Japan’s Kikai Caldera Is Rebuilding a Massive Magma Reservoir

A vast magma reservoir beneath Japan’s Kikai caldera appears to be filling again, giving scientists rare insight into how some of Earth’s largest volcanoes recover after catastrophic eruptions.

The discovery, led by researchers at Kobe University, could improve scientific understanding of giant caldera systems such as Yellowstone in the United States and Lake Toba in Indonesia. It may also help researchers identify underground changes that occur before future large-scale volcanic eruptions.

Kikai Caldera Is Capable of Enormous Eruptions

Kikai is a mostly submerged volcanic caldera located south of Japan. Approximately 7,300 years ago, it produced the largest known volcanic eruption of the Holocene, the current geological epoch that began about 11,700 years ago.

A caldera forms when a massive eruption removes so much magma that the ground above the underground reservoir collapses. Rather than leaving behind a typical cone-shaped volcano, the eruption creates a broad, relatively shallow depression.

The scale of a giant eruption is difficult to imagine. The amount of magma involved could cover New York’s Central Park to a depth of approximately 12 kilometers.

Kikai, Yellowstone, and Toba are prominent examples of giant caldera volcanoes. Scientists know that these systems can erupt repeatedly, but the processes that allow them to accumulate enormous volumes of magma remain poorly understood. This uncertainty makes their long-term behavior difficult to predict.

“We must understand how such large quantities of magma can accumulate to understand how giant caldera eruptions occur,” says Nobukazu Seama, a geophysicist at Kobe University.

Scientists Used Seismic Waves to Study the Ocean Floor

Kikai’s underwater location may appear to be a challenge, but it provided researchers with an important advantage. Because much of the caldera lies beneath the sea, scientists were able to conduct broad, systematic surveys across the volcanic structure.

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

The Kobe University research team collaborated with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) to study the crust beneath Kikai caldera.

Researchers used airgun arrays to generate controlled seismic pulses and placed seismometers on the ocean floor to record how the waves traveled through the underlying rock.

Seismic waves change speed and direction depending on the materials they encounter. By analyzing these changes, scientists can create detailed images of underground structures and identify areas that may contain partially molten rock.

A Large Magma Reservoir Lies Beneath Kikai Caldera

The findings, published in Communications Earth & Environment, revealed a substantial magma-rich region directly beneath the part of Kikai responsible for the enormous eruption 7,300 years ago.

The researchers also estimated the reservoir’s shape and extent. Its size and location indicate that it occupies the same underground volcanic system that supplied magma to the ancient eruption.

“Due to its extent and location, it is clear that this is in fact the same magma reservoir as in the previous eruption,” Seama says.

However, the evidence does not suggest that the magma has remained underground unchanged since the ancient eruption. Instead, it indicates that new molten material has entered and replenished the reservoir over time.

Fresh Magma Is Rebuilding the Volcanic System

A lava dome has been forming near the center of Kikai caldera for approximately 3,900 years. Lava domes develop when thick magma rises slowly to the surface and accumulates around a volcanic vent instead of flowing easily outward.

Chemical analyses show that material from this lava dome and other recent volcanic activity differs from the magma released during Kikai’s giant eruption. This contrast suggests that the caldera’s current magma supply comes from a newer source.

“This means that the magma that is now present in the magma reservoir under the lava dome is likely newly injected magma,” Seama says.

The findings support a broader model in which fresh magma gradually enters and rebuilds underground reservoirs beneath giant calderas after major eruptions. Understanding this process could help scientists determine how these volcanoes prepare for later periods of activity.

What Kikai Can Reveal About Yellowstone and Toba

The magma-reinjection model may apply to other giant volcanic systems around the world. Scientists have also identified large, shallow magma reservoirs beneath major calderas, including Yellowstone and Toba.

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

By studying how quickly fresh magma enters these systems, where it accumulates, and how it changes the surrounding crust, researchers may eventually improve their ability to distinguish normal volcanic activity from signs of a much larger eruption.

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

Does the Discovery Mean Kikai Will Erupt Soon?

The discovery does not indicate that Kikai caldera is about to erupt. Instead, it provides important evidence that the underground system responsible for the ancient eruption remains active and continues to receive new magma.

The research was funded by Japan’s Ministry of Education, Culture, Sports, Science and Technology (MEXT) through the Third Earthquake and Volcano Hazards Observation and Research Program, as well as by the Japan Society for the Promotion of Science through grant 20H00199.

The study was conducted in collaboration with researchers from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).

Source: www.sciencedaily.com