南西北海道,羊蹄火山の完新世噴火史の再検討

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We carried out the geological and petrological investigations around the summit area of Yotei Volcano. Four eruptive crater groups are recognized: Summit, Hinangoya, Niseko and Kitayama in ascending order. They erupted six pyroclastic deposits (from S-6 to S-1 in ascending order) and five lava flows. Lava effusion occurred from the Hinangoya, Niseko and Kitayama crater groups. These activities were mainly Strombolian. Whole-rock chemistry of juvenile materials is distinct among four crater groups, indicating distinct magma system has been active beneath different craters. The radiocarbon age from unit S-2 fallout deposit from the Kitayama crater group was obtained as 4010±30cal. yBP. On the basis of estimated accumulation rate of soil layers, the latest (S-1) and S-4 eruptions from Kitayama group are considered to have occurred in 〜2,500yBP and >5,000yBP, respectively. Eruptive deposits of Kitayama crater group do not show the evidence of interval more than several thousand years between unit S-4 and S-5. Thus, it could be concluded that the Kitayama group started its activity from mid of Holocene. Eruptive volumes of each eruptive group except for the Summit crater group range from 0.1 to 0.18km^3. After the last magmatic eruption in 2.5ka, there are no evidences of eruptions from the Kitayama group, indicating that activity of the group has finished. However, considering newly revealed eruption history of the summit area, it should be noted that next eruptive group with distinct magma system might start its activity from another crater.
2011-06-30

南西北海道,羊蹄火山の完新世噴火史の再検討

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