Morpho-textural and dynamic analysis of ash particles and ash aggregates at Sakurajima volcano (Japan)

Morphological, textural and granulometric studies of volcanic ash particle provides important insights into the mechanisms of fragmentation, transport and deposition in the context of low-to-mid intensity activity, and particularly during those eruptions showing high-transients in the style of activity. A comprehensive study of volcanic ash from Vulcanian activity of variable intensity at Sakurajima volcano (Japan) is here presented together with a detailed analysis of ash aggregates collected and filmed during the same eruptive sequences. Bulk tephra deposits from different events (July-August 2013, October 2014 and November 2019) and high-speed video of falling ash aggregates were collected directly during the fallout. Tephra samples, resulting from the different phases of activity, were analyzed using an optical particle analyzer which allowed to characterized the grain size distribution and to quantify the shape of a large set of particles. A set of objective parameters were used to constrain the shape of ash grains. This helped to better characterize different phases of activity also in the light of the magma fragmentation process and to evaluate the role played by the fragmentation process in controlling the eruption dynamics. SEM analyses of representative ash grains allowed distinguishing four principal types of ash fragments basing on morphological, surface and groundmass features: Blocky Irregular (BI), Blocky Regular (BR), Vesicular (V). A comprehensive textural analysis of grains belonging to either the different classes and phases of activity was provided in order to better resolve the complex relationships between the processes occurring before and during magma fragmentation and secondary processes affecting ash characteristics, like the intra-crateric recycling of ash. This helped also to shed light on the different processes of ash production and link them with the resulting dynamics of activity in the context of unsteady eruptions. On the other hand, the analysis of the high-speed video depicting ash aggregates, and aggregates collected during the same eruptive periods revealed important information about the influence of ash aggregation in controlling the depositional dynamics of Vulcanian eruptions. Three main types of ash aggregates were recognized to occur into all the Sakurajima samples: Ash Clusters, Coated Particles, Cored Clusters. Using image analysis techniques of SEM images, collected aggregates were characterized in terms of dimension, grain size of the aggregating ash, and shape features of the aggregated ash, pointing out important differences between the different types. Analysis of high-resolution, High-speed Camera video recordings, allowed finally to collect an important set of measurements of terminal velocity, bulk density, and size of a large number of observed falling aggregates. The resulting data reveal the strong influence of aggregation processes in controlling ash deposition processes at Sakurajima, and also represent a valuable dataset useful for validation and calibration of numerical models.