Glaciers form when multiple snowfalls in mountainous or polar regions turn into ice that flows across the land, powered by gravity. They are present throughout the year.
Glaciers are so powerful they can change the shape of mountain valleys. As a glacier flows down a valley it wears away the rock and changes it from a typical V-shape, created by river erosion, to a U-shape. This characteristic U-shape makes it easy to spot ancient glacial valleys.
Scientists are particularly interested in merged large glaciers called ice caps or ice sheets in Greenland and Antarctica. They are investigating how manmade climate change is affecting these vast reservoirs of fresh water.
Image: Wright Glacier in Alaska, extending from the Juneau Ice Field to a glacial lake (credit: Gregory G. Dimijian, M.D./SPL)
See amazing footage of a glacier moving towards the sea.
Frozen Planet programme maker Jeff Wilson describes a fast moving glacier moving towards the sea.
Manmade tunnels lead to the Svartisen glacier's beautiful underside.
Dr Iain Stewart follows manmade tunnels that take him deep below the Svartisen glacier in Norway to see why moving sheets of ice are so powerful.
Sound recordings from deep within the ice reveal movement.
Dr Iain Stewart introduces sound recordings made deep within glaciers. It is possible to hear the glaciers creak and groan as they move downhill.
Ice that flows across the land shapes our planet's surface.
Dr Iain Stewart explains how annual snowfall accumulations are gradually compacted to form layers of hard ice that in turn form glaciers.
Many of the world's glaciers are retreating.
Dr Iain Stewart describes the retreat of many of the Earth's glaciers and the break up of polar ice sheets.
A glacier (US // or UK //) is a persistent body of dense ice that is constantly moving under its own weight; it forms where the accumulation of snow exceeds its ablation (melting and sublimation) over many years, often centuries. Glaciers slowly deform and flow due to stresses induced by their weight, creating crevasses, seracs, and other distinguishing features. They also abrade rock and debris from their substrate to create landforms such as cirques and moraines. Glaciers form only on land and are distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.
On Earth, 99% of glacial ice is contained within vast ice sheets in the polar regions, but glaciers may be found in mountain ranges on every continent except Australia, and on a few high-latitude oceanic islands. Between 35°N and 35°S, glaciers occur only in the Himalayas, Andes, Rocky Mountains, a few high mountains in East Africa, Mexico, New Guinea and on Zard Kuh in Iran.
Glacial ice is the largest reservoir of freshwater on Earth. Many glaciers from temperate, alpine and seasonal polar climates store water as ice during the colder seasons and release it later in the form of meltwater as warmer summer temperatures cause the glacier to melt, creating a water source that is especially important for plants, animals and human uses when other sources may be scant. Within high altitude and Antarctic environments, the seasonal temperature difference is often not sufficient to release meltwater.
Because glacial mass is affected by long-term climate changes, e.g., precipitation, mean temperature, and cloud cover, glacial mass changes are considered among the most sensitive indicators of climate change and are a major source of variations in sea level.