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[SVC30-06] What have been erupted from the 2020 eruption of Nishinoshima?
Keywords:violent stronbolian eruption, andesite magma, bimodal volcanism
Nishinoshima, a submarine volcano in the Ogasawara Arc, erupted in 2013 after 40 years of dormancy. Andesites erupted from Nishinoshima that were produced in the recent history of the volcano have been generated by olivine fractionation of primary andesitic magmas (Tamura et al., 2018). These primary andesitic magmas are derived from the partial melting of hydrous mantle (plagioclase peridotites) at relatively low pressures because the lithosphere is currently thin in this region of the Ogasawara arc. A “stockpile” of continental crust (andesitic magma) might be produced when most crust was thin (Tamura et al., 2016).
The eruption mode of Nishinoshima changed from a Strombolian characterized by effusive andesite lava flows to a violent Strombolian in the middle of June, 2020. The eruption plume reached a maximum altitude of ~8 km on July 4th. Does the sudden and drastic change of the eruption styles suggest some advance warning of a large volume violent eruption in the near future?
Caldera-forming eruptions sometimes happen in oceanic arcs (e.g. Yuasa et al., 1991). Sumisu-jima in the Izu arc and West Rota volcano in the Mariana arc, for example, have large submarine calderas of ~10 km in diameter, which have been resulted from a large volume eruption of pumices. The alternations of black and white tephra layers were observed by using submersibles and ROVs along the sections of caldera walls just below the top of caldera walls consisting of large pumice layers (Tamura et al, 2005; Shukuno et al., 2006; Tani et al., 2008; Stern et al., 2008). Thus, these violent caldera-forming eruptions were preceded by bimodal volcanisms. Had the 2020 eruption of Nishinoshima been bimodal in composition or not?
Volcaniclastic rocks erupted in 2020 were sampled on two separate occasions from December 2020 to January 2021. Samples at water depths between 1,000 m and 1,500 m were collected by a Box Corer using R/V Kairei during KR20-E06 from December 15th to December 29th, 2020. Fresh scorias (Fig. 1) and pumices (Fig. 2) were collected by the Box Corer. Northern submarine flank of Nishinoshima, ~15 km north of the island at the water depths of ~2,500 m, has been observed and sampled using the remotely operate submersible HyperDolphin and the R/V Shinseimaru during KS-21-2 from January 21st to January 30th, 2021. Small faults on the surface of submarine flank showed outcrops, suggesting bimodal volcanism of the recent eruption (Fig. 3), in which white layers consist of dacitic pumices having 67 % SiO2.
The 2020 violent Strombolian eruption of Nishinoshima was a bimodal volcanism consisting of basaltic andesite and dacite. It is possible that the magma system of Nishinoshima has changed from andesitic to basalt-dacite bimodal volcanisms, so it is essential to monitor the future course of the activity from the view point of disaster reduction.
The eruption mode of Nishinoshima changed from a Strombolian characterized by effusive andesite lava flows to a violent Strombolian in the middle of June, 2020. The eruption plume reached a maximum altitude of ~8 km on July 4th. Does the sudden and drastic change of the eruption styles suggest some advance warning of a large volume violent eruption in the near future?
Caldera-forming eruptions sometimes happen in oceanic arcs (e.g. Yuasa et al., 1991). Sumisu-jima in the Izu arc and West Rota volcano in the Mariana arc, for example, have large submarine calderas of ~10 km in diameter, which have been resulted from a large volume eruption of pumices. The alternations of black and white tephra layers were observed by using submersibles and ROVs along the sections of caldera walls just below the top of caldera walls consisting of large pumice layers (Tamura et al, 2005; Shukuno et al., 2006; Tani et al., 2008; Stern et al., 2008). Thus, these violent caldera-forming eruptions were preceded by bimodal volcanisms. Had the 2020 eruption of Nishinoshima been bimodal in composition or not?
Volcaniclastic rocks erupted in 2020 were sampled on two separate occasions from December 2020 to January 2021. Samples at water depths between 1,000 m and 1,500 m were collected by a Box Corer using R/V Kairei during KR20-E06 from December 15th to December 29th, 2020. Fresh scorias (Fig. 1) and pumices (Fig. 2) were collected by the Box Corer. Northern submarine flank of Nishinoshima, ~15 km north of the island at the water depths of ~2,500 m, has been observed and sampled using the remotely operate submersible HyperDolphin and the R/V Shinseimaru during KS-21-2 from January 21st to January 30th, 2021. Small faults on the surface of submarine flank showed outcrops, suggesting bimodal volcanism of the recent eruption (Fig. 3), in which white layers consist of dacitic pumices having 67 % SiO2.
The 2020 violent Strombolian eruption of Nishinoshima was a bimodal volcanism consisting of basaltic andesite and dacite. It is possible that the magma system of Nishinoshima has changed from andesitic to basalt-dacite bimodal volcanisms, so it is essential to monitor the future course of the activity from the view point of disaster reduction.