Reconstructing Eruption Energetics From Volcanic Ash Morphology and Geochemistry

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Volcanic eruptions are driven by the expansion of bubbles in magma as it rises through the "plumbing" system beneath a volcano. Gases (mostly water) dissolved in the magma at depth come out of solution as the magma rises to lower pressures near the Earth's surface because the amount of gas that can be dissolved in the magma depends on pressure. When the amount the magma can hold reduces below the amount dissolved, new bubbles are formed, and gas from adjacent magma migrates through the magma into the bubbles. As they grow, they push the overlying magma up and out of the volcano, which causes the overlying bubbles to grow faster, leading to ascent rates at the surface near the vent of the volcano that can be so fast in the case of highly explosive eruptions, that the gases do not have enough time to slowly diffuse through the magma into existing bubbles, and new bubbles form again just prior to eruption. This results in a set of original, large bubbles, and a second set of much smaller and more numerous bubbles. This project is directed toward better determining the threshold of explosivity that can produce this second set of bubbles, so that if future eruptions of individual volcanoes behave like the past ones that produced volcanic ash that can be observed, a better prediction of eruption energy and thus volcanic hazard can be obtained.The study will involve examination of ash from various well observed volcanic eruptions across a spectrum of energies, commonly characterized by "Volcanic Explosivity Index" (VEI). By detailed study using a Scanning Electron Microscope (SEM) the imprints of both populations of bubbles can be observed. There should be a threshold of eruption energy above which late-stage nucleated bubbles (syn-eruptive) are found in ash particles, and below which, only the initial set of large bubbles (pre-eruptive) exist. This threshold will be quantified in this study. In eruptions in which water (as the dominant dissolved volatile) does not have time to diffuse into pre-eruptive bubbles and syn-eruptive bubbles are nucleated, quenching of the erupting magmatic foam as it fragments to ash is expected to leave a concentration gradient of water still dissolved in glass that depends on distance from both pre-eruptive and syn-eruptive bubbles. This water concentration can be observed using Micro-Raman Spectroscopy, which is sensitive to the presence of O-H bonds in the glass of individual ash particles. The resolution of the technique is sufficient to map water concentration across an ash particle and also as a function of depth within the particle. This technique has never been applied to individual ash particles, so the project will also result in a new tool for the scientific community to use in future related projects. By determining the threshold for the nucleation of syn-eruptive bubbles, and further by exploring dissolved water concentration gradients in individual ash particles, this project should result in better understanding of both volcanic hazards, and the processes that drive magma ascent and the formation of ash during explosive volcanic eruptions.