Santorini was once a complete island with a typical volcanic peak. Then, in about 1650 BC, during the final throws of a major eruption, the subterranean roof of the emptied magma chamber collapsed, taking with it the flanks of the volcano and most of the island. This created a breach to the sea, which resulted in the flooding of the massively enlarged caldera, forming one of the largest on earth and creating the basic topography we see today.
There is evidence of at least twelve major explosive eruptions in the last 200,000 years on Santorini. However, the Minoan eruption (1650 BC) was one of the world"s largest volcanic events in the last 10,000 years, estimated to have been ten times the magnitude of the Indonesian Krakatoa eruption. About 30 cubic km of magma and ash was ejected covering a large area of the eastern and southern Mediterranean. The collapse and subsequent explosion resulted in a catastrophic tsunami and the ash and gases released, caused major metrological and environmental damage throughout the eastern Mediterranean and in fact across the whole northern hemisphere. This gigantic tsunami may have led to the collapse of the Minoan civilization on the island of Crete. Another popular theory holds that the Minoan eruption is the source of the legend of Atlantis, as described in the account by Plato, written in circa 375 B.C.
After the Minoan eruption, there was a period of 1500 years, during which the volcano once again remained dormant. Then the volcano came alive with other three eruptions that created the island of Hierra, Theia and Palea Kameni (palea=old and kameni=burnt).
It once again erupted in 726 A.C. increasing the size of Palea Kameni and again in 1457 and 1508 adding further material.
In the 17th century eruptions in a new area of the lagoon created a further island, called Nea Kameni, holding the distinction of being the youngest island in Greece. In 1956 Santorini suffered a major earthquake measuring 7.8 on the Richter scale, although not followed by any volcanic activity; it caused severe damage on the island and resulted in the death of 49 people.
The many historical eruptions mean that the Santorini of today exhibits a complex stratification of overlapping "shield volcano" layering. Basalt and andesite lava flows that make up the shields can be seen as banded layers in the cliff below the town of Fira.