IDENTIFY THE WAYS THAT WEATHERING AND EROSION CHANGE THE EARTH'S SURFACE.

Now that we have looked at the forces that come from inside the earth, let's take a look at external forces. The surface of the earth is changed by weathering and erosion, which is caused by water, wind, and temperature fluctuations. All three factors wear away or destroy the crust of the earth and have a big impact on the earth's surface.

Water

The Grand Canyon was formed by water erosion as the Colorado River slowly wore away the rock. (“Nankoweap.”Grand Canyon National Park: Park Management Information. http://www.nps.gov/ archive/ grca/ photos/images/D0484.JPG)

The beautiful formations of Bryce Canyon in Utah were also formed by water erosion. (Photo courtesy of Jupiterimages Corporation)

Can you think of some ways that water changes the earth? The power of running water can result in amazing landscapes. Have you ever seen the Grand Canyon? Its towering walls were carved by the water of the Colorado River. Even a small stream can dig a deep trench in the earth if given enough time.

Water combined with temperature fluctuations causes rocks to break down into smaller pieces. Porous rock absorbs water, and then freezing temperatures cause the water to expand; this puts enough pressure on rocks to crack and break them. Eventually, large rocks can be broken or worn down into very small pieces.

Other factors also wear down rocks. Substances in the air and water can react with the rock and create chemical compounds that eventually dissolve the rock.

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