Evolution of an explosion caldera as revealed by subsurface structure and eruption deposits of Nigorikawa volcano, southwest Hokkaido, Japan

Nigorikawa volcano, southwest Hokkaido, Japan, is characterized by a caldera-scale volcanic depression underlain by a funnel-shaped subsurface structure. Although this structure has previously been interpreted as an explosion-related volcanic depression, the processes responsible for its formation and the relationships among eruption sequence, vent evolution, and subsurface structure remain poorly understood. To reconstruct its syn-eruptive evolution, we integrated geological analyses of the eruption deposits with drilling-core data. The deposits of the ~15 ka caldera-forming eruption consist of seven pyroclastic units (Ng-1 to Ng-7, from bottom to top), with a total bulk volume of ~8.7 km3. The total volume of ejected lithic fragments is estimated to be 1.4 km3. These fragments are derived predominantly from the shallow basement and pre-caldera lavas. The eruption began with numerous explosive magma–water interactions, producing lithic-rich pyroclastic fallout deposits (Ng-1), which led to enlargement of the shallow conduit. Ng-2 to Ng-5 comprise alternating lithic-rich pyroclastic density currents (PDCs) (Ng-2, -4) and pyroclastic fallout deposits (Ng-3, -5), indicating a transition toward dominantly magmatic activity. The climactic Ng-6 phase produced the largest eruptive volume, including concentrated PDCs. The increasing involvement of crystal-rich felsic magma during the climactic phase likely enhanced pressure gradients within the conduit and promoted large-scale shallow conduit failure. Ng-7 is interpreted as a waning, water-influenced dilute PDC phase of the same eruptive episode. The vent-fill deposits may be correlated with Ng-6 to Ng-7, based on the similarity of their lithic assemblage to that of the extra-caldera deposits of Ng-2 to Ng-7 and the occurrence of accretionary lapilli. This correlation suggests that they formed and accumulated during the Ng-6 to Ng-7 eruptive phases. We interpret the Nigorikawa depression as an explosion caldera formed by explosive excavation, shallow conduit-wall failure, and syn-eruptive fallback/infill during a large-volume felsic PDC-forming eruption. Although its funnel-shaped subsurface geometry resembles that of a diatreme, Nigorikawa differs fundamentally from typical maar–diatremes in eruption scale, vent-fill architecture, and dominant formation mechanism. Our results demonstrate that caldera-scale volcanic depressions can be produced primarily by explosive excavation without magma-reservoir roof collapse, resulting in surface morphology resembling that of small collapse calderas despite fundamentally different subsurface structures and formation processes. These findings provide a framework for distinguishing explosion-related volcanic depressions from collapse calderas and contribute to improved interpretation of ancient volcanic systems and assessment of hazards associated with future large explosive eruptions.