Kikai Caldera's Magma Chamber Refilling After Holocene's Largest Eruption

About 7,300 years ago, the Kikai Caldera volcano, located off Japan's Kyushu island, experienced what is considered the largest known eruption of the Holocene epoch.

A new study details that this volcano's large magma chamber is currently refilling, providing insights into eruption cycles and potentially enhancing the prediction of future volcanic events.

The Akahoya Eruption and Its Impact

The Akahoya eruption, 7,300 years ago, ejected approximately 160 cubic kilometers of dense rock equivalent. This volume surpasses that of the 1912 Novarupta eruption by over 11 times and the 1991 Pinatubo eruption by 32 times. The event distributed material across 4,500 square kilometers and produced pyroclastic flows extending up to 150 kilometers from the epicenter. Tephra deposition occurred across Japan and the Korean peninsula.

While no subsequent eruption has matched the scale of the Akahoya event, the Kikai Caldera remains active, with minor eruptions occurring in recent decades. Previous research had also indicated new volcanic activity and the formation of a lava dome beneath the caldera. The Akahoya eruption is believed to have significantly impacted the Jōmon people, who inhabited ancient Japan.

Given current population densities, a future eruption, even a smaller one, could have substantial consequences.

Giant Calderas Worldwide

Kikai Caldera is one of several known giant calderas globally, alongside North America's Yellowstone (last caldera-forming eruption around 640,000 years ago) and Indonesia's Toba (largest volcanic eruption in recorded history around 74,000 years ago). These volcanoes are known to reawaken after long dormant periods, but the mechanisms of their long-term cycles are not fully understood, complicating eruption predictions.

According to co-author Seama Nobukazu, a geophysicist at Kobe University, understanding how large quantities of magma accumulate is crucial for comprehending giant caldera eruptions.

Research Methodology and Findings

The Kikai Caldera is mostly submerged, which both limits access and preserves evidence of past eruptions, facilitating scientific investigation. Researchers from Kobe University and the Japan Agency for Marine-Earth Science and Technology conducted surveys using research boats, an air-gun array, and ocean-bottom seismometers. By generating seismic pulses and measuring their travel through the Earth's crust, they mapped the subsurface structure.

This research identified a large magma chamber beneath the caldera, which appears to be the same reservoir that supplied the Akahoya eruption. Chemical analyses, however, suggest that the magma inside this chamber is newly injected rather than residual material from the past eruption. Previous studies support this, indicating a new lava dome has been forming within the caldera over the last 3,900 years.

New Magma Re-injection Model

Based on these findings, the researchers propose a new general model for the refilling of magma chambers beneath giant calderas. This model aligns with the presence of large shallow magma reservoirs found under other giant calderas like Yellowstone and Toba.

The research team aims to refine these methods to better understand magma re-injection processes, with the ultimate goal of improving the monitoring of indicators for future giant eruptions.

The study was published in Communications Earth & Environment.