The Quaternary Research (Daiyonki-Kenkyu)

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We established the stratigraphy of rhyolite tephra beds in Toshima Island between 30-10 cal ka BP, and correlated them to those in the Izu-Oshima and Niijima Islands based on the mafic mineral composition, refractive index, and chemical composition of glass shards. Five rhyolite tephra beds (Tos1 to Tos5 in ascending stratigraphic order) are found from the volcanic ash soil and organic soil in Toshima. Tos1, Tos3, Tos4, and Tos5 are correlated to Kz-CbA, a unit of Nj-AkG, Od-1, and Nj-Mt, respectively. Tos2 (Iz-Tos2) consists of two units; upper vitric ash bed and lower lapilli bed. Iz-Tos2 is correlated to the upper part of O86 member in Izu-Oshima. Although the source volcano of Iz-Tos2 remains unclear, its chemical composition and refractive index of glass shards imply that Iz-Tos2 was scattered from a volcano in the Izu Islands. The ¹⁴C ages and stratigraphy indicate that the eruption age of Iz-Tos2 is between 30-19 cal ka BP, around the Last Glacial Maximum. The eruption which produced Iz-Tos2 probably occurred on the former terrestrial volcano(es) corresponding to the island shelves or the tops of the submarine hills around Toshima at present.

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