1986 Volume 31 Issue 2 Pages 85-94
Fission track ages of zircon from the Unzen volcanic rocks in western Kyushu have been dated using the external surface re-etch method. The ages, including standard errors, of the basal volca-niclastic rocks, the Tatsuishi formation, are 0.28±0.05 Ma and 0.25±0.05 Ma. The next oldest Takadake lavas range form 0.26 to 0.20 Ma: the Yatake lava 0.26±0.04 Ma, the Takaiwayama lava 0.24±0.06 Ma, the Kinugasayama lava 0.23±0.08 Ma, and the Takadake lava 0.20±0.05 Ma. The following Kusenbudake lavas fall in a narrow range form 0.19 to 0.17 Ma: the Sarubayama lava 0.19 ± 0.03 Ma, the Azumadake lava 0.18±0.06 Ma, and the Kusenbudake lava 0.17±0.06 Ma. The most recent Fugendake lavas are younger than 0.07 Ma: the Mayuyama lava 0.07±0.02 Ma, the Fugendake lava 0.07±0.02 Ma, and the Myokendake lava 0.06±0.03 Ma. These lavas are probably too young to be accurately dated using the fission track method. It is inferred from the age determination and geological evidence that the volcanic activity of Unzen volcano began about 0.30 Ma ago and has continued into Recent time, with a period of dormancy between 0.17 and 0.07 Ma, during which the Unzen Graben acquired its present form.
