Hydrothermal sealing process in a volcanic conduit interpreted from the characteristics of subvolcanic hydrothermal alterations at Tokachidake volcano, Japan

At Tokachidake volcano, Japan, inflation of the crater area was observed alongside a decrease in fumarolic plume height during 2006–2017, which was considered to be due to conduit sealing. The accumulation of excess pressure owing to continuous sealing can lead to phreatic eruptions and edifice collapses. To reveal the sealing process, this study investigates the characteristics of hydrothermal alterations in the volcanic conduit by analyzing altered materials from the edifice interior and the associated changes in porosity and permeability. The weakly altered rock retained some of its primary minerals and can be interpreted as having formed in an environment where the supply of volcanic gases was limited. In contrast, the conduit-filling lava and scoria fragments were intensely leached and cemented by fine alunite and amorphous silica, resulting in consolidated aggregates (strongly altered rock). Porosity and permeability tended to increase as hydrothermal alteration advanced. However, the strongly altered rocks were originally unconsolidated aggregates of lava and scoria fragments. Our data suggest that the consolidation associated with hydrothermal alterations reduced the permeability of the conduit by three to four orders of magnitude, leading to conduit sealing. This decrease in permeability would create excess pressure within the conduit, with our estimate ranging from a few MPa to 43 MPa. Such conduit sealing is likely to occur repeatedly in the future. Therefore, to elucidate the present state of the volcano and support hazard mitigation, it is essential to correctly interpret the phenomena occurring beneath the crater area based on our results.