Aegean Sea: Undersea volcanic eruption 520,000 years ago

Santorini

A massive volcanic eruption occurred on the seabed off Santorini 520,000 years ago. © Olga Gavrilova/ iStock

There is an active volcanic area beneath the Aegean Sea, which was responsible, among other things, for the Bronze Age eruption of the Santorini volcano. But now geologists have discovered traces of an even larger volcanic eruption in this area. In drill cores from the seabed around Santorini, they discovered a thick layer of pumice that was deposited 520,000 years ago. It indicates the strongest known eruption of the Christiana-Santorini-Columbo volcanic field to date. It generated powerful pyroclastic flows and left deposits over an area of ​​around 3,000 square kilometers. It was one of the strongest eruptions in the entire Aegean, as the researchers report.

The Eastern Mediterranean is one of the most active volcanic regions in the world. Colliding plate boundaries have created several volcanic areas here, including in Sicily, southern Italy and the Aegean region. “The South Aegean volcanic arc lies in the heart of Europe and its submarine volcanoes are a major potential threat,” explain Tim Druitt from the University of Clermont-Auvergne and his colleagues. The Christiana-Santorini-Kolumbo volcanic field is also located in this volcanic arc, which has triggered at least twelve Plinian eruptions characterized by explosive ash and lava eruptions in the area around the Greek island of Santorini in the last 360,000 years. The most famous eruption of this volcanic field was the Thera eruption, which covered the entire eastern Mediterranean with ash and tsunamis around 1600 BC and caused the collapse of the Minoan Empire. However, how active the predominantly submarine volcanoes in this field used to be and still are today has only been partially researched.

150 meter thick layer of pumice stone

That's why Druitt's international research team has now examined the past of the Christiana-Santorini-Columbo volcanic field in more detail. To do this, they took drill cores from the seabed at twelve locations in the volcanic field as part of the international deep drilling program IODP. The drillings reached a depth of 90 meters and included deposits from the last 23 million years. When examining the layers contained in the drill cores, the researchers discovered a layer of pumice and other volcanic material up to 150 meters thick - evidence of a previously unknown, violent volcanic eruption. According to dating, this eruption, dubbed Archaeos, must have occurred around 520,000 years ago at an underwater volcano near Santorini. “An initial dating and estimation of the water depth at which the eruption took place was possible directly on board thanks to micropaleontology,” reports co-author Steffen Kutterolf from the GEOMAR Helmholtz Center for Ocean Research Kiel. The crucial clues were provided by microfossils in the form of foraminiferal shells.

How strong the eruption was at the time was revealed, among other things, by the thickness of the volcanic deposits in the drill cores: “The newly discovered tuff deposit has a volume of more than 90 cubic kilometers and a thickness of up to 150 meters,” reports Kutterolf. “This is six times larger than the pyroclastic flow deposits of the Minoan eruption and ten times larger than those of the Hunga Tonga-Hunga Ha'apai volcanic eruption of January 22, 2022.” The porous tuff alone had a volume of at least 89 cubic kilometers, probably significantly more more, as the researchers determined. In total, the mostly submarine volcanic deposits extend over an area of ​​over 3,000 square kilometers. They also discovered several meter-thick pumice deposits on the neighboring islands of Santorini, Christiani and Anafi. To eject such a large amount of volcanic material, the Archaeos eruption must have been extremely violent.

Gas-rich magma and pyroclastic flows

Based on the amount, distribution and nature of the submarine volcanic deposits, Druitt and his team reconstructed how the Archaeos volcanic eruption took place. Accordingly, gas-rich magma shot out of the undersea volcanic vent at high speed and mixed with the water. A hot mixture of ash, gas and porous pieces of pumice was created, which raced down the volcano's flanks as a pyroclastic flow. These hot, increasingly saturated streams flowed up to 70 kilometers into the surrounding ocean basins. However, part of the eruption broke through the surface of the water, producing an ash cloud and pyroclastic flows. These transported ash and pumice to the surrounding islands. According to the research team, the scale of this eruption exceeds that of the Kos Plateau around 161,000 years ago - previously considered the strongest known eruption in the South Aegean volcanic arc.

“Our new findings change our previous ideas about the South Aegean volcanic arc: They reveal a greater capacity for highly dangerous submarine eruptions than previously thought,” write Druitt and his colleagues. Despite this explosive history, it is very unlikely that the volcanic field will experience such a large eruption again in the near future. “Knowing the history is also an indispensable component for predicting the future,” says Kutterolf.

Source: Tim Druitt (University of Clermont-Auvergne, France) et al., Communications Earth & Environment, doi: 10.1038/s43247-023-01171-z)

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