Eruptive history of Asahidake Volcano, central Hokkaido: New study of the stratigraphy and eruption ages of the Asahidake late stage products.

Abstract

The eruptive history and eruption style of Asahidake volcano, the youngest volcanic edifice of the Taisetsu Volcano Group, central Hokkaido, Japan, are investigated to evaluate its long-term volcanic hazards. The Asahidake edifice consists of a pyroclastic cone whose growth is associated with lava effusions since the late Pleistocene. The eruption rate was relatively high, with an estimated 1.0 km³ dense rock equivalent (DRE)/ky from 15 ka to 9 ka that rapidly decreased to 0.03 km³ DRE/ky from 9 ka to present. After the latest magmatic eruption ca. 5 ka, there was a 2 ky dormant period that was followed by a large-scale phreatic eruption ca. 2.8 ka, which possessed the following eruption sequence. The sequence began with edifice collapse that produced a debris avalanche and formed the Jigokudani horseshoe-shaped crater, followed by phreatic explosions. Lahar flows then effused from many small craters and fissures that had formed in and at the opening part of the Jigokudani crater. This eruptive activity decreased after the eruption sequence. We recognize that the most recent small-scale phreatic eruption occurred ca. 0.7 ka. Although the current fumarole activity is remarkable, it appears that the eruptive activity has declined considerably since the 2.8 ka eruption. Considering the temporal change in eruptive activity, it is possible that the activity of Asahidake volcano has monotonically decreased over the past 10,000 years. However, in the context of volcanic hazard mitigation, it should be noted that small-scale phreatic explosions and/or effusions of lahar similar to the 0.7 ka eruption might potentially occur and endanger tourism activities.