Evolutionary history of the Nasu volcano group, Northeast Japan arc.

The Nasu volcano group is composed of six volcanic edifices, Kasshiasahidake, Sanbon-yaridake, Asahidake, Minamigassan, Futamatayama, and Chausudake, located in the volcanic front of the Northeast Japan arc.     Kasshiasahidake (ca. 0.54-0.42 Ma), Sanbon-yaridake (ca. 0.36-0.27 Ma), Minamigassan (ca. 0.21-0.03 Ma) have similar geological sequences. Alternation of thin basaltic andesite lava flows and associated pyroclastics developed in the lower part, whereas thick andesitic or dacitic lava flows and minor pyroclastic flows developed in the upper part. Between these two stages, caldera collapse sometimes occurred. On the other hand, Asahidake (ca. 0.21-0.06 Ma), Futamatayama (ca. 0.14 Ma), and Chausudake(ca. 0.04-0 Ma) are composed of andesitic lavas, lava domes, and pyroclastic flows, lacking thin basaltic andesite sequences. From evolutionary historical point of view, Kasshiasahidake, Sanbon-yaridake, and Minamigassan (including Asahidake and Chausudake edifices) edifices construct individual stratovolcanoes, which have similar evolutionary histories. Futamatayama is distinct from these.     Volume and eruption duration of the three stratovolcanoes are as follows; ca. 200 k.y., 16.2 km3 for Kasshiasahidake, ca. 150 k.y., 7.2 km3 for Sanbon-yaridake, ca. 200 k.y., 17.8 km3 for Minamigassan (including Asahidake and Chausudake). These volumes and eruption rates are less than those of large stratovolcanoes (Akagi, Haruna, Hakone volcanoes etc.) in near the triple junction of plate boundary, but are comparable to those of small stratovolcanoes (Quaternary volcanoes in Shin-etsu Highland).