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

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.

NanoSIMS results from olivine-hosted melt embayments: Magma ascent rate during explosive basaltic eruptions

“…In the case of the October 14 phase, it was observed that the distribution of bubbles could be separated into two modes: one mode that was characterized by large but relatively rare bubbles and a second mode with small but very numerous bubbles. These two bubble modes have been interpreted to represent two vesiculation events that occur at a relatively greater depth (large mode) followed by a second event (small mode) at a later stage of ascent (Genareau et al, 2013). Without pressure estimates for this transition in nucleation style, we cannot thoroughly apply this to our model; however, these observations do lend support to multiple stages of decompression during magma ascent.…”

“…A recent study has revealed a bimodal distribution of bubble sizes in eruptive products from the October 14 eruptive phase of 1974 Fuego eruption (Genareau et al, 2013). Bimodal distributions of bubble sizes and variations in the complexity of bubble shape have generally been thought to represent separate nucleation events during explosive eruptions (Blower et al, 2003).…”

Erupción del volcán de fuego (Guatemala, 1974): ¿Evidencia de una fragmentación terciaria?

“…La distribución en tamaño de las vesículas es bi-modal, lo que sugiere al menos dos eventos de nucleación durante la erupción. Aun cuando el modo más grande tiene la menor densidad en número de vesículas, este representa el grueso del volumen total de las vesículas (Genareau et al, 2013).…”

“…Esta propuesta concuerda con la distribución bi-modal tamaño de vesículas reportada por Genareau et al (2013), sugerentes de un mínimo de dos eventos de nucleación: uno con vesículas más grandes que ocurre a profundidad en el conducto, con un crecimiento difusivo y de descompresión, y quizá con coalescencias; y un segundo evento, con vesículas más pequeñas, poco tiempo antes de la fragmentación.…”

“…La posibilidad que SEC1 produzca SEC2, a partir de los resultados de esta gráfica, no se puede comprobar por que la pendiente es muy cercana a cero. Estas consideraciones concuerdan con los hallazgos reportados en Genareau et al (2013), donde se indica que las muestras tomadas del volcán Fuego contienen más partículas complejas (con evidencia de múltiples vesículas, algunas sin explotar) y más grandes (relativas a las de Mount St. Helens), además de mantener una fracción mayor de vesículas grandes. Esos autores interpretaron este hallazgo como resultado de una tasa de descompresión menor y/o una mayor viscosidad del magma de Fuego, en línea con una fracción acumulada de vacío (cumulative void fraction) de 0,749 ± 0,051, opuesto a la de 0,864 ± 0,030 obtenida para Mount St. Helens.…”