Glaciers shape high altitude and latitude landscapes in numerous ways. Erosion associated with glacial processes can limit the average height of mountain ranges, while creating the greatest relief on Earth and shaping the highest mountain peaks, but glaciers can also shield pre-existing topography. Glacial erosion processes, though still enigmatic, are central to the evolution of landscapes, particularly since the onset of the Pleistocene. Glacial erosion comprises three fundamental processes: (1) abrasion, (2) quarrying and (3) the direct action of subglacial water flow in sediment transport and bedrock erosion. Glacier sliding and the hydro-mechanical conditions at the ice--bed interface drive erosion processes, and are themselves controlled by the morphology and state of the subglacial drainage system. Although widely acknowledged, the direct and indirect controls of subglacial water flow on glacial erosion have been largely neglected in previous studies. This thesis focuses on exploring these controls using numerical models with an emphasis on sub-annual to annual timescales. This work has three primary objectives: (1) to investigate how the state and morphology of the subglacial drainage system indirectly drive abrasion and quarrying, (2) to devise the first model of direct bedrock erosion by subglacial water flow and (3) to develop a framework for sediment transport in subglacial channels over a rigid bed. The results show that well-known seasonal variations in subglacial hydrology drive patterns of glacial erosion. Abrasion is favoured where the drainage system is the most dynamic, whereas quarrying calculated using a recently published law is hindered; the latter result is at odds with previous theories. Direct erosion by subglacial water flow can explain bedrock channel excavation, but the resulting erosion rates remain negligible compared to expected basin-wide glacial erosion rates. The models predict a bottleneck in sediment transport near the glacier terminus that is inherent to channel dynamics. The resulting sediment accumulation provides a process-based explanation for esker deposition, and could shape proglacial sediment yields. In focusing on spatial and temporal scales commensurate with subglacial processes, this study challenges some of the common assumptions made in glacial erosion studies and provides a starting point for refining models of landscape evolution.
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Thesis advisor: Flowers, Gwenn E.
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