Glacier

Where snow persists from one year to the next, successive layers accumulate and become compressed. Snow that survives a melt season can become firn, a dense intermediate material between snow and glacial ice. Further burial and compaction transform these layers into ice. A glacier develops when enough ice accumulates for the mass to deform and flow under gravity. The conditions that allow glaciers to form vary with temperature, precipitation, elevation and local geography; cold alone does not guarantee glacier growth if snowfall is scarce.

Glaciers move because their ice deforms under pressure and gravity. Ice within the glacier can slowly creep, while some glaciers also slide over water or soft sediment at their base. The relative importance of internal deformation and basal sliding varies with conditions beneath and within the ice. Mountain or valley glaciers generally follow the terrain downslope; ice sheets spread outward from their interior. Features can include crevasses, which are fractures formed where flowing ice is strained, and a terminus, the lower end of a glacier.

A glacier gains mass through accumulation, especially snowfall and snow that is redistributed by wind or avalanches. It loses mass through ablation, which can include melting, sublimation and, where ice enters a lake or ocean, iceberg calving. The annual mass balance is the net difference between gains and losses. A glacier may gain or lose mass while continuing to flow. Its terminus can advance or retreat depending on how ice delivery and loss change; retreat of the terminus does not mean the ice has stopped flowing downhill.

Glacier behavior reflects climate as well as local factors such as topography, precipitation, debris cover and contact with water. Scientists measure changes using field observations of snow and ice, stakes, repeat photographs, satellite imagery and measurements of elevation or ice thickness. Measurements of area or terminus position provide useful evidence but are not interchangeable with mass balance: a glacier's shape and front can respond on different timescales. Long-running, consistent observations help distinguish short-term variation from longer-term trends.

Glaciers shape land by eroding bedrock and transporting and depositing sediment. Meltwater from glaciers contributes to streams and rivers, with importance varying by region and season. Loss of land ice also contributes to global sea-level rise. Glacier-fed lakes can sometimes be held by ice or sediment barriers that may fail and release floodwater. These connections make glacier monitoring relevant to Earth science, water management, hazards and coastal change.