2. Origin of Volcanoes on Island Arcs in Relation to Plate Tectonics(I. Introduction)

Abstract

Three different views have been currently proposed for the role of an underthrusting oceanic slab played in the generation of parent magmas. 1. An interaction of the slab with the mantle under the arc gives enough heat to generate the magmas. 2. The slab provides also water to the mantle, so that the melting point may be lowered. 3. A part of the slab itself is molten and becomes the initial liquid for parent magmas. Any combinations of these three are possible. The author prefers to have the first view and substanciates his model here. Two important differences from the other opinions are pointed out. One point is that basaltic magmas may be parent. In this connection, the multiple correlation coefficients with the focal depths are compared between basaltic rocks and other types of rocks. The other point is that the partially molten zone may be certainly above the middle surface of the slab, based on a seismological evidence. It would mean that any considerable part of the slab should not be molten.