The size range of bubbles that produce ash during explosive volcanic eruptions

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

A New 3D Method of Measuring Bubble Size Distributions from Vesicle Fragments Preserved On Surfaces of Volcanic Ash Particles

We have developed a novel method of measuring bubble size distributions from their remains expressed on the surfaces of volcanic ash particles. The morphology of the ash fragments retains a record of bubble size at the time of fragmentation in the curvature of the convex surfaces on the ash fragments. This curvature can be measured using stereo scanning electron microscopy (SSEM), and morphology can be represented using a digital elevation model (DEM) of ash particle surfaces. Due to the vagaries of the sensitivity of SSEM imagery to surface roughness, a three-point fit technique produces more robust results for curvature than a least-squares approach for curve-fitting of ellipsoids of revolution to ash surfaces. The method allows measurement of vesicles within a size range from one to over a million cubic microns. The inferred bubble size distributions so obtained can potentially provide valuable insights regarding magma dynamics and vesiculation that lead to the explosive eruptions that produce ash. An error analysis of the methodology indicates reasonably accurate reconstruction of bubble geometries and bubble size distributions (BSD). Accuracy is constrained primarily by the size of ash particles themselves since the mode of the BSD should be at least a standard deviation smaller than the dominant dimensions of the particles for robust results

3-D reconstruction of ash vesicularity: Insights into the origin of ash-rich explosive eruptions

Explosive volcanic eruptions are characterized by highly variable degrees of magma fragmentation, even during a single eruptive event. The increasing amount of fine pyroclasts is often uncritically related to explosive magma–water interaction (i.e., hydromagmatic fragmentation). Here we report examination of two examples of major explosive eruptions from the Quaternary Vulsini Volcanic District (central Italy), in which the fine-grained nature of deposits, even in near-vent settings, indicates negligible effect of transport and implies the eruption of highly fragmented magmas. SEM morphoscopy of the juvenile products rules out extensive ash production due to hydromagmatic fragmentation. We apply a recently developed Stereo-Scanning Electron Microscopy (SSEM) technique (Proussevitch et al., 2011) to determine vesicularity features (e.g., bubble size distribution and bubble number density; hereafter BSD and BND, respectively) of ash particles. SSEM analysis provides new insights into magma vesiculation history and fragmentation mechanism leading to major ash-rich eruptions. We conclude that extensive ash production was related to essentially magmatic processes involving high degrees of decompression in shallow magma reservoirs

Whole object surface area and volume of micro-scale 3-D models with “hidden surface”

Micro-scale 3D models, important components of many studies in science and engineering, are often used to determine morphological characteristics such as shape, surface area and volume. The application of techniques such as stereoscopic scanning electron microscopy on whole objects often results in \u27partial-view\u27 models with a portion of object not within the field of view thus not captured in the 3D model. The nature and extent of the surface not captured is dependent on the complex interaction of imaging system attributes (e.g. working distance, viewing angle) with object size, shape and morphology. As a result, any simplistic assumptions in estimating whole object surface area or volume can lead to significant errors. In this study, we report on a novel technique to estimate the physical fraction of an object captured in a partial-view 3D model of an otherwise whole object. This allows a more accurate estimate of surface area and volume. Using 3D models, we demonstrate the robustness of this method and the accuracy of surface area and volume estimates relative to true values

La Correspondencia de España : diario universal de noticias: Año XIV Número 1064 - 1861 agosto 24

Explosive volcanic eruptions are characterized by highly variable degrees of magma fragmentation, even during a single eruptive event. The increasing amount of fine pyroclasts is often uncritically related to explosive magma–water interaction (i.e., hydromagmatic fragmentation). Here we report examination of two examples of major explosive eruptions from the Quaternary Vulsini Volcanic District (central Italy), in which the fine-grained nature of deposits, even in near-vent settings, indicates negligible effect of transport and implies the eruption of highly fragmented magmas. SEM morphoscopy of the juvenile products rules out extensive ash production due to hydromagmatic fragmentation. We apply a recently developed Stereo-Scanning Electron Microscopy (SSEM) technique (Proussevitch et al., 2011) to determine vesicularity features (e.g., bubble size distribution and bubble number density; hereafter BSD and BND, respectively) of ash particles. SSEM analysis provides new insights into magma vesiculation history and fragmentation mechanism leading to major ash-rich eruptions. We conclude that extensive ash production was related to essentially magmatic processes involving high degrees of decompression in shallow magma reservoirs

Sizing up the bubbles that produce very fine ash during explosive volcanic eruptions

Explosive volcanic eruptions emit large proportions of very fine ash (\u3c30 μm) into the atmosphere, posing hazards to aviation, infrastructure, and human health. Here we present an analysis of bubble size distributions at the point of fragmentation during the 18 May 1980 eruption of MSH through the examination of simple ash particles in distally deposited fall samples. The external surfaces of individual fine ash grains preserve the morphology of the bubbles that burst to form the ash, so bubble sizes can be measured using stereo-scanning electron microscopy. Simple ash particles are those that allow the measurement of a single vesicle imprint per individual grain. These simple ash particles are the finest component of the tephra, and can thus travel great distances from the source volcano. Analyses of samples provided bubble volume distributions with a dominant peak between 560 and 5600μm3, corresponding to equivalent vesicle diameter modes of 10–22 μm, and these values were consistent for all examined sample locations. Determination of syn-eruptive bubble sizes thus makes it possible to glean information regarding conduit dynamics at the point of magma fragmentation from observed ash deposits, to parameterize numerical eruption models in ways not previously possible, and to quantify the size of bubbles that burst to create the ash component most hazardous to the aviation industry and human health