Eruptive dynamics and fragmentation mechanisms during cyclic Vulcanian activity at Sakurajima volcano (Japan): Insights from ash texture analysis

Quantitative morpho-textural analysis of volcanic ash is one of the most effective tools to characterize the style of ash-dominated volcanic activity, and to investigate the complex interplay between conduit processes and the associated eruptive dynamics. In this framework, many questions still remain unanswered about the role of conduit processes, particularly of the magma fragmentation processes, in controlling the eruptive dynamics of the unsteady and highly-transient Vulcanian ex- plosions. For this reason, we analyzed ash samples collected during a five days-long eruptive sequence at Sakurajima volcano (Japan), to derive information about ash morphometry and textural features over the entire sequence. During the observed sequence, eruptive activity showed high unsteadiness in the modalities of ash emission, which included all the main different eruptive styles typical of the recent period of Sakurajima activity. Three main intra-eruptive phases (Phase1, Phase2 and Phase3) were recognized based on visual observations and thermal data. Quantitative information about external ash morphometry (i.e., shape) and internal textures were measured for the particles associated with the different phases and discussed in terms of the observed eruptive variability. We quantified crystallinity and vesicularity of ash grains, the crystal size distribution (CSD) of microlites and the microlite number density (MND) of the groundmass for a representative set of ash particles. We discussed the links between the eruptive dynamics and the dominant processes of magma fragmentation, as shown by the combination of the morpho-textural features of ash throughout the whole eruptive sequence and the observed variations of the eruptive phases. All the evidence presented in this work confirm the constant presence at Sakurajima of a highly stratified and degassed magma within the conduit, suggesting that the transient dynamics of the eruption were strongly controlled by variations in the process of magma fragmentation driving the eruptions during the different phases. In particular, the morpho-textural characteristics of ash suggest that Vulcanian eruptions at Sakurajima can be controlled by the progressive pressurization of the upper portion of the magma conduit (between 10 and 50 m in depth). Moreover, using crystal textures, we inferred that the time needed for conduit refilling during intra-eruptive stages is comprised between 1 and 10 month. The resulting information on the eruptive dynamics of Sakurajima is of primary importance for a more exhaustive comprehension of the low-to-mid intensity, ash-dominated explosive activities.