Abstract
Two types of magmas exist at Nishiyama volcano, Hachijojima Island. One is plagioclase-controlled and the other is differentiated magma. The bulk chemistry of the first magma type is controlled by abundance of plagioclase phenocrysts with a constant liquid composition. That of the second type is possibly controlled by fractionation of plagioclase and mafic minerals from a basaltic magma.
Predominant is the plagioclase-controlled basalt at Nishiyama volcano. Summit eruptions invariably tap the plagioclase-controlled magma. Flank eruptions supply not only the plagioclase-controlled basalt, but rarely the differentiated basaltic andesite. These phenomena are similar to those observed at Izu-Oshima volcano. There must be something in common between the magma plumbing systems of Nishiyama and Izu-Oshima volcanoes.
