Fountain-like collapse of an eruption column during a large explosive eruption

Fountain-like collapse of eruption columns has been observed in numerical simulations of explosive eruptions, but such a style of collapse is rarely documented by field observations. The contrasting characteristics of the proximal and distal pyroclastic current deposits from the 34 ka Sounkyo eruption at the Ohachidaira caldera, Japan, likely reflect complex processes of column collapse. The features of the distal welded tuff are most consistent with emplacement by a pyroclastic current from a fountain of the jet core, in which air entrainment was negligible so that the collapsing mixture remained sufficiently hot and dense to allow welding. In contrast, the nonwelded and stratified nature of the proximal equivalent deposits may have required substantial entrainment and expansion of cool air into the mixture. Such enhanced air entrainment might have been related to the margins of the jet, while its core formed a fountain and the distal tuff. The topographic relations of the deposits imply a fountain height and an impact distance of >3 km.