東北日本，秋田駒ケ岳南部カルデラの形成史

抄録

Akita-Komagatake volcano in northern Honshu, Japan, has an elliptical caldera 3 km×1.5 km in diameter (southern caldera) on its southern slope. Although it has been speculated that the AK13 and AK12 tephra units (15‒12 ka) are related to the formation of the caldera, the details of these processes have not been clarified. We divided the caldera-forming activity into three stages on the basis of geological and petrological evidence developed during this study. Stage 1 activity began with a vulcanian eruption that produced Obonai pyroclastic flow, followed by a plinian eruption that produced an eruption column. These eruptions effused basaltic andesite and andesitic scoria to form southern portion of the caldera. This activity, which produced AK13 tephra, involved an eruption of volcanic explosivity index (VEI) 4 and occurred at approximately 15,100 calibrated radiocarbon years before present (cal. BP). Stage 2 involved the formation of a small pyroclastic cone, Minamidake cone, at the southern margin of the caldera. Distal tephra and lava flows derived from the cone have not been recognized. Stage 3 involved phreatomagmatic eruptions of dacitic magma that produced Kunimi pyroclastic flow, which covered the floor of southern portion of the caldera and the Minamidake cone. The activity then shifted to a plinian eruption, during which time juvenile materials changed from dacitic pumice to mixtures of basaltic andesite scoria, pumice and banded pumice. This activity terminated with repeated phreatomagmatic and Vulcanian eruptions. This stage, which produced AK12 tephra, involved an eruption of VEI＝4 and occurred at approximately 13,400 cal. BP. We conclude that the southern caldera is a composite caldera that was formed by two sequential explosive eruptions in the latest Pleistocene. Juvenile materials from Stages 1 and 3 form distinct linear trends in many silica variation diagrams, suggesting that each stage was derived from a different magma system.