Synthesizing Field and Experimental Techniques to Investigate the Enigmatic Processes Occurring at the Base of Pyroclastic Currents
Date of Final Oral Examination (Defense)
Type of Culminating Activity
Doctor of Philosophy in Geosciences
Brittany Brand, Ph.D.
Dorsey Wanless, Ph.D.
Olivier Roche, Ph.D.
Damiano Sarocchi, Ph.D.
Pyroclastic currents are the deadliest hazard associated with explosive volcanic eruptions. These gravity-driven currents consist of volcanic gases and solid particles that range in size from fine ash to boulders. The dangers associated with pyroclastic currents stem from their unpredictability and ability to travel extremely long distances, sometimes in excess of 100 km. To mitigate the risk to populations and infrastructure, we must understand the processes that control the runout distance of pyroclastic currents. The runout distance depends on the complex interplay of processes related to sediment transport, erosion, and deposition. Historically, studies focused on understanding sediment transport and deposition, but studies within the last 15 years demonstrate the important effect of erosional processes on the behavior of pyroclastic currents.
This dissertation research builds on recent studies to investigate how pyroclastic currents interact with the bed via erosion and mixing processes. I seek to answer questions related to the mechanisms by which erosion occurs, how the properties of the bed affect erosion and mixing processes, and how interactions between the flow and the bed affect flow behavior and runout distance. To address these questions, I combine detailed field studies of pyroclastic current deposits with analogue laboratory experiments that simulate pyroclastic currents in a controlled environment. Synthesizing these two approaches, field and experimental, allows for even greater insight into basal processes than either approach could provide on its own.
Ultimately, I show that erosion occurs via a fluid-like mixing process as a result of granular shear instabilities formed at the flow-bed interface. The mixing process generates wave-like structures at the contact between the flow and the bed, and the structures can be preserved in the deposits of both natural and experimental flows. The dimensions of the structures recorded in the deposits directly relate to flow parameters, such as velocity and thickness, at the time the structures formed. I apply scaling relationships derived from experimental data to sedimentary structures observed in the deposits of the pyroclastic currents produced during the May. 18, 1980 eruption of Mount St Helens. This approach produces quantitative estimates of the flow velocity and thickness, important flow parameters that were unconstrained prior to this study. Additionally, the experiments suggest that the erosion and mixing processes decrease the runout distance of pyroclastic currents relative to non-erosive flows, which has important implications for hazard mitigation. Finally, the datasets produced both from the field and experimental studies can be used to test and refine numerical models of pyroclastic currents with the ultimate goal of improving the accuracy of risk assessments for these hazardous flows.
While this dissertation research improves our understanding of the erosion and mixing processes that occur at the flow-bed interface in pyroclastic currents, the final conclusions also beget new questions. Future studies should investigate other mechanisms by which erosion occurs because the mechanism discussed here is not likely to be the single way in which pyroclastic currents entrain bed material. Continued work to synthesize experimental and field studies has the potential to produce additional methods to derive quantitative information from natural pyroclastic deposits. Finally, the next major goal moving forward in the study of pyroclastic currents must be to obtain in situ measurements of flows in real time. Such a dataset will provide the means to test many of the hypotheses set forth regarding the internal processes that govern the behavior of these dangerous volcanic phenomena.
Pollock, Nicholas M., "Synthesizing Field and Experimental Techniques to Investigate the Enigmatic Processes Occurring at the Base of Pyroclastic Currents" (2019). Boise State University Theses and Dissertations. 1599.