What does the snow that forms a glacier change to ice?

The snow that forms a glacier changes to ice through a process called compaction and recrystallization. As new layers of snow accumulate, the weight of the overlying snow compresses the lower layers, causing the flakes to lose air and compact into firmer granules called firn. Over time, with continued pressure and the effects of temperature changes, the firn transforms into solid ice as the ice crystals grow larger and more interconnected. This process can take several decades to centuries, depending on environmental conditions.