Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XIX, No.4


The 1741-1742 Activity of Oshima-Ōshima Volcano, North Japan

Katsui, Yoshio;Yamamoto, Masatsugu

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Oshima-Ōshima, a small volcanic island in the Japan Sea, had been in one cycle of eruption during 50 years from 1741 to 1790, after about 1,500 years of dormancy. On about August 18, 1741, the activity started with ejection of andesitic pumice and ash, which was followed by eruption of basaltic tephra. In the early morning on August 29, a violent eruption occurred. Simultaneously a great earthquake (M>7.5) is estiamated to have occurred in the sea bottom near Oshima-Ōshima, which generated a destructive tsunami and triggered a sector collapse of the volcanic body of Nishi-yama. Then, within the new horseshoe-shaped caldera a central cone was produced by eruption of pyroclastics and lavas of basalt. After the intense eruption in 1741-1742, the activity much decreased. A compositional change in reverse order, from andesitic to basaltic, is evident within a short period of eruption, which can be interpreted in terms of a zoned magma chamber.