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If you make a snowball out of freshly fallen snow and hold it tightly in your hands for awhile, the warmth of your body will cause the snow to melt. Snow is actually a solid form of water. You may also notice that some of the snow pressed together by your hands forms ice. The same thing happens when new snow falls on top of old snow. The pressure of the piled-up snow causes some of the snow to change into ice. In time, a glacier forms. A glacier is a huge mass of moving ice and snow.

Glaciers form in very cold areas, such as high in mountains and near the North and the South poles. Because of the extremely cold temperatures in these areas, the snow that falls does not melt completely. As more snow falls, it covers the older snow. As the snow builds up, the pressure on the older snow squeezes the snow crystals together. Eventually ice forms. When the layers of ice become very thick and heavy, the ice begins to move.

Glaciers contain about 2 percent of the available fresh water on the Earth. As sources of fresh water become more scarce, scientists are trying to develop ways to use this frozen supply of fresh water.

Valley Glaciers

Long, narrow glaciers that move downhill between the steep sides of mountain valleys are called valley glaciers. Usually, valley glaciers follow channels formed in the past by running water. As a valley glacier moves downhill, it bends and twists to fit the shape of the surrounding land. The valley walls and the weight of the ice itself keep the glacier from breaking apart. But on its surface, the ice cracks. Cracks on the surface of glaciers are called crevasses (krih-VAS-sehz).

As a valley glacier slides downward, it tears rock fragments from the mountainside. The rock

Valley Glacier in the Alps

fragments become frozen in the glacier. They cut deep grooves in the valley walls. Finer bits of rock smooth the surfaces of the valley walls in much the same way as a carpenter's sandpaper smooths the surface of a piece of wooden furniture. Mountains located anywhere from the equator to the poles can contain glaciers. Many glaciers are found in the United States. Mount Rainier in Washington

State and Mount Washington in New Hampshire contain small glaciers. Glaciers can also be found in many mountains of Alaska.

As a valley glacier moves, some of the ice begins to melt, forming a stream of water. This water is called melt water. Melt water is usually nearly pure water. Some cities use melt water as a source of their drinking water. Boulder, Colorado, uses melt water from the nearby Arapaho Glacier. Melt water is also used in some places to generate electricity in hydroelectric plants. But some problems arise in the use of melt water in these ways.

Valley glaciers are long narrow glaciers that move down hill between mountain valleys. They are literally frozen rivers. This picture shows a valley glacier in Alaska

Building channels or pipelines to transport melt water from glaciers to cities can be costly. And the construction ofhydroelectric plants in the underdeveloped areas where glaciers are located could alter the surrounding environment.

Continental Glaciers

In the polar regions, snow and ice have built up to form thick sheets. These thick sheets of ice are called continental glaciers, or polar ice sheets. Continental glaciers cover millions of square kilometers of the

Contineantal glaciers such as the Mertz Glacier in Antarctica cover millions of square kilometers

Earth's surface and may be several thousand meters thick. Continental glaciers move slowly in all directions. Continental glaciers are found in Greenland and Antarctica. Nearly 80 percent of Greenland is covered by ice. More than 90 percent of Antarctica is covered by ice. These huge glaciers are more than 3200 meters thick at the center. In the future, continental glaciers could be another source of fresh water.

Icebergs

At the edge of the sea, continental glaciers form overhanging cliffs. Large chunks of ice, called icebergs, often break off from these cliffs and drift into the sea. Some icebergs are as large as the state of Rhode Island! The continental glaciers of Greenland and Antarctica are the major sources of icebergs in ocean waters.

Icebergs can pose a major hazard to ships. In 1912, the ocean liner Titanic sank after smashing into an iceberg in the North Atlantic Ocean. Many lives were lost as this ship, thought to be unsinkable, plunged to the ocean bottom on her first voyage. Today, sea lanes are patrolled constantly by ships and planes on the lookout for icebergs. Much fresh water is frozen in icebergs. Attempts have been made to develop ways of towing icebergs to areas that need supplies of fresh water, such as deserts. But transporting icebergs from Greenland and Antarctica poses several problems.

Icebergs, which often have spectacular shapes, are large chunks of ice that break off glaciers and drift into the sea. Only a small part of an iceberg rises above the water's surface. Can you explain the phrase "tip of the iceberg"?

First, the effects of an iceberg on local weather conditions must be evaluated. Second, the cost and time involved in moving the iceberg must be considered. Third, scientists would have to find a way of preventing the iceberg from melting during the ocean journey. Can you think of ways to use icebergs?