From a geological point of view the solid phase of water can be regarded as a mineral, in fact the most abundant mineral on the Earth’s surface. Glacier ice is a composite of grains, of which each represents a single ice crystal. In the upper layers of the ice sheets the orientations of the symmetry-defining axes (c-axes) of the single crystals is almost randomly distributed. With depth and ongoing deformation the grains are subject to recrystallisation and recovery processes, which together with the deformation itself influence their shape, size and orientation. This leads for example to an alignment of the symmetry axes of the single crystals with depth and thus the development of a strong anisotropy.
Observation and mapping of grain microstructure and substructures within grains provide information about inter- and intragranular deformation and recrystallisation mechanisms of the polycrystal as well as of the crystal lattices. These sub-microscopic processes add up to the flow of ice observable on the large scale. For the modeling of ice flow on the scale of ice sheets and glaciers it is essential to better understand the underlying processes on the small scale.
Another aspect of ice microstructure is the entrapment of air in bubbles and the development of their shape and size with respect to density and air entrapment during the transition from firn to ice, as well as their transformation to air clathrate hydrates at greater depths.