Abstract
Several volcanic rocks from O-shima, Kozu-shima and Hachijo jima of the Izu Islands, from Chichi jima and Haha jima of the Bonin Islands and from Iwo jima of the Iwo Islands were dated by K-Ar method. The rocks from the Izu Islands give the ages of less than 2-3 m.y., while the ages of the rocks from Chichi jima and Haha jima are about 30-40m.y. The rock from Iwo-jima indicates a very young K-Ar age of about 0.03m.y. These results confirm that there is a clear difference in age of volcanic activity between the Izu Islands and the Bonin Islands. This difference has also been suggested geologically.
