Oliver Lockhart, age 10, of Missoula, Mont., for his question:
HOW DOES A GLACIER FORM?
A glacier is a huge mass of ice that flows slowly over land. You'll find them in the cold polar regions in high mountains. They begin to form when more snow falls during the winter than melts and evaporates in summer. The excess gradually builds up in layers.
As the snow builds up in layers, its increasing weight causes the crystals under the surface to become compact, grainiike pellets. At depths of 50 feet or more, the pellets are further compressed into dense crystals of ice. These crystals combine to form glacial ice.
Glacial ice enventually becomes so thick that it begins to move under the pressure of its own weight.
Glaciers are affected by seasonal variations in snowfall and temperature. Most glaciers increase slightly in size during the winter because snow falls over much of their surface. The cold temperatures promote the building up of snow.
They also limit the melting of lower parts of the glaciers as the ice masses move down slope. In areas away from the poles, the size of glaciers decreases in summer because the rising temperatures cause the lower parts to melt.
In the always frigid polar regions, glaciers shrink for other reasons. When glaciers reach the sea, for example, huge chunks of ice break away from them. These chunks fall into the water and become icebergs.
Glaciers may also increase or decrease in size as a result of changes in climate that occur over long periods. For example, the ice sheet that covers much of Greenland is growing smaller because of a gradual rise in temperature in the area since the early 1900s.
A glacier moves down slope because of the pull of gravity. The ice crystals deep within the 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.
The melting and refreezing of the ice crystals along the base of a glacier also help it slide down slope. Heat from friction and from the earths 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 refreeze, froming new ice crystals.
The surface of a glacier 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.
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 glaciers at times flow more than 50 feet per day.
The 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 position.