Largest volcanic eruption in recorded history happened 7300 years ago

The largest volcanic eruption of the current geological epoch happened underwater off the southern coast of Japan about 7300 years ago. The blast produced more than three times as much material as the largest modern eruption known, that of Mount Tambora, which exploded in Indonesia in 1815 and caused such drastic climate changes that it led to the “Year Without a Summer” in 1816.

The new record-holder, the Kikai-Akahoya eruption, came from a submerged caldera in a region of sea near the Japanese island of Kyushu.

The eruption’s devastating consequences for humans living on nearby islands have been documented by geologists and archaeologists, and analysis of deposits of volcanic ash had indicated that the blast was one of the largest eruptions of the current geological epoch, the Holocene, which began 11,700 years ago.

However, the blast’s origins and size were uncertain because of the difficulty in accessing the undersea caldera, the crater that formed after the volcano erupted, and volcanic deposits on the seabed.

Now, Nobukazu Seama at Kobe University in Japan and his colleagues have calculated that the Kikai-Akahoya eruption produced a far larger amount of rock and ash underwater than previously thought, at around 70 cubic kilometres. Combining this with previous estimates from volcanic rock deposited over Japan, the total amount of material pumped out from the volcano equates to more than 300 cubic kilometres of material. This is equivalent to twice the amount of water in Lake Tahoe in the western US. “It is quite big, more than we expected,” says Seama.

It is still significantly behind the massive eruption of the Toba supervolcano in Indonesia some 74,000 years ago, though, which released more than 2500 cubic kilometres of magma.

To assess Kikai-Akahoya, Seama and his colleagues did a seismic survey to map out the underwater region around the caldera roughly 200 metres below the surface. From this, they could see the layers of material around the volcano, but they didn’t know which were from the eruption itself.

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So the researchers collected deposits from the sea floor using a remote-controlled drilling robot and took core samples from the rock below, identifying a layer that contained distinctive volcanic glass. The data allowed them to isolate the volcanic layer from the seismic survey and calculate the total volume of material that the volcano produced.

“We know that these very large, caldera-forming eruptions are rare, but we also know that there are more of these events in the geological past that we’ve found evidence for,” says David Pyle at the University of Oxford.

The main reason that the eruption’s size has taken until now to be determined is because it is difficult to find and measure calderas that lie deep underwater, he says.

The Kikai-Akahoya caldera still has a large magma chamber underneath it. If this explodes, it could produce another eruption, but we don’t know how large it would be because it depends on the size of the magma chamber, says Seama. While the chances of it erupting are small, he says, his team is working on measuring the dome more accurately to improve our understanding of the risk.

Combining historical information from past eruptions like Kikai-Akahoya with studies of more recent underwater eruptions, such as the Hunga Tonga eruption in 2022, could help us produce better models for predicting future eruptions, says Pyle.