Joan Buresh, age 12, of Greenville, S.C., for her question:
WHY DO GLACIERS MOVE?
Glaciers are huge masses of ice that flow slowly over land.The reason glaciers move is because of the flow of gravity. The ice crystals deep within a glacier glide over one another as a result of the pressure of the surface layers. These small movements of the individual crystals cause the entire ice mass to move. There is a melting and refreezing of the ice crystals along the base of a glacier, and this also helps it slide downslope. Heat from friction and from the earth's interior melts some of the crystals of the glacier's bottom layer. The water from the dissolved crystals flows down into nearly open spaces in the layer and refreezes, forming new ice crystals.
A glacier's surface is stiff and rigid, unlike the mass of ice below. It often fractures and forms deep cracks called crevasses as the glacier flows over uneven or steep terrain. Crevasses also develop because the upper layers of a glacier move faster than the lower layers.
Most glaciers flow extremely slowly and move less than a foot a day. But sometimes a glacier may travel much faster for several years. Some have been known to move more than 50 feet in a single day. Various parts of a glacier move at different speeds. The center and upper areas of a valley glacier flow the fastest. The sides and bottom move more slowly because they rub against the walls and floor of the valley.
Scientists measure a glacier's speed by driving stakes into the ice at various points and recording the changes in their positions. As glaciers pass over an area, they help shape the features of the land.
Glaciers greatly altered the surface of large parts of Europe and North America during the Pleistocene Ice Age, which ended from about 10,000 to 15,000 years ago. Best known of the world's glaciers today are those in the French and Swiss Alps. These include the Mer de Glac on Mont Blanc and the Aletsch Glacier near the Jungfraw. Major glaciers also cover regions of northwestern North America. The largest and most famous is the 840 square mile Malaspina Glacier on Yakutat Bay in Alaska.
Other important glaciers include those in Banff National Park in Alberta, in Glacier National Park in Montana and on Mount Rainier in Washington. Largest glacier on the European continent is the Jostedal Glacier in Norway which covers about 300 square miles.