How Glaciers Form and Move

Glaciers form in places where more snow falls than melts or sublimates. As the layers of snow pile up, the weight on the underlying snow increases. Eventually, this weight packs the snow so tightly that glacial ice is formed.

Even though glaciers may appear to be stationary, they are constantly moving. The force of gravity pulls the ice downhill. The ice flows slowly, like honey dripping down a spoon. Sometimes the pressure is great enough to melt the ice at the base of the glacier. This melting can aid the motion of the glacier, with the ice sliding along the bottom. Like a river, a glacier flows fastest in the middle and slowest along the sides.

There are two types of glaciers: continental glaciers and valley glaciers. A continental glacier is a thick sheet of ice that covers a huge area, such as a continent or large island. Most of Earth's fresh water is frozen in the continental glaciers that cover Antarctica and Greenland. A glacier that occurs in a high mountain valley is called a valley glacier. Like the glacier in Figure 18, a valley glacier usually begins near a mountain peak and winds down through a valley formed originally by a stream.

Figure 18 This valley glacier in Alaska has bands that look like ripples. The dark bands, which include rock debris, show the glacier's slower growth in summer. The light bands form in the winter.

Interpreting Photos Which part of the glacier moves faster, its center or its sides?