Investigation of depositional processes in pyroclastic surges : a large-scale experimental approach : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Earth Science at Massey University, Manawatū, New Zealand
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2022
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Massey University
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Abstract
Dilute pyroclastic density currents (dilute PDCs or pyroclastic surges) are frequent and highly lethal phenomena found on volcanoes. They are hot flows of particles and gas that are capable of inflicting significant damage to life and infrastructure. The study and interpretation of sedimentary structures of their natural deposits have been invaluable in recognizing and characterizing their internal flow dynamics. Traditionally, these field based observations and interpretations are largely based on concepts from sediment transport mechanisms developed for fluvial and aeolian systems. Nonetheless, how well these analogies capture sediment transport in PDCs is still unclear, due to a lack of direct measurements inside these currents because of their inherent hostile nature. This thesis presents the results from large-scale experiments, where scaled hot dilute PDCs were synthesised on a variable substrate roughness, in order to investigate sedimentation processes and lateral evolution of sedimentary structures. Proximal to distal evolution of the deposit emplaced by the experimental currents display high resemblance to deposits emplaced by natural PDCs. The proximally emplaced regressive structure composed of massive, poorly sorted lithofacies is comparable to proximal breccias observed in real world deposits. At medial to distal runout lengths, the experimental flows emplace a deposit with a characteristic ‘tripartite’ geometry that is often found in deposits of blast-like surges. This study finds that the tripartite geometry of experimental dilute PDCs reflects the passage of a flow with i) a head (responsible for the rapid deposition of massive layer A); ii) a body (responsible for tractional bedload aggradation of the stratified layer B); and iii) a tail and buoyant ash cloud (responsible for grain-by-grain aggradation of layer C in weakly tractive conditions). The experimental flows confirm the existence of a non-depositional flow front. Deposits emplaced by the flows propagating on contrasting roughness substrate are very comparable. Analogous deposition despite differing roughness conditions are due to the flow’s ability to deposit and subsequently travel on its own deposit which acts as a roughness substrate. Insights from the internal flow structure (velocity, density, and dynamic pressure) reveal that non-deposition and erosional phases are characterised by the passage of coherent turbulent structures, where episodes of elevated sedimentation rates represent periods between the passage of coherent turbulent structures. This research will help better understand the relationship between the observed sedimentary structures and the internal flow structure of by-passing dilute PDCs, which will ultimately add more understanding to the interpretations of natural dilute PDC deposits.