Intraoceanic islands, east pacific ridge, island arcs: Volcanism and upper mantle

Abstract

Two classes of volcanic rocks can be distinguished with respect to petrochemistry, namely oceanic and continental classes. Lavas of island arcs belong to the continental type. Lavas of oceanic ridges are close not to the oceanic, but to the continental type. On petrochemical and geophysical bases the author has drawn the following conclusions: 1. (1) the foci of volcano feeding lie beyond the limits of the earth's crust—in the upper mantle. 2. (2) the composition of the upper mantle under continents, oceans, island arcs and oceanic ridges is somewhat different. 3. (3) the role of assimilation on the passage of magma to the surface is very limited as a rule. According to these conclusions, volcanism may be considered as a certain indicator of the composition and state of the upper mantle matter. A scheme for the evolution of volcanism is suggested on this basis (as a reflection of the evolution of the upper mantle) which is in an agreement with geological and geophysical facts.