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Tunnels in glaciers offer unique opportunities for examining basal processes. At Suess Glacier in the Taylor Valley, Antarctica, a 25 m tunnel excavated into the bed of the glacier provides access to a 3.2 m thick basal zone and the ice–substrate contact. Measurements of ice velocity over two years together with glaciotectonic structures show that there are distinct strain concentrations, a sliding interface and thin shear zones or shear planes within the basal ice. Comparison of ice composition, debris concentrations and the shear strength of basal ice samples suggest that strength is controlled by ice chemistry and debris concentration. The highest strain rates occur in fine-grained amber ice with solute concentrations higher than adjacent ice. Sliding occurs at the base of the ice that experiences the highest strain rates. The substrate and blocks of the substrate within basal ice are characterized by brittle and slow ductile deformation whereas ice with low debris concentrations behaves in a ductile manner. The range of structures observed in the basal ice suggests that deformation occurs in a self-enhancing system. As debris begins to deform, debris and ice are mixed resulting in decreased debris concentrations. Subsequent deformation becomes more rapid and increasingly ductile as the debris and sedimentary structures within the debris are attenuated by glacier flow. The structural complexity and thickness of the resulting basal ice are considerably greater than previous descriptions of cold glaciers and demonstrate that the glacier is or was closely coupled to its bed.

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