Skip to content

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

Yuki Yasuda

Abstract

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 tuffs (~6.5 km3 in volume) from the 34-ka Sounkyo eruption at the Ohachidaira caldera, Japan, may reflect complex processes of column collapse. The features of the distal welded tuff are most consistent with emplacement by pyroclastic currents 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 and retention of fines. In contrast, the fines-depleted, nonwelded, and stratified nature of the volumetrically minor proximal tuffs may require substantial entrainment and expansion of cool air into the mixture. Such enhanced entrainment of air might have been related to the margins of the jet, while its core formed a fountain and the distal tuff. The distal tuff contains more abundant pumice (relative to scoria) clasts than the scoria-richer proximal tuffs, and this might reflect heterogeneous distribution of the two components in the columns. Before fragmentation, a silicic magma-rich core was likely surrounded by the mafic magma-rich margins due to viscosity segregation. Such a process is consistent with the fact that some wall-rock lithic clasts are thinly coated by scoria or incorporated within scoria clasts but never associated with pumice. The distribution of the deposits and their relations to the substrate topography imply a fountain height and an impact distance of >3 km.