Glaciers form where snow accumulation exceeds what process?

Glaciers form where snow accumulation exceeds ablation. Accumulation adds mass through snowfall, avalanches, frost, and refreezing, while ablation removes mass through melting, sublimation, wind erosion, and calving.

A glacier begins when snow survives from one season to the next. Repeated burial compresses the snow, squeezes out air, and transforms it first into granular firn and eventually into dense glacial ice. As the ice thickens, gravity causes it to deform and flow downhill or outward.

A glacier's mass balance compares accumulation with ablation. When accumulation remains greater over time, the glacier can grow and its terminus may advance. When ablation exceeds accumulation, the glacier develops a negative mass balance and generally retreats. A retreating glacier is still flowing; its terminus is simply losing ground faster than ice is being replenished.

The boundary between the accumulation zone and the ablation zone is called the equilibrium line. Confusing ablation with melting alone is a common mistake: melting is important, but ablation also includes sublimation, wind removal, and the breaking off of icebergs.