Roughly 7,300 years after one of the largest volcanic eruptions of the Holocene epoch, Japan's Kikai caldera is slowly refilling its underground magma reservoir, according to a new study in Communications Earth & Environment from researchers at Kobe University — a finding that offers scientists a rare window into how supervolcanoes rebuild themselves after a catastrophic eruption.
Kikai, which lies mostly beneath the ocean south of Japan's main islands, is one of the handful of calderas on Earth, alongside Yellowstone in the United States and Toba in Indonesia, capable of producing eruptions so large they collapse the ground above the emptied magma chamber into a broad depression. Its last giant eruption released enough magma to bury Central Park under roughly 12 kilometers (7.5 miles) of material, and remains the largest known eruption of the current geological epoch.
Turning an Obstacle Into an Advantage
Scientists have long understood that caldera volcanoes can erupt more than once, but have had little direct evidence of how the enormous volumes of magma needed for another giant eruption actually accumulate underground, making it extremely difficult to anticipate when a future eruption might occur. Kikai's underwater location, which might seem to complicate research, instead gave Kobe University geophysicist Nobukazu Seama and his team an advantage: open seafloor that allowed for systematic, large-scale surveys impossible at a land-locked caldera like Yellowstone.
Working with the Japan Agency for Marine-Earth Science and Technology, the researchers used airgun arrays to send seismic pulses through the crust beneath the caldera, then recorded how those waves traveled using ocean-bottom seismometers. Because seismic waves move differently through solid rock than through zones containing molten material, the pattern of those waves let the team map out a substantial, actively accumulating magma reservoir beneath Kikai. "We must understand how such large quantities of magma can accumulate to understand how giant caldera eruptions occur," Seama said of the motivation behind the survey.
What It Means for Yellowstone-Watchers
The finding does not suggest Kikai is near another eruption — supervolcano reservoirs can take thousands to tens of thousands of years to build toward eruptive volumes, and the researchers have not estimated a timeline for this one. But the survey technique itself, and the basic process it reveals of magma slowly reaccumulating in a post-eruption chamber, gives volcanologists a template for probing other dormant giants, including Yellowstone, where similar underground processes are far harder to observe directly given the caldera's position entirely on land. For now, Kikai offers a rare, real-time case study of a supervolcano in the long, quiet middle of its recovery.