recrystallization

Recrystallization is a process where a substance forms new crystals. In the context of ice, it refers to the transformation of snow into glacier ice. This occurs through several stages, beginning with snow crystals in the atmosphere, which are typically hexagonal plates, needles, or stars.1

When snow is deposited, these intricate shapes are unstable. Molecules evaporate from sharp points and condense in hollows, causing the ice grains to round and fit more closely together. This process, along with wind packing, increases the snowpack's density.1

As the snowpack densifies, recrystallization under stress from overlying snow becomes dominant. This causes grains to change in size and shape to minimize stress, with larger or more favorably oriented grains growing at the expense of others. When the density reaches about 830-840 kilograms per cubic meter, the air spaces are sealed off, marking the transition from firn to glacier ice.1