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Trilobites

At Site of Japanese Volcano’s Supereruption, an Immense Lava Dome Lurks

Satsuma Iojima Island in the East China Sea straddles the western edge of the Kikai Caldera, which sits on top of an immense lava dome, scientists say.Credit...The Asahi Shimbun, via Getty Images

Some 7,300 years ago, a supereruption devastated the southern islands of what is now Japan, burying most of the archipelago in thick ash. Known as the Akahoya eruption, the blast was so powerful it caused the volcano’s magma chamber to collapse, leaving a 12-mile wide scar called Kikai Caldera, which is mostly underwater.

Now in a study published Friday, scientists have discovered that a dome of lava lurks beneath the caldera. By studying its magma plumbing, volcanologists could gain insight into the entire caldera system, which could help them better predict when another eruption in the Japanese archipelago might occur.

“The most serious problem that we are worrying about is not an eruption of this lava dome, but the occurrence of the next supereruption,” said Yoshiyuki Tatsumi a volcanologist at Kobe University in Japan and lead author of the study that appeared in the journal Scientific Reports.

Dr. Tatsumi’s previous work has suggested that the chances of a supereruption happening in the Japanese archipelago in the next century are only about 1 percent. But if a volcano in this area erupts, it could eject nearly 10 cubic miles of magma, covering almost all of the country and its 120 million people in nearly eight inches of thick ash, he found.

He and his colleagues at the Kobe Ocean Bottom Exploration Center conducted three surveys of the caldera, during which they used remotely operated vehicles to observe the depression. On their trips they investigated the caldera using seismic analysis as well as geological and electromagnetic tests. They found the lava dome using an acoustic survey.

The dome — a trapped buildup of viscous lava — is estimated to have a volume of about eight cubic miles, a diameter of about six miles and a height of almost 2,000 feet. While other volcanic remnants, like the Yellowstone caldera and the Long Valley caldera, have also been brewing with activity, the paper noted that this dome has a much more immense volume of lava.

This site has experienced at least three supereruptions. One 140,000 years ago, another 95,000 years ago, and then the Akahoya eruption. The scientists are not sure when exactly the current dome began to form, whether it was immediately after the eruption or gradually in the thousands of years that followed.

“The lava dome is chemically different from the supereruption, suggesting that a new magma supply system had been developed after 7,300 years ago,” said Dr. Tatsumi.

They did find that the lava dome was made of similar magma to what is seen in volcanoes on nearby islands. Dr. Tatsumi said it was interesting to find a lava dome in the caldera because usually the period following the birth of a caldera is calm. But the finding shows considerable activity at the Kikai Caldera, similar to the preparation stage for the next supereruption.

Dr. Tatsumi said another survey in March will gather high-resolution images of the underground magma system.

Janine Krippner, a volcanologist at Concord University in West Virginia who was not involved in the study, pointed out that the team has not determined whether what it found is a single large dome or multiple smaller buildups of lava. She stressed that the finding does not mean there will be another large eruption anytime soon and that calderas have a spectrum of activity and hazards that require further research to understand.

“Calderas occur around the world and the more we know about the differences and similarities,” she said, “the more we can understand the hazards and how to prepare for potential eruptions in the future.”

A version of this article appears in print on  , Section D, Page 2 of the New York edition with the headline: Magma Plumbing: At Site of Big Eruption, A Dome of Lava Lurks. Order Reprints | Today’s Paper | Subscribe

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