The Quaternary Zao volcano is situated on the Tohoku Backbone Ranges, Northeast Japan. It is known that activity of the volcano started about 1 Ma and continues to the present. The volcanic history of the Zao volcano can be divided into four stages as follows. Stage 1: A relatively small-scale volcano was formed. It is composed predominantly of pyroclastic materials of basalt and basaltic andesite. Stage 2: A stratovolcano was constructed by lavas and pyroclastic materials of andesite and dacite. The stage 2 is further subdivided into two substages, 2 a and 2 b. Stage 3: Lavas and pyroclastic materials from two vents situated near the summit widely distributed around the flank of the volcano. Rocks consist of basaltic andesite and andesite with a small amount of basalt. Stage 4: After formation of caldera, Goshikidake pyroclastic cone was formed in the caldera. The eruptive products comprise pyroclastic materials and lavas of basaltic andesite.
The rocks of stage 1 belong to low-K series, and all the others to medium-K series. The medium-K rocks from different stages show contrasting trends; the rocks of stage 3 have slightly higher K 2O than those of stage 2 and stage 4 at the same SiO2 content. The basalt of stage 3 is medium-K and a contrast to the low-K basalts of stage 1.
