Glaciers

Glaciers are thick masses of ice that originate on land from the accumulation, compaction, and recrystallization of snow. Glaciers make up the cryosphere, the frozen part of the hydrosphere. The frozen water is temporarily removed from the hydrologic cycle. Glaciers occur more commonly at places on earth where temperatures are such that the glacial ice remains frozen during a sufficient part of the year, preventing the glacier from melting completely. These cold places occur at high elevations and at high latitudes.

Glaciers fall into two broad categories: alpine or valley glaciers; and continental glaciers, ice sheets or ice caps. Alpine glaciers occur at high elevations, on mountain tops. Continental glaciers occur at high latitudes, close to the poles. Alpine and continental glaciers are actually two ends of a spectrum. Many glaciers have characteristics of both.

Alpine glaciers exist on the tops of mountains, and therefore are subject to the force of gravity, which constantly pulls the glacier down the slope. As the glacier slowly moves down the side of the mountain, new glacial ice forms on the mountain top (accumulation). As the glacier reaches the higher temperatures that occur on the lower parts of the mountain, the ice melts (ablation or wasting). This system is like a conveyor belt where the glacier forms in the area of accumulation, then constantly slides down the mountain to the area of ablation where it melts and becomes outwash streams. The downslope movement is due in part to flow within the glacier, and also to sliding over the ground surface. The sliding produces significant erosion resulting from the abrasion of bedrock and the transport of the eroded material to the area of ablation where it is deposited as a moraine.

The moraine may act as a natural dam resulting in a glacial lake. Alpine glaciers are common today in many mountain belts such as the Alps, the Northern and Canadian Rockies, the Andes, and the Himalayas. An alpine glacier exists on top of Mt. Kilamanjaro (19,340 feet), illustrating that glaciers can exist close to the equator if the ground elevation is high enough.