Volcanic ash: study methods, characteristics and meanings

Explosive eruptions commonly generate and disperse large quantities of volcanic ash, fragments less than 2 mm in diameter produced by the mechanical fragmentation of magma and/or wall rocks. Ash-producing eruptions cover a wide range of styles and are controlled by magma properties such as composition, crystal content, temperature and dissolved gases, and by possible interaction with external fluids such as groundwater or surface water. Different eruptive styles at volcanoes with different magma compositions are characterized by a large variability in terms of ash components. This is due to its fine nature, capable to capture the spatial variability of the textural and compositional characteristics of the magma, revealing some evolutionary aspects that coarse products may not notice. After entry into the atmosphere, ash rapidly quench, preserving information about the dynamics of the magma within the volcanic conduit, where fragmentation occurs and the style of the eruption is decided. To associate external features of volcanic ash with their internal texture and geochemistry, we developed a methodological procedure capable of capturing all this information on individual fragments. Approximately 50–100 ash fragments in the 0.5–1 mm size range are randomly hand-picked under the stereomicroscope, and then mounted on double-sided tape on a glass slide. This grain size is representative of the sample variability and allow to perform reliable analytical work on individual clasts. Fragments are photographed both using a digital camera under a stereomicroscope and under a scanning electron microscope (SEM) for the description of the external morphology. The slide is then embedded in epoxy resin and polished until a surface containing approximately the maximum diameter of the fragments is exposed. These mounts are used for SEM inspection, backscattered electron (BSE) imaging of textural features and for analytical purposes (SEM-EDS, EMPA, LA-ICP-MS). Ash components can be divided into juvenile, lithic, recycled clasts and loose crystals. The presence of recycled glass fragments can hide primary magmatic information, introducing distortions into interpretations of ongoing magmatic and volcanic activity. High-temperature experiments at atmospheric pressure have been performed on natural samples to study the effects of heating on the morphological, textural, and compositional characteristics of volcanic ash. By comparing experimental and natural samples, we have proposed the main general criteria to be used for the discrimination of recycled material. I will show examples of application of the above described study methods to reconstruct the dynamics of eruptive processes at both italian (Etna, Vesuvius, Stromboli) and worldwide volcanoes (Eyjafjallajökull). To attend online (Zoom meeting): https://u-paris.zoom.us/j/87339456320?pwd=bVhScDFuMnBCem5OU2JSK0FLYzl6UT09 Meeting ID : 873 3945 6320 Password : 458794