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Most of the world's glaciers are found near the Poles, but glaciers exist on all of the world's continents, even Africa. Australia doesn't 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 islands 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 isn't lost (by melt, evaporation, or calving) during the summer. Such conditions typically prevail in polar and high alpine regions. There are two main types of glaciers: valley glaciers and continental glaciers (known as ice sheets).
The amount of precipitation (whether in the form of snowfall, freezing rain, avalanches, or wind-drifted snow) is important to glacier survival. In areas such as Antarctica, where the low temperatures are ideal for glacier growth, very low annual precipitation causes the glaciers to grow very slowly.
A 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, ablation, or calving. If the accumulated snow survives one melt season, it is considered to be firn. The snow and firn are compressed by the overlying snow, and the buried layers slowly grow together to form a thickened mass of ice.
The pressure created from the overlying snow compacts 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 dense glacial ice usually looks somewhat blue.
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 glaciers (ice sheets) flow outward in all directions from a central point.
Glaciers move by internal deformation and/or by sliding 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. Basal slip 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, internal stresses build up in the ice which cannot be relieved by deformation alone, and cracks (called crevasses) form at the surface of the glacier.
Glaciers dramatically impact their surrounding environment by reshaping the underlying and surrounding landscape as they move, through both erosion and deposition.
Glaciers erode the rock underneath them. A glacier can "carve" a valley, wearing away rocks and soil through abrasion and plucking up and moving large pieces of rock and debris. The glacier pushes this earth and rock forward as it advances, almost like a conveyor belt, and dumps it to the side along the way or at the end of the glacier (deposition). Depositional features include moraines, drumlins, and eskers.
The glacier story concludes as the glacier stops growing and actually begins to move in retreat.
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