glacier

A glacier picks up large boulders and rock debris from the valley and deposits them at the snout of the glacier when the ice melts. Some deposited material is carried great distances by the ice to form erratics.

The Saskatchewan Glacier in the Columbia Icefield, Alberta, Canada. The Columbia Icefield contains about thirty glaciers, and covers an area of 325 sq km/125 sq mi.

A glacier in the Columbia Icefield, viewed from the Icefields Parkway, Alberta, Canada. The Parkway is a 230 km/143 mi stretch of highway in the Canadian Rockies, between Jasper and Banff.

A view of the Muldrow Glacier in Mount McKinley National Park, Alaska, USA. A glacier will begin to form if more snow falls over its point of origin than can melt away or evaporate in warm weather.

Riggs Glacier, at one end of Muir Inlet, which is an extension of Glacier Bay, southeastern Alaska. The area lies within the Glacier Bay National Park and Preserve, to which the only access is by seaplane, tourist cruise-liner, or private yacht. The larger and better known Muir Glacier is a few kilometres to the northeast.

Yahtse Glacier at one end of Icy Bay, Alaska. The Alaska-Canada (Yukon Territory) border travels directly from north to south for 1,000 km/630 mi. If it continued in a straight line, this is the approximate point at which it would have reached the sea. Instead, it turns abruptly eastwards and down the Pacific coast.

Glacier in Patagonia, Argentina. Deep cracks known as crevasses can be seen in the ice. These are formed when a glacier moves over an uneven surface. When the glacier reaches the sea, or as in this picture, a lake, the crevasses will fracture the ice to form icebergs.

Glacier in the northern Himalayas, Tibet. Glacial erosion produces a great variety of geographic features and landscapes.

Glacier on Mount Kilimanjaro, Tanzania. The main peaks of Kilimanjaro, the highest mountain in Africa, are three extinct volcanoes: Kibo, Mawensi, and Shira. Glaciers on Kibo's southwestern slopes terminate at 4,270 m/14,000 ft.

Glaciers in Tibet. The Tibetan Plateau is the largest and highest-elevated region in the world, with an average height of 4,500 m/15,000 ft above sea-level. Glaciers are found on the snow-capped mountain of Muztag Ulu, which has an elevation of 7,282 m/23,892 ft.

Glacier National Park in Montana, USA. The park's mountainous terrain features typically glacial landforms, including pyramidal peaks and U-shaped valleys such as the one seen here between the mountains in the middle distance.

Body of ice, originating in mountains in snowfields above the snowline, that moves slowly downhill and is constantly built up from its source. The geographic features produced by the erosive action of glaciers (erosion) are characteristic and include glacial troughs (U-shaped valleys), corries, and arêtes. In lowlands, the laying down of debris carried by glaciers (glacial deposition) produces a variety of landscape features, such as moraines and drumlins.

Glaciers form where annual snowfall exceeds annual melting and drainage (see glacier budget). The area at the top of the glacier is called the zone of accumulation. The lower area of the glacier is called the ablation zone. In the zone of accumulation, the snow compacts to ice under the weight of the layers above and moves downhill under the force of gravity. As the ice moves, it changes its shape and structure. Partial melting of ice at the base of the glacier also produces sliding of the glacier, as the ice travels over the bedrock. In the ablation zone, melting occurs and glacial till is deposited.

When a glacier moves over an uneven surface, deep crevasses are formed in the upper layers of the ice mass; if it reaches the sea or a lake, it breaks up to form icebergs. A glacier that is formed by one or several valley glaciers at the base of a mountain is called a piedmont glacier. A body of ice that covers a large land surface or continent, for example Greenland or Antarctica, and flows outwards in all directions, is called an icesheet.

Research by members of the geological sciences department of the University of Colorado who collected data on hundreds of glaciers in different parts of the world, excluding Antarctica and Greenland, indicates that reserves of glacial ice are melting more quickly than previously thought, almost certainly as a result of global warming. A 2001 satellite survey of 2,000 glaciers worldwide showed that most are shrinking: for example every mountain glacier in Patagonia, the Himalayas, the Alps, and the Pyrenees has shrunk.