Formation of scoria cone during explosive eruption at Izu‐Oshima volcano, Japan

Abstract

Scoria cones have been regarded to be formed by accumulation of ballistic bombs ejected by mild eruptions. However, more recent geological investigations show that some scoria cones could be formed during explosive eruptions. Here, we demonstrate how the scoria cone of the 1986 Izu‐Oshima eruption was formed during the explosive eruption. We measured particle fractionation of the cone and propose a theoretical model to explain the observation. The model considers lateral transport of particles by turbulent eddies; particles that reached characteristic column radius, L, are laterally transported to ωL where they starts to free‐fall. We obtained ω = 1.2 and 2.5 for larger and smaller particles, respectively, which is consistent to the observation. In the model, extensive fallout takes place at the base of the column where it expands rapidly. We suggest that lateral particle projection and the rapid column expansion are the key processes to form the cones.