Box 41 Physical properties of ice

When the annual amount of snow and ice exceeds ablation, there is net accumulation; successive layers of snow build up and the deeper layers are transformed into ice as the volume of air-filled pores is reduced and density increases (Paterson, 1994). New-fallen snow has a density of 0.02-0.2gem 3, while firn (snow that has survived one melt season) has a density of 0.4-0.83gem-3. Glacier ice has a density of 0.83-0.91 g cm-3, while pure ice has a density of 0.917gem-3. The transformation processes, and the time it takes for transformation, depend on climate. Where melting is rare, like in cold polar regions and at high altitudes, the most important factors are wind transport, crystal movement, changes in crystal size and shape, and internal crystal deformation.

The hardness of ice increases with decreasing temperature. At 0°C the hardness is 1.5 (Mohr's scale) and increases to 6 at —70°C. Ice is considered to belong to the hexagonal system because the crystals are oriented with six more or less regular edges. An ice crystal is built up of parallel layers where the molecules are in hexagonal rings. Glacier ice is polycrystalline, where the crystals lie more or less disorganized with the c-axis pointing in all directions. Glacier ice behaves like a plastic body. The rate of deformation of an ice crystal increases rapidly with the amount of pressure affecting the ice crystal. As a result, a glacier stretches out and thins on steep slopes, and becomes thicker in flatter areas.

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