Mighty Glaciers
*** Written by Ned Jensen
<<< Introduction
The earth's surface constantly changes. Wind, water, and heat wear down mountains and crack stone. But some of the most dramatic changes are caused by giant pieces of ice and rock called glaciers. Glaciers move slowly over the land. They carve out cliffs, valleys, and prairies as they go. When glaciers melt, they leave behind lakes, rivers and hills of soil and boulders.
<<< Types of glaciers
Two common types of glaciers are continental glaciers and valley glaciers. Continental glaciers form at the north and south poles of the earth. These huge sheets of ice cover vast stretches of land. Continental glaciers can be so thick that only the tops of mountains stick out above them. Much of Greenland and Antarctica are buried beneath continental glaciers.
Valley glaciers form on high mountains that rise above the snow line. Above the snow line, snow covers the ground all year. Even in the tropics, valley glaciers can form on the tallest peaks. Like rivers of ice valley glaciers move through long, narrow valleys.
<<< How Do Glaciers grow?
In some places, the temperature stays below freezing for most of the year. Snow piles up much faster than it melts. Each new laver of snow presses down on the snow beneath it. The lower layers of snow become squashed, or compressed.
The compressed snowflakes become ice crystals called firn. The firn crystals squash together as more and more snow presses down on them. Eventually, the firn turns into a huge slab of ice known as a glacier.
<<< How do glaciers move?
Glaciers begin to move when they reach about 30 meters (100 ft) thick. That's about as high as a 12-story building. The glaciers become so heavy that gravity pulls them downhill. Gravity is a force that pulls things toward the center of the earth. It makes objects fall and roll down slopes. The movement of a glacier is called flow.
The heavy ice at the top of a glacier compresses the ice below it. Even above the snow line, ice melts when it is tightly compressed. The melted ice becomes slippery, allowing the glacier to slide. Sometimes, the top layers of ice move faster than the bottom layers. The ice cracks. A crack in a glacier called a crevasse, can be hundreds of meters deep.
Continental glaciers form in the middle of continents. They flow toward the coasts. Valley glaciers flow down mountains. A fast glacier can flow 20 meters (70ft) in one day. But that is very unusual. Most glaciers travel only a few centimeters (less than 1 in) to 30 centimeters (1 ft) a day.
<<< Changing the earth
Like giant bulldozers glaciers push piles of rocks and soil as they slide forward. These piles are called moraines. Some of the rocks and soil get stuck in the ice. The rocks scrape the land beneath the glacier, leaving huge scars.
Some glaciers scrape off entire mountainsides. Valley glaciers carve deep U-shaped valleys with steep cliffs. When a glacier flows below the snow line, the bottom melts, forming a river. Some glaciers carve valleys that reach all the way to the sea. Seawater fills these valleys, creating fjords. Continental glaciers flatten the land.
As glaciers reach the ocean, they crumble and slide into the water. Sometimes, big chunks of ice fall into the sea. The broken-off pieces are called icebergs. Some icebergs in the Antarctic ocean are nearly 80 kilometers (50 mi) long.
<<< When Glaciers Melt
Some glaciers melt, or recede, and eventually disappear. Glaciers recede when the weather gets warmer or less snow falls. Receding glaciers leave their moraines behind as long ridges of rocks and soil. This soil is called till. It can be good for growing food. Glaciers may also drop huge boulders when they melt. Sometimes glaciers create lakes and ponds when the ice melts on the land.
<<< Conclusion
Glaciers are one of many things that give the earth its beautiful and interesting shapes. In the past 100 years, many glaciers have receded. The earth's temperature may be getting warmer. Scientists are watching glaciers to see how they are affected by temperature changes. They can learn a lot about our planet from these enormous rivers of ice.
