A team of geoscientists from Kobe University recently uncovered evidence that a massive volcanic eruption that took place 7,300 years ago in southern Japan was the largest eruption to occur on Earth within the past 11,700 years. Their findings shed new light on mega-eruption dynamics and the influential role volcanoes have played in Earth’s climate history.

The research focused on the submerged caldera of the Kikai volcano, located near the southern tip of Japan’s Kyushu Island. Calderas are huge crater-like landforms created when a volcano partially collapses after releasing tremendous amounts of ejecta during an explosive eruption.

Besides lava, volcanoes also belch out enormous quantities of pumice, ash, and gases at rapid speeds in currents known as pyroclastic flows. The underwater sediments left behind by these flows around Kikai offered the scientists a valuable record of the volcano’s past behavior.

Using seismic reflection surveys and sediment sampling aboard the Fukae Maru research vessel, the team generated extremely detailed images of the sediments hundreds of meters below the seafloor. The high-resolution data allowed them to clarify the distribution, volume, and transport mechanisms of the volcanic ejecta.

They determined that the 7,300-year-old blast spewed out a staggering amount of volcanic materials over an area exceeding 4,500 square kilometers. With an equivalent dense rock volume estimate between 133-183 cubic kilometers, the event surpasses all other known volcanic eruptions worldwide within the Holocene epoch (the current geological period encompassing the last 11,700 years) in terms of size.

In analyzing the offshore sediments and nearby island deposits together, the researchers confirmed they originated from the same source.

Mapping the ash distribution surrounding the vent also shed light on interactions between the powerful pyroclastic surge and surrounding sea water.

Lead author Dr. Seama Nobukazu stresses the broader value of studying mega-eruptions, even though modern civilization has yet to experience one. Huge volcanic blasts shape sediment records we rely on to characterize past events, but erosional losses often make it difficult to accurately estimate their explosive intensities, he explains. Because giant caldera eruptions also impacted ancient climates and thus human history, unraveling eruption dynamics has societal importance.

From this point of view, it is fascinating to think that the event that created a caldera the size of a modern capital city was, in fact, the largest volcanic event since humans have been spread across the planet.


Sources

Kobe University | Satoshi Shimizu, Reina Nakaoka, et al., Submarine pyroclastic deposits from 7.3 ka caldera-forming Kikai-Akahoya eruption. Journal of Volcanology and Geothermal Research, doi.org/10.1016/j.jvolgeores.2024.108017


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