Snow metamorphism is a set of processes occurring in it, which lead to the transformation of snow structure - change in shape, size and number of ice crystals and bonds between them.
There are four types of snow metamorphism: isothermal, temperature-gradient, metamorphism of melting/freezing and dynamic metamorphism. Usually all types of a metamorphism work at the same time and their division is a little conditional.
The main processes of isothermal and temperature-gradient metamorphism are sublimation (from the solid to the gas phase without liquid) and desublimation (opposite process).
At an isothermal metamorphism due to a difference of water vapor pressure over convex and concave sites ice crystals are removed by the sublimation from convex sites, move to concave sites and there they are deposited. Such redistribution of substance leads to rounding of snow grains and growth of larger grains at the expense of fine grains.
At a temperature-gradient metamorphism also larger grains grow at the expense of fine grains, but only under the influence of the temperature gradient. Ice crystals sublimate from warmer surfaces, water vapor migrates and is deposited on colder surfaces of the closest crystals. The highest intensity of this process is observed in the beginning of winter in those cases when a small thickness of the snow cover is combined with low air temperature. Under these conditions, the bottom layer of fresh snow can become the layer of deep rime fully for one to two weeks.
Metamorphism of melting/freezing is associated with the melting of crystals and the freezing of water in the snow. With the penetration of water into the snow horizons with a negative temperature the partial freezing of the water films occurs on the surface of the grains and around the contacts between them. Also the shape and size of grains and the links between them are changed.
Processes of change of snow structure under the influence of external mechanical efforts are called a dynamic metamorphism.
1. Snow/snowflakes, chemical sediment composed of ice crystals (90% air, density < 0.1 g/cm3).
2. Granular ice (50% air, density 0.3 to 0.5 g/cm3) as a consequence of compaction and melting/refreezing.
3. Firn (20-30 % air, density 0.5 to 0.7 g/cm3)
4. Firn becomes glacier ice, when the granular ice crystals are frozen together so that air can no longer penetrate the structure (20% air as bubbles, density >0.7 (about 0.97) g/cm3).
Flakes accumulate - Buried by later falls - Compression expels air - Pressure causes melting and recrystallization - Snow turns into granular firn - Over time, firn melds into interlocking crystals of ice.
