Direct Observation of Basal Sliding and Deformation of Basal Drift at Sub-Freezing Temperatures

Abstract

In a tunnel at the base of sub-polar Urumqi Glacier No. 1, China, three new mechanisms of glacier flow at sub-freezing temperatures have been observed.Taken individually or in combination, these modes of flow can account for nearly all (60-80%) of the overall glacier motion and, yet, they act only within the lowermost 1-2% of the effective glacier thickness. These mechanisms are:(I) enhanced deformation of the frozen and ice-laden subglacial drift; (2) motion across discrete shear planes or shear bands within the frozen drift or at the ice-drift interface; and (3) basal sliding at an ice-rock interface at a temperature of nearly -5°C. The ice-laden drift has an effective viscosity of more than one hundred times less than that measured in the overlying ice, thus allowing very rapid shear deformation. The observed rate of basal sliding at the ice-rock interface agrees favorably with that predicted by the recent work of Shreve (1984) if proper account is taken of the measured surface roughness and reduced ice viscosity.

The movement mechanisms of Ürümqi Glacier No. 1, Tien Shan mountains, China

“…In conclusion, it appears that the large mean shear strain rate of the debris-laden ice in situ found by Echelmeyer and Wang (1987) must be explained by discontinuous slip as mentioned in section 5.…”

“…This speed was approximately 60% of the surface speed, showing that a large part of the glacier movement at this point was due to deformation of the iceladen till bed, which was ~ 0. this layer was less than 2% of the total thickness of the glacier. Echelmeyer and Wang (1987) treated the till as a thin boundary layer and compared the mean rate of shear strain across its 0.35 m thickness with that of the adjacent layer of ice. They assumed that the shear stress was approximately the same in the basal ice and the till, and thereby concluded that the mean effective viscosity for the ice is more than 100 times that of till.…”

“…In contrast, the grain-size distribution of the ice-laden till had a larger percen tage of coarse debris (<I> < 4) (cf. Figure 4 and Echelmeyer and Wang, 1987, fig. 3).…”

“…The ice moving over the boulders was clean. Echelmeyer and Wang (1987) have measured a sliding rate of 0.5 mm dol over a solid rock surface in tunnel I under a shear stress of approximately 60 kPa. In addition, they have also observed discontinuous motion across interfaces between clean ice and underlying ice-laden till in some sections of tunnel I.…”

Strain‐rate effects on the strength of debris‐laden glacier ice

“…Previous research on the mechanical behaviour of basal debris-laden ice has produced a range of results. In general, observations of the behaviour of naturally occurring debrisladen ice suggest that near the melting point, the presence of sediment softens the ice, probably as a result of a liquid water effect (Duval 1977;Echelmeyer & Wang 1987;Cohen 2000). However, there is some evidence that at colder temperatures, debris-laden ice is stronger than clean ice (Lawson 1996).…”