Master of Science
Earth and Environmental Sciences
Sahagian, Dork L.
Other advisers/committee members
Kodama, Kenneth P.; Bebout, Gray E.
Energetic volcanic eruptions are driven by early bubble nucleation. In some cases, during magmatic ascent, decompression rates near the vent become sufficiently high to oversaturate inter-bubble melt enough to trigger a second phase of nucleation. This process creates a bi-modal bubble size distribution: pre-eruptive (50-100 μm) and syn-eruptive (10-50 μm). A scanning electron microscope (SEM) was used to examine bubble imprints preserved in volcanic ash particles and determine an explosivity threshold, represented by the volcanic explosivity index (VEI), below which decompression rates are too slow for a second nucleation event to occur. This threshold is manifested in ash particles as a lack of syn-eruptive bubbles and was found to be between VEI 3 and 4. Furthermore, examination of ash from six eruptions (Redoubt, Spurr, Augustine, Okmok, Novarupta-Katmai, and St. Helens) indicates that eruptions with a higher VEI (e.g., Novarupta-Katmai, VEI 6) have a higher percent of observed particles displaying syn-eruptive bubbles than lower VEI eruptions (e.g., Redoubt, VEI 3). This may reflect a decompression rate gradient horizontally across the conduit during magmatic ascent. For eruptions with higher bulk decompression rates, a larger cross-sectional area of magma within the conduit was able to overcome the slowing force of shear created by this gradient, and decompress rapidly enough to nucleate syn-eruptive bubbles. Furthermore, Micro Raman data show the presence of water in ash from some eruptions (Spurr, Okmok), indicating that magmatic water is still present in some systems upon eruption, and also alluding to complex diffusion and nucleation mechanics. This study advances understanding of the relationship between eruptive products and eruption energetics; this provides a tool to better constrain eruption energetics of ancient eruptions, and thus to better characterize volcanic activity in the past in order to project it to eruption hazards in the future.
Clark, Megan Grace, "Morphological and Geochemical Characteristics of Volcanic Ash: Insights into Eruption Energetics" (2017). Theses and Dissertations. 2555.