Supervolcano under Japan is refilling with fresh magma

One of the world's most powerful supervolcanoes is slowly refilling its huge underground magma reservoir, 7,300 years after one of the largest eruptions in recent geological history, according to scientists in Japan.

Researchers at Kobe University found evidence that fresh magma is building up beneath the Kikai caldera, which lies mostly under the sea off the southern Japanese island of Kyushu. Its eruption about 7,300 years ago was the largest known eruption of the Holocene, the geological period that began about 11,700 years ago and continues today.

What is a caldera?

Some eruptions release so much magma that the ground above the emptied reservoir collapses, leaving a broad, relatively shallow basin known as a caldera. The volumes involved can be enormous: enough, the researchers say, to bury New York's Central Park under a layer of rock about 12km thick. Yellowstone in the United States and Toba in Indonesia are other famous examples.

Scientists know that giant calderas can erupt more than once, but understand little about how so much magma builds up underground again, which makes it very hard to judge when another major eruption might happen.

Listening beneath the sea

Because Kikai is underwater, the team, working with the Japan Agency for Marine-Earth Science and Technology, could carry out large, systematic surveys from ships. They used powerful airguns to send seismic pulses into the seabed, and seismometers on the ocean floor recorded how the waves travelled through the rock. Waves move differently through solid rock and through areas containing molten material, revealing structures that cannot be seen directly.

The results, published in Communications Earth and Environment, revealed a large region containing a high proportion of magma directly beneath the site of the ancient eruption. "Due to its extent and location, it is clear that this is in fact the same magma reservoir as in the previous eruption," said the Kobe University geophysicist Nobukazu Seama.

New magma, not old

A new lava dome has been growing near the centre of the caldera over the past 3,900 years or so, and chemical analysis shows its rock differs from that thrown out in the giant eruption. That suggests the magma now in the reservoir has been injected from deeper in the Earth, rather than being left over from the eruption. "This means that the magma that is now present in the magma reservoir under the lava dome is likely newly injected magma," Professor Seama said.

The researchers stress that the refilling does not mean another catastrophic eruption is imminent. Instead, it shows how a reservoir can be recharged over thousands of years, a process scientists need to understand before they can judge the long-term behaviour of supervolcanoes.

"This magma re-injection model is consistent with the existence of large shallow magma reservoirs beneath other giant calderas like Yellowstone and Toba," Professor Seama said. The team now hopes to refine its methods. "Our ultimate goal is to become better able to monitor the crucial indicators of future giant eruptions."

The ancient Kikai eruption spread ash across much of southern Japan and is thought to have had a lasting impact on the Jomon people living there at the time.