1:45 PM - 3:15 PM
[SVC34-P12] Discovery of drift pumice clasts originating from Nishinoshima at Sanohama, Izu-Oshima
Keywords:Fukutoku-Oka-no-Ba, Nishinoshima, pumice, 2021, Izu-Oshima
The eruption of Fukutoku-Oka-no-Ba (FOB), a submarine volcano in the Ogasawara Islands on August 13, 2021, provided a large amount of drift pumices (DPs). A group of pumice clasts drifted like a raft on the countercurrent of the Kuroshio Current, and in October 2021, they washed ashore on the Daito Islands, Amami Island, and the Okinawa Islands. From late November onwards, DPs were reported to have washed ashore in the Kanto region, the Izu Islands, and overseas such as Taiwan and the Philippines. Various types of pumice were found in the pumice clasts thought to have originated from the 2021 eruption of FOB, which was washed ashore on the main island of Okinawa in the early stage (Ishimura et al., 2022). Most of the pumice washed ashore was well-foamed gray pumice which were characterized as trachyte (Yoshida et al., 2022). Main gray pumice includes black enclaves (Yoshida et al., 2022), in addition to clinopyroxene and plagioclase and a small amount of olivine phenocrysts.
On the afternoon of November 17, 2021, during tracking survey of DPs in various places, a substantial amount of pumices washed ashore on the southeastern end of Sanohama, the southwestern part of Izu-Oshima Island. We measured roundness for all pumices collected and compared it to pumices washed ashore on Toi, Shizuoka. Furthermore, we picked up 15 pumices from DPs at Sanohama and DPs of Yo Coasts on Amami Islands which was correlated to DPs from the eruption of FOB and analyzed them by the colorimeter and electron probe micro analyzer (EPMA). Color indicated that DPs at Sanohama were clearly different from that of FOB. In addition, DPs at Sanohama contain orthopyroxene in addition to clinopyroxenes and lack olivine, and the major element composition of the matrix is dacite, indicating that it is not pumice originating from FOB (Aoki et al., 2022).
In order to identify the source volcanoes of these pumice, we selected 4 pumices from the 15 pumice clasts mentioned above, and performed whole-rock chemical composition analysis using XRF (major element) and ICP-MS (trace element). As a result, it is highly likely that these pumice clasts originated from Nishinoshima belonging to the Ogasawara islands (Tamura et al., 2018).
Nishinoshima has continued to erupt intermittently since November 2013, and most of the ejecta have been believed to be lava, but volcanic ash fall were also observed during the eruption of June 2020 (Yanagisawa et al., 2020; Maeno et al., 2021). No eruptive activity was observed for some time after June 2020, but on August 14, 2021, the day after the 2021 eruption of FOB, it was observed to erupt again for the first time in about a year. There have been no reports that the ejecta produced by this eruption drifted over a wide area of the ocean. The pumice washed ashore on Sanohama on November 17, 2021 that presumed to have originated from Nishinoshima, indicated that the eruptive activity accompanied by pumice eruption would occurred on Nishinoshima, and a certain amount of pumice would be flowed out from there.
Hiramine et al. (2020) pointed out that pyroclastic materials deposited on land flow into sea areas through various processes and that pumice is washed ashore even in normal times when there are no eruptive events. It is necessary to consider the possibility that the pumice at Sanohama may originate from the deposited pumice layer on Nishinoshima collapsed due to the last eruptive activity and flowed into the ocean in addition to the possibility of the production by the eruption on August 14, 2021.
References
Ishimura et al., (2022) Chiri; Yoshida et al. (2022); Island Arc; Aoki et al.(2022) JpGU abstract; Tamura et al. (2018) Island Arc; Yanagisawa et al. (2020) Bull. Volcano. Soc. Japan; Maeno et al. (2021) F. Earth Sci.; Hiramine et al., (2020) Proceedings of the General Meeting of the Association of Japanese Geographers.
On the afternoon of November 17, 2021, during tracking survey of DPs in various places, a substantial amount of pumices washed ashore on the southeastern end of Sanohama, the southwestern part of Izu-Oshima Island. We measured roundness for all pumices collected and compared it to pumices washed ashore on Toi, Shizuoka. Furthermore, we picked up 15 pumices from DPs at Sanohama and DPs of Yo Coasts on Amami Islands which was correlated to DPs from the eruption of FOB and analyzed them by the colorimeter and electron probe micro analyzer (EPMA). Color indicated that DPs at Sanohama were clearly different from that of FOB. In addition, DPs at Sanohama contain orthopyroxene in addition to clinopyroxenes and lack olivine, and the major element composition of the matrix is dacite, indicating that it is not pumice originating from FOB (Aoki et al., 2022).
In order to identify the source volcanoes of these pumice, we selected 4 pumices from the 15 pumice clasts mentioned above, and performed whole-rock chemical composition analysis using XRF (major element) and ICP-MS (trace element). As a result, it is highly likely that these pumice clasts originated from Nishinoshima belonging to the Ogasawara islands (Tamura et al., 2018).
Nishinoshima has continued to erupt intermittently since November 2013, and most of the ejecta have been believed to be lava, but volcanic ash fall were also observed during the eruption of June 2020 (Yanagisawa et al., 2020; Maeno et al., 2021). No eruptive activity was observed for some time after June 2020, but on August 14, 2021, the day after the 2021 eruption of FOB, it was observed to erupt again for the first time in about a year. There have been no reports that the ejecta produced by this eruption drifted over a wide area of the ocean. The pumice washed ashore on Sanohama on November 17, 2021 that presumed to have originated from Nishinoshima, indicated that the eruptive activity accompanied by pumice eruption would occurred on Nishinoshima, and a certain amount of pumice would be flowed out from there.
Hiramine et al. (2020) pointed out that pyroclastic materials deposited on land flow into sea areas through various processes and that pumice is washed ashore even in normal times when there are no eruptive events. It is necessary to consider the possibility that the pumice at Sanohama may originate from the deposited pumice layer on Nishinoshima collapsed due to the last eruptive activity and flowed into the ocean in addition to the possibility of the production by the eruption on August 14, 2021.
References
Ishimura et al., (2022) Chiri; Yoshida et al. (2022); Island Arc; Aoki et al.(2022) JpGU abstract; Tamura et al. (2018) Island Arc; Yanagisawa et al. (2020) Bull. Volcano. Soc. Japan; Maeno et al. (2021) F. Earth Sci.; Hiramine et al., (2020) Proceedings of the General Meeting of the Association of Japanese Geographers.