OPP-99-11559 Iverson - Iowa State University OPP-00-02230 Cohen - Yale University
This is a collaborative proposal with Iowa State, Yale University, and Svartisen Glaciological Observatory in Norway. The influence rates of basal motion are believed to affect the speed of ice masses, but the mechanism is very poorly understood. The sediment, which is entrained in basal ice and sometimes separates glaciers from rock, may be an important factor by causing local friction between basal ice and rock, but the magnitude of this friction or the factors that control it are unknown. Till, in contrast, may lubricate the bed, but its mechanism of flow is also uncertain. This uncertainty looms because there have been few field interpretations.
Two types of experiments will be conducted at Engabreen, a hard-bedded, outlet glacier in Norway, where the Svartisen Glaciological Observatory provides access to the bed beneath 210 m of temperate ice. These experiments will be extensions of the Principal Investigators' earlier subglacial work on hardbed sliding at Engabreen, borehole work on soft-bedded glaciers elsewhere, and laboratory studies of till deformation.
The first experiment will test contrasting models of debris[bed friction. A panel that contains a smooth rock tablet will be installed flush with the bed surface. Friction between the tablet and sliding dirty ice will be studied by measuring the shear stress on the tablet, sliding speed, basal melt rate, ice pressure on the bed, and local basal water pressure. The basal melt rate will be independently varied over a wide range. These experiments will provide a complete characterization of the factors that control local debris/bed friction and rates of glacial abrasion.
In the second experiment, a prism of till will be placed at the icelbed interface. To evaluate the factors that control the motion distribution in the till, instruments will record the till shear strain, dilation and contraction, and pore-water pressure, while the basal water pressure is varied independently. The final position and orientation of sensors and strain indicators will be measured by dissecting the till prism at the ends of tests. Also, clast fabric development and mixing between tills of contrasting lithology will be studied to develop sedimentological criteria for evaluating bed shear strain. These experiments will be more comprehensive and less ambiguous than borehole studies conducted from the surfaces of glaciers.
