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

T. Miwa,Y. Iriyama,F. Nanayama,M. Nagai

doi:10.1186/s40645-019-0316-8

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.

Highlights

Explosive eruptions generate and disperse tephra that deposits around a volcano
By combining a time series of ash falling rate and characteristics of grain size distributions (GSDs), we have been able to demonstrate that secondary thickening can occur during ashfall generated by Vulcanian eruptions, with the process being triggered by the settling of ash aggregates
We quantified the sedimentation process of ash particles during a Vulcanian eruption at Sakurajima volcano, Japan, using GSD analysis and high-speed camera imaging obtained during the eruption sequence at a site 3.5 km from the vent

Summary

Introduction

Explosive eruptions generate and disperse tephra that deposits around a volcano. The characteristics of these tephra deposits reflect their sedimentation process, which is controlled by eruptive style, wind conditions, and the aerodynamic properties of the particles (e.g., Iriyama et al 2018; Koyaguchi and Ohno 2001a; Walker 1971; Wohletz et al 1989). We investigated temporal variations in the sedimentation process of falling ash particles during a Vulcanian eruption at Sakurajima volcano, Japan. We conducted a detailed grain size analysis of ash samples collected at high temporal resolution and observed airborne ash particles using high-speed camera imaging. The combination of these analyses provides detailed insights into the sedimentation process of ash particles, including the effects of ash aggregation, during a shortlived Vulcanian eruption. Grain size analysis In order to examine in detail the sedimentation process of falling ash particles, we analyzed the GSD of 22 ash samples obtained at regular intervals throughout the ashfall. The Md and Sd values of the subpopulations were expressed using the Φ scale (= −log2d; d is particle size in millimeters)

Results and discussion

Conclusions

Methods