CONVERSION OF SNOW TO GLACIAL ICE
Glaciers form where summer melting of snow and ice removes less than the amount added by winter snowfall. The excess snow is buried progressively deeper year-by-year and is converted to blue glacial ice.
The melting point of ice at normal atmospheric pressure is 0° C, but even a little pressure can lower this temperature. This is why snow can be packed into a dense snowball that will sting your brother's ear when it hits! The pressure of your squeezes melts the snow a little and when you release the pressure it re-freezes into a harder mass. The pressure generated by new snow piling up above old snow increases the pressure on the latter causing it partially to melt.
The melting takes place at points of contact -- specifically the elongate "arms" of the hexagonal ice crystals (#1 in the diagram). As the tiny projections melt, surface tension pulls the water inward toward the center of the crystal. As soon as it is away from the point of contact it refreezes (#2 in the diagram). The process continues (#3), serving to concentrate the mass of the water closer to the center of the original snowflake. At some point (#4) the ice has lost its flake-like original shape and has become a well-rounded granule of ice. A layer of this type of ice is called FIRN. With greater pressure (deeper burial) the firn grains fuse together and become a solid mass of crystalline GLACIAL ICE (#5).
Because snowflakes and other ice are naturally occurring, inorganic, crystalline solids they are minerals in the strictest sense of the word. Because snow crystals form by precipitating from a dissolved (evaporated) state in the atmosphere, snow is a chemical sedimentary rock. Because the transformation of snow to firn and finally glacial ice results from increased pressure (and heat transfer) a glacier is a metamorphic rock. A drink "on the rocks" is exactly that!
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