Brent Devlin, age 13, of Freeport, I11., for his question:
HOW DOES A GLACIER MOVE?
A glacier is a very large mass of ice that actually flows slowly over land. A glacier flows down a slope because of the pull of gravity.
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 mass to move. The melting and refreezing of the ice crystals along the base of the glacier also help it slide downslope.
Glaciers form in the cold polar regions and also in certain high mountains. The low temperatures in these places allow large amounts of snow to build up and turn into ice. Most glaciers range in thickness from about 300 to 10,000 feet.
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 nearby open spaces in the layer and refreezes, forming new ice crystals.
The surface of a glacier is stiff and rigid, unlike the mass of ice below which often fractures and forms deep cracks, or "crevasses," as the glacier flows over uneven or steep terrain. Crevasses also develop because the upper layers of a glacier move faster than its lower layers.
Most glaciers flow extremely slowly and move less than a foot per day. But sometimes a glacier may move much faster for several years. For example, some glaciers at times flow more than 50 feet per 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.
As a glacier passes over an area, it helps to 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 about 10,000 to 15,000 years ago.
Glacier erosion happens when an advancing ice mass scoops up rock fragments and drags them along its base. In doing so, the glacier grinds the bedrock, producing a polished but often scratched surface. As the glacier decreases in size, it leaves behind broad humps of hard bedrock.
A glacier in a mountain valley may produce a rounded hollow, called a "crique," near the peak of the mountain. It can also gouge out a U shaped depression in a river valley.
Glaciers sometimes pile up clay, rock and sand in uneven ridges called "moraines."
Glaciers also form "drumlins," which are oval shaped hills that usually consist of rock debris. They can also form "eskers" which are long, narrow ridges of sand and gravel deposited by a stream of water that originally flowed in a tunnel under a melting glacier.