Glacier

 A glacier is a large mass of ice, formed at least in part on land, which shows evidence of present or former movement. It is formed by the compaction and recrystallization of SNOW into ice crystals and commonly also contains air, water and rock debris. Movement is downslope or outward in all directions, caused by the glacier's stress system. Internal deformation and basal slippage of ice is common. Glaciers may end on land, in the ocean (as an ice shelf) or in a lake.

Types

 The 2 principal types are valley glaciers and ice caps. Valley glacier movement is controlled by topography. Although velocity varies, most average less than a metre per day. However, some valley glaciers, called surging glaciers, can achieve speeds in excess of 60 m per day. Tens of thousands of valley glaciers now exist worldwide. In Canada they are found mainly at higher elevations of the western mountain systems and in the mountains and highlands of the arctic islands, eg, AXEL HEIBERG, ELLESMERE, DEVON and BAFFIN islands. Many are less than a kilometre long. Others are much longer, eg, the Hubbard Glacier in the Yukon and Alaska, which is over 100 km long.

 ICE CAPS or ice sheets (if they are over 50 000 km2) are dome shaped and not greatly impeded by topography; thus, they are able to move outward in all directions. Generally, velocities of ice caps and ice sheets are lower than those of valley glaciers. Canada has several ice caps, located in the Cordillera and arctic islands.

Features

 Many features commonly produced by glaciers can be observed on or near the Athabasca Glacier in the Rocky Mountains of JASPER NATIONAL PARK. This glacier, fed by the COLUMBIA ICEFIELD, has been retreating for several years. Various features can be observed on the glacier surface, including crevasses, fissures that form from tensile stress in the glacier surface; icefalls, resulting from crevasses formed where the glacier hangs over a bedrock protuberance; and a medial MORAINE, composed of debris and ice, which is formed where 2 valley glaciers coalesce.

Other features that were formed during the retreat of the glacier (and can be seen nearby) include lateral and recessional moraines, formed by debris deposited along the glacier terminus. In addition, glacier meltwater carries and deposits debris, forming such features as deltas and glacial-outwash plains composed of sand and gravel.

See also GLACIATION.