness (on the islands of the Thira group) from 10 to about 600 cm (this includes a transition unit to the second eruption phase). Most sections measured consist of massive white pumice fall, with a pink hue

Surface of deposit

/

5•'?': :': ?'i!(:'""(;•"'(!'3•5'3' '?';?'::" !?;•:?'7'?: ;?..'•.-'.;.i:;"-'.L.'.:;.';:?::•.::.;'3;i.'-:.;*..-';•.;2:?.:.'.:•i;•, ..-..'.',.-.,.; . ;:;.v'.,.' .-0.;.•,_.',.,:,.?r. :.'.';....;. ß :.:i'.'.,..'i.3'•'?.•:•.'.;.' ;'.•;.'.:.'.•'-.','.'.':.:;:':•':.:•

••• Phreatomagmatic /'•••]••L • 'break' , •. 1

Fig. 10. Generalized stratigraphy of the Minoan Tuff, showing the four eruption phases.

caused by hematite stain derived, in part, from small lithic fragments that have pink halos. The open framework structure of the unit is composed of 95% or more white, angular pumice lapilli, coarse ash, and bombs. The coarsest bombs seen were 25 cm long, although most were considerably smaller. No bread-crusted surfaces were seen on any of these bombs, implying that they were broken from a highly viscous, vesicu- lated magma and not vesiculating, expanding fluid bombs.

Most beds are reversely graded; the lower two-thirds consist of mostly 0.5- to 5-cm pumice lapilli, and in the upper one-third there are coarse lapilli and bomb-size pumice pyroclasts. Near the base of the unit are only traces of lithic fragments (most less than 1 cm long), and within the upper third, 1-2% lithic fragments (up to 20 cm long), and 1-2% grey, andesitic, vesicle-poor bombs, mostly le ss than 4 cm long.

Near the top of this unit (on most of the islands) is a thin (2-40 cm thick) layer of Very fin e white ash, a layer that may have been deposited during the first significant :interaction of the magma with seawater; it is designated the phreatomagmatic "break" (Figure 11). AboVe':the break is a massive open- framework pumice deposit that appears, in the field, to be a Plinian pumice fall; it ranges, in thickness, from 10 to 30 cm.

Not all of the pumice fall deposits consist of single, reversely graded units. Within the southeastern corner of Thira there are units with two layers; a normally graded pumice bed and a reversely graded pumice sequence, as described above.

Pumice fall drapes the irregular topography of Minoan Thira, preserved in most places by deposits of the second eruption phase. Near Cape Therma, the pumice fall is well preserved on inward dipping slopes (30 ̧ ) of the'Lower Pumice series caldera. However, within distal portions of tuff sequence, the pumice bed has been eroded by surges and mudflows of the second and third phases.

The isoPach map, showing total thickness of the first phase, indicates a vent area somewhere near the location of the postcaldera shield of Nea Kameni (Figure 11). On the Minoan reconstruction, this would have been located on either the Skaros or Megalo Vouno shield volcanoes (Figure 8). This is more or less in agreement with the interpretation of Bond and Sparks [1976]. Data do not allow siting of a Point source for the vent but indicate a zone along which the eruption may have begun, The pumice fall extends southeast; winds driving this plume must have been strong, as deposits on Thirasia, upwind from the source, are very thin. This 4deposit has been traced across the eastern Mediterranean in marine sediments [Watki ns et al., 1978; McCoy, 1981].

The fine:grained white ash that makes up a transition sub-

8452 HEIKEN AND McCoY.' CALDERA DEVELOPMENT DURING THE MINOAN ERUPTION

• MINOAN TUFF-FIRST PHASE

/.?> ..

k / ,..% 1.

/ '•.

Stratigraphic Section ß

Isopach Maps (thickness in cm) Plinian Phase (total thickness) ..... Phreatomagmatic "break" . .......... •00• [ Upper Plinian Phase --

J

";% ß '• k' '•.. N /

f,•. . v. • / '

•7• • • • '. s... ......... •o

Fig. 11. Isopach maps of the total thickness of the first (Plinian) eruption phase, showing total thickness and thickness of two subunits, the phreatomagmatic "break," and upper Plinian phase (above the break).

unit near the top of the first phase deposit appears to have been deposited as a density current (base surge); thicknesses (Figure 11) appear to have been controlled, in part, by topography. The subunit exhibits no structure. Pumice fall above the fine-grained subunit (the phreatomagmatic break) is similar to the main body of the unit. An isopach map of the upper Plinian phase does not, however, follow the pattern of the entire unit, but is a deposit of increasing thickness to the southeast, away from the vent.

Pumice pyroclasts from this phase have mostly slightly elongate, blocky forms; they consist of colorless rhyodacitic glass with only traces of small plagi'oclase and pyroxene phenocrysts. Vesicles are well developed, making up about 60%, by volume, of each pumice pyroclast. There are two main types of vesicles: (1) The first are present as ovoid "pockets" up to 4 mm in diameter. The "pockets" consist of well- developed ovoid vesicles up to 1000 #m long that formed from multiple, coalesced large vesicles; thin ridges visible on vesicle walls are remnants of vesicles broken during coalescence. (2) Thin, highly elongate, parallel vesicles; most are flattened ovoids, 10-50 #m wide, in cross section and are several milli- meters long. All vesicle walls are thin, in the range of 0.5-5 #m. The shard to pumice ratio of this deposit is about 1:9 [Heiken, 1983].

Second Phase of the Minoan Eruption (Phreatomagmatic)

The second and third phases of activity were entirely phreatomagmatic. The large base surge dunes, characteristic of the second phase, were noted and first interpreted as phreatomagmatic by Giinther and Pichler [1973]. The second phase deposits range in thickness from 10 to 1200 cm (Figure 12).

Regardless of the thickness, these deposits always consist of well-bedded white pumice lapilli-bearing (and sometimes bomb-beai'ing) fine ash. Proximal portions of the deposit are characterized by large base surge dunes; beds of fine ash alter- nate with beds of subrounded to rounded pumice lapilli. There are occasional concentrations of subrounded pumice pyroclasts in leeside dune deposits. Wavelengths of these dunes range from a few meters to nearly 50 m, but most are in the 5- to 8-m range, with amplitudes of 1-1.5 m. Current directions of the dunes indicate that base surges moved radially from a vent area or areas 1 or 2 km west of Cape Tourlus from the Minoan shield volcanoes, Skaros or Megalo Vouno. Within proximal areas, base surges climbed 200- to 400-m high ridges. They•"also left dunes on 10ø-30 ø slopes with indicated current directions pointing uphill.

Within distal regions, the deposit is finer grained with only traces of rounded pumice lapilli. Within these regions, distri-


Fig. 12.

n•.o,.•'•"•'""-•OAN TUFF-SECOND PHASE

ß ' •2 0 •9 • ..... P1•410 • ß : ß

/ t .soo EXPLANATION

••1050 / Mostly Dunes d • ] Mixed Dunes and -- 5•0•

• Plane •eds '" •i d"'..... ' Mostly Plane Beds PI

yoo ..... ::. ....... / •.• ".....

• '.... Dune Current Directions -- •800 Impact Direction, a Bedding Plane Sag " •• •:m .0 m '"....... •

:. .o /

• ,•oo. •0•••00

..... ..... ::....-" ........ .t. •- •500 PI ........ '" '• "' eo .... e250 :' ': " . -- '"W oo.

' " •' • I 500 • Second (phrcatomagmatic) eruption phase, Minoan Tuff, showing total thickness, fades, and directional features

of base surge deposits and asymmetric ballistic •pacts (bedding plane sags).

bution of the deposits was controlled by topography, leaving valley fills. There are plane beds and mixed plane beds and small dunes. Within the excavation of Minoan Akrotiri, mas- onry walls that extended above the pumice beds of the first phase were flattened by the base surges of the second phase.

There are significant vertical variations within this deposit. The lowest third or half is as described above, with 1-2% small lithie fragments. In the middle of the deposit (sometimes within the lower third or, in distal regions, absent) there are between 3% and 20% small lithic fragments that impart a faint pinkish orange color to the deposit.

The upper third to half of the deposit is similar to the rest but broken by occasional bedding plane sags, where blocks greater than 50 cm in diameter ballistically impacted dune surfaces. Asymmetric impacts indicate a source vent or vents near the Skaros volcano.

Contacts with the overlying third phase, which is also phreatomagmatic, are easily spotted by a transition to poorly bedded, lithic fragment-rich white fine ash. Within proximal areas this contact is gradational and marked by numerous bedding plane sags and large lithic blocks. On the south facing slope of Megalo Vouno, where this phase was deposited on a 36 ø slope, dipping into the caldera, a 5 x 4 x 1.5 m block of grey, platy lava was impacted into the surface of the deposit; the block disintegrated on impact, with ash of the second phase forced into cracks between the fragments. At a spot

between the Skaros and Mikro Profitis Ilias volcanoes (NE side of the caldera), the contact between the second and third phases is marked by a large block of welded red scoria, 0.3-2 m thick and about 15-20 m long. It had shattered on impact, marking the contact for some tens of meters with red scoria fragments.

Contacts between the second and third phase in distal areas are sharp and often erosional, with distal mudflows of the third phase cutting into the second phase and, in some localities, eroding it completely.

Tephra within the second and third phases of the eruption were similar and characteristic of silicic phreatomagmatic eruptions [Self and Sparks, 1978; Heiken, 1983]. In contrast with coarse-grained pumice fall deposits of the first phase, these tephra are fine grained with median grain sizes of 100 #m to 2 mm [Bond and Sparks, 1976; Self and Sparks, 1978]. Vitric pyrocleats are mostly angular and elongate but slightly curved shards. The shard to pumice ratios (based on pyrocleat counts) range from 7:3 to 8:2. Shard sizes range from < 10 to 70 #m (long axis) with vesicle wall thicknesses of 5-10 #m. There are also small pumice pyrocleats. Most shards were formed by chilling and comminution of a vesicular melt; most vesiculation had occurred at depth and was nearly complete when rising magma came into contact with seawater. Nearly all of the shards and small pumice pyrocleats could have been derived from comminution of a pumiceous magma similar to


pumices described for the first phase of activity. Although most pumice lapilli and bombs within the fine ash of this deposit are subrounded to rounded, most shards show minor chipping and abrasion on only the thinnest (< 1 #m) pyroclast edges.

Third Phase of the Minoan Tuff (Phreatomagmatic)

On land, this is the most voluminous and enigmatic phase of the Minoan eruption; there is ample evidence to link it with beginnings of caldera collapse. As was discussed earlier, it has been called a pyroclastic flow and a mudflow. Field and laboratory evidence supports the hypothesis that it is neither (in its entirety) but a phreatomagmatic deposit with multiple facies.

Third-phase deposits are is easily identified in the field. On first glance it is a massive, lithic fragment-rich (2-40% by volume), white pumice lapilli and block-bearing fine ash. It ranges in thickness from 1 to 55 m and makes up 57% of the total volume of the Minoan Tuff on the islands. The deposit has been divided into four facies, all of which grade into each other: (1) ballistic (proximal tuff ring), (2) base surge (proximal), (3) mudflow (distal), and (4) slumps (distal-•on steep slopes).

Ballistic facies (proximal tuff ring). The thickest deposits

of the third eruption phase are within this facies (up to 55 m); they are also closest to the vent(s) (Figure 13). Many subdivi- sions of this unit can be made on the basis of variations in

volume of lithic fragments, pumice bombs, and (rarely) beds of pumice lapilli. It is a massive deposit but with very faint bedding that can occasionally be seen because of trains of lithic clasts. Lithic clasts are, however, usually scattered evenly throughout the white ash matrix. Near the top of some sections, concentrations of lithic clasts can be found at the base

of poorly developed channels or as a lag deposit. Upon careful examination of the ash matrix around larger

(> 1 m) lithic clasts, bedding plane sags were visible, having disrupted bedding, as defined by trains of small lithic clasts. These impact craters are intact; none have been disrupted by flow, which would have been the case if this unit were a mudflow.

There is no topographic control of this facies. Thicknesses of the massive deposit do not change significantly over the "Athinios Ridge" (southeast Thira), from the "low" in the caldera wall between Thira and Athinios (elevation 200 m) to the ridge between Athinios and Profitis Ilias (elevation 300 m) and back down to the Akrotiri Peninsula (elevation 180 m). This draping of topography is easily seen from within the caldera complex.

MINOAN TUFF-THIRD PHASE

30 25 12+

20+. B

24 1-5. ß 2+ .2

•/•25 • 1. 25;+:• Thickness (in m).8 .,, 3+%. 2+ Bedding Plane •g(s)•s

10+ '• FACIES Near-Vent (Ballistic) .:.:•

Near-Vent (Dilute-Dunes) .'!--•']

Distal (Mudflow) r"-'l

10+ Distal (Slumps) ['• ..•

0 1 2km ß ß

7+

12+

18-2, 2'

lO

.'$• * 2-6

5+

.1.5+

B

'8

.•.

10+

.2+ 3+

3+

.1+

.8+

,6+

.0-10

e7+ "" ß ß

15-20 . - - ' -

// /

! absent I absent I

4+ t I

Fig. 13. Third (phreatomagmatic) eruption phase, Minoan Tuff, showing total thickness and facies distributions.

HEIKEN AND McCoY: CALDERA DEVELOPMENT DURING THE MINOAN ERUPTION 845.5

Base surge (proximal). In the southwestern corner of the caldera complex, east of Akrotiri, the massive ballistic facies grades laterally (west) into well-bedded, lithic-rich base surge deposits. This facies is limited to a narrow, 2-km-wide zone at a low point on the caldera wall (Figure 13). When traced laterally, bedding planes become more apparent as they grade into well-bedded (20-30 cm thick) lithic fragment and pumice block-bearing white fine ash and layers of pumice lapilli; this lateral transition occurs over a distance of about 100 m. Near

the top of this well-bedded facies are concentrations of cobble- size lithic fragments similar to those found near the top of the ballistic facies. The well-bedded facies is made up of thick plane beds and is believed to be of base surge origin.

Mudflow (distal). On the outer flanks of most of central Thira, much of the Akrotiri Peninsula and all of Thirasia, the third-phase tuffs are mudflow deposits. In general appearance this facies is similar to that of the ballistic facies: massive lithic

fragment-rich, pumice bomb and lapilli-bearing white fine ash. This facies is confined to arroyos, valleys, and coastal

plains. Distribution of the deposit is definitely topographically controlled. Individual beds within this facies are bounded by concentrations of subrounded and rounded lithic fragments (up to 1 m) at the bottom of each channel-filling mudflow. These mudflows had considerable erosive power, cutting into

underlying phases of the Minoan Tuff and, in some locations, into older rocks. Contacts with the second- and first-phase deposits are sharp, not gradational. In several areas a sequence of channels are visible, with younger mudflows cutting into older ones.

The mudflow facies within ravines located on the flanks of

the volcanic field grades into the near-vent tuff ring (ballistic facies). As in the other facies, the uppermost deposits contain an abundance of lithic fragments (up to 60%) concentrated into lenses similar to stream gravels. The top of the third phase appears to have been reworked, as flood deposits, before eruption of the fourth phase, an interpretation already made by Bond and Sparks [1976].

Slumps (distal, on steep slopes). Massive third-phase deposits are anomalously thick at the base of north facing slopes of Mount Profitis Ilias, the highest point on the island (elevation 564 m). Lithic fragments are scattered evenly throughout the ash matrix or form trains (as described in the ballistic phase); they have not been concentated into lenses, as would have been the case if washed from the slopes of Mount Profitis Ilias. The increased thickness at the base of this mountain

appears to have been related to slumping of the Minoan Tuff (mostly the third phase) after deposition on steep (>25 ø ) slopes of Mount Profitis Ilias. Deposits such as this are rare

MINOAN TUFF-FOURTH PHASE 12+ 3 (eroded)

'15

2(?) 25(?

35-40

35

12 ...•\ 10+

l•?!i::iiiiiiiiiiii::•.'.."::. ..:•9. . :.:::. ...•51•4.5 (channel) <" :'Z.:.'.'.'" ',v..Z.Z.'.: :':'Z Z'; .' ','1

"' • e"' ?..,,

•.:-:.:•. •

*":•::i:::::';; .•,:• 1'• EXPLANATION . .i-Z.'/ I .o

Ignirnbrite Thickness, in rn-.8

•:?• Distribution of the ignimbrite- • 30+

,20+

1,5'

1+

2.5

Fig. 14. Fourth (ignimbritic) eruption phase, Minoan Tuff, showing distribution and thickness.


elsewhere, but in a small area on the south side of this moun-

tain the entire Minoan Tuff deposit is deformed by convolute bedding related to slumping.

Tephra descriptions. The third phase is the only phase of the Minoan eruption where many of the pumice bombs have bread crust surfaces. These bombs increase in number with

increasing volume of lithic fragments. Tephra are very similar to those of the second-phase ac-

tivity. The ratio of shards to pumice is 9:1. Most shards are slightly curved, and 5-20 #m thick; angular edges are sharp with no evidence of pitting or chipping. Small pumices have a varied appearance, from those with straight, highly elongate vesicles to those with slightly elongate vesicles separated by thick vesicle walls.

Third-phase deposits appear to have been formed during large-scale, continuous phreatomagmatic activity [Heiken et al., 1983]. The mass ratio of seawater and magma and degree of confinement in the zone of magma-water interaction may have determined the mode of emplacement of each facies. Ac- cordingly, the proximal ballistic facies resulted from initial explosions accompanying caldera collapse during which water-melt interaction was minimal and adiabatic conditions

controlled ejection of pyroclasts. As collapse allowed more water into the vent area, explosive activity may have increased, and isothermal conditions resulted in the formation of

base surges that moved over low terrain and emplaced deposits grading into the ballistic facies. Distal mudflows formed during water-rich eruption conditions; wet surges contained steam that condensed before separation from the tephra. These mudflows may have contained as much as 30 wt% water and continued to move downslope after their initial deposition [Heiken et al., 1983].

Fourth Phase (lgnirnbrite)

The fourth and last phase of the Minoan eruption produced pyroclastic flows that left thin veneer deposits around the caldera rim and thick (up to 40 m) deposits on the surrounding coastal plains, overlying the third-phase deposits (Figure 14). In contrast with the phreatomagmatic deposits of the second and third phases, deposited at temperatures below the Curie point, tephra of the fourth phase were deposited at temperatures of over 500øC; they acquired a thermoremanent mag- netization (TRM) upon deposition [Wright, 1978].

Along the coastal plains, ignimbrites form fan-shaped de-

IvIINOAN TUFF (Bo 1)

EXP.LANATION

o

lO%

20%

30%

Per cent lithic fragments

Akrotlrl-type daotlc tuffs and lavas scoria

dark brown, frae-grained tuffs • ø:,":'••r•'• other

hydrothermally altered black and gray la s f '••'•glas sy, black va andesltlc lavas

aphamtlc black and gray andesltlc lavas

THIRA

? SOOm o ] 2•.• I

Fig. 15. Lithic fragment distribution, first eruption phase, Minoan Tuff.

HEIKEN AND McCoY' CALDERA DEVELOPMENT DURING THE MINGAN ERUPTION 8457

posits that are thin along fan boundaries and thickest near the fan center (up to 40 m). Central portions of these deposits are composed of massive, light tan lapilli- and pumice-bearing ash. Bedding is poorly developed and visible due to concentrations of small lithic fragments. The faint layering appears to be due to subtle grading of lithic fragments within individual flow deposits. There is an increase in the volume of small (mostly <2 cm) lithic fragments near the top of this subunit.

Along the fan boundaries the deposit is thinner (10 m or less), with distinct lithologic units. Interbedded with pyroclastic flows are tuffaceous epiclastic sediments, graded pumice layers, and wedge-shaped concentrations of rounded, cobble- and boulder-size lithic fragments. There is also an increase in channeling, with the youngest pyroclastic flows filling channels cut into the earliest flows. Matrix material of the

channel-filling flows consists of subrounded to rounded pumice lapilli in medium to coarse ash.

Gravel lenses and epiclastic sediments within the deposit appear to be located mostly in channels related to floods between pyroclastic flows. Bond and Sparks [1976] interpreted

the coarse gravel lenses interbedded with the ignimbrites as having been deposited by flash floods between pyroclastic flows.

Along the east, south, and west rims of the caldera complex are thin (0.7-2 m) tan ash deposits correlative with the much thicker deposits of the coastal plains. These deposits consist of small pumice lapilli, coarse ash, and small (< 2 cm) lithic fragments in an ash matrix. There are some low dunes present within this deposit. The thin, well-bedded caldera rim deposits are similar to "ignimbrite veneer" deposits [Walker et al., 1981]. Pyroclasts in the ignimbrites are heterogeneous in size and shape. The shard to pumice ratio is 9:1 (as it is in the second and third phases of the Minoan eruption), but pumices are somewhat larger than those of the second and third phases. They are 200-300 #m long with 20- to 60-#m-wide elongate vesicles that have thin (3-5 #m) vesicle walls. Overall vesicularity is lower (35%) than in the previous phases (60%). Some vesicles are polygonal in cross section, with nearly flat vesicle walls. Angular, curved, and flat shards in the 5-30 #m range make up most of the ash and were derived from smooth, nearly flat vesicles.

MINOAN TUFF (Bo 2) EXPLANATION

THIR

Ak•ot•i-type dacetin tuffs and lavas

da•k b•own, fme-g•amed tuffs .• hydmthe•mally altered black and g•ay lavas •" •

o

lO%

20%

30%


scoria

••nl•, øther glassy, black andesitic lavas

aphanitlc black and gray andesitic lavas

THIRA

? 500m I 0 • 2•m

Fig. 16. Lithic fragment distribution, second eruption phase, Minoan Tuff.


Lithic Fragments Within the Minoan Tuff All stratigraphic sections of the Minoan Tuff were described

and measured after the Minoan paleogeology and pale- ogeography were studied (Figure 8). The purpose was to determine if, through a study of lithic fragments present in deposits of different eruption phases, the location and size of vents could be determined. This was possible due to the diversity of lithologic types present on Minoan Thira. Within exposures of 2 x 2 m, all lithic fragments were described (field descriptions only), with characterizations of lithology, size, percentage of the total population, angularity, shape, and degree of weather- ing. The coarsest (> 4 cm) fraction was used for the comparison whenever possible (Figures 15-17). When necessary, the less than 4-cm fraction was described and used for this part of the study; a smaller area of 20 x 20 cm was used for describ- ing populations of small lithic fragments.

Up to 12 types of lithic fragmeots were found at many localities; for comparison, however, they were divided into seven categories: (1) aphanitic, black and grey andesitic lavas, (2) hydrothermally altered, aphanitic black and grey lavas, (3) dark brown fine-grained tuffs, (4) green and white tuffs, grey dacite lavas typical of the older volcanic sequence of the Akro- tiri Peninsula, (5) red scoria, (6) "other," including welded tuff,

marble, phyllites, sandstones, and schist clasts, and (7) glassy, black porphyritic lavas.

First phase. The Plinian phase deposits are characterized by only a trace of lithic fragments at the base to 1-2% lithic fragments near the top (Figure 15). There are mostly black or grey aphanitic lava clasts (both angular and fresh and rounded and weathered forms) and an abundance of lava dasts showing some effects of hydrothermal alteration. Traces of green and white tuffs and grey dacites (all with some alteration products) are present, which are typical of older (pre-l.0 m.y.) volcanic sequence now visible in the Akrotiri Peninsula. Brown fine-grained tuffs typical of the Middle Tuff Sequence are present. There are also traces of prevolcanic rocks in this deposit, including limestone, quartz monzonite, and phyllite dasts.

Most of the lithic fragments in this deposit are aphanitic, porphyritic lavas best represented (volumetrically) in the eastern lava shields (Megalo Vouno and Skaros). Many are hydrothermally altered, rounded, and weathered, indicating that they may have been derived from an older vent or vents on one of the shields and that the Minoan eruption began at one of these older vents.

Second phase. The second phase phreatomagrnatic deposit

THIRASIA

MINOAN TUFF (Bo 3) EXPLANATION

o

lO%

20%

30%


Akrot,r,-type daot,c tuffs and lavas scor,a

h

hydrothermally altered black and gray lavas/- • . ack andesltlc lavas

aphan,t•c black and gray ande$,t,c lavas

THIRA

0 500m 0 I 2km

Fig. 17. Lithic fragment distribution, third eruption phase, Minoan Tuff.


Lower Pumice

Caldera (flooded)

Vent, PI,n,an Phase

• 2km

MINOAN TUFF --PHASE I MINOAN TUFF - PHASE II

Fig. 18a Fig. 18b

Vent or Vents, First 'Phreatomagmatic Phase

• 2kin

In,t,at,on of Caldera Collaps•

• 2kin

MINOAN TUFF - PHASE III MINOAN TUFF - PHASE IV

Fig. 18c Fig. ! 8d

Fig. 18. Vent development and caldera collapse during the four phases of the Minoan eruption.

Possible Source

• 2kin

contains 2-10% lithie fragments. The variety and percentages of rock types are similar to the first phase, with a slight increase in tuffs from the Middle Tuff Sequence (Figure 16). The increase in volume of lithie fragment populations might indicate vent erosion by phreatomagmatic activity.

Third phase. A dramatic change from earlier eruption phases is evident within the third phase. Lithie fragments make up 5-40% of the deposit. They are different from the lithie fragments of earlier phases, consisting mostly of angular, fresh blocky black, glassy porphyritic lavas similar to dacites

of the Thirasia shield located on the northwestern flank of the volcanic field (and the youngest flow on the Skaros shield) (Figure 17). The abrupt change, from a few percent clasts derived from the eastern part of the volcanic field, in the first two eruption phases, to a large volume of clasts in the third eruption phase, derived mostly from the northwestern part of the field, may indicated the beginning of caldera collapse (in the west) and massive interaction of magma and seawater. Ten percent qf the clasts were derived from older rocks now seen only on Akrotiri. As the eruption appears to have been from


THIRA: TOPOGRAPHY AND BATHYMETRY (1975 A.D.) (100m contours)

Fig. 19c

Fig. 19. Topography and bathymetry, Thira volcanic field (a) before the Minoan eruption, (b) immediately after the Minoan eruption, and (c) today.

vents in the north-central part of the volcanic field, perhaps a NE trending ridge of the older, Akrotiri-type rocks was present below the lava shields of the north; since the volcanic field appear s to have developed along a NE trending graben, this is possible.

Fourth phase. There is yet another distinct change in the lithic clast populations from 'the third to fourth eruption phases. Lithic clasts within the ignimbrites are characterized by their small size (except in the interbedded flood deposits) and diversity. The most common rock types are lavas similar to those seen in the first two eruption phases. Other clasts Present are tuffs typical of the older volcanic sequence of Ak- rotiri, marble, schist, tuffs from the Middle Tuff Sequence, and

purple and white banded lavas. Most clasts appears to have been derived from the northeastern part of the volcanic field. Due to the small size of lithic clasts in the ignimbrites, no map similar to those for the first three phases was prepared.

SUMMARy AND DISCUSSION

Extension and normal faulting within the Aegean plate during the last 10 Ma are consistent with a NE-SW trending graben along which the Thira volcanic field has developed. From the oldest volcanic rocks of the Akrotiri Peninsula, overlain by 1-Ma-old tuffs, to the youngest rocks. of the Kameni Islands (3 2 years old), eruptions have been from ils- sure•s located within this graben.


The first evidence for existence of an older, flooded caldera

present on Thira before the Minoan eruption was a discrepancy between the volume of tephra erupted during that eruption (•-13-18 km 3) and the volume of material involved in caldera collapse, based on an earlier reconstruction of Minoan Thira (60 km3).

Based upon a paleogeologic and paleotopographic reconstruction of Minoan Thira, before the eruption of 1400 B.C., there existed in the southern half of the volcanic field an •-5- to 6-km-diameter caldera. It was, most likely, formed during eruption of the Lower Pumice series, 100,000 years B.P. (dated by Seward et al. [1980]). This caldera is still visible, is located south of the Kameni Islands, and has an average depth (today) of 280 m. The original depth is not known, as there has been no drilling to determine the thickness of the Minoan Tuff within the older caldera.

Eruptions between 100,000 years B.P. and 1400 B.C. were affected by the presence of a flooded caldera. Within the northern half of the volcanic field there were primarily subaerial eruptions of lava, scoria, and several ignimbrites; three small shield volcanoes were formed during this period. In the southern half of the field there were mostly phreatomagmatic eruptions; eruptions from within the flooded caldera plastered well-bedded fine-grained tephra onto caldera walls. Most of these deposits have not been cut by collapse of the Minoan caldera, as had been proposed in earlier studies; instead, most of the cliffs in the southern part of the caldera complex were formed by wave erosion.

On the basis of the physical characteristics, lithic fragment distributions, and areal distribution of deposits of the four eruption phases of the Minoan Tuff, the following sequence of vent growth and caldera collapse is proposed:

1. Fine phreatic ash erupted from somewhere on the Skaros volcano and was carried southeast.

2. The Plinian pumice fall was erupted from an old vent, carrying along weathered and hydrothermally altered lava fragments with it. This vent was located somewhere on the ancient Skaros or Megalo Vouno volcanoes (Figure 18a).

3. Extension of the vent into a flooded older caldera initi- ated the second (phreatomagmatic) phase of activity (Figure 18b). Vent widening progressed, with increased volume of lithic fragments being carried along in base surges.

4. A sharp increase in the volume of lithic fragments, a change in their composition, and indications of a massive, continuous phreatomagmatic eruption indicate that the third eruption phase may have been associated with the beginning of caldera collapse. Lithic fragments in these deposits are mostly from the western part of the lava shields. Collapse may have been, during this phase, like a trapdoor, with subsidence in the western part of the island (Figure 18c).

5. Eruption of ignimbrites during the fourth phase (although with a phreatomagmatic component) was mostly from subaerial vents in the east (Figure 18d). Collapse during or after this phase extended from west to east; the final product was an 8 x 9 kin, polygonally shaped caldera with a depth (today) of 380 m below sea level (•_ 700 m below the summits of the shield volcanoes). It is located north of the Kameni Islands and is characterized by steep walls that extend down- ward from the summits of the Skaros and Thirasia volcanoes to a caldera floor 380 m below sea level. It overlaps the older, Lower Pumice series caldera; together they form the caldera complex visible today (Figures 19b and 19c).

Slumping of large blocks into the caldera from the NW and west have left narrow, deep grabens that extend beyond the

island's coastline. Both slumps are located away from dike swarms and may have collapsed due to lack of structural support normally supplied by the frameworks of dike-sill complexes.

The volume of the Minoan caldera, based upon the difference between the pre-Minoan and post-Minoan, pre-Kameni paleotopographic reconstructions is 19 km 3, reasonably close to the volume of magma erupted during the Minoan eruption, as estimated by Watkins et al. [1978] (Figures 19a and 19b).

The Kameni Islands were formed during 11 eruption epi- sodes over the last 2000 years [Georgalas, 1962] along N50øE trending fissures. These vents are located in the same general area as the vents for the Minoan eruption (and for the even earlier Skaros volcano). Huijsrnans and Barton [1983] note that the rhyodacitic lavas erupted to form the Kameni Islands came from a depth of about 3 km. These lavas may represent late-stage, volatile-poor lavas from the Minoan magma chamber and are being erupted from near or concomitant with the Minoan vents. There is no evidence for structural

resurgence within the Minoan caldera; the 2.5 km 3 of the Kameni Islands appears to have been formed entirely by eruptions of lava and ash.

Acknowledgments. The work on Thira was supported in part by a sabbatical leave from the Los Alamos National Laboratory and by grants from the National Geographic Society and National Hellenic Research Foundation. Collaborations with Michael Moutsoulas, Na- tional Center for Space Research, Athens, and Michael Barton and Joep Huijsmans, State University of Utrecht, are greatly appreciated. Permission to work in Greece was made possible by the Institute of Geology and Mineral Exploration, Athens. Reviews of this paper by S. Sparks, K. Wohletz, and H. Sigurdsson were most useful and are greatly appreciated.

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(Received July 26, 1983; revised January 13, 1984;

accepted February 22, 1984.)

caldera development during the minoan eruption, thira, cyclades, greece

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