Variations in glacier horizontal and vertical motion occur at a variety of intra-annual timescales: monthly, daily and even hourly. These variations have been identified from measurements made both beneath and on the surface of glaciers. They must be associated with variations in basal motion rather than changes in internal ice-deformation rates. Variations in basal motion result from changes in sliding rates over a 'hard bed' (i.e., rigid bedrock) or changes in deformation rates within a 'soft bed' (i.e., unlithified permeable till). Changes in both sliding and bed deformation rates are related to variations in subglacial water pressures and therefore depend critically on the structure of the subglacial drainage system and the hydraulics of individual drainage passageways. Thus changes in subglacial drainage system structure and drainage passageway hydraulics can cause intra-annual variations in glacier motion. However, intra-annual variations in glacier motion will also be influenced by variations in longitudinal stress gradients as a result of changes in the rate of longitudinal extension and compression.
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