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Mechanical behaviour and structure of the debris-rich basal ice layer

Introduction

Basal ice can be defined as a relatively thin layer of ice that forms at the glacier bed through a combination of thermal and mechanical processes which link a glacier to its substrate. These thermal and mechanical processes result in several characteristics including layers and/or lenses of debris entrained from the bed, deformation structures, relatively high solute concentrations and unusual gas composition that distinguish the ice from ice that has formed solely by the firnification of snow at the glacier surface (Hubbard and Sharp 1989; Knight 1997). Basal ice is thought to be an important component in the rheological behaviour of glaciers.

Most models of glacier behaviour are founded on the assumptions of homogenous deformation behaviour that is described by Glen's flow law and on the presence of a sharp ice-substrate interface. These assumptions are increasingly at odds with field and laboratory observations that demonstrate that basal is characterised by compositional and mechanical properties that distinguish it from glacier ice formed by precipitation alone. These observations suggest that unconsolidated deposits beneath glaciers are common and that these deposits may deform in response to shear stresses imposed by the flowing ice. The separation between modelling efforts and field and laboratory observations is compounded by a tendency to conceptualise the behaviour of basal ice and the glacier substrate as independent entities when such a separation may not be supported by field observations.

Despite a call for enhancement of our understanding of the response to stress of ice that actually exists at glacier beds by Theakstone in 1967, and restatement of the same imperative in a landmark review of basal ice formation and deformation by Hubbard and Sharp (1986) we have not yet achieved a complete understanding of the formation and deformation of basal ice. Until such a comprehension is achieved there seems little prospect that the significance of the behaviour of the basal ice layer can evolve from the treatment of ice as an isotropic material.

The purpose of this chapter is to examine recent developments of the understanding of the deformation at the base of glaciers in the light of field observations and experiments conducted beneath cold-based glaciers in the McMurdo dry valleys. This paper attempts to achieve these objectives by focusing on whether the existing conceptual separation between the behaviour of basal ice and an unconsolidated glacier substrate is consistent with observations made in the field.

Summary

  1. Examination of the literature suggests that there is confusion concerning the treatment of debris-rich basal ice and ice-rich subglacial sediment. What is described as subglacial permafrost by some researchers is treated as debris-rich basal ice by others.
  2. Observations of subglacial conditions in cold-based glaciers in the McMurdo dry valleys demonstrates that in several cases there is no single clear boundary that separates basal ice from a frozen substrate. There is abundant evidence to suggest that deformation of the glacier substrate has resulted in both en masse entrainment of the glacier substrate and mixing of material from the glacier substrate with basal ice.
  3. Deformation measurements made at the base of glaciers in the McMurdo dry valleys demonstrate that the strain fields close to glacier beds are much more complicated that would be expected from Glen's flow law. Such measurements glaciers, together the close examination of the structure of basal zones, suggests that treatment of a glacier bed as a simple ice-substrate boundary is flawed.
  4. In order to advance our understanding of the behaviour of subglacial materials we need to move away from the artificial separation and treatment of basal ice and the glacier substrate and recognise a continuum of material properties and deformation processes which is variable in time and space.

Reference

Fitzsimons, S.J., 2005. Mechanical behaviour and structure of the debris-rich basal ice layer. In: Knight, P. (ed.) Glacier Science and Environmental Change. Blackwell, U.K., 251-259.

 

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© 2009 Department of Geography, University of Otago, Dunedin, New Zealand