Glaciers, Retreating

THERE ARE GLACIERS in all areas of the globe. Most glaciers are found in the polar regions, but many of the Earth's tallest mountains also have glaciers. Glaciers go through life cycles. They have growing years, moving years, and retreating years. For glaciers to form, very specific climatic conditions are necessary. They are usually found where there is enough snowfall for a snow pack to permanently accumulate, where summers are not warm enough for all of the snow to melt. In some regions, such as at the North Pole, the intense cold squeezes out moisture as rain or snow before it advances to the far north. This makes the Arctic region a cold desert because the cold prevents the accumulation of snow. However, in warmer areas, the greater amount of moisture combined with sufficiently low temperatures encourage snow and the possibility of glacier formation.

There are two major kinds of glaciers: continental glaciers and valley glaciers. Continental glaciers are often called ice sheets—the glaciers in Antarctica are continental ice sheets. For practical purposes, the [Page 448]entire continent is locked in a single combined glacier. It is also a slow growth glacier because the low level of moisture adds only small amounts of snow each year. Alpine glaciers form in high mountain areas. In Africa, Mount Kilimanjaro has a glacial field. There are many other glacial areas such as those in the mountains of western North America and South America. New Zealand's South Island has glaciers, as do the Himalayas and the Alps in Europe.

Glaciers form because every year more snow remains than there was the previous year, because not all snow melts during the summer. As the winter snow accumulates, its weight turns the snow beneath it into ice. The snow that remains is called firn. As more snow falls, it and the snow above it compress the firn into buried layers that become thickened masses of ice crystals. In the second phase of glacier life, they move or flow. The sheer weight of the ice causes continental glaciers to move outward from a central point. As the ice is deformed internally by the weight of the ice above it, a thin layer of water forms at the bottom and the glacier then follows gravity's pull down the path of least resistance. The water may be from summer melt or from the pressure.

If the mountain slopes upon which a glacier has formed are steep, its movement may be rapid. The moving ice has internal stresses that cause fracturing into cracks (crevasses) that form at the surface of the glacier. Moving glaciers also have a dramatic impact on the environment. They scour the surface beneath them and scrape the sides of the mountain where they are flowing like frozen rivers. They crush rocks into pebbles and soil. As a glacier passes, there is erosion and then deposition of the glacial debris as moraines, drumlins, and eskers. Glaciers eventually melt and leave behind rocky debris. They also leave scoured areas, valleys, and mountainsides. The causes of glacial retreat are increases in temperatures, evaporation rates, and wind scouring that promotes sublimation. In the summertime, glaciers undergo a natural retreat in the form of ablation. A glacier will grow and move as long as more snow accumulates than melts. When there is more melting than accumulation, the glacier will eventually decrease in size or disappear.

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