Santorini, also known as Thera, is the southernmost of the Kyklades islands in Greece. It is unique among the Mediterranean islands as it is the remains of some of the most dramatic volcanic eruptions known. A giant central lagoon, more or less rectangular and measuring about 12km by 7km, is surrounded by 300m high sheer cliffs on three sides. The island slopes downward from the cliffs to the surrounding Mediterranean sea. On the fourth side, the lagoon is separated from the Mediterranean by another much smaller island called Therasia, also with cliffs. The lagoon is joined to the sea in two places, in the northwest and southwest. The water in the centre of the lagoon is nearly 400m deep, so it is an ideal safe harbour for even the biggest ships. The island's ports are all in the lagoon and there are no ports on the outside of the island. The towns of Santorini cling to the top of the cliff looking down on the lagoon.

Geology

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 Minoan Eruption

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!

After the Minoan Eruption

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 Paleo 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.

Life Returns to the Island

After the eruption, nothing could have lived on the island for quite some time. But nature does not stand still. Studies of other volcanic islands show that plants would have started to grow on the island within a few hundred years. In 900 BC, the island was re-inhabited by Dorian Greeks. They called the island Thera. The remains of one of their ancient towns, now known as Ancient Thera, have been excavated on the slopes of Profitis Ilias Mountain. The island has been continuously occupied since then. Due to its central location in the Eastern Mediterranean, it was occasionally used as a stronghold. The Egyptian Greeks used it in the 3rd Century BC.

In the 13th to 15th Centuries, the island was under the control of the Venetians, one of the great seafaring nations of the Mediterranean. They called the island Santa Irene, literally Saint Irene. This name rapidly changed into the present form of Santorini. Despite the fact that it's been called Santorini for the best part of seven centuries, many writers and tourists insist that the island's 'real' name, and the name of its principal town, is Thera. These people refuse to call the main town Fira, even though that's what the people who live there call it¹.

By the 20th Century, Santorini had become a forgotten island. Due to the lack of water, the locals survived on a diet of dried bread, tomato paste, fava beans and wine. There were no roads, only tracks suitable for mules. There was really very little contact with the outside world. This changed in 1956, when a violent earthquake shook much of Greece. Humanitarian aid was provided by the Americans, who were still tidying up Europe after World War II. Rather than rebuilding the port of Oia, which was destroyed in the earthquake, they built an entirely new port at Athinios. This is the modern port where all goods and most visitors now enter the island. For the first time, there was a road that cars and lorries could drive on, and Santorini got its first motor vehicles.

In the late 1970s, tourism took off, and Santorini is now probably the number one destination for tourists in Greece.

Thera Theories

The End of the Minoans

The Minoan civilisation on Crete came to a sudden end in the middle of the 2nd Millennium BC when all the Minoan palaces were burnt down and never rebuilt. In 1939, Spyridion Marinatos, a Greek archaeologist, came up with the theory that the eruption of Mount Thera brought about the end of the Minoan Civilisation. At the time, the dates of the palace burning and the eruption were not known very accurately. Marinatos's theory was that when the mountain collapsed into the caldera, a giant wave would have been formed which would struck the north coast of Crete, which is only 125km away. Since most of the Cretan civilisation was along the coast, this would have been destroyed. Looters then finished the job by burning what was left.

This theory is very popular and is often quoted in guidebooks about Crete. Unfortunately for the theory, the dates of the two events are now known more accurately. The burning of the Minoan palaces happened around 1450 BC. There are two possible dates for the Minoan eruption, but they are both before 1600 BC. An analysis of tree growth rings sets the eruption at 1628 BC, while counting the seasonal layers in cores of ice from the Greenland ice-cap sets the date at 1645 BC. So the Minoan civilisation didn't fall until nearly 200 years after the eruption.

A Single Eruption

Up to about 20 years ago, it was thought that the entire Santorini caldera was formed in a single massive eruption. Examination of the remains of a Minoan town which was covered by ash during the eruption show that parts of it were built on the side of the cliffs going down into the caldera. This shows that the caldera already existed in some form before the Minoan eruption. Detailed geological studies of the rocks around the caldera have shown a series of at least seven massive eruptions over the last 200,000 years, the Minoan one being the most recent.

Atlantis

Plato's story of Atlantis was intended to be a moral tale, illustrating principles of ethics and politics. He described a tyrannical civilisation which lived on an island 'beyond the pillars of Hercules'. The city in the middle of the island was surrounded by a series of ringed walls. The whole island of Atlantis collapsed into the sea.

There are definite parallels with Santorini here. We don't know exactly what shape the island was before the eruption, but the present ring shape was certainly there to some extent. If the ancient Minoans built a city on the volcano in the middle, it would have been surrounded by at least one ring. Perhaps remains of previous calderas made a few other rings. We have no way of telling. While the island did not sink into the sea, the central part certainly did.

But the phrase 'the Pillars of Hercules' normally means the Straits of Gibraltar. In this view, Atlantis was situated in the Atlantic Ocean. In fact, the Atlantic Ocean is named after the story of Atlantis. Proponents of the 'Santorini is Atlantis' theory say that Plato set the tale in a far-off place to make it hypothetical, allowing him free reign on the moral aspects.

There's not much more to say about this theory, but it has certainly struck home with the tourists. You'll see references to Atlantis all over Santorini, from the names of tavernas and bars to brands of wine.

Exodus

The Bible describes in the book of Exodus the journey of the Israelites out of Egypt to their promised land in Palestine. First the country of Egypt was afflicted with ten plagues, including darkness, hail, frogs and dying livestock. Then when the Israelites were crossing the desert, they were shown the way by a pillar of cloud by day and a pillar of fire by night. When they reached the Red Sea, the waters parted and they walked across. The waters came together behind them, drowning the pursuing Egyptians.

Of course, every one of these divine manifestations can be attributed to the eruption of Mount Thera, and of course the theory does not stand up to scrutiny. The column of pumice and ash above the volcano would indeed have looked like a pillar of cloud during the day, but it is not in the right direction to lead the tribes of Israel to the promised land. We are all familiar with the eerie glow of lava coming from a volcano, but it is unlikely to have been visible by night hundreds of kilometres away. Undoubtedly, Egypt was showered with all sorts of debris during the eruption, but 'a plague of frogs'? Unless the Minoans were in the business of farming frogs in a big way, that's one plague that is not volcanic in origin. Seismic tremors in the wake of the caldera formation can do strange things to the sea, and the sea bed can become dry land temporarily, but not for the amount of time it would take a crowd of one million people to cross.

More Information

• Decade Volcano for detailed geological information.
• Santorini - A Tourist Guide for visits to the island.