Glaciers: Continental

The term glacier originates from the Latin word glacies, meaning “ice,” and refers to any body of perennial ice originating on land through the densification and recrystallization of snow that is massive enough to be flowing under its own weight. Generally, glaciers form in areas where the annual accumulation of snow, through precipitation and deposition, is greater than the annual mass loss due to melting, sublimation, and the calving of icebergs at the glacier margin. Glaciers provide important stores of water to more than 40% of the world's population, maintain the temperature of the planet through a high albedo (they reflect sunlight back to space), and, in the case of the largest bodies of ice, even affect local and regional climate.

Glaciers form where the snow accumulates for hundreds to thousands of years. As the snow accumulates, older layers of snow are buried beneath more and more mass and are eventually compressed and recrystallized into dense glacial ice (density = 917 kilograms per cubic meter). If enough mass accumulates, the ice begins to flow downhill and outward toward the margins, where mass is lost.

Mass loss on a glacier is known as ablation. Anywhere on the glacier or ice mass where annual ablation exceeds accumulation is included in the ablation zone. Areas where accumulation exceeds ablation are included in the accumulation zone. The equilibrium line separates the accumulation and ablation zones and marks where accumulation and ablation are equal. This occurs at the equilibrium line altitude, which typically varies with latitude and the prevailing climatic conditions. On some glaciers, the highest elevation area may experience no melt at all. This area is referred to as the dry snow zone.

Glaciers occur in many places and in many different morphologies. The largest ice masses are ice sheets. Ice sheets cover continent-sized areas and are typically dome shaped. The surface morphology of ice sheets, unlike that of mountain glaciers, is relatively unaffected by the underlying topography. The flow of ice from these domes is outward from the summit, terminating in many outlet glaciers along the ice sheet margins. These outlet glaciers typically are constrained by the topography.

Figure 1 Map of Antarctica showing areas of sea ice, ice shelves, and the two ice sheets

Source: Adapted from Central Intelligence Agency. (2009). The world factbook. Washington, DC: Author.

At present, there are three large ice sheets on Earth, namely the West Antarctic Ice Sheet, the East Antarctic Ice Sheet (Figure 1), and the Greenland Ice Sheet (Figure 2). Combined, these ice sheets contain more than 90% of the estimated 33,000,000 km3 of ice contained in all the glaciers in the world. The ice thickness on the East Antarctic Ice Sheet exceeds 4,200 m (meters) in places, while the West Antarctic Ice Sheet is grounded mostly below sea level and may be up to 2,500 m below sea level in places.

Figure 2 Blue Marble image of the Greenland Ice Sheet showing accumulated melt area from 1979 to 2007 from passive microwave data

Source: NASA/Goddard Space Flight Center Scientific Visualization Studio. Next Generation Blue Marble data courtesy of Reto Stockli (NASA/GSFC).

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