Characterisation of reverse grading in ignimbrites through image analysis and experimental granular currents

Johnson, Matthew

Abstract

Pyroclastic density currents (PDCs) are hot, density-driven fast-moving flows of gas, rock and ash produced by volcanic events such as explosive eruptions, the fallback of eruption columns or the collapse of lava domes. They are deadly geological hazards which have caused >90 000 deaths since 1600 AD. We must improve our understanding of PDCs and their deposits to improve our ability to prepare for future events. PDCs are rarely observed up close due to their hazardous nature and as such real time analysis is difficult. Through the use of models and the interpretations of deposits, known as ignimbrites, we can improve our understanding of the flow dynamics of PDCs. The deposits of PDCs can provide important information about how these deadly volcanic hazards behave in time and space. Reverse grading of clasts is often observed in these deposits and can be interpreted in different ways such as growing eruption intensity where larger clasts are introduced over time. Alternatively, it could record kinematic sorting (the ‘muesli effect’) where small grains percolate downwards and large grains rise. The link between current dynamics and reverse grading is previously untested in aerated granular currents.
This study used aerated granular currents created in an analogue flume to investigate how reverse grading may be related to kinematic sorting. These experiments are complemented by sedimentological characterisation of ignimbrites through image analysis along with static tests of kinematic sorting. Our results show that aerated currents are stratified through kinematic sorting whereby larger grains are carried towards the top of a current and smaller grains are closer to the base. Stratification of the current controls the composition of the flow boundary zone and therefore the clasts which are able to deposit. Through quantitative analysis, we show that kinematic sorting during flow is directly linked to creating reversely graded deposits.

Citation

Johnson, M. Characterisation of reverse grading in ignimbrites through image analysis and experimental granular currents. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4320614

• Thesis Type: Thesis
• Deposit Date: Jun 30, 2023
• Publicly Available Date: Jun 30, 2023
• Keywords: Geology
• Public URL: https://hull-repository.worktribe.com/output/4320614
• Award Date: Sep 1, 2022