2015 Volume 68 Issue 1 Pages 29-39
High school earth sciences deal with the crystallization differentiation model as the main reason for the variety of igneous rocks. Recent igneous petrology, however, indicates that magma mixing and the partial melting of crust material are more important factors than previously though in producing the island arc igneous rocks. In this paper, we propose a practical educational program for verification of crystallization differentiation and magma-mixing models by using variable igneous rocks of Usu Volcano, Hokkaido. A small amount, less than 20% in weight, of felsic magma is expected from the differentiation model of basaltic primary magma. Mass balance calculations of individual igneous rock from Usu Volcano are conducted using a topographic map and a geologic map. Usu Volcano consists of a large amount of felsic rocks ～65%, which means there is an inconsistency with the simple crystallization differentiation model. An observation program for thin sections of felsic rocks using a polarization-microscope tells us much about the evidence of the magma-mixing model, such as basic inclusions in the felsic rocks, and resorption morphology of plagioclase phenocrysts.
