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Hachijo Island, located on the volcanic front of the Izu-Ogasawara (Bonin) arc, consists of two stratovolcanoes : Nishiyama and Higashiyama. Nishiyama (Alt. 854.3 m) is a scarcely dissected cone called “Hachijo-fuji”. Nishiyama began its volcanic activities about 10, 000 years ago. Many lateral volcanoes exist around Nishiyama, and one of them, Kandoyama (Alt. 194.9 m) is a tuff cone which was formed by a phreatomagmatic eruption. This study discusses the stratigraphy of deposits, and the eruptive types and the eruption ages of Nishiyama after the formation of Kandoyama. The results are summarized as follows.1) This study names four air-fall pyroclastic materials “Ny1-4”, which erupted from Nishiyama after the pyroclastic surge eruption of Kandoyama (ca. 2, 500 y.BP). All of them are scoriaceous.2) Ny2 erupted from a lateral volcano on the west of Kandoyama. This scoria cone is composed of agglutinate.3) Ny3 erupted from the summit crater of Nishiyama ca. 1, 100 y.BP. Ny3 was widely distributed on the southeast flank of Nishiyama, but it did not reach the foot of Higashiyama. Prior to the eruption of Ny3, a large amount of lava flowed down the southeast flank of Nishiyama.4) Ny4 erupted from a crater row along the southeast side of Nishiyama. From the description found in archives, it is clear that the eruption of Ny4 corresponds to A.D. 1605 (Keicho 10th) eruption. Ny4 deposited around the vents as agglutinate, and air-fall materials were widely distributed on the southeast side of Nishiyama. However, the total volume of Ny4 was the same or less than that of the Miyakejima 1983 eruption and the Izu-Oshima 1986 eruption.5) There are descriptions in the archives of at least four eruptions and about ten occurrences of earthquakes and tsunami in and around Hachijo Island. The fissures and tsunami deposits which followed historic earthquakes were found at the Yaene site located on the south side of Nishiyama. The pyroclastic materials from Nishiyama date these past earthquakes.

Stratigraphy and Eruption Ages of Deposits at the Southeast Side of Nishiyama Volcano, Hachijo Island during the Last 2, 500 Years