Japan’s Buried Supervolcano Is Quietly Refilling With Fresh Magma 7,300 Years After Its Giant Eruption

Scientists at Kobe University say fresh magma is slowly accumulating beneath Japan’s Kikai caldera, about 7,300 years after the volcano produced the largest known eruption of the Holocene, according to ScienceDaily. The study appears in Communications Earth & Environment.

The reservoir’s refilling does not mean another catastrophic eruption is imminent, the report said. Instead, it offers a rare look at how giant volcanic systems rebuild their magma supplies over thousands of years.

What is a caldera, and why is Kikai special?

When an enormous volume of magma escapes during an eruption, the rock above the emptied reservoir can collapse and leave a broad, relatively shallow depression called a caldera. ScienceDaily gave Yellowstone, Toba and Kikai as famous examples.

Most of Kikai lies beneath the ocean. Its last giant eruption, about 7,300 years ago, was the largest known volcanic eruption of the Holocene, the geological epoch that began roughly 11,700 years ago and continues today. To illustrate the scale of such eruptions, the report said the magma released can be enough to bury New York City’s Central Park beneath a layer roughly 12 kilometers (7.5 miles) thick.

How did researchers look inside the volcano?

The Kobe University team worked with the Japan Agency for Marine-Earth Science and Technology. They used airgun arrays to send seismic pulses through the crust beneath the ocean floor, and ocean bottom seismometers recorded how the waves traveled through the rock.

Seismic waves move differently through solid rock and through regions containing molten material, so the recordings let scientists study structures that cannot be seen directly. Kobe University geophysicist Nobukazu Seama said the underwater setting is an advantage because it allows systematic, large-scale surveys.

What did they find?

The survey revealed a substantial region with a high proportion of magma directly beneath the site of the ancient eruption. From the measurements, the team estimated the reservoir’s approximate size and shape. Seama said its extent and location make it clear that this is the same magma reservoir involved in the earlier eruption.

Is this leftover magma or new magma?

The researchers think it is new. Earlier investigations found that a lava dome has been growing near the center of the caldera over approximately the past 3,900 years, and chemical analyses showed that material from the dome and other recent activity differs in composition from what the giant eruption released.

That suggests the magma now stored under the volcano was supplied by new injections of molten rock from deeper in the crust or mantle, rather than remaining unchanged since the big eruption. Seama said the magma under the lava dome is likely newly injected magma.

Why it matters: what does this mean for other supervolcanoes?

Scientists know giant caldera volcanoes can erupt more than once, but ScienceDaily said they still understand little about how the huge quantities of magma needed for such events build up underground. That gap makes it hard to anticipate when another major eruption might occur.

Seama said researchers must understand how such large quantities of magma accumulate to understand how giant caldera eruptions happen. The report said the findings could help explain how systems such as Yellowstone in the United States and Toba in Indonesia rebuild their magma supplies, and potentially help scientists improve their ability to recognize warning signs of future eruptions.

For now, the work adds a data point on how a reservoir can be recharged over thousands of years, a process the report described as important to understand before the long-term behavior of supervolcanoes can be reliably assessed.

Around Prague, based on reporting by ScienceDaily

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