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The Life of a Glacier
Most of the world’s glaciers are found near the poles, but glaciers exist on all of the world’s continents, even Africa. Australia does not have any glaciers; however, it is considered part of Oceania, which includes several Pacific island chains and the large islands of Papua New Guinea and New Zealand. Both of these countries have glaciers.
Glaciers require very specific climatic conditions. Most are found in regions of high snowfall in winter and cool temperatures in summer. These conditions ensure that the snow that accumulates in the winter is not lost during the summer. Such conditions typically prevail in polar and high alpine regions.
The amount of precipitation, whether in the form of snowfall, freezing rain, avalanches, or wind-drifted snow, is important to glacier survival. For instance, in very dry parts of Antarctica, low temperatures are ideal for glacier growth, but the small amount of net annual precipitation causes the glaciers to grow very slowly, or even to disappear due evaporation of the ice (sublimation).
Growing yearsA glacier forms when snow accumulates over time, turns to ice, and begins to flow outwards and downwards under the pressure of its own weight.
In polar and high-altitude alpine regions, glaciers generally accumulate more snow in the winter than they lose in the summer from melting, evaporation, or calving. If the accumulated snow survives one melt season, it forms a denser, more compressed layer called firn. The snow and firn are further compressed by overlying snowfall, and the buried layers slowly grow together to form a thickened mass of ice.
Each year’s new snowfall continues to compact the underlying layers, and the snow grains become larger ice crystals randomly oriented in connected air spaces. These ice crystals can eventually grow to become several centimeters in diameter.
As compression continues and the ice crystals grow, the air spaces in the layers decrease, becoming small and isolated. This compaction compresses more air spaces out of the snowpack, and compacts the remaining air into bubbles. At greater depth (hundreds of meters) the air in these bubbles is squeezed into the crystal structure of the ice. Thus dense glacial ice has no air bubbles, but contains trapped air nevertheless.
Under the pressure of its own weight and the forces of gravity, a glacier will begin to move, or flow, outwards and downwards. Valley glaciers flow down valleys, and continental ice sheets flow outward in all directions.
Glaciers move by internal deformation of the ice, and by sliding over the rocks and sediments at the base. Internal deformation occurs when the weight and mass of a glacier causes it to spread out due to gravity. Sliding occurs when the glacier slides on a thin layer of water at the bottom of the glacier. This water may come from glacial melting due to the pressure of the overlying ice or from water that has worked its way through cracks in the glacier. Glaciers can also readily slide on a soft sediment bed that has some water in it. This is known as basal sliding and may account for most of the movement of thin, cold glaciers on steep slopes or only 10 to 20 percent of the movement of warm, thick glaciers lying on gentle slopes.
When a glacier moves rapidly around a rock outcrop or over a steep area in the bedrock, internal stresses build up in the ice. These stresses can cause cracks, or crevasses, on the glacier surface.
Glacier retreat, melt, and ablation result from increasing temperature, evaporation, and wind scouring. Ablation is a natural and seasonal part of glacier life. As long as snow accumulation equals or is greater than melt and ablation, a glacier will remain in balance or even grow. Once winter snowfall decreases, or summer melt increases, the glacier will begin to retreat.
As they flow, glaciers plow up or push aside rocks and debris, which is then left behind when the glacier recedes. Then, as large glaciers retreat, the underlying ground surface is typically abraded of most materials, leaving only scars and debris on the underlying bedrock surface.
Over the past 60 to 100 years, glaciers worldwide have tended to retreat. Alpine glaciers, which are typically smaller and less stable to begin with, seem particularly susceptible to retreat. Over 90 percent of the measured alpine glaciers in the world are retreating, in almost every major glaciated region. The causes of this widespread retreat are varied, but the underlying primary causes are a warming climate and the effects of increased soot and dust in areas of higher agricultural and industrial activity.
NSIDC's Glacier Glossary - Search and browse terms related to glaciers in NSIDC's comprehensive cryospheric glossary.
NSIDC Glacier Photograph Collection - NSIDC archives a Glacier Photograph Collection of historical photos, which includes both aerial and terrestrial photos for the 1880s to 1975. The photos are primarily of Alaskan glaciers, but coverage also includes the Pacific Northwest and Europe.