Pamela Harris, age 12, of Barre, Vt., for her question:

WHY DO GLACIERS MOVE?

Glaciers are huge masses of ice that flow slowly over land. They move because gravity pulls them.

Ice crystals deep within a glacier glide over one another as a result of the pressure of the surface layers. These small movements of individual crystals cause the entire ice mass to move.

The ice crystals melt and refreeze along the glacier's base, and this also helps it slide down slope.

Heat from friction and from the earth's interior melts some of the crystals of the glacier's bottom layer. Water from the dissolved crystals flows into slightly open spaces and refreezes, forming new 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 flaws over uneven or steep terrain. Crevasses also develop because the upper layers of a glacier move faster than the lower layers. The sides, which rub against surrounding rock, may also move at different speeds than other parts of the glacier.

Most glaciers flow 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.

Scientists measure a glacier's speed by driving stakes into the ice at various points and recording the changes in their positions.

As glaciers move, they shape the features of the land. For instance, glaciers greatly altered the surface of large parts of Europe and North America during the Pleistocene Ice Age, which ended between 10,000 and 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 Jungfrau.

Major glaciers also cover parts 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.

The largest glacier in Europe is Jostedal in Norway: it covers about 300 square miles.