Kikai caldera magma is slowly replenishing beneath one of Earth’s most powerful supervolcanoes, according to researchers who found evidence of fresh molten rock accumulating under the seafloor south of Japan. The discovery comes thousands of years after the Kikai system produced one of the largest eruptions of the Holocene, roughly 7,300 years ago.
The work provides a rare window into how giant volcanic systems rebuild their magma stores after cataclysmic events. Scientists say the findings could inform understanding of other supervolcanoes, including Yellowstone in the United States and Toba in Indonesia, improving the ability to recognize long-term warning signs.
Kikai caldera magma and how supervolcanoes store it
Some eruptions release volumes so vast they reshape entire landscapes. In the most extreme cases, the erupted magma volume could bury an area the size of New York City’s Central Park beneath a layer about 12 kilometers, or 7.5 miles, thick.
When such reservoirs are drained, the overlying crust can collapse to form a broad, shallow depression known as a caldera. These structures mark some of the most powerful eruptions in Earth’s history.
Familiar examples include Yellowstone, Toba, and Japan’s mostly submarine Kikai caldera. The Kikai system last produced a colossal eruption about 7,300 years ago, regarded as the largest known event of the Holocene, the epoch that began roughly 11,700 years ago and continues today.
Although giant caldera volcanoes can erupt multiple times, scientists have not fully resolved how the immense magma volumes required for these events accumulate. That uncertainty makes it difficult to anticipate when another major eruption could occur.
“We must understand how such large quantities of magma can accumulate to understand how giant caldera eruptions occur,” said Kobe University geophysicist Nobukazu Seama.
Seafloor surveys reveal a large reservoir beneath Kikai
Kikai’s submarine setting, rather than hindering research, allows for broad seafloor surveys across the volcanic complex. “The underwater location allows us to implement systematic, large-scale surveys,” Seama said.
Researchers from Kobe University and the Japan Agency for Marine-Earth Science and Technology used powerful airgun arrays to send seismic pulses through the crust beneath the ocean floor. Ocean bottom seismometers recorded the returning signals. Because seismic waves move differently through solid rock compared with partially molten zones, scientists can map hidden structures using these measurements.
Results published in Communications Earth & Environment identified a substantial zone with a high melt fraction directly beneath the site of the 7,300-year-old eruption. Analysis of the signals indicates the approximate size, shape, and position of the reservoir, consistent with the storage system implicated in the ancient event.
“Due to its extent and location it is clear that this is in fact the same magma reservoir as in the previous eruption,” Seama said.
Fresh injections are recharging the supervolcano
The team reports that the magma now present is unlikely to be solely a remnant of the prehistoric eruption. Previous work documented a lava dome forming near the caldera’s center over about the last 3,900 years. Lava domes grow when viscous magma accumulates at a vent rather than flowing away.
Chemical studies show that recent materials from the dome differ from deposits produced by the giant eruption. That contrast suggests the current reservoir has been fed by new injections of magma rising from deeper in the crust or mantle.
“This means that the magma that is now present in the magma reservoir under the lava dome is likely newly injected magma,” Seama said.
Combined seismic and geochemical evidence points to a long-term recharging process that gradually rebuilds the chamber after a major eruption. Researchers emphasize that replenishment does not imply an imminent large eruption. Instead, it clarifies how magma can accumulate over millennia, a critical factor for assessing the long-term behavior of supervolcanoes.
Implications for Yellowstone and other giant calderas
The proposed re-injection model may help explain why other vast caldera systems, including Yellowstone and Toba, host large, relatively shallow reservoirs. According to the researchers, after a giant eruption, new magma can slowly refill an existing chamber, shaping long-term cycles of storage, unrest, and major activity.
The team plans to refine the seismic methods demonstrated at Kikai to track magma movement and storage in greater detail. “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,” the researchers said.
The study advances understanding of how fresh magma accumulates beneath a caldera responsible for one of the planet’s most extraordinary eruptions, bringing scientists closer to decoding the processes that make such events possible.
Funding was provided by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) through The Third Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research), and by the Japan Society for the Promotion of Science (grant 20H00199). The research involved collaboration with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).
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