Problems of volcanism and plutonism. Volcano-plutonic formations

There is no clean-cut boundary within the upper crust between volcanic and plutonic phenomena, nor between their associations. Numerous examples from the comparatively young circum-Pacific belt — and a number from older associations throughout the world — are presented to illustrate this point of view. Most of the examples are of volcanoes with deeply eroded cores consisting of crystallized porphyritic to even-grained rocks, or of "batholiths" capped by genetically related effusive and pyroclastic rocks, especially ignimbrites. The ever-growing roster of such examples indicates that they are the rule rather than the exception. Modern systematics of igneous rocks should have three, rather than two, main divisions: volcanic, plutonic, and volcano-plutonic. These three rock types reflect the overall progress of igneous activity within the diversity of natural processes. Only by conceding the presence of companion volcanic phenomena for shallow to intermediate plutonic phenomena, and vice versa, can one gain an understanding of magmatic phenomena actually existing under certain geologic conditions. The controlling tectonic mechanism for alternation of comagmatic volcanic and plutonic series remains obscure. Tectono-physical analysis of such associations must be supported by field data. Eventually, it should be possible to set up a general structural and genetic systematics of igneous rocks and to work out a classification of volcano-plutonic rocks now underway. — R. C. Epis.