Terrence Banks, age 13, of Milwaukee, Wis., for his question:
WHAT IS A GLACIER?
Best known glaciers in the world today are the ones located in the French and Swiss Alps. Included is the Mer de
Glace on Mont Blanc and the Aletsche Glacier near the Jungfrau. Largest on the European continent is the Jostedal
Glacier in Norway. It covers about 300 square miles. In North America the largest is the 840 square mile Malaspina
Glacier on Yakutat Bay in Alaska.
A glacier is a large mass of ice that flows slowly over the land. Glaciers form in the cold polar regions and also on high mountains. Low temperatures in these places make it possible for large amounts of snow to build up and turn into ice. Most glaciers are between 300 and 10,000 feet thick.
There are two types of glaciers: valley glaciers and continental glaciers. Valley glaciers are long, narrow bodies of ice that fill high mountain valleys. Many of them move down sloping valleys from bowl shaped hollows located near the mountain peaks. They are found in the Southern Alps of New Zealand, in the northern Andes of South America, in mountain ranges near the poles and at lower elevations of the European Alps.
Continental glaciers are broad, very thick and cover vast areas of land near the earth's polar regions. This type of glacier builds up at the center and slopes outward toward the sea in all directions..
A glacier will form when more snow falls during the winter than melts and evaporates in summer. The excess snow gradually builds up in layers. Its increasing weight causes the snow crystals under the surface to become compact, grainlike pellets. At depths of about 50 feet, the pellets are further compressed into dense crystals of ice. Finally the ice will become so thick that it will start to move under the pressure of its own weight.
The cold temperatures of a glacier actually promote the buildup of snow. Glaciers often increase or decrease in size as a result of changes in climate that occur over a long period of time. 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 flows downslope because of the pull of gravity. The slight movement of individual ice crystals inside a glacier cause the entire ice mass to move. Heat from friction and from the earth's interior also 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.
Most glaciers move less than a foot a day, although some have been known to travel more than 50 feet a day for various periods of time.