2008 年 37 巻 5 号 p. 131-136
During the latest phase of eruptive activity by the Mutsu-Hiuchidake volcano, Northeast Japan, large pyroclastic flows were estimated by previous geological and tephrochronological studies to have occurred at about 0.3 Ma. In this re-examination of the latest volcanic history, we present new fission-track ages for two zircons from the pyroclastic flow deposits from the volcano and two from the associated basement rocks. FT ages determined for the pyroclastic flow deposits are 0.81 ± 0.14 Ma and 0.77 ± 0.10 Ma compared to the 2.6 ± 0.2 Ma and 3.0 ± 0.3 Ma ages for the basement rocks. These results confirm that the lavas at the top of the volcanic edifice are younger than the pyroclastic flow deposits. In addition, the volcano is estimated to have been active from 1.2 to 0.5 Ma, based on other K-Ar ages, TL ages and stratigraphic relationships.