Mt. Eboshidake stands at the western part of the Eboshi volcano group, in Johshin district, central Japan. The K-Ar ages of 0.77-0.76 Ma are obtained for three lava samples collected from Ofuji, Kanahara lavas along with the lowermost part of Eboshidake lavas in the southwestern flank of Mt.Eboshidake. These studied lavas are stratigraphically situated between the IwayakanÅfnon lavas and the upper part of the Eboshidake lavas which have been dated to be 0.97Ma and 0.35Ma, respectively. The Kanahara lavas which have previously been regarded as a part of the Eboshidake lavas, are newly defined because of their distinct inclination. Our results indicate that the volcanic activity of the Eboshi volcano group have initiated at least 0.8Ma, and possibly 1Ma, instead of the formerly regarded age of ca. 0.4Ma. On the basis of the new age data, the life span of the Eboshi volcano group was longer and consequently its eruption rate was lower than they used to be estimated.

Key words:
K-Ar age, Eboshi volcano group, Johshin district, central Japan, Quaternary