Introduction Polycrystalline aggregates in pure shear dynamic recrystallisation Grain shape and preferred orientation changes Grain shape and preferred orientation changes Fabric Evolution of glacial ice during deformation Movies References List of figures Acknowledgements Home
Introduction

Experimental modelling to illustrate the development of microstructures at high metamorphic grade is inherently difficult due to the paucity of information concerning the rheology of polyphase rocks in the earths crust. Conversely, the rheology of ice, even close to its melting point, is very well known and provides an excellent material to model a dynamic system.

A proper interpretation of the origins of microstructure and microstructural dynamics is fundamental to our understanding of the flow in rocks and in ice masses such as glaciers. Glacial ice and rock at high metamorphic grade deform according to the same non-linear flow laws. Ice is therefore an ideal analogue for the study of crustal deformation. Folds, faults, boudinage structures and shear zones can be observed in glaciers at both the mesoscale and macroscale. It is also possible to directly measure the strain rate associated with the development of structures in glaciers. The integration of strain rate measurements with constitutive flow laws and an understanding of material parameters allows us to interpret the flow of polycrystalline materials.
 
Movie 1 Image
Movie 1 - Pure shear deformation in polycrystalline ice over About 2.5 days. Click on the image to see the movie. It takes a few minutes to download. So please be patient.
This is a deforming ice mass, that is viewed under a microscope and photographed in plane polarised light using time lapse photography.

The processes we see occur as solid state transformations and involve submicroscopic interactions between dislocations, and differently oriented grain boundaries in a variety of misoriented grains with respect to the applied stress axis.

These are referred to as intracrystalline processes and involve the generation of defect structures in the crystal lattice as a result of the deformation.

The flow of rock and on the scale of a crystal is dominated by the anisotropic nature of the crystal structure; in hexagonal minerals such as quartz (see fig. 1.1.1) and ice (see fig. 1.1.2) the most commonly reported slip system is in the basal plane (0001). Basal glide will only occur if the critical resolved shear stress on the potential glide plane is sufficiently low to activate three-dimensional glide. Diffusional flow is another process that operates at high temperatures.
 
Figure 1. 1. 1
Figure 1.1.1. Quartz crystal showing location of main crystal faces and principal crystallographic axes.
The table lists some of the potential glide systems in quartz.
 

It should be noted that not all slip systems will be activated as the critical resolved shear stress of many of these systems will be too high to prevent activation of the system.

As the critical resolved shear-stress is very low on (0001) then deformation is dominated by slip on the basal plane.

Additional independent glide can occur on non-basal systems and include various combinations parallel to an a-axis, c-axis or one of the vector sums c+a and these are particularly important in the higher temperature deformation of minerals such as quartz (see fig. 1.1.1).
 
Figure 1. 1. 2
Figure 1.1.2. Schematic representation of the basal glide plane (0001)
in ice in relation to non-basal glide planes and glide directions.
 
Because of their low crystallographic symmetry and their limited glide systems it is possible to make comparisons between the microstructural behaviour of quartz (see fig. 1.1.1) and ice (see fig. 1.1.2) when undergoing plastic deformation. For instance if a single crystal is flattened it will shear by intracrystalline glide on its basal plane; this produces a shape transformation and a rotation of its c- axis (see fig. 1.1.3). A crystal can be considered as a matrix of elastic-plastic glide planes belonging to the one slip system, which will follow elastic-plastic behaviour and will only be activated if it reaches a critical value of resolved shear-stress.
 
Figure 1. 1. 3
Figure 1.1.3. The flattening of a single crystal that accommodates the shortening by shearing on its basal plane in which the initial critical resolved shear-stress will be high as it lies approximately 45° to the compression direction.
 
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Created: August 23, 1999
Last modified: March 15, 2004
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