Geology of Minami-Iwo Volcano, Izu-Bonnin Islands, Japan

Abstract

Minami-Iwo volcano is located at the southern end of Izu-Bonnin volcanic chain, 1300km south of Tokyo. It consists of three major volcanic edifices; older volcano I (OV1), older volcano II (OV2), and younger volcano (YV). At the boundaries between OV1 and OV2 and OV2 and YV, pyroclastic materials are characteristically observed; scoria fall deposit between OV1 and OV2, and scoria fall deposit and pyroclastic flow deposit between OV2 and YV. Older volcanoes, OV1 and OV2, are wholly composed of lava flows with both aa and pahoehoe surface textures. Younger volcano (YV), on the other hand, consists mainly of scoriaceous agglutinate which made up the steep slope of the main Minami-Iwo volcano. The older volcanoes are cut by numerous dikes (115 all told) which are dominant at the south and southwest sea cliffs. Four specimens collected from OV1, OV2 and dikes show normal thermoremnant magnetic orientation suggesting the oldest age less than a few hundred thousand years. Rocks of Minami-Iwo volcano are ankaramite, olivine basalt, augite-olivine basalt and plagioclase phyric basalt. The petrographic features and the chemical composition show that the rocks of Minami-Iwo volcano are of alkali rock series. The basalt of alkali rock affinity is the first discovery for volcanoes located at the volcanic front in Japan.