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We need to have some understanding of the process called "erosion." The Grand Canyon is a perfect example of erosion. Any wearing away of the earth, however, is called "erosion."

The wearing away of rocks into smaller particles that make up soils takes place in a variety of ways. Depending upon the locality of your school, you can:

(1) observe the wind carrying and depositing dirt and dust;(2) observe a stream wearing away the earth, carrying sediments, and depositing them in other places;

(3) observe how ocean shores are continually being worn away by water action;

(4) observe how plants crack rocks.

a. Wind and water.

Examine different kinds of rocks varying in degree of hardness. Scratch one rock with another to see how rocks can be worn into soil.

Rub various other rocks together over a sheet of black paper and observe the soil-like grains as they fall.

Wind and water carry abrasive materials that wear away softer materials in a similar way. Wind and water, alone (without other materials to cause abrasion) can cause

Find pictures of areas eroded by wind or water. Now we're back to the Grand Canyon!

b. Temperature changes.

Soak a rough piece of sandstone in water for about an hour, then pour the rock and water into a milk carton. Set the carton in a freezer. When it is frozen, tear away the carton, and let the ice melt naturally in a fine sieve or cloth. Examine the remaining rock and other particles.

Make an analogy to rocks that get wet naturally, then freeze and thaw.

Find a rock that has a small crack in it. Turn the rock so that it will hold water in that crack, and place it in the freezer. Water expands when it freezes, and if the freezer is cold enough, the ice in the rock's crevice should increase the size of the crack. Let the ice melt, and measure the water. Now, put water in the rock again, and see if it will hold more than the amount that melted out. Repeated "treatments" of freezing and melting will eventually break the rock. This is one way that erosion makes little ones out of big ones.

Now heat small pieces of soft rock such as sandstone in a pan over a hot plate for several minutes, turning the rocks over frequently to distribute the heat evenly. Drop the heated rocks into a bowl of cold water and observe what happens. (Use a metal bowl for this.)

The temperature changes in these activities simulate the far slower temperature changes found in nature, changes such as those between night and day, and those between winter and summer.

Temperature changes can break down rocks, contributing to the building of soil.

c. Plants.

Soak some seeds in water overnight, then pack them tightly in a small plastic bottle with a screw cap. Cap the bottle, and observe it daily.

You will see that as the seeds grow, the pressure they exert is great enough to break the bottle.

Look for places in rocks, cement, or asphalt, to find plants that are breaking down materials in a similar way.

While plants are a part of erosion, they also help prevent erosion where it isn't wanted. For example, notice the plantings on road-cuts in hilly areas, or in cities near where there is a freeway interchange. A thick growth of plants can hold the soil in place and keep it from washing away.

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