FLOW IN POLYCRYSTALLINE ICE

Part 2 - Background information

By Chris Wilson and Brett Marmo

2.2 Glaciers

Discharge of ice from the Antarctic and Greenland ice sheets or valley glaciers (Kamb 1959) is controlled primarily by rapid moving ice streams and outlet glaciers. The evolution of glacial ice produces mechanical heterogeneities that change the way an ice mass responds to applied stresses and temperature variations. In order to understand the way a glacier responds to varied stress, strain and temperature histories, it is first necessary to examine the behaviour of ice as a material. In general, deformation in both single crystals and polycrystals is described by Glen's Law, where the strain rate, , is related to stress by a power law of the form:

----------------------------------------equation (1)

A and n are constants (Glen 1955). However, Glen's empirical flow law does not consider the strong plastic anisotropy due to easy glide on the basal plane.

A flow law that includes a geometric term is therefore required to model anisotropic flow. An anisotropic flow law has been implemented in a finite difference model to estimate the pervasiveness of fabrics in the Framnes Mountain outlet glaciers.

Figure 2.2.1: Simulated fabric development in flow lines from an outlet glacier in the Framnes Mountains, Antarctica (after Marmo & Wilson 1999). The fabric is based on the accumulation of strain over a period of 100 years where the initial c-axis distribution was random. The geometric factor that relates the intensity of the fabric is shown, as is the value for the flow parameter A that correlates to the geometric term. Areas of plastic flow from visco-plastic models described by Marmo & Wilson (1999) are also shown. Note the development of a thrust plane below the Rumdoodle Grid in Line A.