Holocene Eruptive Activity Recorded as Volcanic Glass in the Sediments of Beppu Bay, Central Kyushu, Japan

Abstract

The eruptive history around Beppu Bay, central Kyushu, Japan, is investigated using a 15 m-long sedimentary core obtained with a piston corer. A standard method to determine the down-hole content of volcanic glass was used in this study. This method consists of two steps: (1) systematic separation of the volcanic glass particles from samples taken at close intervals, and the determination of their relative concentration; (2) precise measurement of the refractive indices of the separated glass particles. A recently developed measuring system (RIMS 86) based on the thermal immersion method permitted quick and accurate measurement of the refractive index of a large number of samples. Five discrete tephras erupted since 7, 500 y B.P. occur within the sedimentary succession underlying Beppu Bay. The oldest tephra is correlated with the widespread K-Ah tephra originating from the Kikai Caldera is southern Kyushu Island. The other four tephras (ca. 5, 000 y B.P., 4, 000 y B.P., 2, 000 y B.P. and 1, 500 y B.P.) are considered to represent eruptives of more proximal volcanoes located in the area surrounding Beppu Bay.