Ancient Volcanic Eruption
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The violent eruption was centred on a small island just north of the existing island of Nea Kameni in the centre of the caldera. The caldera itself was formed several hundred thousand years ago by collapse of the centre of a circular island caused by the emptying of the magma chamber during an eruption. It has been filled several times by ignimbrite since then and the process repeated, most recently 21,000 years ago. The northern part of the caldera was refilled by the volcano and then collapsed again during the Minoan eruption. Before the Minoan eruption, the caldera formed a nearly continuous ring with the only entrance between the tiny island of Aspronisi and Thera. The eruption destroyed the sections of the ring between Aspronisi and Therasia, and between Therasia and Thera, creating two new channels.
On Santorini, there is a deposit of white tephra thrown from the eruption; it is up to 60 metres thick overlying the soil marking the ground level before the eruption. The layer is divided into three fairly distinct bands indicating different phases of the eruption.
A series of warning earthquakes must have been alarming enough and early enough before the eruption for all the residents to pack up and move out, as not a single body has been found at the Akrotiri site, and only one body has been found on Therasia. Differences in pottery styles between the beginning of the evacuation and the catastrophic eruption, and preserved gullies eroded in the ash layers indicate that the volcano may have given warning years in advance. It remains to be seen if further excavations will show bodies of people huddled along the coast, too late to get off in a boat to escape the volcano's fury, akin to the finds at Herculaneum, which was buried by the much smaller eruption of Mount Vesuvius in 79 AD.
In a classic Plinian eruption marked by columns of smoke and ash extending high into the stratosphere, the Minoan eruption created a plume 30-35 km in height, and magma coming into contact with the shallow marine embayment would have caused a violent phreatic eruption. The eruption also generated a 35 to 150 m high tsunami (estimates vary) that devastated the north coast of Crete, 70km (45 miles) away. The impact of the tsunami pummelled coastal towns such as Amnisos, where building walls have been knocked out of alignment. The tsunami would also certainly have eliminated every timber of the Minoan fleet along Crete's northern shore. On the island of Anaphi, 27 km to the east, ash layers 10 feet deep have been found, as well as pumice layers on slopes 250 meters above sea level. Elsewhere in the Mediterranean there are pumice deposits that could be caused by the Thera eruption [1]. Ash layers in cores drilled from the seabed and from lakes in Turkey, though, show that the heaviest ashfall was towards the east and northeast of Santorini. (Ash found in Crete is now known to have been from a precursory phase of the eruption, some weeks or months before the main eruptive phases, and would have had little impact [2].)
The volume of ejecta is estimated to have been much more than four times what was blown into the stratosphere by Krakatau in 1883, a better-recorded event. Every vestige of life is likely to have been eliminated or smothered in the ashfall, leaving an island that had essentially been sterilized, as was Krakatau.
(source: wikipedia.org)