16:00 〜 16:15
[SVC34-13] 2021年福徳岡ノ場噴火から約1年間の記録―漂着軽石の量、粒径、粒子形状、付着生物に着目して―
キーワード：2021年福徳岡ノ場噴火、漂着軽石、海底火山、円磨度
On 13 August 2021, an eruption occurred at Fukutoku-Oka-no-Ba (FOB), a submarine volcano in the Ogasawara Islands (Bonin Islands). The eruption lasted for three days, with an estimated volcanic explosivity index of 4 and 0.1-0.3 km^3 of drifting pumice (Maeno et al., 2022). Pumice drift associated with submarine volcanic eruptions has been recorded worldwide and is suggested to contribute to the spread of marine organisms (e.g., Bryan et al. 2012). On the other hand, large-scale pumice drift in the present could be a disaster, causing damage to ship navigation, port facilities, fishery, and tourism. Trace of such pumice rafts is possible if the area of the pumice raft is large enough to be seen on satellite images. However, once they have dispersed to some extent, we cannot trace them, and it is difficult to track them by the time they drift ashore.
In this study, therefore, we conducted a tracking of the 2021 FOB drift pumice mainly on Japanese Islands and collected it on the coasts for about one year from the eruption. We measured the amount, size (20 pumice clasts from the largest in each site), and particle shape (roundness (R) defined by Wadell (1932)) of FOB pumice that drifted ashore. As a result, we found that the amount and size of drift pumice decreased rapidly with time, indicating offshore diffusion and size decrease due to breaking and abrasion in pumice raft on the sea. Comparing the roundness of artificially crushed FOB pumice and the sample collected 9 days after the eruption by Japan Meteorological Agency with those from Daito Islands, where the pumice first drifted ashore, we can see a large increase in R from references to samples in the islands. This is considered to be due to the predominance of rounding as the pumices collided with each other during the two months on the sea in the initial stage. Thereafter, no clear difference in R was observed one year after the event. This indicates that a certain degree of R was reached rapidly during the drift, which could be one indicator of drift pumice. Furthermore, as pointed out in previous studies (Naya et al. 2021; Fujita 2022; Mizuyama and Fujita 2022; Ohno et al. 2022), sessile organisms was also recognized on the 2021 FOB pumice and is expected to contribute to the spread of marine organisms. In the short time from the eruption, the most abundant species were goose barnacles, bryozoans, and rarely bivalves. In addition, corals, balanomorpha, algae, and calcareous tube-dwelling polychaetes were found on drift pumice since June 2022. However, such changes in attachment ratio and species are expected to be complicated by sea currents, surface conditions of the pumice, number of collisions on the sea, season, and other factors, and thus require more detailed discussion.
In this study, therefore, we conducted a tracking of the 2021 FOB drift pumice mainly on Japanese Islands and collected it on the coasts for about one year from the eruption. We measured the amount, size (20 pumice clasts from the largest in each site), and particle shape (roundness (R) defined by Wadell (1932)) of FOB pumice that drifted ashore. As a result, we found that the amount and size of drift pumice decreased rapidly with time, indicating offshore diffusion and size decrease due to breaking and abrasion in pumice raft on the sea. Comparing the roundness of artificially crushed FOB pumice and the sample collected 9 days after the eruption by Japan Meteorological Agency with those from Daito Islands, where the pumice first drifted ashore, we can see a large increase in R from references to samples in the islands. This is considered to be due to the predominance of rounding as the pumices collided with each other during the two months on the sea in the initial stage. Thereafter, no clear difference in R was observed one year after the event. This indicates that a certain degree of R was reached rapidly during the drift, which could be one indicator of drift pumice. Furthermore, as pointed out in previous studies (Naya et al. 2021; Fujita 2022; Mizuyama and Fujita 2022; Ohno et al. 2022), sessile organisms was also recognized on the 2021 FOB pumice and is expected to contribute to the spread of marine organisms. In the short time from the eruption, the most abundant species were goose barnacles, bryozoans, and rarely bivalves. In addition, corals, balanomorpha, algae, and calcareous tube-dwelling polychaetes were found on drift pumice since June 2022. However, such changes in attachment ratio and species are expected to be complicated by sea currents, surface conditions of the pumice, number of collisions on the sea, season, and other factors, and thus require more detailed discussion.