Geonomic Notes on the Bronze Age Eruption of Santorini

THE PROBABLE POSITION OF THE ANCIENT CRATER

The so-called "Libyan Fresco," that was discovered during the excavations at Akrotiri, Thera in the 1972 season, is well-known as a really valuable work of art. In addition to its artistic value it has serious importance from the volcanoogical point of view as well. It enables us, namely, to clarify a very important detail of the Minoan eruption of Santorini. This fresco -known also as The Miniature - was found in the fifth room of the so-called West House. The walls of the room in question were imperfect. The fresco in question decorated the eastern and southern walls. A subtropical landscape was originally seen on the eastern wall and, as its continuation, a town on the southeast corner, followed by a fleet on the south wall and finally another town in the southwest corner (Marinatos 1974). Professor Marinatos was of the opinion that the upper part of the western and northern walls were destroyed by the direct effect of a "volcanic hurricane", that is a nuée ardente (glowing volcanic cloud that moves along the slopes with a speed of a hurricane). It is very important to note that the traces of the hot, tiny grains, that were carried by the nuée ardente, can be seen excellently on the surface of the western part of the fresco. This statement is valid for that part of The Miniature which decorated the southern wall. The former windows of the western wall were very near it (Fig. 1.). In front of this part of the fresco, at a distance of 4 metres, were the windows of the northern wall.

The fastly moving, glowing particles of the nuée ardente had penetrated into the interior of the room through these northern windows and impacted onto the surface of the fresco on the southern wall like the shotgrains of a shotgun. As Professor Marinatos has mentioned (1974, p. 39) : "As we go further away from the windows towards the inner part of Room 5, the traces of the bombardment begin to fade. After the last part of the town. ..the traces of the bombardment are slighter. Further left still... the traces are even less. One of the foremost ships... suffered still damage... Then, all other parts of the frieze, which covered the South wall, are free of traces of bombardment." From this description and the analysis of the photos of the fresco, one can conclude that about a two meters long part of the fresco on the south wall was touched by the grains. These two meters were measured from the southwestern corner of the room along the southern wall.

Professor Marinatos has suggested that taking into account the position of the respective part of the fresco and the site of the northern windows one can estimate the position of the ancient crater from which the nuée ardente was issued. This idea was practically applied by the writer of these lines. Without discussing the details of the work the author would like to report the final result briefly. Accordingly, the direction of the arrival of the grains might have been approximately N 18^o W, provided that the value of two metres is accepted as the length of that part of the fresco which was bombarded by the very hot particles ⁽¹⁾.

The value mentioned means that the active center from which the nuee ardente had started, was - looking from Akrotiri - in the direction of the southeastern edge of Palea Kameni, the older volcanic center within the present-day caldera. This island came into being either in 197 or 194 B.C., or else in 46 A.D., (Georgalas 1962) on the place of the former cone. The active center, on the other hand, must have had a position well above sea-level, otherwise the glowing cloud could not have reached the immediate environment of Akrotiri. The surface of the island of Thera just north of Akrotiri, namely, is situated at a higher level above the sea than the site of the excavations. This supposition corresponds well to the generally accepted view according to which the cone of the ancient volcano was at least 1500 metres high.

The line with a direction of N 18^o W, that is S 18^o E, that begins at the excavated area, passes first through the place of the former crater of the very ancient volcano, called "Akrotiri" (Fig. 2. ) and on the other edge of the caldera it touches the crater of another very old volcano, known as "Simandir". This feature can be found at the northeastern peninsula of the island Therasia, the second largest remnant of Stronghyle. As a matter of fact ancient Stronghyle was not a simple volcano with one and only one top-crater on its cone but rather a cluster of volcanoes of different ages (Williams 1941; Georgalas 1962). In this volcanic complex there were many craters and smaller, individual volcanoes, the traces of some of which were recently discovered under the level of the sea on the bottom of the caldera.

It is very highly unlikely that the nuée ardente would have issued either from the volcano of "Akrotiri" or from "Simandir". At the time of the Minoan eruption, the volcano "Akrotiri" had been perfectly inactive for many hundred thousands of years. Its activity, namely, took place in the very early part of the development of the volcanic cluster as a whole, and after the final phase of the activity of "Akrotiri", the center of the volcanism has shifted towards the northeast and north respectively. At this time newer volcanoes came into being successively, along a great arc, beginning with the group of "Fanari", "Lumaravi" and "Akrotiri" volcanoes at the southwestern part of the present Thera and ending with "Simandir" and "Therasia - Skaros" volcanoes at the inner shore of the  present Therasia island.

As regards "Simandir", this ancient volcano has produced basaltic lavas from which a small and not too high shield was created. The activity of basaltic shield-volcanoes is characterized by a very low index of explosivity - in contrast to the activity of stratovolcanoes - and nuée ardente phenomena are perfectly unknown. Therefore it appears to be certain that neither "Akrotiri" nor "Simandir" were the source of the nuée ardente, the traces of which can be seen on the Libyan Fresco. The only possible source is the supposed crater on or very near the top of the former cone of Stronghyle at a remarkable altitude above the sea and probably on that place where, in our days, the southeastern edge of Palea Kameni island exists.

POSSIBLE AEROPHYSICAL AND CLIMATOLOGICAL AFTER -EFFECTS OF THE MINOAN ERUPTION

To-day it is clear that a very close, causal connection exists between stronger volcanic eruptions and changes in the climate. Many studies on this subject are known, e.g. Humphreys (1940), Lamb (1970), etc. Some great volcanic outbursts, especially that of Krakatau (1883), Mount Pelée (1902), Katmai (1912) and Agung (1963) were studied from this point of view in a very detailed form.

On the basis of the remarkable similarities between Krakatau and Santorini we have an opportunity to come to conclusions about the possible aero-physical and climatological events that took place at Santorini in the Bronze Age, considering the phenomena which accompanied the great eruption of Krakatau. The idea of such a comparison was originally suggested by Mrs. Georgiana Hédervári.

The detonations of the explosions during the paroxysmal phase of the Krakatau eruption (around 10 o'clock, forenoon, 27 August 1883) were heard up to a distance of 4800 km. Accepting the same value for Santorini, the noise accompanying the paroxysm and the collapse of the volcanic mount - provided that the propagation of the sound was isotropic - was heard within the following circle (Fig. 3.): Spitzbergen - Novaya Zemlya - Gulf of Ob - Novosibirsk - Lake Ala-kul (east of Lake Balkhask) -Delhi - Bombay - Mombasa - the central part of Lake Tanganyika - the mouth of the Congo - Monrovia - Dakar - Azores Scoresby of Eastern Greenland.  

After the Krakatau eruption the airwaves made two complete circles around the Earth as a whole. A similar phenomenon can be supposed for the Minoan outburst, too. The top of the Krakatau's eruption cloud reached a maximum height of 80 km. Bond and Sparks (1976) have suggested cautiously "'greater than 20 km" for Santorini's case. On the other hand, van Bemmelen wrote about 100 km (1971). This second figure appears to be the most likely. Within a distance of 200 km from Krakatau, total darkness lasted for 22 hours, within a circle of 80 km it lasted 57 hours. On northern Crete, at a distance of about 120 km from Santorini, the total period of darkness might have been therefore 50 hours or more. According to Wexler (1951) the horizontal extent (length) of Krakatau's ash cloud reached 8000 - 11.200 km. Immediately after the eruption the speed of this cloud was 117 km per hour. Similar values for the length and speed of the cloud can be accepted for Santorini as well.

After the Krakatau catastrophe Bishop's rings and unusual, fearful deep red twilights were observed in many parts of the world. It is very likely that Bishop's rings and strange lights on the sky were observed in the Mediterranean region and perhaps elsewhere, too, after the Minoan outburst of Santorini. Ash fall was observed in Spain after the Krakatau eruption. The distance between the volcano and Central Spain is some 12.400 km. This means that ash from Santorini could have fallen in California, near San Francisco!

According to Lamb (1970), a remarkable deficit in the incoming solar beam was observed as long as 2 - 3 years after Krakatau's eruption and, furthermore, the average temperature of the Northern Hemisphere and even that of the Earth as a whole exhibited a considerable decrease after this event. Similar climatological consequences were probably present after the Minoan eruption as well. That is a general decrease in the world's mean temperature can be supposed, which lasted at least 2 or 3 years or more, as a result of the eruption.

GEOPHYSICAL PRELIMINARIES OF THE MINOAN ERUPTION

According to the generally accepted view, the Bronze Age eruption of Santorini had been triggered by a strong tectonic earthquake. Let us mention in this respect that almost all of this volcano's later eruptions were preceded or accompanied or else followed by at least one strong earthquake as is evident from the comparison of the data, given by Georgalas (1962) and Galanopoulos (1961).

In the opinion of the author such correlations between eruptions and earthquakes are rather common, as was demonstrated for the case of Indonesia (Hédervári 1976). Many examples from other areas of the world were described in the Reports of the Research Group on Planetary and Geophysical Volcanology (RGPGY Reports 1976). Blot (1976) has written a long book about the possibility of such relationships and described many other examples. In addition to these some other authors came also to the conclusion that these geophysical events are, or at least may be, correlated with one another. The author suggests that the phenomenon discovered by Blot should be called tectono-volcanic effect.

In an earlier paper (Hédervári 1972) the writer, on the basis of some reasonable assumptions, concluded that the triggering shock preceded the beginning of the first tephra-fall from Santorini by about ten months only -a value which is in very good congruence with the archaeological facts that were pointed out by Marinatos (1971). The data for the respective triggering shock are not repeated here; they can be found in the mentioned paper (Hédervári 1972).

Recent results of the excavation led to the conclusion that a half century before the great Minoan outburst another strong shock occurred on or near Santorini (Marinatos 1972). It is clear that the earthquake in question had nothing to do with the Minoan eruption. That is, this shock had not a direct triggering effect; the time-interval between the two events was too long.

The catalogue, compiled by Galanopoulos (1961) mentions altogether six great earthquakes, with an intensity of 10 degrees on the 12 degrees scale, that preceded the Minoan eruption. According to Marinatos the respective shock occurred around 1550 B.C. According to Galanopoulos a strong earthquake took place in 1570 B.C. at Knossos and it was accompanied by fire. (Epicentral coordinates, according to Sieberg, were as follows: 35,5^o N, 25,5^o E). It may be that the shock mentioned by Marinatos and this latter mentioned one were the same. If it was a shock with an intensity of 10 degrees on Crete, it might have had a devasting force on Santorini itself, with a probable intensity of 8 or 9 degrees. According to the author's estimate (Hédervári 1972) the intensity of the triggering shock at the epicenter was somewhat smaller, probably 9 degrees and it was felt on Santorini itself with an intensity of 8 degrees.

In an extremely interesting paper, Doumas (1974) has also dealt with the possible earthquakes prior to the eruption. He based his conclusions on archaeological evidence. He didn't speak about a triggering earthquake (of tectonic origin) but rather about slight tremors before the first appearance of volcanic materials on and around the cone of the volcano. "It is unlikely" - wrote Dr. Doumas (p. 110) - "that severe earthquakes were the first in the series of events... Rather, it seems, there were slight tremors which gave the inhabitants time to take precautions and to leave with their more precious belongings. Evidence for this is abundant... People were not caught unexpectedly by sudden and severe earthquakes... More severe earthquakes... came when people had already gone away. But again the evidence is that these quakes were not very severe... Whatever the force of the earthquakes, these must have stopped for a considerable period of time, in which people recovered from the shock and felt safe to return to their ruined city" (p. 111).

In all likelihood these shocks were of volcanic origin, due to the upward movement of magma within the conduit towards the free surface. Such volcanic earthquakes (type A of Minakami) are very frequent before greater eruptions and usually have a depth-range from 1 km to 20 km (Minakami 1974). The majority of them occur at a very shallow depth, only from 1 - 5 km from the surface. They are usually relatively weak shocks with a magnitude of around 2 - 3, and very rarely greater than 6. Immediately preceding the outburst type B earthquakes also can be expected from a very shallow depth beneath the active crater. The focal depth varies from 100 metres to 1 km. In addition to these two kinds of earthshocks, explosion quakes and continuous volcanic tremors also occurred at Santorini. However, there must have been a strong tectonic earthquake before the commencement of the series of these volcanic shocks. This was the powerful earthquake on or near Crete in 1500 B.C., mentioned by Platakis and catalogued by Galanopoulos (1961). The writer is convinced that the traces of this very powerful shock will be or have been discovered at Akrotiri.

In the conclusions, Doumas has emphasized that "It does not seem to be any argument against the idea that the LM Ib destruction, so widely spread in the Aegean, was caused by severe earthquakes. The distribution pattern of the destructions all round the island of Santorini may warrant a suggestion that the epicentre of the earthquakes was Thera itself. If so, these earthquakes might either have generated from the collapse of the roof of the Thera magma chamber, or have resulted in weakening the resistance of that roof which finally collapsed forming thus the enormous caldera of Thera" (p. 114).

The writer perfectly agrees with this view. His energetical calculations (Hédervári 1971) showed that a great deal of the energy had to be used for the creation of so-called "collapse earthquakes" during and after the paroxysm of the outburst, and the epicentral area of these quakes, naturally, was the newly  formed caldera of Santorini.

NOTE

In Table II. and Table III. of the author's paper of 1971 there were serious numerical errors in the last column (E_p = ...). Consequently the results which follow from these data should be corrected. Here the results of the new calculations are published:

In Table II :

E_p = (M₁ g H) + E_w, 10²⁵ ergs

Erroneous value:           h, km              Correct value:

       6,32                          1,000                      3,260

      12,26                         2,000                      4,220

      28,12                         3,000                       7,180

      45,05                         4,000                       9,170

       65,78                        5,000                     11,130

     227,90                       10,000                     20,900

     499,40                       15,000                     31,400

     844,40                       20,000                      40,400

In Table III :

Erroneous value:           h, km              Correct value:

       6,72                          1,000                      3,460

      16,70                         2,000                      5,700

      31,16                         3,000                      7,940

      50,15                         4,000                    10,180

      73,40                         5,000                    12,400

    257,60                       10,000                    23,600

    553,80                       15,000                    34,800

    962,00                       20,000                    46,000

The author asks those persons who wish to make further calculations on the basis of the data presented in the Acta article to please consider the corrected values given above.

- (1). Let us suppose now that the length of the bombarded part of the fresco is only One meter instead of two. In this case the direction will be about N 6,5^o W only. This means that the ancient crater from which the nuee andente in question issued was situated at the western shore of Nea Kameni and not on the eastern or southeastern edge of Palaea Kameni island. The latter would be probable if the angle were N 18^o W.

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