Direct Measurement of Stress at the Base of a Glacier

Abstract Contact stress transducers were placed in subglacial bedrock and used to monitor continuously shear stress and normal pressure changes at the contact with the overriding glacier sole 100 m beneath the surface of the Glacier d’Argentière during periods in summer 1973 and spring 1975. The measured fluctuations in normal pressure and shear stress do not appear to be related to changes in sliding velocity. Analysis of the data reveals short-term fluctuations in normal pressure and shear stress which appear to be related to the passage of individual large debris particles or groups of particles over the transducer. The shear stress appears to be a function of the volume concentration of debris in the ice. The volume concentration at any point appears to be partially dependent on a “streaming” process by which basal debris-rich ice tends to flow around the lateral flanks of hummocks on the glacier bed. Where sub-glacial cavities occur, this streaming effect appears to be dependent on the extent of cavitation and thus on ice overburden pressure and velocity. It is suggested that this process can account for an apparent lag between changes in normal pressure and shear stress. The maximum ratio between shear and normal stress averaged over a period of 10 min was 0.44. This is equivalent to a spatial average over 0.3 cm. Debris concentrations in basal ice of up to 43% by volume occurred. It is suggested that concentrations of this order are common at the base of temperate glaciers and thus that a significant part of the drag at the base of a glacier may be contributed by frictional interactions between the basal-debris load and the bed.