5:15 PM - 7:15 PM
[SVC36-P07] Temporal changes of drift pumice on the beaches associated with the 2021 Fukutoku-Oka-no-Ba eruption
Keywords:Fukutoku-Oka-no-Ba, drift pumice, organism attachment, roundness
A submarine volcanic eruption occurred at Fukutoku-Oka-no-Ba (FOB) in the Izu–Bonin arc on August 13, 2021. Pumice associated with that eruption drifted in the ocean and washed ashore all over Japan in one year (Yoshida et al., 2022; Ishimura and Hiramine, 2025). Although there was little direct damage from this eruption, damage caused by this drift pumice occurred mainly in the coastal areas of the Nansei Islands. In particular, the impact on ports, ship navigation, and aquaculture was significant, with the volume recovered in Okinawa Prefecture alone amounting to 110,000 m3 (Okinawa Prefecture HP). Ishimura and Hiramine (2025) estimated that the mass of drift pumice that drifted ashore along the beaches in the Nansei Islands was very small, less than 1% of the total mass of the ejecta, which is consistent with the results of pumice drift simulations (Nishikawa et al., 2023). Ishimura and Hiramine (2025) also reported that the mass of drift pumice on the beaches decreased with time after the start of washing ashore, indicating rapid dispersion at sea. Thus, the dispersion of drift pumice along the time sequence from the eruption was clearly observed in the case of the 2021 FOB eruption.
On the other hand, the internal structure of the 2021 FOB pumice (Takeuchi et al., 2024) and the results of a floating experiment using the pumice (Ishimura and Hiramine, 2025) indicate that it continues to float for a long time. Therefore, how the mass and size of drift pumice change in a given location needs to be examined for preparing the future pumice drifting event. In addition, from geomorphological and geological perspectives, the change over time of drift pumice on beaches and the preservation process of drift pumice into sediments can be used as a modern analogue when considering the depositional environment at that time.
In this study, the 2021 FOB pumice samples were collected from Ishigaki Island, Okinawa Island, Amami-Oshima Island, and Kikai Island in the Nansei Islands, where a large mass of pumice drifted ashore following the 2021 FOB eruption, in October-December 2021, May-June 2022, October 2022, October 2023, and October 2024. The mass, size, shape, and attached organisms were described. We have now completed analysis of samples through October 2023, and will report a summary of results through October 2024 in the presentation.
On the other hand, the internal structure of the 2021 FOB pumice (Takeuchi et al., 2024) and the results of a floating experiment using the pumice (Ishimura and Hiramine, 2025) indicate that it continues to float for a long time. Therefore, how the mass and size of drift pumice change in a given location needs to be examined for preparing the future pumice drifting event. In addition, from geomorphological and geological perspectives, the change over time of drift pumice on beaches and the preservation process of drift pumice into sediments can be used as a modern analogue when considering the depositional environment at that time.
In this study, the 2021 FOB pumice samples were collected from Ishigaki Island, Okinawa Island, Amami-Oshima Island, and Kikai Island in the Nansei Islands, where a large mass of pumice drifted ashore following the 2021 FOB eruption, in October-December 2021, May-June 2022, October 2022, October 2023, and October 2024. The mass, size, shape, and attached organisms were described. We have now completed analysis of samples through October 2023, and will report a summary of results through October 2024 in the presentation.