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In-situ evidence of ash aggregation during volcanic cloud sedimentation

Simon Thivet 1, Valentin Fréret-Lorgeril2, Allan Fries1, Riccardo Simionato1,3, Carolina Díaz-Vecino1, Jonathan Lemus1,3, Masato Iguchi4, and Costanza Bonadonna1

  • Affiliations: 1Department of Earth Sciences, University of Geneva, Geneva, Switzerland; 2Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F63000 Clermont-Ferrand, France; 3Department of Computer Science, University of Geneva, Geneva, Switzerland; 4Professor Emeritus, Kyoto University, Japan 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 110

  • Programme No: 3.15.15

  • Theme 3 > Session 15

Abstract

Explosive volcanic eruptions are commonly associated with significant ash emissions, resulting in widespread impacts. Volcanic unrest can also result in low-intensity activities that are often neglected, with understudied hazards and processes. This study investigates mild volcanic plumes and clouds (< 2 km above vents) produced in November 2023 at Sakurajima volcano (Japan) and characterized thanks to high-resolution videos captured from distant sites. Associated ash fallouts were sampled and analyzed in situ by combining ground- and drone-based approaches (ash trays and collectors, disdrometer, optical particle counter). This multi-disciplinary approach first reveals that ash aggregation occurs even from mild explosive activity. Grain componentry and size distribution from airborne samples in altitude show the presence of fewer and smaller aggregates compared with samples at ground levels. This indicates that ash aggregation mostly develops during sedimentation, as particles with different settling velocities collide and stick during their fall. Furthermore, sampling under different atmospheric conditions highlights that high relative humidity enhances particle aggregation efficiency in the form of accretionary pellets, instead of particle clusters during dry conditions. This study provides the first in-situ airborne-based measurements for volcanic ash aggregation. It supports previous field, experimental, and numerical investigations on the origin of the different aggregate types, which is crucial for predicting volcanic ash dispersion.