Abstract
This article considers the current petrological hypotheses of the formation of andesitic magma beneath island arcs. It is suggested that there are two most likely mechanisms of formation of andesites: 1) fractional differentiation of a basaltic magma at small depths, and 2) direct melting of the andesitic magmas from amphibolized rocks in the lowermost layers of the crust and adjoining part of the upper mantle. Formation of magmas of the calc-alkalic series occurs with the participation of the volatile phases. The conditions necessary for generation of calc-alkalic magmas are provided by the action of compressive stresses. Undifferentiated basaltic magmas are formed in the melting of the upper mantle material under conditions of extensive stress. —Authors.
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