1992 年 87 巻 10 号 p. 420-429
Mutsu-Hiuchidake (783.1m height) is a Quaternary composite volcano, situated at the northern part of Shimokita peninsula, Northeast Japan. The basement rocks are composed mainly of Tertiary lavas and pyroclastic rocks. Stratigraphic relationships among volcanic ejecta were established using wide-spread tephras and marine terrace deposits as key beds.
The volcanic activity are divided into two main stages. Older Hiuchidake stage: Before 0.10 Ma, the volcanic activity began with eruption of ash fall. Stratovolcano, composed of pyroclastic flows and lava flows, was formed. Younger Hiuchidake: After 0.08 Ma, the volcanic activity began. It is divided into three substages. The first stage of Younger Hiuchidake is characterized by pumice flow deposits, including a large amount of fragments. The first stage probably triggered land slide. The second stage consists of pyroclastic flows, most of which show strongly welding. The third stage, lava flows and lava dome were extruded from several centers.
Rocks of Hiuchidake volcano are mainly augite-hypersthene andesite and hornblende dacite. Hornblende phenocryst often occur in andesite.