The voluminous Quaternary volcanic rocks in the Suwa-Yatsugatake volcanic province (SYVP) of central Japan are known as Enrei volcanic rocks, Yabashira and Yatsugatake volcanic products. These volcanic rocks consist mostly of basalt and andesite, with a small amount of rhyolite. We report on the revised stratigraphy and K-Ar ages of rhyolitic lava domes in the Daimontoge area of the central part of the SYVP, and discuss the duration of rhyolitic volcanism in the SYVP. Our work clarifies the fact that the two rhyolitic lava domes studied were formed independently, ca. 0.24Ma (Utsukushimatsu lava dome) and ca. 1.07Ma (Daimontoge lava dome), and the age of the basement volcanic rocks predates ca. 1Ma. The duration of rhyolitic volcanism in the SYVP is divided into 1.2-0.6Ma and 0.3-0.2Ma, based on the findings of previous studies and this study. The previous studies show that volcanic activity had not occurred post-0.6Ma in the central part of the SYVP. However, our data show that the rhyolitic lava dome (Utsukushimatsu lava) was formed much later, ca. 0.24Ma. The spatiotemporal distributions and whole-rock composition of the rhyolite suggest that rhyolitic volcanism prior to 0.6Ma was related to the formation of Oiwake volcanic graben in the central SYVP. At the time of the effusion of the Utsukushimatsu lava dome, the activity of the Yatsugatake volcanoes that adjoined the eastern side of the Daimontoge area was the most vigorous in their history. Therefore, we suggest that the formation of the Utsukushimatsu lava dome in the Daimontoge area resulted from the increment and expansion of activity of the Yatsugatake volcanoes.

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High-level radioactive waste has to be disposed in a stable geological formation at a depth of more than 300 meters. Infrequent geological and climate processes and their effects, however, can not be perfectly ignored in 10⁵- to 10⁶-years long-term future assessments. Therefore, we must evaluate the hazard area and probability of infrequent disruption which will not be excluded by the selection program of the final repository site. This study deals with gigantic volcanic eruptions, forming large collapse calderas, as one of the assessments and constructs the GIS-tool for its scenario analysis. Our GIS-tool is composed of (1) database of eruptive history of large-caldera volcanoes in Japan during Quaternary time and (2) program to calculate two-type circular hazard areas. The first hazard area is defined as a crustal deformation zone due to growth of the magma chamber using a sill model. The second hazard area shows a run-out distance of a large-volume of pyroclastic flow on the ground using a subcritical flow model (Bursik and Woods, 1996).

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