Sedimentary System of Ash Deposits from Long-Term Vulcanian Activity at Sakurajima Volcano, Japan

Ash deposits from short-lived vulcanian eruptions are ubiquitous at active volcanoes and are important for understanding the development of a volcanic edifice. In this study, we investigated the sedimentary system of ash deposits produced by long-term vulcanian activity from Showa crater of Sakurajima volcano, Japan, based on the grain size characteristics of ash from individual eruptions. We collected falling ash samples from 20 individual vulcanian eruptions between March 3 and 11, 2010. The grain size distributions of the ash samples were unimodal, bimodal, and trimodal, depending on the event. The median particle size of the main subpopulation decreases with distance from vent. The low standard deviation of the grain size of the main subpopulation implies deposition from transient volcanic plumes. For the finest subpopulation in the polymodal samples, the median grain size is very fine ash (<30 μm), indicating that this subpopulation was derived from ash aggregates. Overall, the sedimentary system of the long-term vulcanian activity at Sakurajima volcano is explained by accumulation of ash particles from repeated transient plumes and ash aggregation, and the latter controlled the behavior of the very fine ash.