Glaciers

Glaciers are large land-bound bodies of ice. To be called a glacier, the ice mass must be moving, or show evidence of having moved in the past. Covering about 10% of Earth's surface, glaciers store a significant amount of Earth's supply of freshwater.

Glaciers form from the buildup of snow over time. As snow accumulates, it is compressed under its own weight. Compaction, along with partial thawing and refreezing, converts the original snow to a type of granular ice called firn. As snow continues to accumulate, the firn is buried and further compacted and is eventually converted into glacial ice. Sufficient accumulation of snow and ice is critical to the ability of glacial ice to deform and flow. Pressure from above allows the solid ice to flow at depth.

Glacial ice flows outward from the center of the accumulation and/or downhill under the force of gravity. Plastic flow, the chief mechanism of glacial movement, occurs when individual ice crystals within the center of the mass move very small distances. The cumulative motion of a large number of

ice crystals results in movement of the glacier as a whole. Basal slip is another important mechanism of glacial flow. It occurs when the glacier slides along its base, usually aided by the presence of meltwater between the ice and the land surface. Glacial movement is generally so slow as to be imperceptible to the human eye. Rates of movement typically range from a few millimeters to a few meters per day. Occasionally, however, a glacier may surge, moving tens of meters per day for a period of months to years, before slowing down again.

The two main types of glaciers, classified based on size and location, are continental and alpine glaciers. A continental glacier, as the name suggests, is one that covers a large portion of a continent. These glaciers, also known as ice sheets, flow out from one or more centers of accumulation. Such zones of accumulation are typically more than 1.9 mi (3 km) thick, and continental glaciers cover most of the region's topographical features except for the highest mountain peaks. Continental glaciers are presently found only in Greenland and Antarctica. Over 695,000 mi2 (1,800,000 km2) of Greenland is hidden beneath its ice sheet, while continental glaciers in Antarctica cover 4,885,350 mi2 (12,653,000 km2). During the Pleistocene, continental glaciers were widespread, and massive ice sheets covered much of North America. Depositional and erosional evidence of their existence can be found throughout Canada and the northern portion of the United States. Smaller versions of continental glaciers, known as ice caps, cover less than 19,300 mi2 (50,000 km2). The small continental glacier covering much of Iceland is an example of an ice cap. Ice caps are also found in the polar regions of Canada, including Baffin Island.

Alpine glaciers are those that occur in mountainous regions. Alpine glaciers are much smaller than continental glaciers, and flow from areas of higher elevation to lower areas. Large alpine glaciers in North America occur in Alaska and the Rocky Mountains. As alpine glaciers flow down from mountaintops, they are generally confined to a valley or system of valleys. These valley glaciers are analogous to streams, and often have smaller tributary glaciers that flow into them. Valley glaciers leave behind clear evidence of their existence. River erosion typically produces a valley that is narrow at the bottom, with a V-shaped cross section. Wide, flat-floored valleys with a U-shaped cross section indicate the previous existence of a valley glacier. Matanuska Glacier, located along the highway 90 mi (145 km) northeast of Anchorage, Alaska, is a famous example of a valley glacier.

Glaciers are responsible for reshaping the land by eroding and transporting a great deal of material. Glaciers erode material by plucking and abrasion. Plucking refers to the removal of rock by the advancing ice. Material may be plucked from beneath or along the sides of a glacier, and then picked up and transported by the moving ice mass. Abrasion differs from plucking in that it doesn't involve removing large particles of rock; rather it refers to grinding or filing processes. Highly polished rock surfaces are formed by abrasion. Where large rock fragments are embedded into the bottom of the ice, they may grind into the underlying bedrock forming glacial striations, or scratches. The Pleistocene glaciers formed in Canada and the northern United States have left behind large areas of polished and striated bedrock, exposed when glaciers scraped off the surface sediment. Other erosional features of glaciers include hanging valleys, cirques, arêtes, and horns.

Glaciers eventually deposit the material they accumulate. Material deposited by glaciers includes glacial erratics, large boulders transported by ice and deposited far from their source; glacial till, unsorted deposits of sediment and rocks; and stratified drift, layered, sorted deposits that originated from glacial streams. Extensive deposits of sediment from glacial streams are known as outwash plains. Depositional land-forms created by glaciers include moraines, drumlins, kanes, and eskers.

See also Glacial landforms; Glaciation; Polar Ice