0100487275

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Accepted Manuscript

Submarine pyroclastic deposits from 7.3 ka caldera-forming Kikai-Akahoya eruption

English (英語)

Journal of Volcanology and Geothermal Research

448

108017

Elsevier

2024-02-01

2026-02-01

Extensive submarine pyroclastic deposits were discovered around the Kikai caldera, Southwest Japan, which originated from the caldera-forming Kikai-Akahoya (K-Ah) eruption that occurred at 7.3 ka. This submarine pyroclastic unit originated from a pyroclastic density current (PDC) that entered the sea and was transformed into a subaqueous density current. Our high-resolution marine seismic reflection surveys reveal that the uppermost unit exhibiting prominent seismic facies thins radially away from the caldera and covers >4500 km² of the seafloor. The estimated volume of the submarine deposits is >71 km³ based on the isopach map. Seismic sequence stratigraphic analyses and correlation of shard glass compositions from the piston core and remotely operated vehicle samples with those from subaerial outcrops identified the unit as pyroclastic deposits produced by the K-Ah eruption. The geomorphological features of the unit indicate a long runout distance (i.e., 40 km), exponential thinning with increasing distance from the source, and a depression-filling nature of the deposits suggesting that the unit was formed predominantly by a density current that entered the water. The quantitative comparisons of geometric shape and its thinning rate, including that for non-volcanic density currents, indicate that the subaerial PDC entering into the water was transported as a dilute density current in the water. We propose that the formation of these dilute density currents transformed by PDCs probably requires sufficient water depth, as inferred also from previous analog experiments. Adding on-land ignimbrite of 5 km³ and widespread tephra of 249–374 km³, our estimate of submarine pyroclastic deposits of >71 km³ leads to a total bulk volume of the K-Ah eruption of 332–457 km³ (133–183 km³ in DRE), making it probably the largest eruption of the Holocene.

Seismic reflection

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