八丈西山火山のマグマの分化

概要

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Hachijo-Nishiyama (meaning of the West Mountain) is an active composite volcano situated, next to another Pleistocene composite volcano Higashiyama (meaning of the East Mountain), on the Hachijojima Island, 300 km south of Tokyo. These two volcanoes stand on the volcanic front along the Izu-Mariana arc. Since the beginning stage of Nishiyama, ca. 10,000 yBP, the volcano repeatedly erupted in a shallow submarine environment. The explosively emitted pyroclastic materials covered almost whole the island. By 3,000 yBP, Nishiyama had grown as a subaerial volcano and lavas and fallout scoria were erupted without considerable effects of water. In this study, we presented petrographical and petrochemical descriptions of the lavas of Nishiyama in the latest 3,000 years. Based on these data, we discussed the differentiation processes of Nishiyarna volcano and the genetic relationship between the magma of Nishiyama and Higashiyama. Nishiyama volcano is totally composed of basalt to basaltic andesite with ubiquitous plagioclase, with or without mafic phenocryst minerals such as olivine, clinopyroxene, orthopyroxene, and titanomagnetite. In terms of SiO_2-Al_2O_3 relationship and amount of phenocrysts, we divide the differentiation trends of Nishiyama into three groups. P-group, A-group and T-group.
特定非営利活動法人日本火山学会の論文
2002-05-10