Abstract

Volcanic eruptions can produce ash particles with a range of sizes and morphologies. Here we morphologically distinguish two textural types: Simple (generally smaller) ash particles, where the observable surface displays a single measureable bubble because there is at most one vesicle imprint preserved on each facet of the particle; and complex ash particles, which display multiple vesicle imprints on their surfaces for measurement and may contain complete, unfragmented vesicles in their interiors. Digital elevation models from stereo-scanning electron microscopic images of complex ash particles from the 14 October 1974 sub-Plinian eruption of Volcán Fuego, Guatemala and the 18 May 1980 Plinian eruption of Mount St. Helens, Washington, U.S.A. reveal size distributions of bubbles that burst during magma fragmentation. Results were compared between these two well-characterized eruptions of different explosivities and magma compositions and indicate that bubble size distributions (BSDs) are bimodal, suggesting a minimum of two nucleation events during both eruptions. The larger size mode has a much lower bubble number density (BND) than the smaller size mode, yet these few larger bubbles represent the bulk of the total bubble volume. We infer that the larger bubbles reflect an earlier nucleation event (at depth within the conduit) with subsequent diffusive and decompressive bubble growth and possible coalescence during magma ascent, while the smaller bubbles reflect a relatively later nucleation event occurring closer in time to the point of fragmentation. Bubbles in the Mount St. Helens complex ash particles are generally smaller, but have a total number density roughly one order of magnitude higher, compared to the Fuego samples. Results demonstrate that because ash from explosive eruptions preserves the size of bubbles that nucleated in the magma, grew, and then burst during fragmentation, the analysis of the ash-sized component of tephra can provide insights into the spatial distribution of bubbles in the magma prior to fragmentation, enabling better parameterization of numerical eruption models and improved understanding of ash transport phenomena that result in pyroclastic volcanic hazards. Additionally, the fact that the ash-sized component of tephra preserves BSDs and BNDs consistent with those preserved in larger pyroclasts indicates that these values can be obtained in cases where only distal ash samples from particular eruptions are obtainable.

Highlights

  • Explosive volcanic eruptions can result in significant hazards to people and property due to the generation of pyroclastic density currents, emission of ash into the atmosphere, and deposition of ash at great distances from the source volcano

  • National Institutes of Health, Bethesda, Maryland, USA, http:// imagej.nih.gov/ij/, 1997–2012). Complex ash particles Both simple and complex ash particles were found in all locations (Figures 7 and 8 and Additional files 1 and 2), but since complex ash particles are typically larger than simple ones, the relative proportion of complex particles decreases with distance from the vent as larger particles settle out of the plume faster (Figure 9)

  • Vesicle imprints were observed in hundreds of complex ash particles, and digital elevation models (DEMs) were constructed for many of these

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Summary

Introduction

Explosive volcanic eruptions can result in significant hazards to people and property due to the generation of pyroclastic density currents, emission of ash into the atmosphere, and deposition of ash at great distances from the source volcano. By constraining the sizes of the bubbles that burst during fragmentation, it is possible to glean information regarding magma ascent and vesiculation processes, as well as to establish a correlation between eruption style and the proportion of ash generated during eruption. It is toward these goals that this research is directed

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