Collaborative Research on the Basal Motion of Glaciers: Field Studies of Debris/Bed Friction and Bed Deformation, Engabreen, Norway

 

Of the factors that affect the speed of ice masses, those that influence rates of basal motion are among the most poorly understood. One of these factors is sediment, which is entrained in basal ice and sometimes separates glaciers from rock. Sediment in ice causes local friction between basal ice and rock, but there is no consensus regarding the magnitude of this friction or the factors that control it. Till layers that separate ice from rock, in contrast, may lubricate the bed, but the mechanism of flow is uncertain. Sedimentological characteristics of basal tills in the geologic record have not eliminated this uncertainty because there is little experimental basis for field interpretations.

Experiments were conducted at Engabreen, a hard-bedded, outlet glacier in Norway, where the Svartisen Subglacial Laboratory provides human access to the bed beneath 210 m of temperate ice. A prism of till was placed at the ice-bed interface. To assess the factors that control the distribution of motion in the till, instruments recorded the till shear strain, dilation and contraction, and pore-water pressure, while the basal water pressure was varied independently. The final position and orientation of sensors and strain indicators were measured by dissecting the till prism at the ends of tests. These experiments were more compehensive and less ambiguous than borehole studies conducted from the surfaces of glaciers.