Aerodynamic characteristics and genesis of aggregates at Sakurajima Volcano, Japan

M. C. Diaz Vecino,V. Freret-Lorgeril,C. Bonadonna,A. P. Poulidis,M. Iguchi,E. Rossi,P. Gabellini,A. Fries,S. Pollastri,J. Lemus

doi:10.1038/s41598-022-05854-z

M. C. Diaz Vecino, V. Freret-Lorgeril + Show 8 more

Open Access

https://doi.org/10.1038/s41598-022-05854-z

Copy DOI

Abstract

Aggregation of volcanic ash is known to significantly impact sedimentation from volcanic plumes. The study of particle aggregates during tephra fallout is crucial to increase our understanding of both ash aggregation and sedimentation. In this work, we describe key features of ash aggregates and ash sedimentation associated with eleven Vulcanian explosions at Sakurajima Volcano (Japan) based on state-of-the-art sampling techniques. We identified five types of aggregates of both Particle Cluster (PC) and Accretionary Pellet (AP) categories. In particular, we found that PCs and the first and third type of APs can coexist within the same eruption in rainy conditions. We also found that the aerodynamic properties of aggregates (e.g., terminal velocity and density) depend on their type. In addition, grainsize analysis revealed that characteristics of the grainsize distributions (GSDs) of tephra samples correlate with the typology of the aggregates identified. In fact, bimodal GSDs correlate with the presence of cored clusters (PC3) and liquid pellets (AP3), while unimodal GSDs correlate either with the occurrence of ash clusters (PC1) or with the large particles (coarse ash) coated by fine ash (PC2).

Highlights

Aggregation of volcanic ash is known to significantly impact sedimentation from volcanic plumes
Ash aggregates are usually classified according to Brown et al.[1] and Bagheri et al.[11] who describe them in terms of Particle Clusters (PC), and Accretionary Pellets (AP)
PC1s are irregular aggregates formed only by fine ash; PC2s consist of a large particle partially covered with fine ash; PC3s consist of a core particle covered by a thick shell of particles < 90 μm 1,11; AP1s are spherical to sub-spherical aggregates with poor internal structure; AP2s are spherical to sub-spherical to cylindrical aggregates characterized by a pronounced internal structure; AP3s are rain drops filled with ash particles

Summary

Introduction

Aggregation of volcanic ash is known to significantly impact sedimentation from volcanic plumes. The studies of Bagheri et al.[11] and Gabellini et al.[17] have shown that highspeed camera (HSC) videos of falling aggregates, combined with simultaneous sample collection on adhesive paper for further analysis at the Scanning Electron Microscope (SEM) can provide an accurate description of the overall internal structure of the object, even for aggregates that are typically not preserved in the deposit (e.g., the PC type). This represents the current state-of-the-art of the sampling techniques for the observation of ash aggregates in the field. Vulcanian activity started in 1955 at the Minamidake crater and is still persisting at the time of writing (June 2021), with almost daily Vulcanian explosions occurring mainly at the Minamidake crater and sometimes at the Showa crater

Methods

Results

Discussion

Conclusion