One of Earth’s most powerful supervolcanoes is slowly refilling its enormous underground magma reservoir, thousands of years after producing one of the largest volcanic eruptions in recent geological history. Scientists at Kobe University have discovered evidence that fresh magma is accumulating beneath Japan’s Kikai caldera, which unleashed a catastrophic eruption about 7,300 years ago.
The discovery offers a rare look at how giant volcanic systems rebuild their magma supplies after massive eruptions. It could also provide important clues about the behavior of other supervolcanoes, including Yellowstone in the United States and Toba in Indonesia, potentially helping scientists improve their ability to recognize warning signs of future eruptions.
How Supervolcanoes Store Enormous Amounts of Magma
Some volcanic eruptions are so immense that they dramatically reshape the landscape. The amount of magma released can be staggering, enough to bury all of New York City’s Central Park beneath a layer roughly 12 kilometers (7.5 miles) thick.
When such enormous volumes of magma escape from underground, the rock above the emptied reservoir can collapse, leaving behind a broad, relatively shallow depression known as a caldera. These features are associated with some of the most powerful eruptions in Earth’s history.
Famous examples include Yellowstone, Toba, and Japan’s Kikai caldera, most of which lies beneath the ocean. Kikai last experienced a giant eruption approximately 7,300 years ago, producing the largest known volcanic eruption of the Holocene, the geological epoch that began roughly 11,700 years ago and continues today.
Although scientists know that giant caldera volcanoes can erupt more than once, they still have an incomplete understanding of how the enormous quantities of magma required for these events accumulate underground. This makes it extremely difficult to anticipate when another major eruption might occur.
“We must understand how such large quantities of magma can accumulate to understand how giant caldera eruptions occur,” says Kobe University geophysicist Nobukazu Seama.
Scientists Discover a Massive Magma Reservoir Beneath Japan
Kikai’s underwater location might seem like an obstacle to studying the volcano, but it actually gives researchers an important advantage. The surrounding seafloor allows scientists to carry out extensive geological surveys across the volcanic structure.
“The underwater location allows us to implement systematic, large-scale surveys,” Seama explains.
To investigate what lies beneath Kikai, researchers from Kobe University collaborated with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). They used powerful airgun arrays to generate seismic pulses that traveled through the crust beneath the ocean floor.
Instruments called ocean bottom seismometers recorded how these waves moved through the underground rock. Because seismic waves travel differently through solid rock and regions containing molten material, scientists can analyze their behavior to investigate structures that cannot be observed directly.
The findings, published in Communications Earth & Environment, revealed a substantial region containing a high proportion of magma directly beneath the site of Kikai’s massive eruption 7,300 years ago.
By examining the seismic measurements, the team was able to determine the approximate size and shape of the underground reservoir. Its position and extent suggest that it occupies the same magma storage system involved in 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.
Fresh Magma Is Recharging the Ancient Supervolcano
Perhaps the most intriguing finding is that the magma currently beneath Kikai does not appear to be simply leftover material from its catastrophic eruption thousands of years ago.
Previous investigations revealed that a new lava dome has been developing near the center of the caldera over approximately the past 3,900 years. A lava dome forms when relatively thick, viscous lava accumulates around a volcanic vent rather than flowing freely across the surrounding landscape.
Chemical analyses also showed that volcanic material from this dome and other more recent activity differs in composition from the material released during Kikai’s giant eruption.
That difference provides an important clue. It suggests that the magma now stored beneath the volcano was supplied by new injections of molten rock from deeper within Earth’s crust or mantle, rather than remaining unchanged since the previous eruption.
“This means that the magma that is now present in the magma reservoir under the lava dome is likely newly injected magma,” summarizes Seama.
Together, the seismic observations and chemical evidence help researchers develop a clearer picture of how a giant volcanic system can gradually rebuild its underground magma supply following a massive eruption.
This replenishment process does not mean that another catastrophic eruption is imminent. Instead, it reveals how a magma reservoir can be recharged over thousands of years, an important process that scientists need to understand before they can reliably assess the long-term behavior of supervolcanoes.
What the Discovery Could Reveal About Yellowstone
The findings could have implications far beyond Japan.
Researchers believe the process identified beneath Kikai may help explain why other enormous caldera systems, including Yellowstone and Toba, also contain large magma reservoirs relatively close to Earth’s surface.
“This magma re-injection model is consistent with the existence of large shallow magma reservoirs beneath other giant calderas like Yellowstone and Toba,” says Seama.
The proposed model suggests that, following a giant eruption, new magma can gradually enter and replenish an existing underground reservoir. Understanding how this happens could reveal important patterns in the long-term cycles of magma accumulation, volcanic activity, and major eruptions.
Researchers now hope to improve the seismic techniques used at Kikai so they can investigate the movement and storage of magma 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.”
By revealing how fresh magma accumulates beneath a volcano responsible for one of Earth’s most extraordinary eruptions, the research brings scientists closer to understanding the processes that eventually make such colossal events possible.
This research was funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) (The Third Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research)) and the Japan Society for the Promotion of Science (grant 20H00199). It was conducted in collaboration with researchers from the Japan Agency for Marine-Earth Science and Technology (JAMSTEC).
