LAUSR

Abstract Subvolcanic hydrothermal alteration often leads to volcanic hazards, such as edifice collapse and phreatic eruption. The 1926 eruption of Tokachidake volcano caused a pyroclastic cone to collapse, and phreatic eruptions have occurred repeatedly. Here, we investigate the hydrothermal system of Tokachidake volcano by examining the characteristics of hydrothermally altered rocks and discuss the effects of the hydrothermal system on volcanic hazards. Beneath the crater area, the dense Tairagadake lavas (Ta lava) at the basement of Tokachidake volcano form piles that are >500 m thick. The Ta lava constrains the flows of volcanic gases and thermal water in the subsurface, and strong hydrothermal alteration occurs in both the underlying and overlying deposits that are more permeable. Above the Ta lava, Tokachidake ejecta are altered in low-temperature acidic environments (