The Kikai Caldera Volcano, located 40km off the southern coast of Kyushu, is known to have caused a huge caldera eruption (Akahoya eruption) 7300 years ago, but the amount of ejecta that would have flowed into the ocean and how it was transported underwater is unknown. The process was not clear. This time, a research group from Kobe University carried out seismic reflection surveys to visualize the layered structure beneath the seafloor around the Kikai Caldera Volcano, and analyzed sediment samples taken from the seafloor. It was revealed that ejecta originating from pyroclastic flows was deposited.
Analysis of the ocean floor shows that ejecta from the Akahoya eruption covers a wide area of the ocean floor around Kikai Caldera Volcano, and the layer thickness decreases exponentially with distance from the caldera, and is deposited to fill the unevenness of the ocean floor. That's what I found out. This is thought to be because a large pyroclastic flow released from a caldera eruption plunged into the ocean, mixed with seawater, formed a dilute density current, and was deposited on the sea floor as it traveled more than 40km underwater. It is said that it will be done. This is the first study to show how a pyroclastic flow behaves when it enters the ocean at sufficient depth.
The total amount of ejecta produced by the pyroclastic flow, estimated from changes in the distribution and thickness of the deposits, was more than 71 cubic kilometers. When combined with estimates of the amount of volcanic ash deposited in a wide area in Japan, the amount amounts to more than 332 to 457 cubic kilometers. These results indicate that the Akahoya eruption was the largest eruption on Earth during the Holocene, from 11700 years ago to the present.
It is known that part of the pyroclastic flow from the Akahoya eruption crossed the ocean and caused devastating damage to the Jomon civilization in southern Kyushu, and if such a huge eruption were to occur again in modern civilization, it would be devastating to humanity and society. It poses an immeasurable threat. The findings obtained in this study are expected to contribute to elucidating the processes and mechanisms of giant eruptions that may occur in the near future.
Paper information:【Journal of Volcanology and Geothermal Research】Submarine pyroclastic deposits from 7.3 ka caldera-forming Kikai-Akahoya eruption
