13:45 〜 14:00
[SCG54-11] August 2021 eruption of Fukutoku-Oka-no-Ba Volcano, Izu-Bonin Arc: Understanding the eruption and tephra dispersal mechanisms from rapid response seafloor surveys
★Invited Papers
The Fukutoku-Oka-no-Ba (FOB) volcano is located at the southern end of the Izu-Bonin Arc and erupted on August 13, 2021, for the first time in 11 years, as confirmed by observations from the Himawari-8 weather satellite and the Japan Coast Guard aircraft. During this event, the eruption column soared to over 16 km, reaching the tropopause. Satellite imagery further revealed a concurrent underwater discoloration beneath the eruption column and the emergence of pumice raft on the sea surface. Subsequent aerial observations on August 15 unveiled the formation of a new island approximately 1 km in diameter, alongside the northwest drift of the pumice raft. The eruption persisted for three days, concluding with Surtseyan-style activity.
Post-eruption, we conducted two rapid response research cruises in 2022 using the R/V SHINSEIMARU operated by JAMSTEC. The KS-22-5 cruise, scheduled from April 12 to 23, aimed to gather sediment samples through K-grab and gravity corers, along with rock samples using dredge, to examine volcanic deposits associated with the August 2021 eruption. However, Typhoon No. 1 disrupted the expedition, limiting our stay in the survey area to approximately 15 hours instead of the planned 5 days. Nonetheless, we managed to conduct K-grab samplings at eight sites west of the 2021 vent, collecting volcanic deposits from the eruption.
The KS-22-13 cruise, scheduled between August 25 and September 3, 2022, conducted four successful dives (Dives 2183 to 2186) utilizing the ROV Hyperdolphin 4500 to explore the seafloor of the FOB and its vicinity, and to collect rock, sediment, and benthos samples. During Dive 2183, fine volcanic ash and reddish pumice clasts were retrieved from the seafloor 18 km west of the 2021 vent, corresponding to the area directly beneath the Plinian eruption column and umbrella cloud formed on August 13. These samples likely originate from this phase of the eruption.
Comparisons of bathymetric data collected around the FOB, obtained by the Japan Coast Guard in 1999 and from the KS-22-5 and -13 cruises, revealed no significant topographical changes exceeding 10 m before and after the 2021 eruption. Detailed geochemical and petrological analyses of seafloor deposits indicated systematic differences in textures and chemical compositions compared to the pumice raft, underscoring the importance of collecting erupted materials near the vent for a comprehensive understanding of the eruption. Furthermore, it has become evident that the distribution of volcanic deposits on the seafloor generally corresponds to the downwind area of the eruption column and discolored waters formed during the Plinian eruption, suggesting significant quantities of juvenile magmatic materials within the eruption column.
Post-eruption, we conducted two rapid response research cruises in 2022 using the R/V SHINSEIMARU operated by JAMSTEC. The KS-22-5 cruise, scheduled from April 12 to 23, aimed to gather sediment samples through K-grab and gravity corers, along with rock samples using dredge, to examine volcanic deposits associated with the August 2021 eruption. However, Typhoon No. 1 disrupted the expedition, limiting our stay in the survey area to approximately 15 hours instead of the planned 5 days. Nonetheless, we managed to conduct K-grab samplings at eight sites west of the 2021 vent, collecting volcanic deposits from the eruption.
The KS-22-13 cruise, scheduled between August 25 and September 3, 2022, conducted four successful dives (Dives 2183 to 2186) utilizing the ROV Hyperdolphin 4500 to explore the seafloor of the FOB and its vicinity, and to collect rock, sediment, and benthos samples. During Dive 2183, fine volcanic ash and reddish pumice clasts were retrieved from the seafloor 18 km west of the 2021 vent, corresponding to the area directly beneath the Plinian eruption column and umbrella cloud formed on August 13. These samples likely originate from this phase of the eruption.
Comparisons of bathymetric data collected around the FOB, obtained by the Japan Coast Guard in 1999 and from the KS-22-5 and -13 cruises, revealed no significant topographical changes exceeding 10 m before and after the 2021 eruption. Detailed geochemical and petrological analyses of seafloor deposits indicated systematic differences in textures and chemical compositions compared to the pumice raft, underscoring the importance of collecting erupted materials near the vent for a comprehensive understanding of the eruption. Furthermore, it has become evident that the distribution of volcanic deposits on the seafloor generally corresponds to the downwind area of the eruption column and discolored waters formed during the Plinian eruption, suggesting significant quantities of juvenile magmatic materials within the eruption column.