A glacier is a persistent body of ice formed on land from accumulated snow that becomes sufficiently thick to flow under its own weight. Its movement, driven by gravity, distinguishes it from a stationary snowfield. Glaciers are components of the cryosphere, ranging from small mountain ice bodies to continental-scale ice sheets. They store frozen water, transport sediment, reshape terrain, and preserve evidence of past environmental conditions. (nps.gov)
Formation and mass balance
Glaciers develop where snow survives successive summers and accumulates over many years. Burial, compaction, and recrystallization transform snow into firn, an intermediate granular material, and eventually into dense glacial ice. Formation depends on both snowfall and summer melting: low temperature alone does not guarantee glacier growth in an extremely dry environment. Avalanches and wind redistribution can also supply snow. (nps.gov)
The difference between gains and losses is a glacier’s mass balance. Accumulation adds material, principally through snowfall; ablation removes it through melting, sublimation, and the breaking away of ice. A positive balance increases total ice mass, whereas a negative balance reduces it. Mountain glaciers usually have an upper accumulation zone and a lower ablation zone. The equilibrium line separates areas of annual net gain from areas of annual net loss. (nsidc.org)
Mass balance and the position of the glacier’s terminus are related but are not identical measurements. A retreating terminus does not mean that ice flows backward: ice continues moving toward the margin while losses exceed the supply arriving there. (nsidc.org)
Movement and surface features
Glacial movement occurs through internal deformation and, where conditions permit, sliding at the bed. Under sustained stress, ice deforms and transfers material from accumulation areas toward margins. Basal movement depends on the underlying rock or sediment and the presence of liquid water. Ice frozen to its bed may have little basal motion, although deformation still allows the ice above it to flow. (nsidc.org)
Flow is uneven across a glacier. Resistance from the bed and valley walls commonly makes movement slower near these boundaries than near the surface and center. The relatively brittle surface can fracture as deeper ice deforms or moves over irregular terrain, producing crevasses. These fractures may be hidden beneath snow bridges. (nps.gov)
Where glaciers terminate in lakes or the ocean, pieces can detach through calving, forming icebergs. Some glaciers also undergo episodes of unusually rapid movement called surges, rather than maintaining a nearly steady flow rate. (nsidc.org)
Types and distribution
Glaciers are classified partly by their shape and relationship to terrain. Valley glaciers occupy elongated mountain valleys. Cirque glaciers lie in bowl-shaped mountain hollows. Piedmont glaciers spread into broad lobes where valley-confined ice reaches flatter land. Tidewater glaciers terminate in the sea; this designation describes their setting rather than their size. (nsidc.org)
An ice cap is a dome-like body of glacial ice covering less than 50,000 square kilometers. An icefield is more strongly controlled by the underlying topography and commonly feeds several valley glaciers. An ice sheet exceeds 50,000 square kilometers; the two present-day examples cover Greenland and Antarctica. Their ice can drain through outlet glaciers and fast-flowing ice streams. (nsidc.org)
When grounded glacier ice extends into seawater and becomes afloat, it can form an ice shelf. Such floating extensions differ from sea ice, which forms through the freezing of seawater. (nsidc.org)
Erosion and landforms
Glaciers modify landscapes through erosion, transport, and deposition. Rock fragments embedded in basal ice scrape and polish bedrock through abrasion, sometimes leaving parallel scratches. Plucking removes blocks of rock that become incorporated into the moving ice. These processes can deepen and widen valleys. (nps.gov)
Characteristic erosional landforms include U-shaped valleys, bowl-shaped cirques, sharp ridges called arêtes, and pyramidal peaks called horns. Hanging valleys occur where a tributary valley floor remains above the main glacial trough; streams descending from them may form waterfalls. (nps.gov)
Material deposited directly by ice commonly forms unsorted sediment known as till. Accumulations of glacial debris produce moraines, including ridges along glacier margins or across former terminus positions. Meltwater transports and redeposits sediment beyond the ice. These deposits and erosional features help reconstruct former glacier extent. (nps.gov)
Water, climate, and observation
Glaciers participate in the water cycle by storing snowfall and releasing meltwater. Their runoff supplies rivers and supports downstream drinking-water systems, irrigation, and hydropower. Glacier-associated lakes can also create hazards: failure or overtopping of an ice or moraine barrier may produce a glacial lake outburst flood. (public.wmo.int)
Changes in glaciers are important indicators of climate change. Warmer air and ocean conditions increase ice loss, while surface darkening by dust and soot can increase absorption of solar radiation. Melting land-based ice contributes to sea-level rise. For 2000–2023, a global assessment estimated average glacier losses of approximately 273 billion tonnes annually, excluding the Greenland and Antarctic ice sheets, equivalent to about 0.75 millimeters of global sea-level rise per year. (nsidc.org)
Researchers measure glacier change using snow surveys, stakes installed in ice, aerial photography, lidar, and satellite imagery. Repeated elevation measurements reveal changes in volume, complementing observations of glacier area and surface mass balance. Ice cores provide another record: trapped gases, particles, and variations in water isotopes preserve evidence used in paleoclimatology to investigate atmospheric composition and past temperatures. (usgs.gov)