Volcanic Geology of Yokoate-jima, the Tokara Islands, Kagoshima Prefecture, Japan （Ⅰ）

Abstract

Yokoate-jima is a guitar-shaped island measuring 10 km round in circumference. The island consists of two stratovolcanoes, To-ho (East peak) 512 m high and Sei-ho (West peak) 268 m high. Initial activity would have taken place during the late Pleistocene on the submarine ridge where the present island is situated. The activity of the To-ho is divided into three stages. During the first and second stages, were formed a stratovolcano composed of lavas and pyroclastic materials of two pyroxene andesite and olivine-bearing two pyroxene andesite. During the third stage, eruption of pyroclastic materials of olivine-bearing two pyroxene andesite took place. The activity of the Sei-ho is also divided into three stages. During the first stage, alternate eruption of lavas and pyroclastic materials of two pyroxene andesite built up a stratovolcano. During the second stage, intrusion of lava dome of two pyroxene andesite formed the Sei-ho volcano. Eruption of pyroclastic materials of two pyroxene andesite occurred during the third stage.