Petrology of Zao volcano

Zao volcano which belongs to Nasu volcanic zone covers an area, about 20km in E-W and about 11km in N-S, on the border of Miyagi and Yamagata Prefectures, northeastern Japan. Zao volcano, as well as Ryuzan volcano on the north and South Zao volcano on the south, are lying on a general trend extending from northwest to southeast. The central part of this volcano is built up of crater rim “Umanose”, central cone “Goshikidake” and small crater lake “Okama” showing a typical structure of double volcano expressed by the successive eruptions, explosions and subsidences. The volcanic prolucts of Zao volcano, consisting of lava flows and pyroclastics, lie upon the eroded surface of Pre-Tertiary granodiorite and the Neogene Green Tuff Formations. Most of lava flows are composed of (olivine bearing) augite hypersthene andesites of calc-alkali rock series, those of tholeiitic rock series are rare. Ten new chemical analyses are made and some petrogenetic problems are discussed.