Late Pleistocene-Holocene volcanic history of Asahidake subgroup of Taisetsu volcano group, central Hokkaido, Japan

Abstract

The Asahidake subgroup is the youngest subgroup of the Taisetsu volcano group, which has been active since ca. 1 Ma. The subgroup became active after the formation of the Ohachidaira caldera (ca. 34 ka), and erupted from a source on the southwestern flank of the caldera. The subgroup is composed of three volcanic edifices: Kumagatake, Ushiro-Asahidake, and Asahidake in ascending order. Kumagatake is a cinder cone (0.4 km³ dense-rock equivalent, DRE) with three craters, and Ushiro-Asahidake consists of lava flows and a dome (combined volume of 0.5 km³ DRE). The activity of Asahidake comprised two eruptive stages: early and late. During the early stage (until ca. 5 ka), repeated magmatic eruptions formed a stratovolcano, while the late stage (since 2-3 ka) is characterized by phreatic eruptions. Temporal changes in eruptive scale indicate the early stage can be further divided into two sub-stages: E-1 and E-2. The E-1 sub-stage is characterized by relatively large pyroclastic eruptions and lava effusions (7.1 km³ DRE), while the E-2 sub-stage is characterized by a smaller eruptive scale, and pyroclastic materials and lava flows are restricted to near the summit area (1.7 km³ DRE). This later stage started with edifice collapse, forming a horseshoe-shaped crater (Jigokudani crater). Since then, phreatic eruptions have frequently occurred in the crater and fumarole activity has continued until the present day. The Asahidake rocks range from basaltic andesite to dacite, and usually contain mafic inclusions. Silicic andesite was dominant until the E-1 sub-stage, and mafic andesite was commonly erupted during the E-2 sub-stage.