上信地域烏帽子岳南西麓の溶岩類のK-Ar年代

概要

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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 Iwayakan'non lavas and the upper part of the Eboshidake lavas which have been dated to be 0.97 Ma and 0.35 Ma, 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 has initiated at least 0.8 Ma, and possibly 1 Ma, instead of the formerly resarded age of ca. 0.4 Ma. 0n the basis of the new age data, the life span of the Eboshi volcano group was longer and consequently its emotion rate was lower than they used to be estimated.
特定非営利活動法人日本火山学会の論文
2004-08-31