Development of the Oshima Island

Abstract

Under the Oshima Volcano sits ruin of basaltic volcano named Okata basalts. Under the Okata basalts, it is believed to exist miocene volcanic complex (Yugashima Series) as the fragments of altered andesites and gabbro are contained in the tuff breccia. The Okata basalts is composed in pliocene epoch as strato-volcano. After it is folded, faulted and eroded deeply, the activity of the Oshima Volcano began covering the reef of ruined Okata basalts exposed on the surface of the pacific ocean in quaternary epoch.The volcanism succeeded violently intercalating peaceful time, when the pre-historic man lived at the sea shore, and at last, after the flow out of large quantities of lava, caulderon subsidence of the top of the volcano occurred. Before the subsidence, fault in the direction of N-S was formed along the east coast of the island, which is now represented as cliff of about 100 m in height. And also, accompanying the subsidence, fault was formed at the north part of the caldera in the direction of NE-SW to obscure the oval form of the caldera.After the subsidence central cone Miharayama began its development. The broad crater named “Inner Somma” seems to be formed in the pre-historic time and the historic activities occurred in it to form narrow vents or small cinder cones here and there in each case. The violent activities to flow lava over the wall of “Inner Somma” occurred only two times in the record and the recent activity is the third case.