What processes turn snow into dense glacier ice?

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

Initially, intricate snow crystals in a snowpack become unstable, causing molecules to evaporate from sharp points and condense into hollows, which rounds the grains and allows them to pack more closely. Wind may also break crystal points to increase density. As grains touch, they develop connecting "necks" through sintering, or rotate to fit together more tightly. If liquid water is present, the process accelerates significantly through melting, refreezing, and the compacting force of surface tension.1

Snow that survives a melting season becomes firn. As burial depth increases, the weight of overlying snow causes recrystallization, where grains change size and shape to minimize stress. Once density reaches approximately 830–840 kg/m³, air spaces are sealed off, making the material impermeable. Further compression of remaining air bubbles eventually results in clear, dense glacier ice.12