Ice

Because they share a common composition with their liquid state, ice molecules also consist of the same 2 to 1 ratio of hydrogen and oxygen atoms, the well-known H₂O molecule. The shape of this molecule, the oxygen atom at the center with the two hydrogen atoms separated by an angle of 104.52°, dictates the structure of the solid, crystalline ice. All naturally occurring ice crystals are hexagonal in shape and all snowflakes reflect this basic six-sided crystal habit. The crystal lattice consists of linked hexagonal rings of water molecules with considerable open space in the center of the ring.

Under artificial laboratory conditions of very high pressures and low temperatures, ice can be forced to crystallize in a number of allotropic forms that are stable only under those particular conditions. Crystallization can occur in these laboratory situations in one of several non-hexagonal forms. This is similar to the way that carbon atoms may crystallize to form graphite or, under more extreme conditions, diamond. The conditions under which the alternate forms might be created do not occur naturally on Earth. They may, however, be present on other bodies in space.

The crystalline structure of ice may be deformed by stress, such as the weight of overlying ice on the deeper portions of a glacier. One type of deformation involves shearing of the crystal lattice along parallel planes. Recrystallization, on the other hand, entails the change in the shape and orientation of crystals within the solid. Both of these processes produce the phenomenon known as creep, responsible for the flowing motion of massive ice bodies such as glaciers.