Fragmentation mechanisms revealed trough the ash morphology and texture at Sakurajima volcano (Japan)

Volcanic ash represents a fundamental source of information on eruptive processes occurring both prior and after explosive eruptions. In particular, many studies evidenced how volcanic ash can be used to extract unique information about the style of volcanic activity and the relative fragmentation processes. We present a detailed study of ash collected during classical vulcanian activity at Sakurajima volcano (Japan), aimed at investigating the relationships between eruption dynamics and the key features of the resulting volcanic ash (e.g. shape and texture). Information about fragmentation mechanism is revealed by a comprehensive investigation over a complete sequence of activity observed in summer 2013 and October 2014. Based on SEM imaging of the ash samples, 4 main categories (Blocky Irregular, Blocky Regular, Rough-Vesicular, and Rough) have been defined. These characterize all the different phases observed in the eruptive activity, without showing important changes in concentration or morphology. The ash morphology has been then quantitatively defined trough a set of shape parameters, and compared with textural features (ground mass crystallinity, vesicularity) in order to outline the relations with different styles of activity observed during the whole sequence. An exhaustive quantitative dataset on the shape and textural variability of Sakurajima ash provide important insights into magma fragmentation mechanisms and their relations with the evolution of eruptive dynamics. S01.2

Determination of the largest clast sizes of tephra deposits for the characterization of explosive eruptions: a study of the IAVCEI commission on tephra hazard modelling

The distribution of clasts deposited around a volcano during an explosive eruption typically contoured by isopleth maps provides important insights into the associated plume height, wind speed and eruptive style. Nonetheless, a wide range of strategies exists to determine the largest clasts, which can lead to very different results with obvious implications for the characterization of eruptive behaviour of active volcanoes. The IAVCEI Commission on Tephra Hazard Modelling has carried out a dedicated exercise to assess the influence of various strategies on the determination of the largest clasts. Suggestions on the selection of sampling area, collection strategy, choice of clast typologies and clast characterization (i.e. axis measurement and averaging technique) are given, mostly based on a thorough investigation of two outcrops of a Plinian tephra deposit from Cotopaxi volcano (Ecuador) located at different distances from the vent. These include: (1) sampling on a flat paleotopography far from significant slopes to minimize remobilization effects; (2) sampling on specified-horizontal-area sections (with the statistically representative sampling area depending on the outcrop grain size and lithic content); (3) clast characterization based on the geometric mean of its three orthogonal axes with the approximation of the minimum ellipsoid (lithic fragments are better than pumice clasts when present); and (4) use of the method of the 50th percentile of a sample of 20 clasts as the best way to assess the largest clasts. It is also suggested that all data collected for the construction of isopleth maps be made available to the community through the use of a standardized data collection template, to assess the applicability of the new proposed strategy on a large number of deposits and to build a large dataset for the future development and refinement of dispersal model

Dynamic analysis of ash aggregates revealed through HS-HR imaging at Sakurajima volcano (Japan)

Ash aggregation processes during explosive eruptions can effectively influence volcanic plume dispersal and ash sedimentation. Recently, dedicated experiments have been carried out and numerical models have been developed in order to produce reliable forecasting of the ash dispersals. However, including ash aggregation processes in numerical simulations is to date a problematic task for volcanologists, because of the lack of solid field-based datasets required to scale, validate and calibrate models. A field-based dynamical investigation of ash aggregates collected at Sakurajima (Japan) with a High-Speed, High- Resolution camera is here presented. Three main types of ash aggregates are recognized to occur into all the examined samples (Single Particles, Coated Particles, Cored Clusters). Using image analysis techniques, clusters were characterized in terms of average dimension, grain size and shape features of the aggregating ash, pointing out important differences between the different cluster types. Dynamical analysis of falling aggregates allowed a significant set of measurements of terminal velocity, bulk density, and size of a large number of observed falling aggregates to be collected. The resulting data reveal the strong influence of aggregation processes in controlling ash deposition processes at Sakurajima

Conduit geometry and evolution of effusion rate during basaltic effusive events: Insights from numerical modeling

The dynamics of effusive events is controlled by the interplay between conduit geometry and source conditions. Dyke-like geometries have been commonly employed for describing conduits during effusive eruptions, but their depth-dependent and temporal modifications are largely unknown. Here we present a novel model that describes the evolution of conduit geometry during effusive eruptions by using a quasisteady state approach based on a 1D conduit model and appropriate criteria to model the geometric evolution of the conduit due to fluid shear stress and elastic deformation. Such approach provides time-dependent trends for effusion rate, conduit geometry, exit velocity and gas flow, among other output variables. Fluid shear stress leads to upward widening conduits, whereas elastic deformation becomes relevant only during final phases of the eruptions. Since the model is able to reproduce different trends of effusion rate, it was employed for addressing the effects of magma source conditions and conduit properties on the main characteristics of the resulting effusive eruptions (e.g. duration, erupted mass, maximum effusion rate). We show that the total erupted mass is mainly controlled by magma reservoir dimensions and their conditions before the eruption (i.e., initial overpressure), whereas conduit processes and geometry are able to affect the magma withdrawal rate and thus the eruption duration and effusion rate. The resulting effusion rate trends were classified in different types, and associated to the curves described in the literature for different volcanic events. Results well reproduce these trends and provide new insights for interpreting them, highlighting the importance of reservoir overpressure and the initial dimensions of the feeding dyke on the resulting effusion rate curve

Chlorine as a geobarometer for alkaline magmas: Evidence from a systematic study of the eruptions of Mount Somma-Vesuvius

International audienceDefining the magma storage conditions of a volcanic system is a major goal in modern volcanology due to its direct implications for the style of a possible eruption, and thus on the associated risk of any crisis and the necessary management and mitigation strategies. Below 200 MPa and at equivalent depths, the strongly non-ideal behaviour of the H-CO -S-Cl-F system in the silicate melt causes unmixing of the fluid phase to form an H 2 O-rich vapour and a hydrosaline phase in equilibrium with the silicate melt, both responsible for buffering the chlorine (Cl) concentration. Following this equilibrium, the Cl concentration in melts may be used as a geobarometer for alkaline magmas. Systematic application of this method to the main explosive eruptions of Mount Somma-Vesuvius highlights two main magma ponding zones, at ~180–200 and ~100 MPa. At these pressures, the maximum pre-eruptive H 2 O contents for the different magma compositions can be estimated; the results obtained, largely in agreement with the current literature, therefore confirm the validity of the method. The Cl geobarometer may help scientists to define the variation of the magmatic reservoir location through time and thus provide strong constraints on pre-eruptive conditions, which are of utmost importance for volcanic crisis management