How does grain size distribution impact the mobility and deposition of analogue pyroclastic density currents?

Johnston, Thomas

Abstract

Pyroclastic density currents (PDCs) are hot, density-driven flows of gas, rock and ash generated during explosive volcanic eruptions or from the collapse of lava domes. PDCs are able to travel for tens of km, traversing topographic barriers hundreds of metres high. They are notably more mobile than other gravity currents of comparable size. Gas fluidisation has been attributed as a major contributor to this high mobility. Experimentation on non-fluidised granular flows has assessed the influence of grainsize on mobility, finding that the finer the grains, the larger the mobility of the mass. Recent advances in analogue models of gas-fluidised granular currents have revealed the impact of aeration on current mobility, and how flow behaviour can control deposit architecture and morphology. However, these experiments have so far largely used only a single grain-size.
The impact of grain size variations on the mobility of aerated granular currents remains untested. Therefore, this project investigated the impact of grain size distribution on current velocity and run-out distance in a series of analogue experiments using an aerated flume.
The experiments demonstrate that the mobility of these dense granular currents is related to the proportion of fines within the current but is primarily controlled by the initial sorting of the current. The more well sorted currents have a lower velocity but a greater run-out distance than the poorly sorted currents. The proportion of fines is found to have very little control on current velocity, but does have some control on run-out distance and degree of non-uniformity in velocity. Deposition during the analogue experiments occurs by a combination of gradual and stepwise aggradation.
This work contributes our understanding of PDC mobility and how mobility can be interpreted from ignimbrites, with implications for numerical modelling and hazard mapping of PDCs.

Citation

Johnston, T. How does grain size distribution impact the mobility and deposition of analogue pyroclastic density currents?. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4224880

• Thesis Type: Thesis
• Deposit Date: Feb 24, 2023
• Publicly Available Date: Feb 24, 2023
• Keywords: Geology
• Public URL: https://hull-repository.worktribe.com/output/4224880
• Additional Information: School of Environmental Sciences
• Award Date: 2022-11