One of Earth’s most explosive volcanoes is quietly refilling with magma

About 7,300 years ago, a volcano on the Japanese island of Kyushu released the largest known eruption of the Holocene, our current geological era.

In a new study, researchers show how this volcano’s vast magma chamber is now slowly refilling, potentially shedding light on its eruption cycle and similar volcanoes — and thus supporting humanity’s ongoing efforts to predict future eruptions earlier and more accurately.

The Kikai caldera volcano ejected about 160 cubic kilometers (38 cubic mi) of rock during its Akahoya eruption 7,300 years ago, 11 times the amount ejected by Novarupta in 1912 and 32 times that of Pinatubo.

The violent explosion spread material over 4,500 square kilometers, an area several times larger than London, and sent pyroclastic flows up to 150 kilometers (93 miles) from the epicenter. Tephra fell over a swath of Japan and the Korean peninsula.

The volcano has not done anything dramatic since then, but it is still active, producing a scattered array of small eruptions in recent decades.

Previous research has found evidence of new volcanic activity beneath the Kikai caldera, pointing to the formation of a lava dome and raising concerns about its potential to erupt again.

Despite little evidence and the absence of written records, the Akahoya eruption is believed to have wiped out the Jomon people, who lived in present-day Japan between about 14,000 BC and 300 BC.

A lot has changed in the last seven millennia, and given the current population density in the region another eruption – even a relatively minor one – could be very devastating.

Besides Kikai, famous calderas (huge, shallow craters left behind) include Yellowstone in North America, where the last caldera-forming eruption occurred about 640,000 years ago, and Toba in Indonesia, which produced the largest volcanic eruption in recorded history about 74,000 years ago.

These powerful volcanoes are known to reawaken and erupt after long intervals, although the mechanics behind these long-term cycles remain largely mysterious, hampering our ability to predict their next cataclysmic eruption.

“To understand how giant caldera eruptions occur, we need to understand how such large amounts of magma can accumulate,” says co-author Sema Nobukazu, a geophysicist at Kobe University in Japan.

The Kikai Caldera is now mostly submerged beneath the ocean, limiting access but also preserving remnants of past eruptions and facilitating their modern study.

“The underwater location allows us to implement systematic, large-scale surveys,” Sima says.

Colleagues from Shima and Kobe universities and the Japan Agency for Ocean-Earth Science and Technology deployed research boats to probe the area using an air-gun array and several dozen sea-bottom seismometers.

The researchers generated seismic pulses with an air gun and then used seismometers to measure how the pulses traveled through the Earth’s crust, revealing valuable information about what lies beneath.

This exposed a large magma chamber that appears to have supplied Akahoya.

“Because of its extent and location, it’s clear that it’s actually a magma reservoir like in previous eruptions,” Sima says.

There doesn’t seem to be any magma left inside, though; Chemical analyzes suggest that its composition is different from that of the Akahoya material. Previous studies also indicate that a new lava dome has been forming in the caldera in the last 3,900 years.

“This means that the magma in the magma reservoir beneath the lava dome is probably newly injected magma,” Sima says.

Based on these findings, the researchers propose a new general model for refilling magma chambers beneath giant calderas, providing insight into Kikai and other volcanoes around the world.

“This magma re-injection model is consistent with the existence of large shallow magma reservoirs beneath other giant calderas such as Yellowstone and Toba,” Seema says.

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“We want to refine the methods that have proven so useful in this study to understand the re-injection processes more deeply,” adds Seama. “Our ultimate goal is to be better able to monitor important indicators of future giant eruptions.”

The study was published in 2015 Communication Earth and Environment.

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