大島火山の地質と岩石

抄録

O-sima rises from a submarine ridge extending from the Izu Peninsula, central Honsyu, to Mariana Arc. In the vicinity of O-sima, the ridge is probably made up of older Miocene volcanic rocks (Yugasima Group). Basalt lava and pyroclastic rocks probably of Pliocene age lie on the Miocene volcanic rocks and form the direct basement of O-sima Volcano. These rocks are now exposed in two separate areas, one on the northern coast of the island and another on the southeastern, and are named the Okata Basalt Group and the Hude-sima Basalt Group respectively. The Pliocene rocks are tholeiitic olivine basalt having compositions more mafic than the lavas of the volcano. O-sima is a flat stratovolcano with a summit caldera within which lies an active cone Mihara-yama. The main body is composed of thin flows of olivine-bearing tholeiitic basalt and pyroclastic layers (Somma lava). There are more than ten parasitic spatter cones, scoria cones, and pit craters arranged roughly in three narrow zones radiating from the center of the main body; namely from the center to NW, NE, and SE. Probably they represent rift zones comparable to those found in Huzi Volcano and Hawaiian volcanoes. The eruption of the Somma lava started probably in late Pleistocene or early Holocene and closed with the formation of the caldera which took place within the last one or two thousand years. Mihara-yama is also a stratovolcano with a summit crater which has been active during the historic time. The Mihara-yama lava (Central Cone lava) is tholeiitic pyroxene basalt free from olivine, representing a more advanced stage of fractionation of basaltic magma than the Somma lava.