The volcanic tsunami that broke an empire

3700 years ago, Mount Thera exploded with a force that ruptured the fabric of an entire civilisation. Humans had been coping with the area’s angry tectonics for centuries, but for the Minoans of Crete, something made this eruption truly devastating.

In 2000, scientists found tsunami traces in sediments from Crete and Turkey. Now, researchers have uncovered the full horror of the waves that crippled the Aegean’s most powerful traders.

Today the island of Santorini, as Mount Thera is now known, is a peaceful tourist paradise of white walls and turquoise water. But in 17th century BC, it was a great furnace of lava, ash and steam.

Studying Santorini’s rocks reveals that the eruption built in stages. First, small explosions littered the south of the island with a thin ash layer. Then, as the magma chamber under the volcano emptied, it began to slump and bigger explosions sent phyroclastic flows – superheated avalanches of boulders, gas and pulverised rock – charging down its flanks.

When the volcano collapsed completely, all hell broke loose as sea water flooded into the cauldron of magma. Imagine the effect of throwing water on a chip pan fire (see here for an example) then multiply by a billion. Two giant pyroclastic flows, both 55 metres thick and carrying 30 cubic kilometres of material, swept into the sea, followed by a fourth phase of activity as gas rushed out of the mamga chamber. Then it rained.

The team of researchers from Greece and Hawaii used computer models to estimate marine effects from both pyroclastic flows and the mountain’s collapse. By adding sea floor topography data from Thera and the surrounding islands, together with estimates of the pyroclastic flow volume and water displaced by the slump, they were able to predict the tsunami wave size.

Results published in Geophysical Journal International suggest the waves were gigantic. Pyroclastic flows hitting the sea sent ripples fanning out from the south of the Island. By the time they reached Crete, some towered 90 feet (28 metres) over the Minoan harbours.

But the tsunami generated when the volcano collapsed was truly terrifying. Output from the rather inappropriately named FUNWAVE model suggests some crests were a staggering 160 feet (50 metres) high – the size of Nelson’s Column.

With boats smashed, harbours in ruins and livelihoods gone, the Minoans abandoned their coastal settlements, leaving themselves vulnerable to attack. In just a few generations they’d succumbed to the Mycenaeans and twelve centuries of civilisation came to an end.

All that’s left of Thera now is the beautiful crescent of Santorini, with its beaches of black volcanic sand. And while everything appears calm, the recent creation of sulphurous islands in the centre of Santorini’s lagoon is the first clue that the mountain is rebuilding. Some day, Thera will explode again.

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