How does snowfall change into glacier ice?

The transformation of snowfall into glacier ice is a gradual process of densification driven by pressure, temperature, and time.1

When snow accumulates, the intricate shapes of individual crystals are unstable. Molecules evaporate from sharp points and condense into hollows, causing the grains to round and fit together more tightly. This process is accelerated by wind packing, temperature gradients, and the presence of liquid water, which facilitates melting and refreezing.1

As the snowpack deepens, the weight of overlying layers exerts pressure, leading to:
* Compaction and Recrystallization: Grains change size and shape to minimize stress, often with larger grains growing at the expense of smaller ones.12
* Formation of Firn: Snow that survives a melting season becomes firn, an intermediate granular stage with a density typically exceeding 500 kilograms per cubic meter.12
* Transition to Glacier Ice: Once the density reaches approximately 830 to 840 kilograms per cubic meter, air spaces are sealed off, making the material impermeable to fluids.12

Ultimately, further compression of air bubbles and recrystallization under stress turn the material into clear, dense glacier ice.1