Before proceeding to a more theoretical discussion of the dynamics of glaciers, it will be useful to present a brief introduction to the voluminous literature on deformation or creep of ice. We will begin by looking at deformation processes on an atomic scale and then introduce empirical and semi-empirical relations that provide a macroscopic description of the deformation. Finally, we will show how principles of linear fracture mechanics can be used to predict crevasse depth.

Crystal structure of ice

There are nine known crystalline forms of ice, but seven of them are stable only at pressures in excess of about 200 MPa, and the eighth, a cubic form, ice Ic, is stable only at temperatures below about -100°C (Figure 4.1). As the highest pressures and lowest temperatures in glaciers on Earth are about 40 MPa and -60°C, respectively, these eight forms need not concern us. We thus restrict our attention to the common form of terrestrial ice, ice Ih.

The structure of ice Ih is shown, in stereoscopic view, in Figure 4.2a. It is a hexagonal mineral (hence the “h”) with a rather open structure in which every oxygen atom, represented by the large circles in Figure 4.2a, is bonded to four additional oxygen atoms at the corners of a tetrahedron. The tetrahedra are joined together in such a way that the oxygens form hexagonal rings with the O=O bonds zigzagging lightly up and down as one progresses around the ring (Figure 4.2b); three of the oxygens thus lie 0.09 nm above the other three.