Japan's Kikai Caldera Magma Reservoir Refilling with Fresh Magma 7,300 Years After Major Eruption

Scientists have determined that a large magma reservoir beneath Japan’s Kikai caldera is slowly accumulating fresh melt thousands of years after one of the largest volcanic eruptions of the Holocene epoch. The findings come from a seismic refraction survey led by researchers at Kobe University in collaboration with the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), published in Communications Earth & Environment (nature.com).

Kikai caldera, located mostly underwater south of Kyushu, produced the Kikai-Akahoya eruption approximately 7,300 years ago. That event ejected an estimated 133–183 cubic kilometers of dense-rock-equivalent material, making it probably the largest Holocene eruption, according to the study and related reporting from Kobe University (kobe-u.ac.jp). The eruption left a broad caldera depression after the overlying rock collapsed into the emptied reservoir.

Using airgun arrays to generate seismic pulses and 39 ocean-bottom seismometers along a 175-kilometer survey line, the team imaged the subsurface structure. They identified a pronounced low-velocity anomaly indicating a large magma-rich region at depths of 2.5 to 6 kilometers directly beneath the caldera. In cross-section the reservoir appears roughly trapezoidal, with a width at least matching the inner caldera, and a melt fraction estimated at 3–6 percent (up to about 10 percent), as detailed in the paper (nature.com). One volume estimate for the structure is around 220 cubic kilometers.

“Due to its extent and location it is clear that this is in fact the same magma reservoir as in the previous eruption,” said Kobe University geophysicist Nobukazu Seama, according to the university’s announcement and ScienceDaily coverage (sciencedaily.com).

Importantly, the magma now present is not simply residual material from the 7,300-year-old event. A central lava dome has been forming inside the caldera for the past approximately 3,900 years, with a volume exceeding 32 cubic kilometers. Chemical analyses show that material from this dome and other recent activity differs in composition from the magma ejected during the giant eruption. “This means that the magma that is now present in the magma reservoir under the lava dome is likely newly injected magma,” Seama summarized (scitechdaily.com).

The researchers propose a melt re-injection model in which new magma from deeper sources gradually replenishes the same shallow reservoir that fueled the earlier caldera-forming eruption. This process is consistent with the presence of large shallow magma reservoirs observed beneath other giant calderas, including Yellowstone in the United States and Toba in Indonesia, Seama noted (sciencedaily.com).

The study emphasizes that the observed recharge does not indicate an imminent major eruption. Instead, it illustrates how giant caldera systems can rebuild magma supplies over millennia following catastrophic events. The team aims to refine the seismic methods to better monitor re-injection processes and potential indicators of future large eruptions. The research was supported by Japan’s Ministry of Education, Culture, Sports, Science and Technology and the Japan Society for the Promotion of Science.