Some Comments on the Minoan Eruption of Santorini

The writer then deals with the damaging effects of the various phases of the Minoan eruption of Santorini as distinguished by Bond and Sparks.

Comparison with known Icelandic eruptions leads to the conclusion that the thickness of the Minoan tephra on Crete is not per se likely to have caused extensive damage on the island. Fluorine poisoning may have been more serious, but on the whole it seems somewhat doubtful that the Minoan eruptions played a decisive role in the fall of the Minoan power on Crete, although it is likely to have contributed to its decline. Yet the tremendous eruption must have made a profound impression on people in the then Minoan archipelago, an impression almost certainly handed down through many generations. As there is a tendency in legends to link under one head important events, although they are separated by some time interval, it seems quite natural that two so fateful events as the Minoan eruption of Santorini and the fall of the Minoan power on Crete had - even when separated by some decades - merged into one in Plato's Atlantic myth.

What role did the Minoan eruption of Santorini play in the decline and fall of the Minoan civilisation on Crete and elsewhere? This is a question difficult to answer, and the answers still vary greatly.

Before making some comments on this fascinating problem I think it might be elucidating for those students of the problem who are not geoscientists for me to deal shortly with the damaging factors of volcanic eruptions in general and their effects.

The main types of volcanic eruptions, their damaging factors, and the range of their effects are put together in Table 1. As seen in that Table I have found it appropriate to distinguish between short-reaching (local) and far-reaching (extensive) effects. The border between these is put at 25 km, maybe somewhat arbitrarily, but I think it should anyhow be put somewhere between 20 and 30 km. Extremely widespread effects I have termed "global".

Following the usual practice in textbooks on volcanology I distinguish in Table I between three main types of volcanic eruptions: Effusive, which are wholly or predominantly lava-producing; mixed, producing both lava and tephra; and explosive, purely tephra-producing. To the first type belong the eruptions of the Hawaiian shield volcanoes and the Icelandic crater rows; the eruptions of Vesuvius are typically mixed, and the Krakatoa eruption in 1883 was wholly explosive.

In Table I we can read e.g. that the damaging factors in effusive eruptions are: Lava flows, tephra falls, gases, jökulhlaups (glacier bursts) and volcanic earthquakes. We also find that the effects of these factors are usually short-reaching, rarely far-reaching, and that two of them have in extremely rare cases a "global" range. The only effusive eruption that may possibly fit into that category is the Lakagigar eruption in 1783, the biggest lava eruption on earth in historical times.

The Minoan eruption of Santorini was, like the Krakatoa eruption in 1883, wholly explosive and tephra-producing. It should be kept in mind that the entire island of Thera was well within the range of the short-reaching effects of the damaging factors listed in Table 1. According to that table the possible damaging factors of this eruption are the following (jökulhlaups obviously excluded): Tephra fall, gases, tephra flows, lahars, base surges, (volcanic) earthquakes and tsunamis.

Table I: Volcanic eruptions and their damagind factors

During the last decades geologists, mainly English and American, have developed methods for the study of explosive eruptions which can be termed generally as tephrometric. Using parameters based on isopach -and isoplethmaps, as well as on granulometric studies of the tephra layers, and by distinguishing between various components of the tephra, it has proved possible to characterize and identify various types of explosive eruptions and eruption phases. Bond & Sparks (1976) have in an important paper applied these methods to the Minoan eruption of Santorini and been able to distinguish the following sequence of distinctive phases: Plinian phase, phreato-magmatic Surtseyan-type activity, mud-flows, tephra flows and, finally, caldera collapse.

Characteristic of Plinian eruptions is a tremendous production of airfall tephra. Bond & Sparks estimate the volume of the Plinian phase Santorini tephra at 3 - 5 km³.

As seen in Table I, the effects of tephra falls are usually far-reaching, and exceptionally the effects are "global". The near-effects of a tephra fall are first and foremost due to the usually great thickness of the tephra layer found near the volcano. With its weight, especially when it absorbs rain, it can break down buildings and sometimes bury them completely; such as was the case on Heimaey in 1973 (Fig. 1). Foremost among the far-reaching effects is usually the blanketing of fields and pastures by tephra for various lengths of time.

Judging from the paper of Bond & Sparks (op. cit.), the pumice layer due to the Plinian phase was not thick enough to bury buildings in peripheral parts of Thera, but certainly it was thick enough to make nearly the entire island uninhabitable for a very long time. Another near-effect of tephra fall in violent explosive eruptions is the falling of bombs and lithic blocks big enough to kill people and animals, and near the volcano the bombs can be hot enough to set buildings on fire. This can also have happened on Thera.

As to far-reaching effects of the Plinian phase tephra on Crete and elsewhere I shall here make some use of experience mainly from Iceland.

Figs. 2 and 3 are isopach maps of two Hekla - tephra layers, H₁ and H₃, produced by typical Plinian eruptions, H₁ in 1104 A.D., H3 about 2800 B.P. Both have been mapped in great detail (cf. Fig. 2 and Thorarinsson, 1970, Fig. 3). On these maps I have placed Knossos (K) and Zakro (Z) at the same distance from Hekla as their distance from the Santorini volcano and I have placed them both on the axis of maximum thickness of the tephra layer, which means in the wind direction from the volcano during the tephra fall, and at angles to that direction, c. 52° and ca. 28° respectively. According to the map of Ninkovich & Heezen (1965, Fig. 162), showing the distribution of the Santorini tephra on the sea floor, these were the angles between the prevailing wind-vector during the Plinian phase of the eruption, and the two cities of Knossos and Zakro (Fig. 4).

The figures in the upper half of Table II show the thickness of the tephra layers at the locations of Knossos and Zakro relative to Hekla as placed on the two isopach maps, both in their present compressed state and as freshly fallen, which means about 70 % greater thickness. As aforesaid, Bond & Sparks (1976) assume that the volume of the Plinian phase Santorini tephra is 3 - 5 km³. Calculated volume figures for H₁ and H₃ are 2,5 km³ and 12 km³ respectively, and the figures in the lower half of Table II are the H₁ - figures X 2, and the H₃ - figures X 5/12, so as to fit approximately a Plinian phase Santorini layer of 5 km³. The bold figures in the lower half of Table II correspond to what may realistically be assumed for the thickness of the Plinian phase layer on Knossos and Zakro as freshly fallen.

Of course these figures should be taken cum grano salis. We do not know the wind direction with certainty and the angles used are possibly somewhat too large (cf. Bond & Sparks 1976, Fig. 1). Yet their maps show that the angles used on the H₁ and H₃ maps are not far from correct. The thickness of a tephra layer at various distances from its source is not strictly proportional to the volume of the tephra layer. The wind velocity is a factor to count with and we do not know how strong a wind prevailed during the Plinian phase of the Minoan eruption.

Both the H₁ and H₃ maps indicate a rather strong wind, and it is not likely that it was stronger during the Plinian phase of the Santorini eruption. It should also be noted that the figures in the lower half of Table II are rather similar whether calculated from layer H₁ or H₃, which supports the assumption that they are reasonable.

We may therefore assume as likely that the fresh fallen tephra layer of the Plinian phase was 1 - 2 cm at Knossos and about 3 cm at Zakro. In the compressed state a thickness of 0.5 - 2.0 cm could thus be expected on Eastern Crete in places where the layer has been completely preserved in situ. Judging from granulometric measurements of the layers H₁ and H₃, the Santorini layer on Crete probably was fine-medium sandy, particle diam. 0.1 - 0.6 mm, and therefore very easily wind drifted. No wonder that it is now rather difficult to find the layer on Crete, although there is no doubt about its existence there (cf. Vitaliano & Vitaliano 1974).

In my lecture at the 1st Congress on the volcano of Thera I concluded that by summing up the comparative evidence from Iceland we find that when a tephra fall has caused abandonment of rural settlements lasting decades, the thickness of the tephra layer as freshly fallen was about 20 cm in highland districts and 30 to 50 cm in more fertile lowland areas. When the abandonment lasted 1 to 5 years the thickness was 15 cm or more and at least 10 cm thickness was needed to cause abandonment lasting less than a year (Thorarinsson 1971, p. 228).

Even if the wind direction during the Plinian phase of the Santorini eruption was directly Thera - Knossos or Thera - Zakro - and almost certainly this was far from being the case - the maximum figures in the lower half of Table II show a thickness less than that which causes 1 - 5 years evacuation of settlements in Iceland.

It seems unlikely that later phases of the Santorini eruptions, as described by Bond & Sparks (1976) could have added considerably to the thickness of the tephra layer on Crete.

Although one should be careful in applying experience from one country to a different environment, I think it justified on the strength of the Icelandic examples to regard it as very unlikely that the thickness per se of the Santorini tephra on Crete could have seriously affected the conditions of life on the island for more than a year. But it must have made a traumatic impression on the minds of people on Eastern Crete, as it was in all probability covered in complete darkness for hours and possibly for days. A possible factor that could have seriously added to the damage caused by the tephra fall, by taking a heavy toll of grazing livestock, is fluorine poisoning (Thorarinsson 1971, 230).

The Surtseyan-type phase of the Santorini eruption was, according to Bond & Sparks (1976), characterized by s.c. base surges. A base surge (cf. Fig. 5) is, according to J.G. Moore, (1967) "a ring-shaped basal cloud that sweeps outward as a density flow from the base of a vertical eruption column". The range of destruction is short, hardly exceeding 10 kilometers, but within a short range base surges can cause great damage. The base surges accompanying the eruption of Taal Volcano in Luzon on the Philippines in 1965 damaged small bushes and trees up to a 6 km distance from the eruption column and removed totally all trees from ½ to 1 km from the eruption center (Moore, op. cit.).

The damaging effects of the base surges on Thera may have covered the entire island, but most of the island was then more or less thoroughly devastated by the Plinian tephra fall. The base surges certainly did not affect other islands, but some fine grained tephra (ash) of the Surtseyan phase may have been deposited on Crete.

The effects of the lahars, or mudflows, are strictly short-reaching and although disastrous as far as they reach they could have had no effects outside Thera.

Since the devastation of St. Pierre on Martinique on May 9th 1902, when nearly 30.000 persons were killed instantly by nuée ardente from Mont Pelée, nuées ardentes and the accompanying tephra flows have rightly been feared more than other damaging factors of explosive eruptions. The Minoan tephra flow deposits, including ignimbrites, which Bond & Sparks (1976) found to have formed during the fourth phase of the eruption, were certainly devastating - if there was anything left to devastate. But their effects did not affect other islands.

The dangerous gases of volcanic eruptions are mainly sulphur compounds, carbon oxides and fluorine. In explosive eruptions their effects are usually short-reaching with the exception of fluorine, whose effects are mainly far -reaching.

With the exception of the nuées ardentes, where gases are deadly because of their high temperature, gases are not likely to have played a significant role in the devastation of Thera. It is possible, however, that in some places on the island people and animals were suffocated by lack of oxygen in the eruption clouds.

The main collapse of the Santorini volcano, and the formation of the big caldera took place, according to Bond & Sparks (1976) after the tephra flow phase. The caldera formation is likely to have caused large tsunamis, but how large and how destructive is difficult to decide. One should be careful in jumping to conclusions such as the one that, because the Santorini caldera is three times bigger than the caldera formed in the Krakatoa eruption, the resulting tsunamis were three times bigger too. This depends among other things on how long a time it took to form the Santorini caldera. To mention but two examples in recent times: the formation of the Öskjuvatn caldera after the big rhyolitic eruption of Askja in 1875 took about 3 decades, although much of it occurred during the first year after the eruption (Thorarinsson & Sigvaldason 1962). The enlargement of the summit caldera of Isla Fernandina in the Galapagos Islands was nearly finished 12 days after the eruption in 1968 (Simkin & Howard 1970). It seems likely, though, that the main collapse of the Santorini volcano was a process of short duration, and thus violent, and the resulting tsunamis are the factor of the Minoan eruption which is the most likely to have had far-reaching destructive effects. These effects must, however, have been restricted to cities and other settlements bordering on shores.

The volcanic earthquakes preceding and accompanying the Minoan eruption are not likely to have been strong enough to have serious far-reaching effects or to cause tsunamis. But the strongest shocks were probably strong enough locally to shake down buildings on Thera. Usually the interval between strong volcanic earthquakes and the beginning of the following eruption is a matter of a few hours or days, but there are exceptions. According to Seneca, Pompeii, situated at about the same distance from Vesuvius as Akrotiri from the volcanic centre on Thera, was partly destroyed by an earthquake on February 5, 63 A.D., 16½ years before the disastrous eruption that began on August 24, 79 A.D.

During this time interval earthquake shocks were felt now and then, although not strong enough to stop the rebuilding of Pompeii. One cannot quite exclude the possibility, although it is not a likely one, that volcanic earthquakes, strong enough to damage severely cities on Thera such as Akrotiri, occurred decades before the big eruption.

From a volcanological point of view it does not seem impossible that Akrotiri was hit so hard by a volcanic earthquake near the end of LM IA that the city was deserted, and that repeated earthquake shocks kept it more or less uninhabited until the big eruption began decades later, in LM IB, cf. the opinion expressed by Money (1973), but contested by Doumas (1974) the following year. Against the possibility that the eruption occurred in LM IB speak the results of Vitaliano & Vitaliano's (1974) search for the Minoan tephra on Crete as these results seem to suggest that the Minoan eruption could not have occurred much later than the end of LM IA.

As to establishing the absolute dating of the Minoan eruption, the archaeological methods are probably still the most exact ones. Corrections of radiocarbon datings by the dendrochronological time scale and other means may, in a rather near future, lead to more exact C14 -datings of the Santorini tephra than we have as yet. A still more exact dating can possibly be obtained in cores from the Greenland inland ice, minutely studied by Dansgaard and his collaborators, provided it proves possible to identify dust from the Minoan Santorini eruption in these cores.

Summing up, we find that the Minoan eruption of Santorini hit the settlements on the island of Thera (Stronghyle) with nearly all the damaging or destructive effects of an extremely large explosive eruption, and the devastation of all settlements on that island must have been total. But it seems doubtful that the far-reaching damaging effects of that eruption were severe enough to play a decisive role in the fall of the Minoan civilization on Crete. That it contributed to a decline of that civilization can be regarded as reasonable, and doubtless this tremendous natural catastrophe made a very deep and longlasting impression on people in the Minoan world, impression bound to be handed down through many generations.

Finally I want to point out that one ought to distinguish between the role the Minoan eruption of Santorini may have played in the collapse of the Minoan power on the one hand, and the possible or probable share of this eruption in Plato's Atlantis myth on the other. We know with certainty that as a result of the tremendous eruption and the subsequent caldera formation a land mass sank into the sea, greater than any at any time anywhere since the dawn of civilization.

We also know that Minoan cities sank into the sea, or were completely buried by tephra and other deposits, due to the same eruption. It seems very likely, not to say almost certain, that more or less diffuse memories of this tremendous and shocking disaster are woven into Plato's tale of the sinking of Atlantis, regardless of the role the eruption may have played in the collapse of the Minoan power on Crete. There is the tendency, not unusual in legends and oral traditions, to link under one head important events that are relatively near each other in time compared with the time that has elapsed since they occurred. Therefore it seems to me quite natural that these two fateful events, the Minoan eruption of Santorini and the fall of the Minoan civilization on Crete, even if separated by some decades, merged into one in the legend that was told by Plato more than a millenium later.

ADDENDA

In my lecture delivered at the 1st International Scientific Congress on the Volcano of Thera, I stated that additional deep-sea core sampling was urgently needed in order to settle the question whether or not the Minoan tephra fall had seriously affected settlements on Crete (Thorarinsson 1970, 230). When I wrote my paper for the Second International Congress on Thera a year ago I did not know that this additional core sampling had been carried out and I tried therefore to attack the problem from another angle, basing my estimate of the tephra thickness on Crete on detailed isopach maps of 2 Icelandic eruptions and a recent paper on the Santorini tephra by Bond & Sparks (1976).

At the Tenth INQUA Congress in Birmingham in August 1977 C.P.L. Walker presented an important paper on the formation of ignimbrites: The signification of vitric-enriched air-fall ashes associated with crystal-enriched ignimbrites (Abstracts X INQUA Congress, Birmingham, 492). In this paper he shows that the formation of airfall tephra in eruptions producing ignimbrites is very much greater than hitherto assumed. And shortly before my paper for the Second International Congress on Thera was handed to the printers I received a preprint of a highly important paper: Volume and extent of the Minoan tephra from Santorini volcano: new evidence from deep-sea sediment cores. This paper will be published in Nature and its authors are Watkins, Sparks, Sigurdsson, Huangi, Federman, Carey and Ninkovich. To quote the abstract of that paper: "Analyses of tephra in abyssal piston cores collected during cruises of R/V Trident show that the Minoan eruption produced at least 28 km³ of tephra (13 km³ rock equivalent). A layer up to 5 cm thick must have been deposited on eastern Crete". The paper has two isopach maps of the Minoan tephra. With these isopach maps and Walker's above-mentioned paper my attempts at calculating the thickness of the Minoan tephra on Crete are, of course, of a greatly reduced interest. Yet, such calculations are not entirely futile. This statement is supported by the following: in my calculations the angle between the axis of maximum thickness of the Minoan tephra and the direction Santorini - Knossos was 52°, in accordance with Ninkovich's and Heezen's map (Ninkovich & Heezen 1965). According to the new isopach maps of the layer this angle is 75°.

Had I calculated with that angle the bold figures on my Table II would have been ca. 0.34 and 0.04 for Knossos and ca. 1.0 and ca. 0.6 for Zakro. Let us calculate like Watkins et al. (op. cit.) with 55 - 60 km³ as the total volume of the Santorini tephra as freshly fallen (whereas I used 5 km³ in my calculation).

Let us further assume that the percentage of the total volume that fell on Crete was the same as the percentage during the Plinian phase, although the percentage of the total volume is likely to be lower. We then arrive at the thickness figures ca. 0.5 - 4.0 cm for Knossos and 7 - 11 cm for Zakro. These figures are likely to be maximum figures both for the reason just mentioned and because the angle 75^o used in the calculation is likely to be a minimum angle, according to the new isopach map. But the figures are not far from those obtained by Watkins et al. and lead to a similar conclusion to the one I had reached before viz. that the tephra fall of the Minoan eruption certainly affected eastern Crete, but did hardly play a decisive role in the fall of the Minoan power.

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