Mobility of Pyroclastic Density Currents: Integrating Field and Experimental Techniques to Understand the Controls and Consequences of Erosion

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Pyroclastic density currents (PDCs) are the most dangerous hazard associated with explosive volcanism. These unpredictable currents consist of searing hot clouds of gas, ash and rock that travel down the slopes of erupting volcanoes with tremendous force and velocity. The driving force for these devastating currents is their dense nature (due to the mixture of ash and rock) relative to the ambient air. PDCs will travel across the landscape, potentially many miles beyond the volcano flanks, until the ash and rock within the current has dropped out and the current density decreases to match that of the ambient air. Despite the pervasiveness of PDCs and their deadly consequences, many fundamental aspects of their behavior and controls on runout distance remain poorly understood. One of the most important gaps in our understanding of these currents is the mechanism(s) for eroding into the surface over which a PDC flows, and the influence of mixing substrate material into the current on downstream flow dynamics. Given that the primary control on runout distance is a current?s density relative to the ambient air, bulking of the current due to entrainment of the substrate would influence and possibly extend the ultimate runout distance, thereby increasing destructive potential. This work combines field techniques and scaled laboratory experiments to examine the complex relationships between PDC conditions and erosion, and the consequence of erosion on current mobility.This project will explore the control of three main parameters on a current's ability to erode from the substrate: slope, degree of fluidization (pore pressure), and nature of the substrate (particle size, particle density, thickness of erodible bed and substrate roughness). This work will be conducted in three phases. The first phase includes field studies on the well-exposed PDC deposits from the May 18th, 1980 eruption of Mt St Helens (MSH), which builds on the previous work on these deposits of the lead investigator. Field work includes textural, granulometry and componentry studies to determine (or infer) the source of eroded lithics within the PDC deposits, the substrate conditions that favor erosion (e.g., slope, surface roughness) and the influence of erosion on downstream flow dynamics of the eroding PDCs. The second phase involves scaled experiments to explore the general conditions that favor erosion via shear at the base versus underpressure in the head of fluidized currents, which build on the fundamental work of collaborator Dr. Roche. The third phase includes experiments that specifically explore our interpretations and hypothesis developed from the phase one field results from MSH by assessing (1) the influence of topographic obstacles on erosion and downstream flow dynamics, (2) the role of an increased density gradient on basal shear stress and erosion, and (3) the development of fabric in laboratory flows as a function of degree of fluidization and interaction with obstacles. The ultimate goal is to develop a more comprehensive understanding of the controls on PDC damage potential and runout distance, which will enable better assessments and mitigation of the hazards associated with future explosive eruptions.