archeology - tsunami generated by the late bronze age eruption of thera

Pure appl. geophys. 157 (2000) 1227–12560033–4553/00/081227–30 $ 1.50+0.20/0

Tsunami Generated by the Late Bronze Age Eruption of Thera(Santorini), Greece

FLOYD W. MCCOY1 and GRANT HEIKEN2

Abstract—Tsunami were generated during the Late Bronze Age (LBA) eruption of the island ofThera, in the southern Aegean Sea, by both caldera collapse, and by the entry of pyroclastic surges/flowsand lahars/debris flows into the sea. Tsunami generated by caldera collapse propagated to the westproducing deep-sea sedimentary deposits in the eastern Mediterranean Sea known as homogenites;open-ocean wave heights of about 1.9–17 m are estimated. Tsunami generated by the entry ofpyroclastic flows/surges and lahars/debris flows into the sea propagated in all directions around theisland; wave heights along coastal areas were about 7–12 m as estimated from newly identified tsunamideposits on eastern Thera as well as from pumice deposits found at archaeological sites on northern andeastern Crete.

Key words: Tsunami, volcanic eruptions, tephra, pyroclastic surges, pyroclastic flows, debris flows,lahar, calderas, tsunami deposits, pumice, ash, Late Bronze Age.

1. Introduction

An explosive eruption on the island of Thera, also known as Santorini, in theCycladic Islands, about 3600 years B.P. during the Late Bronze Age (LBA) hadprofound effects in the Aegean and eastern Mediterranean region. Local effectsincluded the devastation of the island and a civilization that inhabited that islandleading to the demise of cultural centers throughout the region, such as the Minoanculture on Crete (thus the eruption is often referred to as the ‘‘Minoan’’ eruption)and the Cycladic culture in the Cycladic Islands. Regional effects included wide-spread dispersal of ash, possible climate change associated with the injection of ashinto high altitudes, seismic activity, dispersal of pumice rafts by oceanic surfacecurrents, and generation of tsunami. Evidence comes from both the geological andarchaeological record of the circum-Mediterranean area. Deposits of tephra arewidespread throughout the Aegean and eastern Mediterranean region from the Nile

1 University of Hawaii, Windward Community College, Kaneohe, HI 96744. E-mail: [email protected]

2 Los Alamos National Laboratory, Earth and Environmental Sciences Division, Los Alamos, NM87545.

Floyd W. McCoy and Grant Heiken1228 Pure appl. geophys.,

Delta to the Black Sea (various papers in HARDY, 1990; STANLEY and SHENG,1986; GUICHARD et al., 1993) and provide dramatic documentation of the explosiv-ity of this Plinian-type eruption. Atmospheric temperature decrease of about 0.5°Cin the Northern Hemisphere is estimated from sulphur aerosols injected into thestratosphere (SIGURDSSON et al., 1990). Seismic activity possibly associated withthe eruption caused widespread damage to LBA Minoan buildings (DOUMAS, 1983;various papers in Hardy, 1990). Pumice accumulations in coastal deposits andarchaeological sites throughout the eastern Mediterranean and Aegean littoralsuggest extensive rafting of pumice following the LBA eruption (FRANCAVIGLIA,1990; and others).

That tsunami were generated has been documented by the discovery of adistinctive sedimentary deposit, homogenites, interbedded in deep-sea sediments ofthe Mediterranean Sea (CITA et al., 1984; and others), by chaotic accumulations oftephra and other sediments at archaeological sites in the Aegean region (ASTROM,1978; NEEV et al., 1987; FRANCAVIGLIA, 1990; and others), as well as by newlyidentified deposits within the LBA eruption sequence on Thera (MCCOY, 1997;MCCOY and HEIKEN, 1997). Geological and geophysical studies on Thera havelead to an understanding of the eruptive mechanics, vent placement and topo-graphic changes associated with the eruption, and inferences for the generation oftsunami (BOND and SPARKS, 1976; HEIKEN and MCCOY, 1984; DRUITT et al.,1989; SPARKS and WILSON, 1990; and others). Tsunami generated during theeruption has been assumed by archaeologists as one of the primary destructiveeffects of the eruption, although identifiable damage to man-made structures bytsunami remains elusive (MARINATOS, 1939; PLATON, 1971; DOUMAS, 1983; BAR-

BER, 1987; various papers in HARDY, 1990; and others).This paper reviews evidence for tsunami from the archaeological and geological

record, and presents preliminary data on volcaniclastic deposits found on Therathat are possibly the result of tsunami inundation. Possible tsunamigenic phases ofthe LBA eruption are discussed with inferences for wave propagation into theAegean and eastern Mediterranean Seas; estimates for tsunami wave heights aremade from sedimentary deposits considered to have originated from tsunamipassage at sea and impact on land.

2. Background

2.1 Geologic and Tectonic Setting of Thera

Thera is one of several volcanic centers active during the Quaternary along theHellenic island arc of the southern Aegean Sea (Fig. 1). Five islands surround(Thera, Therasia, Aspronisi) or occur within (Kameni Islands) two water-filledcalderas to form the archipelago of Santorini (Fig. 2). Thera, Therasia, and

Tsunami Generated by the Late Bronze Age Eruption of Thera 1229Vol. 157, 2000

Aspronisi are remnants of a larger island destroyed during the LBA (Minoan)eruption. The Kameni Islands have been constructed by post-Bronze Age volcan-ism, the latest eruption in 1950 on Nea Kameni; these islands today have manifes-tations of volcanic activity with hot springs, fumeroles, and low-level seismicactivity (FYTIKAS et al., 1990).

Volcanic activity during the past 700,000 years constructed pyroclastic conesand lava shields of dominantly dacitic composition, to form the Thera volcanic field(PICHLER and KUSSMAUL, 1980; SEWARD et al., 1980; HEIKEN and MCCOY, 1984;

Figure 1The southern Aegean and eastern Mediterranean Seas showing the locations (circles) of volcanoes alongthe Aegean island arc; the circled location northeast of Santorini is the underwater volcano at ColomboBank. Islands mentioned in the text are identified. Large open arrow indicates the direction of maximumtectonic closure between the African and Aegean/Europe plates. Lines and patterns, from north tosouth, indicate: dashed lines (Aegean Sea)=150 (northern line) and 100 km (southern line) contour onthe subsurface depth of the subducted crustal slab beneath the Aegean arc; dotted lines south and westof Crete=sea floor troughs (Hellenic ‘‘trench’’ system); fine stipple pattern through the centralMediterranean Sea=Mediterranean Ridge, an accretionary wedge of sediment produced by the closurebetween Africa and Europe; thick dark line with sawtooth pattern=approximate position on the seafloor of the downgoing slab. Tectonic indicators are from LE PICHON and ANGELIER (1979), RYAN et

al. (1982), KASTENS et al. (1992), and MEIJER and WORTEL (1997).


Figure 2Generalized geologic map of Santorini. ‘‘Basement’’ refers to Mesozoic metamorphic rocks; the Akrotirivolcanic complex is a sequence of pillow lavas and associated sediments of the Pliocene age. MegaloVouno volcanic complex, the Micros Profitis Elias, Therasia and Skaros volcanoes are eruption centersactive over the past 900,000 years. Data from PICHLER and KUSSMAUL (1980), HEIKEN and MCCOY

(1984), DRUITT et al. (1989), and unpublished field data. The LBA eruption is often referred to as the‘‘Minoan’’ eruption, after the Minoan culture that existed on Crete at the time of the eruption.

DRUITT et al., 1989; BARTON and HUIJSMANS, 1986; FRIEDRICH, 1994; MCCOY

and HEIKEN, 2000). The latest pyroclastic eruption in this sequence was the LBA(Minoan) eruption that excavated a large caldera north of the present-day Kameni


Islands and deposited a thick tephra layer on the islands surrounding the caldera(HEIKEN and MCCOY, 1984) (Fig. 2).

These volcanic deposits are built upon Mesozoic metamorphic rocks andPliocene submarine pillow lavas and associated sediments (Fig. 2). The Mesozoicterrain is part of the Hellenide-Anatolide orogenic belt deformed during the earlyCenozoic closure of the African and European plates (MCKENZIE, 1972; LE

PICHON and ANGELIER, 1979; FYTIKAS et al., 1984). An extensional crustal regimeduring the Pliocene, in response to this closure, formed the Aegean basin with theislands of the Aegean basin being the subaerial portions of a series of Paleogene-Miocene nappes (MAKRIS, 1978; BONNEAU, 1984; HALL et al., 1984; ROBERTSON

and DIXON, 1984; ARMIJO et al., 1992). Contemporary plate-closure rates are about10–40 mm/yr., with intra-arc extension rates of about 3–8 cm/yr. (DEWEY et al.,1973; LIVERMORE and SMITH, 1985; ORAL et al., 1995; KAHLE and MULLER,1996). A northwesterly-dipping Benioff zone occurs beneath the arc (Fig. 1)(AGARWAL et al., 1976; BATH, 1983; SPAKMAN, 1986). Seismic activity in responseto this tectonic activity, as well as to volcanism, is centered in the southern AegeanSea in the vicinity of Thera, often with destructive tsunami (BATH, 1983; TAYMAZ

et al., 1990).

2.2 Generation of Tsunami by Volcanism and Slumping

Tsunami created during historic volcanic eruptions have been documented,particularly those generated during the 1883 eruption of Krakatau. Accounts fromthis eruption come from both survivors and from later studies (see SIMKIN andFISKE, 1983, for an excellent compilation; see also SIGURDSSON et al., 1991; CAREY

et al., 1996, 2000). Tsunami were produced by collapse of the caldera (WHARTON,1888), entry of pyroclastic surges and flows into the ocean (SELF and RAMPINO,1981; LATTER, 1981; SIGURDSSON et al., 1991; CAREY et al., 1996, 2000), slumpingof parts of the underwater volcanic edifice (SELF and RAMPINO, 1981), in additionto the explosive interaction of magma and seawater (not considered as significant asother effects in generating tsunami by SELF and RAMPINO, 1981). Wave heightsexceeded 40 m; runup distances reached 11 miles inland over low slopes (on theorder of 1–5°) of coastal benches (SIMKIN and FISKE, 1983).

Tsunami produced during the 1815 eruption of Tambora in Indonesia weregenerated by pyroclastic flows entering the ocean (SELF and RAMPINO, 1981);coastal inundation reached up to 4 m elevations (VAN PADANG, 1971; WALKER,1973). The 3500 yBP eruption of Aniakchak volcano in Alaska produced tsunamiwith runups as high as 15 m over low slopes (coastal peat bog), apparently also byentry of pyroclastic flows into the ocean (WAYTHOMAS and NEAL, 1997).

Tsunami generated by sea-floor displacement due to gravity-driven slumps andslides are well documented. The collapse of volcanic edifices to produce tsunami arethe focus of numerous papers in this volume and elsewhere (MCGUIRE et al., 1996).


Massive underwater slides generating tsunami are documented, the best studiedperhaps being the Storegga slides off Norway about 7000 yBP (BUGGE et al., 1987;JANSEN et al., 1987) leaving deposits in Scotland (DAWSON et al., 1988, 1993) andNorway (BONDEVIK et al., 1997). Runup heights from this tsunami are estimated atup to an elevation of 11 m (BONDEVIK et al., 1997). An underwater slide in theAleutian trench south of Alaska in 1946 produced devastating tsunami with runupsreaching 35 m in Alaska and 17 m in Hawaii, and with estimated maximumwave-heights in Hawaii up to 15 m (MACDONALD et al., 1947). Tsunamis generatedby debris avalanches accompanying or following volcanic activity have producedwaves of the order of 9–20 m in height (reviewed in MCGUIRE, 1996).

In the Aegean, there are accounts describing historic volcanic eruptions andconsequent tsunami. In 1650, as one example, Jesuit priests on Thera described withsome awe a submarine eruption at Colombo Bank northeast of Thera (Fig. 1) ‘‘theocean burned . . . , flames leaping from the sea . . . fiery clouds lowered upon us . . .cast up into the heavens enormous rocks, which fell to earth again some twoleagues away . . . in a field we saw a boulder spewed from the bowels of the earth,of a size that 40 men could not move it . . . the sea covered with pumice . . .sulfurous steam’’ (GALANOPOULOS and BACON, 1969). Tsunamis associated withthis eruption were apparently generated by caldera collapse; recent bathymetricsurveys of Colombo Bank indicate collapse was on the order of 400 m (PERISSO-

RATIS, 1985). On Thera, waves deposited pebbles and dead fish as much as twomiles inland in valleys (GALANOPOULOS and BACON, 1969); tsunamis were reportedto be 16 m high on the island of Ios immediately north of Thera (LUCE, 1969); atPatmos, northeast of Thera ‘‘the sea rose 50 m and 30 m on the west and eastcoasts, respectively’’ (GALANOPOULOS and BACON, 1969); on the northern coast ofCrete, ships were torn from their moorings and sank in the harbor at Iraklion (thetown was under siege by Turkish forces at the time and their ships were destroyedby the tsunami, thus ending the siege) (GALANOPOULOS and BACON, 1969;GALANOPOULOS, 1960; DOUMAS, 1983).

2.3 Obser6ations of Tsunami Wa6e Heights

Accounts describing tsunami often use two different observations: (1) waveheight, as would be observed at the coastline with the tsunami approaching, and (2)elevation or distance onshore of runup by the wave, usually easy to distinguish bylimits of damage or of displaced debris inland. Wave height is a difficult measure-ment, potentially dangerous to the observer, and is reliably measured by high-waterlevels or damage levels marked on surviving structures after inundation—suchmeasurements are rare; good data on tsunami wave heights are correspondinglyrare. Runup is not a reliable indicator of wave height, rather is a function ofnumerous variables: slope and topography offshore and onshore; soil/sediment androck types offshore and onshore; roughness factors determined offshore by reefs,


bars, banks, etc., and determined onshore by rivers, hills, vegetation, buildings, etc.;in addition to wave characteristics such as wave type, height, angle of approach,onshore velocity, etc. (LOOMIS, 1976; COX, 1979). Experimental studies have foundamplification factors of runup over wave height as high as 5.7 (SPIELVOGEL, 1973).

3. The LBA Eruption of Thera

3.1 Archaeological Setting

In the archaeological chronology of the southern Aegean derived from pottery-sherd types, the eruption occurred, and defines, the boundary between the LateMinoan IA and Late Minoan IB of the Late Bronze Age (LBA) (see various papersin HARDY, 1990). Dates center around two intervals, 1630 BC and 1500 BC (seeMANNING, 1995, for discussion of the controversy concerning these dates). Archaeo-logical evidence provides a rich account of the effects of the eruption on civiliza-tions in the region, particularly the Cycladic culture on Santorini and the Minoanculture on Crete. On Santorini, an archaeological excavation at Akrotiri (Fig. 3)uncovered what appears to have been a prosperous small city (DOUMAS, 1983)buried within tephra of the LBA eruption.

In the aftermath of the eruption, both the Cycladic and Minoan cultures werereplaced about two centuries after the volcanic disaster by the Mycenaean culturefrom mainland Greece. Whether this cultural change was due entirely to theeruption or to other cultural factors, such as war or economics, continues to beargued by archaeologists and historians—that widespread devastation did occurconcurrently with the eruption from earthquakes, fire, and possibly tsunami,however, is clear in the archaeological record (see, for example, MARINATOS, 1939;GALANOPOULOS and BACON, 1969; LUCE, 1969; HOOD, 1971; PLATON, 1971;BARBER, 1987; DOUMAS, 1983). Certainly the consequences of an eruption of suchmagnitude near the center of two thalassocratic LBA societies would have had amajor impact on their future. One need only compare the effects of equivalentlylarge Plinian eruptions on post-Bronze Age civilizations to see the potentially hugeeffect of the Thera eruption and its regional effects on these LBA societies (see, e.g.,MCCOY and HEIKEN, 2000).

3.2 Explosi6ity, Magnitude and Tephra Volume

The explosivity index of the LBA eruption of Thera is estimated at 6.9 on theVolcanic Explosivity Index (VEI; NEWHALL and SELF, 1982) and as such representsone of the largest eruptions in the past few millennia (only seven eruptions havehad higher VEI values in the past four millennia; SIMKIN and SIEBERT, 1994;DECKER, 1990; VOGEL et al., 1990).


Volcanic plume heights are estimated to have been up to 36 km (SIGURDSSON

et al., 1990). Injection of ash to these heights, in conjunction with estimates of ahigh sulfur content within the plume, suggests that global climatic cooling for a fewyears was a likely consequence of the eruption (SIGURDSSON et al., 1990). Theduration of the major part of the eruption ranged from six hours to four days withno extended interruptions in this activity, erupting at an intensity of 1.4–4.2×108

kg/sec.; accumulation rates of tephra were as high as 3 cm/min. (PYLE, 1990;SPARKS and WILSON, 1990; SIGURDSSON et al., 1990). Volume of caldera collapseis calculated at 18–39 km3 (HEIKEN and MCCOY, 1984; PYLE, 1990); moderndepths in the caldera are 390 m (PERISSORATIS, 1985) indicating at least thisamount of collapse during the eruption, but certainly much more, perhaps as muchas 690 m depending upon reconstructions of the pre-eruption topography of theisland (PICHLER and FRIEDRICH, 1980; HEIKEN and MCCOY, 1984; FRIEDRICH etal., 1988; DRUITT and FRANCAVIGLIA, 1992).

Figure 3Map of the southern portion of Thera showing the Akrotiri region and the archaeological excavation.


Figure 4Stratigraphic section of the deposits from the LBA eruption (adapted from MCCOY and HEIKEN, 2000;nomenclature follows RECK, 1936; HEIKEN and MCCOY, 1984; and DRUITT et al., 1989). Arrows in theright column indicate suggested tsunamigenic phases of the eruption. At lower right in the lithologicsection, a man-made wall is depicted to indicate phases of the eruption that preserved (BO1 and lower

BO2) then destroyed (succeeding phases) archaeological structures.

3.3 Eruption Sequence and Tsunamigenic Phases

The LBA eruption occurred in four major phases, preceded by a minorprecursor phase; eruption activity during three of these likely produced tsunami(Fig. 4). The discussion below is summarized from HEIKEN and MCCOY (1984),unless otherwise noted.

A precursor eruptive phase of magmatic and phreatomagmatic activity someweeks or months prior to the major phases of the eruption deposited a thin tephralayer extending to 8 cm over southern Thera (Fig. 5a), and may have providedadvance warning of the impending disaster to local inhabitants (HEIKEN andMCCOY, 1990). Bedding structures in the precursor deposit are suggestive of air-falldeposition with local dispersal by atmospheric winds (HEIKEN and MCCOY, 1990).Although a significant lithic component in the ash might indicate smaller collapsenear the vent, there is no evidence of larger collapse structures during this precursorphase. This evidence in combination with the lack of major destruction to man-made structures (DOUMAS, 1983) and the inferred lower level of eruptive activity,suggest no tsunami generation during this initial eruption phase.

Intense magmatic activity of the first major phase of the eruption (BO1/MinoanA) deposited up to 7 m of pumice and ash, with a minor lithic component, to the


Figure 5Depictions of vent positions (suture line) and tephra deposits (fine stippled pattern) from the precursorand major phases of the LBA eruption of Thera; also shown is the modern coastline (solid line) and the

approximate LBA pre-eruption coastline (dotted line) (modified from HEIKEN et al., 1990).Figure 5a. Precursor eruption phase (BO0).

southeast and east (Fig. 5b). Dispersal was by atmospheric and stratospheric winds;dispersal patterns and isopachs imply a vent north of the modern Kameni Islands,perhaps an extension and enlargement of the vent of the precursor phase. Archae-ological evidence indicates burial of man-made buildings with limited damagemainly from collapsed roofs. Tephra characteristics indicate fragmentation anddischarge of magma by magmatic gases in a setting where there was no waterinteraction at the vent. Such depositional mechanisms, in combination with theinferred vent position on land and a lack of evidence for major collapse of thevolcanic edifice, are not those mechanisms and settings noted for tsunami genera-tion during historic volcanic eruptions. Accordingly it is inferred that tsunami werenot generated during this first major phase of the LBA eruption.

Pumice deposited on the sea around Thera during this first phase of theeruption may have formed enormous rafts of floating pumice that drifted onsurface currents throughout the Aegean and eastern Mediterranean Seas. For


Figure 5bFirst major eruption phase (BO1/Minoan A).

example, pumice rafts following the 1883 eruption of Krakatau, a less explosiveeruption than LBA Thera (VEI=6.0; SIMKIN and SIEBERT, 1994) were as much as15 ft thick and floated across the Indian Ocean transporting skeletons and trees(SIMKIN and FISKE, 1983).

With the second phase (B02/Minoan B), however, the eruption characterchanged to phreatomagmatic activity with numerous thin (B1 m) pyroclasticsurges and flows leaving a deposit of up to 12 m; thickest accumulations are onsouthern Thera (Fig. 5c). Changes in bedform characteristics indicate an increasingvelocity of surges and flows as second phase activity continued, which is reflected inthe complete destruction of any man-made structures not buried within first-phaseor early second-phase deposits (MCCOY and HEIKEN, 2000). Flow directions andthe pattern of thickness variations suggest a vent sited south of the first-phase ventin a setting where water had unlimited access into the vent.

By comparison with tsunamigenic phases of historic eruptions, such asKrakatau in 1883, it is entirely likely that tsunamis were generated during thesecond phase of the LBA eruption, particularly during later portions of this phase,wherever surges and flows entered the sea (assuming surge/flow bulk densities


Figure 5cSecond major eruption phase (BO2/Minoan B); also shown are pyroclastic surge and flow directions(from MCCOY and HEIKEN, 2000), with inferred sites of tsunami generation and directions of wave

propagation.

greater than sea water to diminish buoyancy and allow flow into the water ratherthan on the sea surface). Thicker accumulations along southern Thera indicatepyroclastic activity focused in this direction and thus tsunami perhaps directedmore towards the south and southeast, although tsunami would have been gener-ated anywhere these surges and flows entered the ocean along the outer perimeterof the island. Tsunami generated within the center of the island in the southernembayment would have had limited entry into the Aegean Sea through the narrowchannel in the pre-collapse landscape (Fig. 5c).

Pyroclastic flow activity continued into the third phase of the eruption (BO3/Mi-noan C) with the initiation of caldera collapse (Fig. 5d). The deposit from thisphase of the eruption is a massive unit (up to 55 m thick on central-southeasternThera) within which individual pyroclastic-flow units are not distinguishable. Ahigh proportion of lithic fragments, some many meters in size, and the compositionof these fragments indicate major collapse of the north-central portion of the LBA


Figure 5dThird major eruption phase (BO3/Minoan C); also depicted is the area of initial caldera collapse(modified from MCCOY and HEIKEN, 2000, and HEIKEN et al., 1990), with inferred sites of tsunami

generation and directions of wave propagation due to entry of pyroclastic flows into the sea.

island to form the caldera. Thickness variations and ballistic trajectories of lithicblocks indicate a vent situated approximately in the same position as that for thesecond-phase of the eruption. Late in this phase, lahars and debris flows wereproduced.

As during second-phase activity, tsunamis may have been generated whereverpyroclastic flows entered the sea around the perimeter of the island(s). Tsunamiscould have been larger than those produced earlier in the eruption because thecharacteristics of third phase deposits on land suggest a few massive slurry-likemixtures that were thick enough to flow over caldera walls close to 400 m high(HEIKEN and MCCOY, 1984; SPARKS and WILSON, 1990)—the entry of such apyroclastic slurry into the ocean could have produced huge tsunami propagated inall directions. In addition, large tsunamis might have been produced by sea waterfilling the collapsing caldera, particularly if the two peripheral fault blocks to thecaldera (Fig. 2) had partially collapsed in this phase of the eruption, thus wideningthe ocean channels to the north and east-southeast and focusing tsunami in thesedirections.


Figure 5eFourth eruption phase (BO4/Minoan D); also depicted are areas of final caldera collapse, pyroclasticflow directions (modified from Heiken et al., 1990, and MCCOY and HEIKEN, 2000), and inferred sites

of tsunami generation. The position of the tsunami deposit at Pori is identified.

The final phase of the LBA eruption (BO4/Minoan D) was marked by variedactivity: lithic-rich base surge deposits, lahars, debris flows, and some co-ignimbriteash-fall deposits (Figs. 5e and 5f). That tsunami was generated by this activity,towards the east by both a pyroclastic flow and a lahar/debris flow entering theocean, is documented by tsunami deposits within the fourth phase deposit (de-scribed below; (MCCOY, 1997, 1997b). This phase also marked the completion ofcaldera collapse (BOND and SPARKS, 1976; HEIKEN and MCCOY, 1984; MCCOY

and HEIKEN, 2000). Vertical collapse of at least 400 m in the caldera (perhaps upto 700 m assuming about 300 m elevation to the LBA island that was collapsinginto the caldera), and collapse of the two fault blocks peripheral to the caldera by300 m, also produced tsunami (vertical displacements relative to modern sea levelusing bathymetric data from PERISSORATIS, 1985). This is documented by sedimen-


Figure 5fFourth eruption phase (BO4/Minoan D); also depicted are flow directions of lahars, debris flows andlithic-rich base surges (modified from HEIKEN et al., 1990, and MCCOY and HEIKEN, 2000), inferred

sites of tsunami generation, and directions of wave propagation.

tary deposits within the deep-sea stratigraphic succession of the eastern Mediter-ranean Sea that have been identified as tsunami deposits produced by wavespropagating to the west and west-southwest (KASTENS and CITA, 1981; CITA et al.,1984; HIEKE, 1984).

4. Sedimentary Deposits Produced by Tsunami Generated during the LBAEruption of Thera

4.1 Homogenites: Deep-sea Tsunami Deposits

The formation of a distinctive deep-sea sedimentary deposit found in cores fromthe eastern Mediterranean Sea, homogenites, has been attributed to tsunami


(KASTENS and CITA, 1981; CITA et al., 1984; HIEKE, 1984). A single homogenitelayer can be up to 9 m thick. Homogenites are homogeneous in color (thus thename; usually a light to medium brown-grey), soupy, contain a microfossil assem-blage of mixed ages, have a thin basal zone of silt or sand, show normal sizegrading through much of the layer, and have no sedimentary structures except forburrows in the upper few centimeters (Fig. 6). Homogenites are distinctive withtheir homogeneous appearance, thickness, and color that contrasts with the pelagicstratigraphic section of the eastern Mediterranean of turbidites (white to brown),sapropels (dark grey to black), volcanic ash layers (light tan to grey), burrowedpelagic muds (light green to dark brown) and oozes (white).

Homogenites are found only in cores taken of sediments from floors of enclosedbasins on the sea floor. Homogenites appear in seismic reflection profiles as anacoustic transparent layer constrained to the basin floor (Fig. 7). They occur at thestratigraphic level of the ash layer from the LBA eruption; the LBA ash layer is notpresent in cores containing a homogenite. Cores from surrounding slopes of thebasin show an unconformity/paraconformity at the stratigraphic interval of theLBA ash layer.

Figure 6Typical sedimentological characteristics of homogenites from two cores taken southwest of Crete on the

Mediterranean Ridge; thickness of both homogenite layers is close to 5 m (from CITA et al., 1984).


Figure 7Near-bottom 4 kHz seismic-reflection profile showing restriction of the homogenite layer (acoustically

transparent interval) within and on a basin floor (from KASTENS and CITA, 1981).

These factors suggest slumping of sediments onto basin floors from the sur-rounding walls at the time of the LBA eruption, triggered by the passage of tsunamigenerated during the LBA Thera eruption (KASTENS and CITA, 1981) (Fig. 8).Tsunami would have had wavelengths adequate to provide near-bottom currentvelocities sufficient to erode and slump deep-sea sediments. In the intervening 3600years since the eruption, compaction of the turbid bottom water ponded within thebasin has formed the homogenites.

The occurrence of homogenites only in ocean-floor basins southwest of Crete,and the alignment of these basins with pathlines calculated for tsunami propagationto the west from Thera (Fig. 9), infer wave generation by collapse of the calderaand the fault block west of the caldera (third and fourth phases of the eruption;Figs. 5e,f). Collapse of the latter would have widened and deepened the channelinto the Aegean Sea, focusing wave propagation to the west.

4.2 Tsunami Deposits on Thera

A 3.5-m thick volcaniclastic deposit intercalated between third and fourth phasedeposits of the LBA eruption (see Fig. 4), near the village of Pori on the east coastof Thera (Fig. 5), has been interpreted as tephra reworked by tsunami generatedduring the eruption (MCCOY, 1997; MCCOY and HEIKEN, 1997). The deposit is amixture of pumice, ash and lithic (lava) fragments, characteristic components of allphases of the eruption, in two distinct layers (Fig. 10). The lower layer has a


minimum thickness of 1.5 m and extends inland 36 m to an elevation of 6 m on a2–16° slope. It has a lenticular shape open towards the ocean but which thinsinland; internal bedding structures include planar and cross-bedded layers, somewith normal size-grading, clastic tails in the lee of larger clastic particles, and layersof lithic-rich gravels, often in sharp contact with erosional features. The basalcontact of the lower layer is sharp with erosional scour into the underlying tephra.

The upper layer appears to be about 2 m thick; the upper contact is gradationaland indistinct. It is a chaotic mixture of particle sizes and composition with nointernal bedding structures, but with a higher content of coarser lithic fragmentsthan found in the lower layer. The contact between the lower and upper layer isdistinct with some features indicative of erosion. Detailed sedimentological work onthe deposit continues and will be reported elsewhere.

A tsunami origin for this deposit rather than one related to eruptive mechanismsis suggested by: (1) the lens-shaped geometry of the deposit open towards the oceanwhich is a unique geometry within the volcanic succession, (2) sedimentary struc-tures in the lower layer of the deposit that are indicative of a water-laid depositunder oscillatory flow, in contrast to structures in surrounding pyroclastic andvolcaniclasitc deposits, and (3) the chaotic upper layer of the deposit which appearsto be reworked rather than the product of deposition from pyroclastic activity,lahars or debris flows. A fluvial or debris flow origin for the deposit is discounted

Figure 8Diagrammatic representation of sediment disturbance and slumping into isolated basins on the Mediter-ranean Ridge to produce homogenite layers with passage of tsunami generated during the LBA eruption

of Thera (from CITA et al., 1984).


Figure 9Pathlines of inferred tsunami propagation. Solid arrows trace tsunami possibly generated by the collapseof the caldera and a large fault block west of the caldera (see Figs. 6e and 6f). Shaded areas in theeastern Mediterranean Sea identify basins on the deep-sea floor where cores have sampled homogenites(11 shaded areas identify 35 different basins). Dashed arrows indicate pathlines for tsunami possiblygenerated by the entry of pyroclastic flows and lahars/debris flows into the sea (from KASTENS and

CITA, 1981).

by these observations: (1) cross-bedding structures indicate oscillatory flow ratherthan unidirectional flow; (2) abundant pumice is indicative of limited abrasionduring flow, inferring short duration and distance of flow; and (3) the deposit doesnot appear constrained within a channel, rather appears to be planar in bedgeometry, indicative of sheet-wash type motion.

The stratigraphic position of the tsunami deposit, in lower contact with pyro-clastic flow deposits of the third (BO3/Minoan C) phase of the eruption, and mixedalong the upper contact with debris-flow deposits of the fourth (BO4/Minoan D)eruption phase, infer tsunami origin related to the entry of pyroclastic flows anddebris flows into the sea. Initial tsunami flooding is marked by the erosional basalcontact. Internal packets of cross-bedded and planar beds within the lower portionof the tsunami deposit might indicate multiple inundation episodes related torepeated entry of pyroclastic flows into the sea, or seiche during maximuminundation. The upper portion of this deposit may represent tsunami generated by


the entry of a debris flow into the sea (lack of internal sedimentary structures;chaotic assemblage of pumice and lithic fragments; higher proportion of angular,pebble and cobble-sized lithic fragments; somewhat gradational upper contact intoa debris flow). If the latter, this is one of few indications of tsunami generated bythe entry of debris flows into the sea during a volcanic eruption.

4.3 Tsunami Deposits Elsewhere

Tsunami deposits related to the LBA eruption have been inferred from numer-ous coastal exposures and archaeological sites where rounded pumice has beenfound, often mixed with seashells and sometimes in a cultural destruction layerdated to the LBA (Fig. 11): Anaphi island east of Thera (MARINOS andMELIDONIS, 1971), Crete (at Amnissos, MARINATOS, 1939; PICHLER and SCHIER-

ING, 1977; FRANCAVIGLIA and DI SABATINO, 1990; at Pitsidia, VALLIANOU, 1996),Cyprus (ASTROM, 1978; MESZAROS, 1978), and Israel (near Tel Aviv, PFANNEN-

STIEL, 1960; other coastal sites, NEEV et al., 1987).However, the use of pumice in coastal exposures as indicators of tsunami

deposition must be done with caution. The disparity between travel times fortsunami generated by the eruption and for pumice rafts drifting on surface currents

Figure 10Sketch (from a photograph) of the tephra stratigraphy and tsunami layer near Pori, Thera. Stratigraphicidentification and approximate scale on the cliff face are shown on the right. Note the lack of internalstratification in tephra layer BO3 and the series of deposits from lahars and debris flows in BO4, incontrast to the lenticular shape and internal stratification of the tsunami deposit. The two layerscomposing the tsunami deposit are shown: a lower layer characterized by numerous planar beds,cross-beds, and gravel-rich beds, most with erosional contacts including the basal contact; an upper layer

characterized by the lack of internal stratification in a chaotic, poorly-sorted volcaniclastic mixture.


Figure 11Regional ash distribution of tephra from the LBA eruption (shaded area). Symbols identify sites andsampling locations where ash (deep-sea cores, lake cores, exposures on land, archaeological sites) andpumice (exposures on land and at archaeological sites) have been found. Numbers next to the twotriangles in Anatolia represent layer thicknesses in cm for ash deposits found in lake sediments; curvedlines over the eastern Mediterranean Sea are isopachs of ash thickness, in cm, of the tephra layer foundin the deep-sea stratigraphic sequence sampled by coring. Data are from: NINKOVITCH and HEEZEN

(1965), NEEV et al. (1987), CADOGAN (1972), RAPP et al. (1973), VITALIANO and VITALIANO (1974),ZEIST et al. (1975), FORNASERI et al. (1975), ASTROM (1978), WATKINS et al. (1978), HOOD (1978),DOUMAS and PAPAZOGLOU (1980), MCCOY (1980, 1981), SULLIVAN (1988, 1990), STANLEY andSHENG, 1986), KELLER (1980), BETANCOURT et al. (1990), MARKETOU (1990), SOLES and DAVARAS

(1990), WARREN and PUCHELT (1990), FRANCAVIGLIA (1990), and GUICHARD et al. (1993).

(5–20 cm/sec., O8 ZSOY et al., 1989) constrains distances from Thera where tsunamiwould have arrived coincident with rafted pumice. As one example, open-oceanvelocities for tsunami of the order of 1000 km/hr would have waves arriving in theLevant (approximately 1000 km, straight line distance from Thera) in about 60–100mm (Fig. 12; YOKOYAMA, 1978). Pumice rafted by surface currents of 5–20 cm/secfor 1500 km (typical surface currents in the eastern Mediterranean Sea; approxi-mate float path following eastern Mediterranean gyres; O8 ZSOY et al., 1989) would


arrive along the Levant coastline in about 85–350 days, long after the eruption anderuption-generated tsunami (assuming a four-day duration of the LBA eruption;SPARKS and WILSON, 1990; SIGURDSSON et al., 1990). Accordingly, pumice foundalong the Levant littoral could not have been deposited by tsunami generated bythe LBA eruption.

In a general case (applying assumptions noted above on surface current patternsand velocity, drift paths and rates for pumice rafts, 4-day duration of the eruption,tsunami generated during late-phase eruptive activity), rafted pumice would be atdistances of only about 70–100 km or less from Thera (north-central coast ofCrete, Anaphi, Melos, Amorgos) after four days of drifting and in position fortsunami arrival and deposition on land.

5. Wa6e Characteristics Inferred for Tsunami Generated by the LBA Eruption ofThera

Wave heights near the eruption site on Santorini (no water depth specified ordistance offshore) have been estimated at 50–86 m (YOKOYAMA, 1978). The lowerwave-height estimate was based upon two pumice deposits about 1 m thick foundby MARINOS and MELIDONIS (1971) on the island of Anaphi due east of Thera(Fig. 1) at an elevation of 40–50 m. It is not clear if the Anaphi deposit represents

Figure 12Tsunami time-distance relationships and pathlines from Thera; contours are in minutes (from

YOKOYAMA, 1978).


a pumice-fall accumulation or a deposit left from runup by tsunami. The upperwave-height estimate was derived by YOKOYAMA (1978) from the occurrence ofpumice 5 m above sea level in a coastal terrace near Tel Aviv (presumed related tothe Thera eruption by PFANNENSTIEL, 1960; see section above), assuming LBAsea-level about 2 m below present levels and a wave height of 7 m offshore of TelAviv, then calculating an original wave height of 86 m near Santorini. By averagingthe 50–86 m wave-height estimate, then ‘‘weighting twice’’ the Anaphi estimatebecause the former ‘‘might be more reliable,’’ YOKOYAMA (1988) refined thewave-height calculation to 63 m near Santorini.

Tsunami directed towards Anaphi and the Levant, including south towardsCrete, likely were generated by the entry of pyroclastic flows and lahars/debris flowsin the sea (as noted previously). Tsunami deposits found near Pori on Thera suggestthese estimates by YOKOYAMA (1978) may be too high. This deposit has an uppercontact 7–8 m above modern sea level, providing a minimum indication of tsunamiwave heights. Adding to this an LBA (3600 yBP) sea level perhaps as much as 2meters lower than modern levels in the Mediterranean and southern Aegean region(VAN ANDEL and SHACKLETON, 1982; FLEMMING and WEBB, 1986; GALILI et al.,1988; STANLEY and WARNE, 1993), and evidence of perhaps up to 2 m ofsubsidence of Thera since the LBA (GALANOPOULOS and BACON, 1969), thentsunami wave heights at coastal impact might have been as much as 11–12 m.Given uncertainties between runup and wave height, wave heights somewherebetween 7 m and 12 m might be suggested. Such an estimate is close to those byPICHLER and SCHIERING (1977) of 8–10 m wave heights along the northern andeastern coasts of Crete at various archaeological sites.

KASTENS and CITA (1981) used YOKOYAMA’s (1978) minimum estimate of a 50m wave near Thera, and assumed this wave amplitude at a 200 m water depth offSantorini, to calculate open-water wave heights of between 1.9 and 17 m, with wavelengths \100 km, in the eastern Mediterranean Sea over the abyssal homogenitesites. Sea-floor current velocities of up to 49 cm/sec would result from such a wave,and would exceed critical erosional velocities needed to erode and cause slumpingof fine-grained pelagic sediments off basin walls onto basin floors to form ho-mogenite deposits (KASTENS and CITA, 1981). Nearshore wave heights as open-ocean tsunami amplitudes of this magnitude entering shallow coastal waters mightbe considerably higher than those suggested from the tsunami deposit at Pori,Thera, a reflection of generation of the homogenite-forming tsunami by calderacollapse rather than by pyroclastic and lahars/debris flows entering the sea.

6. Summary

Tsunamis generated by the LBA eruption of Santorini were generated by (1)caldera collapse and (2) the entry of pyroclastic flows and lahars/debris flows into


the sea. Volcanogenic tsunamis produced by caldera collapse and the entry ofpyroclastic flows into the sea are well documented from historic eruptions; tsunamigeneration by the entry of lahars and debris flows into the sea is poorly docu-mented. Evidence for tsunami comes from deep-sea deposits left by open-oceanpassage of the wave, as well as from deposits on land where inundation has leftpumice accumulations, has produced destruction layers at archaeological sites, orhas left distinctive volcanoclastic deposits within the tephra stratigraphy of theeruption on eastern Thera. Pumice accumulations in coastal exposures or archaeo-logical sites at distances greater than about 100 km likely were not, however,emplaced by tsunami generated during the LBA Thera eruption considering driftrates via surface currents in the Aegean and eastern Mediterranean Seas and traveltimes for tsunami.

Wave propagation following caldera collapse (third and fourth phases oferuption activity) would have been to the west, considering somewhat asymmetriccollapse of the volcanic edifice and the occurrence of tsunami deposits (homogen-ites) on the eastern Mediterranean sea-floor; open-ocean wave amplitudes arecalculated to have been 1.9–17 m. Wave propagation following ocean entry ofpyroclastic flows and lahars/debris flows (second through fourth phases of eruptionactivity) would have been in all directions around Thera, and certainly towards theeast as indicated by tsunami deposits on eastern Thera; wave amplitudes areestimated at about 7–12 m along coastal Thera and Crete. The regional effects oftsunami must have been significant, considering the cataclysmic event whichoccurred within the core of two thalassocratic cultures, Minoan and Cycladic, andwhich destroyed the center of one, the Cycladic culture on Thera.

Acknowledgements

Our thanks to numerous colleagues for discussions, assistance and comments:C. Doumas and the archaeologists at the Akrotiri archaeological excavation, MegWatters, Eleni Argy Murray, and other field helpers and students (especially TomLowther, Mary Lewis, and Ian Robins). Reviews by B. Keating, W. Dudley, W.Friedrich and anonymous reviewers improved the manuscript considerably. Wethank those who made life in the field a pleasant experience (N. Yannaros, A.Kambouris, F. Colombo, K. Damigos, M. Karnavoulis and others). Field workwas permitted by the Institute of Geology and Mineral Exploration (IGME), andsupported by the National Geographic Society, Earthwatch/Center for Field Stud-ies and the Associated Scientists at Woods Hole. Production of graphics wassupported in part by the Colgate Fund of the Associated Scientists at Woods Hole.Portions of this study were done while F. McCoy was Senior Research Associate atthe Wiener Laboratory of the American School of Classical Studies in Athens. Thisis a contribution from the School of Oceanography and Earth Sciences and


Technology, University of Hawaii (no. 4955) and the Associated Scientists atWoods Hole (Mass.).

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(Received June 4, 1998, revised March 5, 1999, accepted May 18, 1999)

archeology - tsunami generated by the late bronze age eruption of thera

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