南九州とスマトラの火山活動と火山

Abstract

The magma of ignimbrite and welded tuff ("Sirasu") might be originated from the remeltof oceanic plate, when the front of plate is subducted to nearly 100 km in depth. The volcanismof these ignimbrite and Sirasu is so violent as to make a big caldera, of which Bouguer anomaliesare about 50 mgal lower than those of the surrounding area, and the big caldera has the diameterof 15-20 km which is distributed along the big faults.After these activity, the dacite and andesite volcanisms occure, which make the centralcone of volcanoes. The magma of this volcanoes is quite different from that of ignimbrite andSirasu in the genesis. This magma must be originated from the partial melt of the upper mantleunder the volcanic belts of Island arc.