Glacier Formation

Glaciers form through the the compaction and recrystallization of snow in areas above a point called the snowline, where at least some amount of snow remains on the ground perennially, with the snowline in polar regions often being located around sea level, while in more temperate regions it occurs at higher elevations.  Though freshly deposited snow is quite low density, it is slowly compacted over time, becoming denser and denser. Over the first year of this process, the snowflakes lose their classic shape, becoming a granular material referred to as firn. Then, after several more years, the firn is compacted into true glacial ice.

Stages of snow compaction into glacial ice. Source: Ritter, Dale F., et al. Process Geomorphology. Waveland, 2011.