The Minoan Eruption of Santorini, Greece

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The Minoan eruption of Santorini, Greece

A. BOND & R. S. J. SPARKS

SUMMARY The Minoan eruption of Santorini produced logical contrasts in the mass flows which pro- the following sequence of deposits- a plinian duced them. Grain size analyses show wide pumice fall deposit, interbedded surtseyan-type ranges in the lithic contents of the different ash fall and base surge deposits, mud-flow types of deposit: ignimbrite (35--6o~), mud- deposits and ignimbrite interbedded with very flows (2o-3o ~) and the pyroclastic fall and coarse, well-sorted flood deposits. The variation base surge deposits (4-x 5 %). The ignimbrite is of thickness and grain size in the plinian deposit enriched in crystals, complemented by deple- indicates a vent x km west of Thera town. The tion in fine air-fall ash beds that interstratify base surges and surtseyan-type activity is in- with the ignimbrite. The gas velocity of the terpreted as the result of sea water entering the plinian phase is estimated as 55o m/s, the erup- magma chamber. The poorly sorted mud-flow tion column height as greater than 2o km deposits and ignimbrite are distinguished on and it is shown that only particles of 2 mm their grain size, temperature and morphological could have reached Minoan Crete. characteristics, which indicate substantial rheo-

I N T H ~. L AT E B R ON Z F. A o E a paroxysmal eruption took place on Santorini Vol- cano, referred to as the Minoan eruption after the Minoan civilisation which inhabited the island at that time. The eruption produced a great volume of pumice and ash and resulted in the formation of the present day caldera which measures x 1- 5 × 8 km and probably had a catastrophic effect on the people living in the southern Aegean. There is still controversy concerning the exact date of the eruption, which is largely based on archaeological finds at the buried town of Acrotiri on Santorini. For convenience the date of I47o B.C. 4-2O years is accepted, following Luce (1967). Santorini is a group of islands lying I2o km north of Crete at the southern end of the Cycladic Group. It is the only active volcano in the eastern Mediterranean to have been copiously active in historic times. The islands of Palea Kammeni and Nea Kammeni in the middle of the caldera have been constructed by post-Minoan eruptions in 197 B.C., A.D. 19, 46, 726, 1570, 17O7- 171 I, 1866--70 , 1925--6, 1928, I939-41 and I95 ° (Georgalas I962) which have been largely small, gas-poor eruptions characterised by weak vulcanian explosions and the effusion of blocky lava flows; the eruption of A.D. 726, however, produced considerable pumice. Santorini has been the subject of several stratigraphical and petrological studies including the classic research of Fouqu6 (i879) and the studies of Pichler & Kuss- maul (I972) and Nicholls (I97I). The volcano is built up on a basement of Triassic limestones, forming the hills of Monte Elias and Platinamos, and Mesozoic schists near the port of Athinion (Pichler & Kussmaul I972 ). Previous workers recognise several phases of activity at different centres in the construction of the volcano, beginning wlth the old and substantially altered Acr0tiri group in the south. Van Padang (t 936) reeognises several series of pyroclastic deposits: i) the lower Pumice 'series,' 2) the middle Pumice 'series' and 3) the upper Pumice

Jl geol. Soc. Lond. vol. x32, x976, pp. I-I6, 7 figs. Printed in Northern Ireland.

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'series' However, there is considerable stratigraphic imbalance in that both the lower and middle Pumice 'series' represent the accumulated products of a large number of explosive eruptions exposed in the wall of the caldera, whereas the upper Pumice 'series' represents the products of one eruption, the t47 o B.C. event. Before the Minoan eruption there was probably only one island, known as Stronghyli (Pichler & Kussmaul I972) with a steep sided central cone which, judging from the present profile of the volcano, rose to between 5oo and 8oo m above sea level. Repeated eruptions of blocky lavas similar to the historic lavas of Nea Kammeni has infilled an old ealdera in the NE. of the island, a cross section of which is preserved in the caldera walls between Thera and Oia. To the south lay a low sloping plain formed by the pyroclastic accumulations from many pre- historic eruptions. The Minoan eruption produced a thick, continuous layer of white pumice and ash that now covers the remnants of the volcano like icing on a cake. The products of the eruption were a plinian pumice fall deposit succeeded by fine ash fall and base surge deposits, ignimbrite, mud-flows and mud-flood deposits. This study describes the relations and characteristics of the products of this eruption and offers volcanological interpretation of an event which has aroused much interest and debate in the fields of geology, archaeology, history and the classics.

I. Products of the Minoan eruption

(A) THE PLINIAN PUMICE FALL DEPOSIT The lowermost unit is a pyroclastic fall deposit composed of white to pale pink pumice clasts and accessory angular lithic fragments. Figure I shows the isopach map and isopleth maps of the average diameter of the five largest pumice and lithic clasts seen at each exposure, and they all indicate a source vent about I to 2 km west of Thera town. The deposit is reversely graded in the central and southern parts of the island. The deposit is poorly stratified, which suggests continuity in the activity (stratification in pyroclastic fall deposits is a result of discontinuity in the activity), and well sorted. The content of lithic clasts increases towards the top of the deposit to form a lithic enriched zone making up between 5 and 4o% of the total thickness, and reaching a maximum I km north of Thera town. The deposit contains several varieties of lithic fragments including a highly ferruginous sandstone, altered tufts and some hFpabyssal rocks. Many of the xenoliths are coated with a veneer of deep red iron oxides, and a red-stained halo, formed by a leaching of iron oxides from the lithics, often extends into the surrounding pumice. In addition to the white pumice a pale grey rounded pumice commonly occurs in the upper parts of the deposit. Many of the white pumice clasts contain irregularly shaped fragments of this grey pumice and in rare examples the grey and white pumice are intimately mixed as streaks. The boundaries between the white and grey pumice are crenu- late and highly embayed. These features are characteristics of the mixture of two magmas of contrasted viscosities (Blake et al. 1964). The grey pumice is a basaltic andesite, containing minor amounts of olivine, whereas the main white pumice is a rhyodacite (Gunther & Piehler i973).

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The Minoan Pumice Deposit is similar to many other deposits which are attrib- uted to plinian-type activity. It has the form of a widely dispersed sheet, is poorly stratified, contains a relatively low proportion of lithics and is reversely graded,

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FiG. I. A. Main place names and outcrops of Mesozoic rock: (x) Mt. Elias, (2) Platinamos. B. Isopach map of the Plirdan pumice deposit in cm. Open stars--thickness in- complete due to erosion. Closed stars--thickness over-estimated due to slippage on steep slope. C,D. Isopleth maps of the average diameter of the five largest fragments in cm for pumice and litldc clasts respectively. The suspected source vent for the Minoan eruption is shown by the large black circle.

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in common with other well described plinian deposits, for example, Fogo A in the Azores (Walker & Croasdale 197 I) ; the A.D. 79 and 1200 B.C. deposits of Somma- Vesuvius, Italy (Liter et al. I973) , and the Granadilla pumice deposit, Tenerife (Booth 1973). Walker (1973) classifies explosive volcanic eruptions on the basis of the dispersal D, defined as the area enclosed within the o.o I Tmax isopach (where Tmax is the maximum thickness of the deposit) and the degree of fragmentation F, defined as the percentage finer than I mm at the o.o I Tmax isopach. He defines plinian deposits as those with D values greater than I ooo km ~.. Although the area of Santorini limits application of this definition, D must be very much in excess of IOOO km 2. Tmax is taken as 550 cm (Fig. I) and the 25 cm isopach covers about 500 km * and so o.o I Tmax (5"5 cm) must cover several thousand km ~. Although the fragmentation F is difficult to ascertain the deposit can be regarded as the product of thoroughly typical Plinian activity.

(B) BASE SURGE AND FINE ASH FALL DEPOSITS Much finer, ill-sorted and well-stratified deposits overlie the Plinian pumice deposit in the caldera wall. These are extremely heterogeneous and the grain size of individual strata varies considerably. Cross-stratification and ripple structures (Fig. 2) are very common. In the 1965 eruption of Taal Volcano, Phillipines (Moore 1967) clouds were produced at the base of the eruptive column which expanded horizontally outwards and are believed to be similar to the base surge clouds observed in thermonuclear blasts. It has now been established that volcanic base surges are common phenomena and that this type of activity produces characteristic cross-stratified deposits and ripple structures (Crowe & Fisher 1973, Schmincke et al. 1973). Base surges have a relatively low temperature compared to magmatic temperatures and form in phreatomagmatic eruptions (where the magma comes into contact with water by eruption in shallow water or where abundant ground waters have access to the vent--Waters & Fisher i97o ). The cross-stratified deposits above the plinian pumice deposit are thought to be formed by base surges rather than by more normal sedimentary processes because: (i) They are composed largely of low density pumice and ash which would float in an aqueous environment. Most of the pumice has indeed been removed from undoubted river and flood deposits formed after the eruption of 147 ° s.c. (ii) The deposits mantle the topography, and even occur on top of Mont Elias (600 m) which indicates a highly expanded gas-particle mixture similar to that observed in recent base surge eruptions. (iii) Bomb sags occur between base surge units showing that volcanic explosions accompanied their formation. The 147o B.C. base surge deposits are composed largely of poorly sorted pumice and ash and the lithic content is generally low. The structures are similar to those observed in other base surge deposits such as Ubehebe Craters (Crowe & Fisher 1973), and Laacher-see (Schmincke et a/. 1973). In sections radial to the vent the mega-ripple structures often show a characteristic profile (Fig. 2a). The structures are asymmetric, and coarse well-sorted lenses of pumice occur on the lee side of the crest as depicted in Fig. 2a. The crests of these structures sometimes can be

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shown to migrate down-current like climbing ripples (Allen 1970). Similar structures have been observed in some older pyroclastic surge deposits on Santorini and in the base surge deposits of Milos, another Aegean Volcano. However, there are many other bedding structures and the Santorini bases surge deposits are complex. Tangential to the flow direction the mega-ripple shuctures are symmetri- cal; both erosional and depositional structures are seen. The deposits decrease in thickness from up to 12 m in the caldera walls to less than 4 m on the coast, and they also rapidly decrease in grain size away from the caldera wall. The end of the plinian phase then is marked by a transition to phreatomagmatic activity indicated by the occurrence of up to 12 m of fine grained base surge deposits and associated fine grained poorly sorted but well stratified air fall ash beds. The latter are thick on Therasia (up to 4 m) and are very similar to the type of deposit formed in surtseyan type activity. Walker & Croasdale (1972) noticed that explosive basaltic eruption in shallow water, such as that observed on Surtsey

SKlTCH SECTION OF 1470 BC DLPOSITS IN GAL DERA WALL ABOYf ATHIAlOlY FIG.2. Two sketch sections of the bedding structures in the base surge deposits formed in the Minoan eruption.

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(1963-67), results in greatly increased fragmentation of the magma and thus produced very fine grained, well-stratified and relatively ill-soIted deposits. Similar types of activity are believed to occur in acidic activity. The extremely fine grain size of the base surge and air fall ash beds that followed the plinian pumice deposit are therefore thought to be the result of water coming into contact with the vesieu- lating magma and resulting in very much increased fragmentation of the material. In the south the upper part of the plinian pumice deposits contains a thin layer of fine grained ash which separates an upper 15-3 ° cm thick pumice layer from the main thick pumice fall deposit. Around Oia and on Therasia this thin fine ash layer thickens into a base surge unit up to 5 ° cm thick. It is surmised from this relationship that the onset of phreatomagmatic activity was broken by a short return to plinian activity represented by the upper pumice layer above this base surge unit, designated Surge I. The upper pumice layer maintains a fairly uniform thickness over all the islands (x 5 and 35 cm) and it is inferred that there was little wind at this time. Surge I is a very fine grained, lithic poor, fine white pumice and ash deposit and its distribution is confined to the north, suggesting that it was perhaps directed by a northward breach in the vent wall. After the upper pumice deposit, surtseyan air-fall ashes are intimately mixed with base surge units. Boundaries between individual base stage units are recog- nised by erosional horizons (Fig. 2b) often characterised by bomb sags and thin mantles of ash fall material. There are at least a dozen individual units but correla- tion is hampered by close similarities in appearance. The lowest two base surge units above the upper pumice deposit (Fig. 2b) are, however, sufficiently distinctive to correlate over wide areas. They are designated Surge II and Surge III and were sampled for sieve analysis. Surge II is a very fine grained, poorly sorted, lithic poor unit of white pumice and ash. Surge III is unusual in being relatively coarse grained, lacking very fine components and in containing abundant distinctive red- stained xenoliths. There are many base surge units above Surge III, and their appearance suggests similar characteristics to Surge II.

(c) MUD-FLOW DEPOSITS Above the base surge deposits, in the caldera wall section and on the steep inner slopes of the volcano, there is a great thickness of unsorted and unstratified pumice and ash deposits. They attain their greatest thickness in topographic depressions (e.g. 40 m at Thera quarry) and contain pumice blocks up to 30 cm and many lithic blocks ranging from I O to over 2oo cm. The deposits are believed to be composed of several units marked by finer grained basal layers which result in a crudely stratified appearance. Gunther & Pichler (I973) consider these deposits to be ash flow deposits, but they are now believed to be mud-flows for the following reasons: fi) There is no grading even of metre sized lithic blocks (P1. IA) although the density of the flows must have been considerably less than that of the blocks. This feature is typical of many water.lubricated debris flows which behave as Bingham fluids (Johnson x97o ) and allow the flow to carry massive fragments by virtue of the high yield strength of the debris flow matrix.

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(ii) They have been deposited on steep slopes, for example on the side of Messa Vouna (slope 2i°), which again indicates a substantial yield strength. (iii) Evidence of low temperature is seen in the form of enclosed fragments of clay rich breccia which are completely unbaked. There are no gas pipes present. (iv) They contain huge irregular masses of brecciated glassy lava up to i o m in diameter. These are sometimes undisturbed despite their delicate structure, and at other times are sheared out into schlieren. These features can only survive in flows which move by a combination of plug flow (with no internal movement) and laminar flow, typical of Bingham fluids. The mud-flows are composed largely of pumice and ash, but contain large blocks of glassy lava and breccia similar to the recent lavas of Nea Kammeni. The high content of fines within the mud-flows suggests that they are derived from base surge and air fall ash deposits further up the slopes of the volcano. Phreatomagma- tic activity could well have built a tuff ring around the vent on the upper steep slope and it is possible that the collapse of such an unstable structure could account for the formation of the mud-flows.

(D) IGNIMBRITE Massive pumice and ash deposits, up to 60 m thick, of a different kind are found on the coastal plain between Cape Kolumbo and Monolithos and between Acrotiri and Exomiti. These deposits are also poorly sorted and generally structureless, but differ fiom the mud-flows in that the largest pumice clast rarely exceeds I o cm and the largest lithic 5 cm. The deposits in detail are composed of a large number of relatively thin flow units and the succession of Monolithos contains at least 4o units. Individual flow units rarely exceed 3 m thick, though one 15 m thick was observed near Acrofiri. Near Monolithos a large number of thin flow units 5 to 50 cm thick occur in one part of the cliff and give the deposit a stratified appearance. A wide variety of grading is manifest in individual flow units. Reverse grading of pumice is occasionally seen and pumice concentration zones found at the top of some flow units contain pumice up to 3 ° cm in size. Only a few units show normal grading of lithic clasts and reverse grading of both pumic and lithic clasts was observed in one 2 m thick flow unit. Most units fine towards their base and have a recognisable basal layer, but one unit was observed where pumice was normally graded and a finer grained base was absent. A detailed description of grading found within individual flow units of the Minoan Ignimbrite can be found in Sparks (I975 in press), in which there is a discussion of the origin of grading structures commonly found in ignimbrite flow units. One impoItant featm e of these deposits is shown schematically in Fig. 3 where the facies variation of the deposits of the Minoan eruption is shown. The pyro- clasfic flows came to rest on the low gradient (i-2 °) slopes of the coastal plain of Santorini which indicates very different theological properties from the mud.flows. During the 1951 eruption of Mount Lamington, Taylor (i 958) observed the pyro- elastic flows flowing off the steep inner slopes of the volcano and thus detaching

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themselves from their source. On Tenerife it has been shown that the pyroclastic flows did not deposit on slopes greater than Io ° (Booth & Walker pers. comm.). On Santorini the slope above which the pyroclastic flows did not deposit must have been less than 5 ° . This morphologically controlled detachment has also been postulated for the ignimbrites of Chile (Francis et al. 1974) and seems an important characteristic of pyroclastic flows. Although the Minoan ignimbrite is non-welded it shows evidence for a temperature of emplacement well above Ioo°C. The flow units are in places interbedded with coarse, well sorted crudely bedded lithic breccias, interpreted (see next section) as flood deposits. At the contact between the pyroclastic deposits and the breccias large numbers of gas fumarole pipes originate and extend for several metres upwards into the ignimbrite (P1. IB). The pipes are enriched in lithie components (Figs. 4, 7) of a size which suggests that they are derived from the breccias, and they are well sorted and coarse, lacking fine grained components. The pipes vary from 2-3 ° cm in diameter, and are believed to be formed by the generation of steam from the wet breccias underlying the hot ignimbrite and the rise of the steam along discrete channels. Fines were winnowed out by the steam. The ignimbrite flow units are occasionally interbedded with very fine grained ash deposits. These are similar to those observed in many other ignimbrites (Sparks et al. 1973). One interesting relation occurs at Exomiti, where an elongate 2oo m high limestone ridge (Platinamos) runs north-south, oblique to the likely flow direction of the pyroclastic flows. Between Cape Exomiti and Acrotiri the cliffs up to 6o m high are composed of Minoan Ignimbrite. To the east of Cape Exomiti and Platinamos, however, the Minoan formation rapidly thins and is represented largely by the breccia deposits and very fine grained air fall ash with a few thin ignimbrite flow units as a minor component. The Platinamos limestone ridge probably acted as a barrier sufficient to channel most of the pyroclastic flows down the depression between Acrotiri and Exomoti. Only a few pyroclastic flows man- aged to surmount the barrier and deposit on the east side of the ridge. The fine

SCHEMATIC PROFILE OF 1470 BC ERUPTION PRODUCTS 5)

Fx o. 3. Stratigraphie and morphological relationships of the deposits of the Minoan eruption. Profile not to scale. I. Plinian pumice deposit; 4. Ignimbrite; 2. Base surge deposits; 5- Flood breccia deposits. 3- Mud-flow deposits;

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ash beds are interpreted as the deposit from the fine grained turbulent dust cloud such as is seen above most nudes, indicating that only this fine grained upper dilute portion of the pyroclastie flows drifted over the barrier.

(E) FLOOD DEPOSITS The coarse breccia beds described above as interbedded with ignimbrites are composed almost entirely of lithic debris with minor amounts of pumice. They have characteristics similar to the deposits of braided rivers, with an irregular distribution of shallow erosional channels and coarse grained lenses possibly re- presenting inter-channel bars. They are moderately well sorted and very coarse with blocks up to a metre in size. Their coarseness and inter-bedding with ignimbrite suggest that they are the results of flash floods during the period of pyroclastic flow activity. The flood deposits are most abundant towards the top of the succession and also near the steep inner slopes of the volcano (Fig. 3). They are very abundant near Messa Voula and thin out rapidly towards Monolithos where they practically disappear.

2. Granulometric studies Sieve analyses (N -- 51) were made of the various deposits formed in the Minoan eruption using sieves at one phi intervals where $ = --Log~ m (m equals grain size in mm). Cumulative curves were drawn on logarithmic probability paper to obtain graphically derived statistical parameters for the distribution. The two most useful parameters are the median diameter Md÷ -- $50 and a÷ which equals ($u--~xe)/2 and represents the graphical standard deviation and is used as a meas- ure of sorting. Selected samples were further analysed by separating each size class down to {mm into the three main components that occur in pyroclastic deposits namely pumice (including fine ash and shards), crystals and lithics. Details of the procedures are described elsewhere (Walker 1971). Figure 4 shows typical frequency histograms of the various Minoan deposits. Some unpublished sieve analyses are also included for comparison, by courtesy of G. P. L. Walker. In order to compare the grain size characteristics of these different deposits the data have been plotted on a Md÷/a÷ diagram (Fig. 5). In the case of most of the Minoan ignimbrite samples, the mud-flow deposits and the plinian pumice fall deposit, the size frequency histograms (Fig. 4) and the approximately straight line cumulative frequency curves show that these grain size populations are unimodal. The deposits themselves are unstratified and appear uniform in grain size on a local scale. In these cases comparison of the deposits using graphical statistics is considered valid. In Fig. 5 it is seen that the ignimbrite is fine grained and poorly sorted whereas the plinian pumice deposit is coarse grained and well sorted. Both these deposits plot in fields characteristic of their particular mode of formation (Walker 1971). The mud-flow deposits are generally more poorly sorted than the ignimbrite. This is largely a result of the presence of large pumice and lithie clasts present in the mud-flow deposits, but not present in the ignimbrite. In highly stratified deposits such as the base surge deposits and the fine surtseyan ashes of Therasia, interpretation of the grain size data must be more cautious. The

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GAS PIPE IN 30 (22) (13) ,ON,Mnn,T, Monolithos IGNIMBRITE IGN IMBRITE 20 Cape Exomiti Acrotiri (1)

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size frequency histograms of the base surge deposits (Fig. 4) and their cumulative frequency curves reflect polymodal distributions, and values of Md, and tr÷ for these deposits have little real meaning. The main reason for the irregularity of these distributions is believed to be the highly stratified nature of these deposits. In most cases it proved impossible to sample individual strata, because often each stratum was only one grain thick. Thus most of our sieve analyses were of mixed populations from several strata. Channel samples were taken through the complete thickness of individual base surge units and these results are more reliable indicators of the overall grain size population. The position of the three lowermost base surge units (I, II and III) is shown on a Md÷/tr÷ plot (Fig. 5) and indicates a wide range in grain size populations of the overriding base surge clouds. It is stressed that individual strata within these base surge units appear much better sorted. Only one sample of the coarse

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' "i' ' i' ' 'i | l u ' 'i " I, " i -4 -3 -2 -1 0 + 1 + 2 + 3 +4 GRAIN SIZE MD~ FIG. 5. Md~]o. diagram showing the variation within and between the different deposits of theq'Minoan eruption. Legend---dark circles: ignimbrite; open circles: plinian pumice deposits; open squares: surtseyan white ashes, Therasia; stars: mud- flow deposits; triangles: core analyses of three base surge units, I, II and III; triangle with superscript C" coarse lee-side lens in base surge dune structure; G: gas pipes in ignimbrite.

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lenses shown in Fig. 2a was sampled without undue contamination from surrounding layers and this plots as a well sorted, coarse deposit in Fig. 5. Interpretation of the data for the surtseyan fine white ash fall deposits is also problematical. In Fig. 5 they plot as fine grained and very poorly sorted deposits in common with basaltic surtseyan deposits (Walker & Croasdale I972 ). Again it is difficult to collect individual strata and the sieve analyses are of a mixture of several strata. Another cause of poor sorting is believed to be that in phreatomagmatic eruptions, explosions are observed to follow one another closely and conse- quently the populations of different explosions are often intermixed during fall-out. An important feature of the eruptive products is the amount of foreign rock debris incorporated in the different types of deposit. In Fig. 6 each sample has been plotted on a pumice-crystal-lithic triangle (P-C-L). The data represent the proportions of the constituents excluding finer than ~t mm size fractions, recalculated to Ioo%. Although the different modes of transport of the various types of deposit and the exclusion of fines beneath ~t mm grain size affect the relative position of the samples, Fig. 6 shows that the ignimbrite flow units have unusually high lithic contents (30-60% by weight). The mud-flows and Surge III also have relatively high contents (2o-3o%), whereas the other surges, the surtseyan ashes and the plinian pumice deposit have much lower lithic contents (4-I5%). Figure 6 demonstrates that the very low content of lithics of 1% estimated for the Minoan deposits (Luce 1969) is incorrect. In many other eruptions of a similar type, ignimbrite is the major component of the eruptive products. The plinian pumice and other deposits can only account for a small proportion of the missing volume of the caldera and if, as is suspected, ignimbrite was the major eruptive product the very high proportion of lithics is a significant factor in calculations of volumes and energetics. The very high lithic content of the ignimbrite in comparison to many other ignimbrites (Sparks I975) may account for its totally non-welded nature: 40-50% of it was composed of cold rock debris. The lithic clasts in the ignimbrite consist of an assortment of basaltic andesite, andesite lavas, sandstones and schists. In contrast the lithics in the mud-flows are dominated by a glassy, flow banded

p FIG. 6. Plots of the weight percentages of pumice (P), //o • \ crystals (C) and lithics (L) for whole samples / Ann : \ of the Minoan deposits, recalculated to / " . \ ,oo ~o excluding the ~ mm size and finer / _ " \ fractions. Legend as for Fig. 5 except // ." \ for dark triangles which are base urge deposits

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dacitic rock similar to the modern lavas of Nea Kammeni and to those directly underlying the Minoan deposits north of Thera. The original volcanic cone is likely to have been composed of this lava type and it is suggested that much of the lithie debris in the mud-flows was picked up on the upper slope. The diverse nature and high content of the lithics in the ignimbrite suggest an origin from within the magma chamber or conduit walls. Figure 7 shows tha~ ignimbrites have much higher lithic contents than other deposits, confirming that the differences are genuine. Walker (i 972) demonstrated that when coarse pumice clasts from ignimbrite are crushed the proportion of P P

IMM

/ ," a , > -\

/..U..'_ ." \ / _.~ •, • . - \ / o>~ ..- \ / ,,.~". :-. - /, i,.- ~b.- ,, Ill. I~ C L C I..

P P /~ 114MM I/8

0"4

k C C FIo. 7. Plots of the weight percentages ofpumice (P), crystals (C) and lithics (L) in the > z, >½, >~: and >~ mm size fractions where the proportions represent those from individual sieve grades for samples of the Minoan deposits. Legend as for Fig. 5 except M--mud-flow deposit, and dark squares, which are fine ash deposits interbedded with ignimbrite. The large dark circle represents the proportions of pumice to crystals in artificially crushed pumice, using the data of Walker (x972). Lines of equal crystal-pumice ratio are shown. The bracketed figures show the crystal enrichment over the artificially crushed pumice for lines of constant crystal-pumice ratio.

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liberated crystals to pumice is very much lower than it is in the same ignimbrite. Walker's data for individual size fractions for crushed pumice are plotted in Fig. 7, and fines of equal C/P ratio are shown. Each line of equal C/P ratio also represents a line of equal crystal enrichment over the crushed pumice, where the enrichment factor E.F. is the ratio of C/P in the ignimbrite to that in the crushed sample. In nearly all cases the ignimbrites are substantially enriched in crystals compared with other types of deposit. The fine ash fall deposits interbedded with the ignimbrite are also plotted in Fig. 7. They are believed to represent the fine fallout from the turbulent ash cloud such as is observed above many historic pyroclastic flows (Sparks et al. 1973) and are depleted in crystals and lithics compared to their associated ignimbrites, a relationship which has been found elsewhere (Sparks x975). Caution must however be employed in interpreting Fig. 7 as the pumice used by Walker (1972) came from a mud-flow deposit near Thera. There is no dramatic change in crystallinity during the course of the eruption, but small changes of phenocryst content in the pumice cannot be ruled out. Unpublished analyses show no change of composition with time. The degree of crystal enrichment is generally less in the base surge deposits than in the ignimbrite.

3. Discussion The destruction of Minoan Crete is widely held by archaeologists to be conDected with the Minoan eruption (Lute 1969). There is, however, a problem of timing as the evacuation of Thera occurred in Late Minoan Ia times (c. 149o B.C.) whereas the widespread destruction of Minoan Crete and many other Aegean centres occurred in Late Minoan Ib times (c. I45 o B.c.--Doumas 1974). The exact sequence of events has recently been debated by Money (1973) and Doumas (i 974) but conflicts with some of the evidence presented in this paper. The Minoan eruption of Santorini has also been widely quoted as an exceptionally violent event and one with an enormous production of kinetic and thermal energy (Herdervari 1968), but these calculations are based on limited data. The Minoan eruption was characterised by a sequence of distinctive phases similar to that recorded in many other eruptions involving silica rich magmas (Sparks et al. 1973): (i) Plinian phase~plinian activity is thought to be the result of a continuous, high intensity gas blast ejecting pyroclastic debris to a height of several tens of kilometres, where high velocity winds spread the ejecta over a wide area. Although the present study is confined to near-vent exposures, by comparison with other and better known examples the Minoan deposit must have had a volume of between 3 and 5 km3- Unpublished calculations (using the data from Figs. 2c, d and a new model of plinian type activity) suggest a gas velocity of 55 ° m/s and an eruption column height of over 2o km. The plinian phase probably could have lasted up to 24 hours, judging from the durations of similar historic eruptions (Lirer et al. 1973)- As can be seen from Fig. 2 the eruption was incapable of ejecting large blocks further than a few kilometres and speculation about the possibility of IO cm bombs reaching

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Crete at supersonic velocities must be rejected (Herdarvari I968). Even if the wind speed had been 5 ° m]s (jet stream velocities) to the south-east, calculations (Wilson I972 ) indicate that the maximum particle size to reach Crete could only have been around 2 mm. (ii) Phreatomagmatic aetivity~this was probably due to sea water having access to the vent. Fine grained ash fall beds and base surges were pro- dueed. The absence of accretionary lapilli may indicate rather higher temperatures for these surges (> Ioo°C), or low water contents. (iii) Mud-flows~these were generated possibly as a result of the collapse of a tuff-ring formed around the vent by the phreatomagmatie activity. (iv) Pyroclastie flows~large numbers of pyroclastic flows were generated which flowed off the steep upper slopes and into the sea. They eventually extended the coastal plains around the island to form the ignimbrite. Towards the close of the pyroclastie flow activity, extensive intra- formational flooding from the upper slopes of the volcano formed flood gravels. Even at this stage collapse of the caldera had still not occurred. (v) Caldera eollapse~there is no evidence on the exact timing, although it must have occurred after the last stages of the eruption because both the mud-flows and torrent deposits require a source of material from above the level of the present ealdera rim. The mud-flows also contain large water-worn lava blocks with pot-holes, such as are found in a mountain stream bed on steep slopes and they suggest the existence of a sizeable volcanic edifice at this stage in the eruption. Two important points arise from this sequence. Firstly the time span of the eruption was probably very short. This is deduced from the shortness of the plinian phase and by comparison with similar historic eruptions (e.g. the eruption of Katmai I912). Secondly, the volume of the plinian phase, the phreatomagmatic phase and the flood deposits is only a small proportion of the total volume of 6o km 3 of material apparently missing from the caldera. This suggests that most of the material was ejected as pyroclastie flows which entered the sea. The domi- nance of ignimbrite as an eruptive product is a common feature of many similar eruptions, for example the Ito pyroclastic flow eruption (Yokoyama 1974). In view of the unknown volume of material in the sea any calculations of the volume of juvenile magma and on the amount of thermal energy released must be speculative.

ACKNOWLEDGEMENTS. We are grateful for financial support from N.E.R.C.; for the help, criticism and support of Dr G. P. L. Walker; and to Dr L. Wilson, Lancaster University for calculating the gas velocity, column height and dispersal of the plinian phase, using an unpublished model of plinian volcanism. 4. References /I~LLEN, J. R. L. t 97 o. A quantitative model of climbing ripples and their cross-laminated deposits. Sedzmentology x4~ 5-26. BLAb, D. H., ELWELL, R. W. D., GmSON, I. L., S~mLHORN, R. R. & WALX~ER, G. P. L. x964 Some relationships resulting from the intimate association of acid and basic magmas. Q. Jl geol. Soc. Lond. x2x, 31-49. BOOTH, B. x973. The Granadilla Pumice deposit of Southern Tenerife, Canary Islands. Proc. Geol. Assoc. 84, 353-7o.

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CRO~/'E, B. M. & FISHER, R. V. 1973. Sedimentary structures in base surge deposits with special reference to cross-bedding. Ubehebe Craters, Death Valley, California. Bull. geol. Soc. Am. 84, 663-82. Dotn~s, C. x974. The Minoan eruption of Santorini Volcano. Antiquity 48, x xo-t 5. FouQu~, F. I879. Santorin et ses eruptions. G. Macon, Paris. F~cts, P. W., ROOBAL, M. J., WALX~a, G. P. L., COBBALD, P. R. & COWARD, M. x974. The San Pedro and San Pablo Volcanoes of Northern Chile and their hot avalanche deposits. Geol. Rdsch. 65, 357-88. G~OROXLA.S, G. G. x962. Greece. Catalogue of the Active Volcanoes of the World including solfatara fields. x2, 4o Napoli. G0~raEa, VOH D. & PICHLEa, H. I973. Die Obere and Untere Bimsstein-Folge auf Santorin. N. rib. Geol. Pal~wnt, Mh. 7, 394-415- ~~RV~, P. x968. Volcanophysical investigations on the energetics of the Minoan eruption of Volcano Santorini. Bull. Volcan. 32, 439-6I. Jom~soH, A. M. 197o. Physical processes in geology. Freeman, Cooper, San Francisco. Lmm~, L., I~TOP.~, T., BOOTH, B. & WAta~R, G. P. L. I973. Two Plinian fall deposits from Somma Vesuvius, Italy. Bull. geol. Soc. Am. 84, 759-72- Luca~, J. V. 1969 . The End of Atlantis. In M. Wheeler (ed.) New aspects of antiquity. Thames & Hudson. MONEY, J. x973- The destruction of Acrotiri. Antiquity 47, 5o-3. MooRE, J. G. 1967. Base surge in recent eruptions. Bull. volcan. 3o, 337-63- Nxca~OLm, I. A. I97 I. Petrology of Santorini Volcano, Cyclades, Greece. J. Petrol. 12, 67-119. Pxcnta~a, H. & Kussm~UL, S. I972. The calc-alkaline volcanic rocks of the Sarttorini Group (Aegean Sea, Greece). N. Jb. Miner. Abh. x x6, 268-3o 7. SCH~Ne~, H.-U., FISn~,a, R. V. & WATEaS, A. C. 1973. Antidune and chute and pool structures in the base surge deposits of the Laacher See Area, Germany. Sedimentology 2o, 553-74. SPARKS, R. S. J. i975. Grain size variations in ignimbrites and implications for the transport of pyroclastic flows. Sedimentology (in press). , SE~, S. & W~.xER, G. P. L. 1973. Products of ignimbrite eruptions. Geology x, x x5-8. TAYLOR, G. A. I958. The 1951 eruption of Mount Lamington. Bull. Bur. Mineral. Resour. Geol. Geophys. 38. VAN PADANG, M. I936. Die Geschichte des Vulkanismus Santorins yon ihren Anfangen bis zum zerstorenden Bimssteinausbruch um die mitte des 2 Jahrtausends v. chr. In Reek (I936) Santorins----der Werdegang eines Inselvulkans und sein Ausbruch. x, 1-93. WAL~R, G. P. L. x97 I. Grain size characteristics of pyroclastic deposits. Jl. Geol. 79, 696-714. , 1972. Crystal concentration in ignimbrites. Contrib. Minerat. Petrol. 36, x35-46. I973. Explosive volcanic eruptions--a new classification scheme. Geol. Rdsch. 62, 431-46. , & CROASDALE,R. 1971. Two Pliniaa eruptions in the Azores. Q. Jl geol. Soc. Lond. 127, 17-55- x972. Characteristics of some basaltic pyroclastics. Bull. volcan. 35, 3o5-x 7. WATERS, A. C. & FISHER, R. V. I97o. Base surges and their deposits: Cape Linhos and Taal Volcanoes. Journ. geophys. Res. 76, 5596-6x4 . WILSOH, L. I972. Explosive volcanic eruptions, II. The atmosphere trajectories of pyroelasts. Geophys. J. R. astron. Soc. 3o, 38t--92. YOKOYAMA, S. I974. Flow and emplacement mechanisms of the Ito pyroclastic flow in Southern Kyushu, Japan. Sci. Rpt. Tokyo Kyoitu Daigaku, Sect. c, x2(t 14).

Received 2 x January 1975; revised typescript received 24 March 1975.

Miss A. Bond, Geology Department, Imperial College, London SW7. R. S. J. Sparks, Lunar and Planetary Unit, Department of Environmental Sciences, University of Lancaster, Lancaster LAI 4YR.

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