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GLACIERS Although glaciers account for much less erosion than running water, in themselves, they are much better eroders than streams. Geological features that are characteristic of glaciation are significantly different than those of running water. A glacier is a thick ice mass that originates on land from the accumulation of, compaction and recrystallization of snow. Glaciers flow, generally downhill, and accumulate, carry and deposit sediment. The characteristics of the sediment carried and deposited by glaciers are very different from those of streams. There are two fundamental types of glaciers Valley or alpine glaciers are the ones we normally think about. Each is a stream of ice bounded by precipitous rock walls that flows down-valley from an accumulation center near its head. Ice sheets are much larger and flow out in all directions from one or more centers and completely obscure all but the highest areas of underlying terrain. For instance, Greenland is covered by an ice sheet that covers 1.7 million square kilometers (80% of the island) with an average thickness of 1500 meters. The Antarctic ice sheet is 13.9 million square kilometers (1.5 times the area of the US) and attains a maximum thickness of 4300 m. Continental ice sheets cover about 10% of the earth's land area. When an ice sheet flows into a bay and no longer sits on land but floats, you get an ice shelf. An interesting point to note is that Antarctica's ice sheet constitutes 80% of the world's ice and 2/3 of the fresh water. If this ice melted, the sea level would rise 60-70 meters, covering most of the densely populated parts of the globe. Formation of Glacial Ice Snow is the raw material from which glacial ice originates. Glaciers form where more snow falls in winter than melts in summer. When temperatures stay below freezing after a snowfall (the fluffy accumulation of hexagonal snow crystals), the extremities of the crystals evaporate in the air around them while the moisture condenses near the centers of the flakes. These smaller granular snow particles pack down under the weight of the snow above, pushing the air out and recrystallizing into a mass of dense grains with the consistency of coarse sand called firn. Once the thickness of the overlying ice and snow exceeds 50 meters, the firn packs into a solid mass of interlocked crystals (glacial ice). Shape and Movement of a Glacier The upper part of the glacier, the zone of accumulation, is the part of the glacier with perennial snow cover. The lower part is called the zone of wastage where ice is lost by melting, evaporation and calving (bits breaking off). The boundary between the 2 zones is called the firn limit. What Glaciers do to Valleys Rivers cut V-shaped valleys, whereas glaciers cut steep-sided U-shaped valleys. A cirque is a semi-circular basin at the head of a glaciated valley formed by frost wedging and plucking. A hanging valley is a tributary valley that enters a glacial trough at a considerable height above the floor of the trough. How do Glaciers Move? Ice behaves like a brittle solid until the pressure or load upon it is equivalent to about 50 meters of ice. Above this limit is the zone of fracture where cracks form which are called crevasses. Below 50 meters, the ice flows plastically (with no cracking). Most glaciers slide by basal slip as a solid sheet of ice moving over the ground below. Because of friction at the base, the motion of ice near the base is always slower than higher up. The surfaces of glaciers provide an indication of the shape of the land over which the glacier is moving. Crevasses are tension fractures which form in the upper brittle part of a glacier (they are no more than 40 meters deep). They form when the glacier is stretched as it begins to ride over steeper terrain. Once the terrain gets flatter again, the crevasses close up. An icefall forms when a glacier rides over very steep terrain, the brittle surface layer becomes highly fractured with large blocks and pinnacles forming a very jumbled surface. Glaciers significantly change the terrain over which they have moved. A retreating glacier will leave behind moraines, poorly sorted material (ranging from fine rock flour to large boulders) which the glacier eroded and carried with it. When the ice melted, the moraine was left behind. End or terminal moraines are the ridges of a moraine left (like a bulldozer pile) at the leading edge of the glacier. More specifically, the terminal moraine marks the maximum advance of the glacier. Lateral moraine is the moraine that forms along the sides of the glacier. Medial moraine forms from the lateral moraine of two glaciers that merge. Eskers are long sinuous ridges of water-deposited sediment apparently formed in tunnels below the glacier front edge. Drumlins are moraines that have been reshaped into hummocks by the overriding glacier. Ice Ages There have many periods in the Earth's history when the surface temperature of the planet is cooler than it is now, causing much of the planet's surface to be shrouded in ice. We all know of the last ice age about 18,000 years ago which covered much of North America and much of Northern Europe with ice sheets. These ice sheets formed from precipitation of snow from water evaporated from the oceans, producing a net drop in ocean level (the mean sea level during the last ice age was nearly 100 meters lower than it is now. A record of sea level versus time since the last ice age shows that the sea level has been fairly static only over the last 6000 years. This last ice age resulted in a large ice sheet covering most of Scandinavia. As we have previously discussed, the increased weight on the land mass causes the area to be pushed down into the upper mantle. Thus, Scandinavia sank somewhat into the mantle during the last ice age. The mantle material that was pushed out of the way by the subsidence of Scandinavia pushed the low lying plains that we now call Holland up higher. After the ice melted, Isostasy started to push Scandinavia up again (it is still slowly rising today). In the process, Holland is now slowly sinking. Hence the need for expensive dikes to keep the sea out. We are currently in a short warm period in one of the Earth's cooler times (taking the last 2 million years). Prior to that, we can see that there have been periodic warm and cold periods that last tens to hundreds of million years. It is inevitable that this warm spell will soon (in geologic time) be over and the ice sheets may again advance over the continents.
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