glacier

A glacier is a large, persistent body of ice that forms on land from the accumulation, compaction, and recrystallization of snow over many years or centuries. Glaciers flow slowly under their own weight, a combination of internal deformation and basal sliding, with movement ranging from centimeters to meters per day in active regions. Snow that survives the summer becomes firn and eventually glacial ice as it is compressed.

Glaciers are classified by geometry and regime. Valley (or alpine) glaciers occupy mountain valleys and flow downslope; continental glaciers form large ice sheets that cover substantial areas, such as those in Greenland and Antarctica; ice caps and piedmont glaciers describe broad, dome-shaped and spread-out terminus configurations, respectively. Each type has a local balance between accumulation in the upper zones and ablation (melting and sublimation) in the lower zones, with an equilibrium line marking the boundary.

Glacial landscapes are shaped by erosion and deposition. As glaciers advance and retreat, they carve U-shaped valleys, cirques, and arêtes, and transport rock fragments, creating moraines, drumlins, and eskers. Fractures produce crevasses and seracs, and termini that reach the sea may calve to form icebergs. Glaciers can generate proglacial lakes, which may pose flood hazards known as glacial lake outburst floods.

Glaciers occur mainly in high-latitude regions and high mountain ranges. They are found in Greenland, Antarctica, the Arctic and subpolar regions, and in the Alps, Himalayas, Andes, and other mountains. They are sensitive indicators of climate change, with many glaciers retreating in recent decades as global temperatures rise, though some have advanced or remained stable where snowfall balances or exceeds melt.