Sedimentation process of ashfall during a Vulcanian eruption as revealed by high-temporal-resolution grain size analysis and high-speed camera imaging

Abstract We here examined the sedimentation process of falling ash particles during a short-lived Vulcanian eruption at Sakurajima volcano, Japan, using high-speed camera imaging of airborne ash particles, as well as grain size analyses of ash samples collected at high temporal resolution. Ashfalls from a sequence of two Vulcanian eruptions at 10:44 and 11:26 JST (Japan Standard Time) on 26 March 2016 were investigated. The field site for imaging and sampling was located approximately 3.5 km from the vent. The ash particles collected exhibit bimodal grain size distributions (GSDs). The median particle size of the main subpopulation decreased throughout the ashfall owing to size-dependent segregation of ash particles from a single source released from a constant height. The volume fraction of the subordinate subpopulation is interpreted to represent a component of ash aggregates and shows an increase in its volume fraction after the eruption onset. High-speed camera imaging reveals that the mean particle density decreased over time as the ashfall continued. These data suggest an increasing contribution of low-density aggregates during the course of the ashfall. The temporal changes in the density of airborne ash particles and in the GSD of ash samples demonstrate that the sedimentation process during a Vulcanian eruption at Sakurajima volcano is characterized by sedimentation of ash particles from transient eruption plumes at constant height, in which ash aggregation increases over time. Moreover, time series analysis of the ash falling rate shows that secondary thickening occurred during these Vulcanian eruptions and that this can be triggered by the settling of ash aggregates.