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 erup- tion, 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, follow- ing 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/1/1/4896862/gsjgs.132.1.0001.pdf by guest on 28 September 2021 A. Bond & R. S. J. Sparks '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 south- ern 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). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/1/1/4896862/gsjgs.132.1.0001.pdf by guest on 28 September 2021 Minoan eruption of Santorini 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, A ~,4#E I~OLIiMM B / ~"To Thickness i N I. .o* ..o o 1 l ~I0,1 ~fXO~IITI -(~ - ,...~j.'""--.~ Maximum D / ~*~"-~,_ Maximum ~a ..2~r "~'~l~s Pumice / ~ ~,~. :'" :!e.= kt7.i / ! ~ ~ ~--~.,,,\- N • ~, "~'"~ " ~4" /1 I "7-4 12-5 14 0 18"4 ~#o ,,~-~ .,,.o .,.~ "~ .12"3 ,J' o, ! =.~ 3~. 3e 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 erup- tion is shown by the large black circle. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/132/1/1/4896862/gsjgs.132.1.0001.pdf by guest on 28 September 2021 A. Bond & R. S. J. @arks 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.