|Figure 1. An unnamed U-shaped valley in the Coast Mountains of Tracy Arm-Ford's Terror Wilderness, Tongass National Forest, AK. Image credit: USGS.|
Glaciers exist on all the continents of the world except Australia. Most of the world's glaciers are found near the North and South Poles (for more information about Arctic glaciers, please see our page on Greenland). A large number of glaciers, however, are found in mid-latitude and tropical regions wherever the right conditions exist.
Glaciers exert a significant influence on a landscape. As glaciers move across the terrain, they pick up rock and debris, carve valleys (see Figure 1), and create landforms. Flowing glaciers erode and scour the ground beneath and to the sides of them. These rivers of ice also pick up boulders, soil, trees, and other debris and carry it along in their flow. Once the glacier begins its retreat, however, this material is deposited wherever the glacier.s ice melts. Kettle lakes are formed when large chunks of ice fall off of retreating glaciers and melt, filling depressions in the ground. For more information on glacial formations, see these links, which will connect you to some sites with photos and descriptions.
There are two main types of glaciers — valley and continental. Valley or alpine glaciers form in mountainous regions where movement is inhibited by valley walls. Continental glaciers, also known as ice sheets, are "dome-shaped mass[es] of glacier ice… greater than 50,000 square kilometers (12 million acres) (e.g., the Greenland and Antarctic ice sheets)" (NSICD).
Glaciers can form in areas characterized by cool summer temperatures and heavy winter snowfall. These conditions allow enough snow to create and maintain the glacier while limiting loss of mass. Growth is largely dependent upon precipitation. In areas with the temperatures necessary for glaciers to form, low precipitation rates will lead to slower growth (such as in Antarctica). A healthy glacier has a mass balance of zero or can be positive. This means that the glacier is accumulating as much or more than it is losing through ablation — melting, evaporation, calving, etc.
In places where winter snowfall survives the melt season, each year's snowfall accumulates over the last. As the years go by and the layers add up, the pressure created by the upper layers begins to turn the lower layers to ice. The glacier becomes denser still as the ice crystals grow, taking up the air space in the layers. Light at all colors of the spectrum are absorbed by this ice except one — blue. This is what makes glacial ice appear blue.
Once a glacier accumulates enough weight, gravity and the pressure of its own mass force it to move downhill, in the case of alpine glaciers, or outward, in the case of continental glaciers. This is called internal deformation. Glaciers can also move due to sliding, or basal slip. A layer of water or soft sediment with some water in it allows the overlying glacier to move over it much faster because it acts like a lubricant. Water can either be in the landscape before the glacier gets there or can come from melt water accumulating on the top of the glacier and leaking through cracks in its layers (see moulins). According to the NSIDC, "Basal slip may account for most of the movement of thin, cold glaciers on steep slopes, or only 10 to 20% of the movement of warm, thick glaciers lying on gentle slopes."
Glacial flow depends in part on the climate — warmer, drier weather leads to glacial retreat, while colder, wetter weather creates the conditions necessary for building glaciers. When temperatures increase or there is significant evaporation due to wind and warmer weather, the glacier begins to retreat. During the past 60 to 100 years, the NSIDC states that glaciers throughout the world have tended toward retreat. The Center explains "Alpine glaciers, which are typically smaller and less stable [than continental glaciers] to begin with, seem particularly susceptible to glacial retreat" (NSIDC).
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