Up to about two million years ago, Santorini was a small non-volcanic island. Remains of this can still be seen at Mount Profitis Ilias in the southeast of the present island, which is made from non-volcanic limestone. About two million years ago, volcanoes under the sea to the west of the island started producing magma, resulting in a number of small islands. Eventually (around 500,000 years ago) there were two giant 'shield volcanoes'. These are mountains in the shape of flat cones. These mountains united with the non-volcanic island to make one big island. Although neither of these mountains exist any longer, geologists have given them names. The northern mountain is called Mount Peristeria while the southern one is called Mount Thera.

About 200,000 years ago, things started hotting up. Mount Thera started to produce vast amounts of magma and ash, eventually completely emptying the magma chamber under the mountain. The structure of the mountain was not able to support itself and it went crashing downwards into the empty magma chamber, leaving a caldera - a wide, deep hole in the ground. This process was repeated in a whole series of eruptions over the next 200,000 years, with both mountains producing magma, collapsing, regrowing and collapsing again, each time deepening the caldera and eventually leaving the island in the shape it is today.

The last massive eruption is reckoned to have taken place in 1645 BC.

The eruption of 1645 BC is known as the Minoan eruption because it occurred during the lifetime of the Minoan civilisation. The Minoans were an ancient people who set up a civilisation on Crete and the surrounding islands, including Santorini. They were living on Santorini at the time of the massive eruption. Remains of their villages have been discovered buried under volcanic ash, both on the main island of Santorini and on the adjoining island of Therasia. Evidently, they were a cultured people, with multiple-storeyed houses, plumbing and painted frescoes. There were no dead bodies discovered under the ash, so the Santorini Minoans may have been given advance warning of the eruption and may have evacuated the island.

The Minoan eruption of Santorini is reckoned to have been one of the biggest volcanic eruptions since the beginning of civilisation. Only the eruption of Tambura, Indonesia in 1815 was bigger. For comparison, Santorini produced about 35 times as much rock and ash as the eruption of Mount St Helens in the USA in 1980 and had an explosivity rating about ten times that of Mt St Helens. The eruption of Krakatoa in Indonesia in 1883 was slightly smaller than that of Santorini. To give some idea of the scale, Krakatoa's eruption produced an ocean wave which killed 36,000 people and the bang was heard over 4,500 km away.

The first phase of the Minoan eruption was when Mount Thera blew its top and churned out pumice, a white rock filled with bubbles of gas which is lighter than water. A column about 35km high of pumice and ash was blown into the stratosphere raining back down on the island. Where it hasn't been removed, Santorini is still covered in a layer of pumice, in places as much as 5m thick. In the next phase, large amounts of magma were given off in explosive blasts by the mountain. This would have produced 'mushroom clouds' similar to those produced by atom bombs. In total, between the pumice and the magma, 30 - 40 cubic kilometres of rock were thrown out of the volcano. In the final stage of the eruption, the magma chamber beneath the surface of the mountain was emptied. The surface of the volcano collapsed, further deepening the existing caldera and leaving the vertical cliffs that can be seen today. The new caldera was deeper than the sea around it, so the sea rushed in and engulfed the centre of the island. This would have been quite some sight!

Mount Thera did not die as a result of the eruption. It started to reform, eventually poking its tip above the water in 197 BC. A number of islands have formed and reformed in the centre of the caldera. At present there are two, known as Palea Kameni (Old Burnt Island) and Nea Kameni (New Burnt Island). These have erupted sporadically over the centuries. Nea Kameni erupted as recently as 1950 and is still smoking even today. These are minor affairs, however, compared with the mind-boggling scale of the previous eruptions. Volcanoes are unpredictable things, but it is likely that the Thera volcano will continue to grow until it is once again a major mountain, and maybe blow its top again.

The Peristeria volcano seems now to be extinct. There is no trace now of where it once stood, at the north end of the present caldera.

Source: BBC.co.uk