Studies on the Basal-Ice Zone of Findelen Glacier, Switzerland

Basal and englacial debris layers have been observed to coincide distinctly with the location of glacier thrust planes or shear zones, e.g. at Shoestring Glacier (Brugman and Meier 1980) and Variegated Glacier (Kamb and others 1985). They are also evident at Findelen Glacier. Field observations strongly suggest that the mechanics of debris-laden ice can be important for understanding the flow response of certain glaciers. In this paper the material properties of basal ice at Findelen Glacier are examined, as studied with the aid of ice-core drilling techniques. Ice cores were taken from near the center line of Findelen Glacier during the summer of 1985, for the express purpose of characterizing the basal ice in terms of observed structures, composition and rheology. Related studies were simultaneously performed on bore-hole and surface strain-rate deformation (Iken and others, unpublished) and bore-hole resistivity (Schütz and Rõthlisberger 1985). The ice core, retrieved from the deepest level possible (approximately 4 m above the bed), contained only a minor amount of fine rock debris. The presence of a debris-containing basal-ice zone is strongly suggested by the evidence that further hot-water drilling in the bore holes resulted in abraded drill stems and in the irregular progress of the drill for the few remaining meters to the glacier bed. Therefore, during 1986 samples of debris-laden basal ice were taken from several locations where the basal material was clearly exposed along lateral ice cliffs in the ablation area of the glacier. Three distinct types of ice at Findelen Glacier were compared: clean bubbly coarse-grained ice taken from above the glacier bed, clean clear coarse-grained ice taken from within a few meters of the glacier bed at center line (170 m depth), and debris-laden ice taken from the glacier bed at the lateral margin. Uniaxial compression experiments were conducted on the glacier-ice samples, using the triaxial testing apparatus of the 1GB at ETH-Höngerberg. Laboratory studies of ice deformation were carried out under the guidance of A. Hofer, P. Hertzog and Professor Huder of IGB. Sample temperature, volume and strain-rate were monitored throughout each experiment. The confining pressure of each experiment was set at 4 bars (1 bar = 100 kPa). The deviatone stresses applied were 1, 2 and 4 bars, and testing temperatures were set at –1.5, -0.8 and –0.4°C for each type of ice sampled (clean bubbly, clean clear, and debris-laden). Preliminary results from experiments conducted on the clean bubbly and clean clear ice indicate that the mechanical behavior of the main bulk of the glacier is well represented by a power-law creep relationship with an exponent of about 4. This value is in close agreement with independent results obtained from bore-hole–tilt studies (Iken and others, unpublished) and within the range of values cited in the published literature (Paterson 1983). The pre-exponential factor is not well represented by an Arrhenius-type relationship, and enhanced deformation is indicated for samples tested nearest the melting point. Additional laboratory and field studies on the ice-crystal fabrics and the behavior of the debris-laden ice are still in progress. In this paper the results of these investigations are discussed, with special emphasis on information gained from the ice-core drilling studies.