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Erosion: The Breakdown and Movement of Crustal Material (page 2)

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Author: Janice VanCleave

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1. How does the amount of surface area affect the rate of weathering? Repeat the experiment making 16 equal-sized mud balls. Compare the changes in the shape of the mud balls and the amount of sediment on the cookie sheet in this and the original experiment. More changes in shape and more sediment indicate a faster rate of weathering.
2. How does the composition of material affect its rate of weathering? Repeat the original experiment mixing 1/4 cup (63 ml) of soil with 1/4 cup (63 ml) of aquarium gravel. Science Fair Hint: Display pictures of areas such as Bryce Canyon in Utah and the Grand Canyon in Arizona that show the results of weathering when rocks of different composition, and hence different resistance to weathering, are in the same place.

1. One way that mechanical weathering occurs is when water freezes inside cracks in rock. The water expands and may push hard enough to split the rock. Design a way to show how the expansion of freezing water cracks materials. Try mixing water with 1/2 cup (125 ml) of soil to make eight equal-size mud balls. Put four of the balls on a saucer and place them in a freezer. Put the remaining balls in a second saucer and set them in a warm, dry place. After 24 hours, observe the surface of the two sets of mud balls for cracks. Did the frozen mud balls crack? If not, why? Take photographs of the mud balls at the start and end of the experiment to represent the results. Find out more about other examples of mechanical weathering. Use the mud ball photographs, photographs from experiments you design, and pictures from books to prepare a display poster representing different types of mechanical weathering.
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1. Rusting is a type of chemical weathering called oxidation (combination with oxygen). Rusting is the combination of iron with oxygen in the air to form iron oxide, or rust. Rust is not as strong as iron. Rocks containing iron erode as the iron rusts and the rust crumbles because of gravity, or is moved by water, wind, or ice. Demonstrate oxidation of iron by moistening a lemon-size piece of steel wool with water. Note: Use steel wool without soap, available where paint is sold. Wear gloves when handling the steel wool to prevent it from cutting your skin.
2. In your gloved hand, rub the wet steel wool between your fingers. Observe the firmness of the steel wool. Place the wet steel wool on a saucer and observe its surface for three to five days for signs of rust. When all or most of the surface appears a reddish brown, pick up the rusted steel wool in your gloved hand and again rub it between your fingers. Compare the firmness of the steel wool before and after it rusts.

3. If water is present, rusting occurs quickly and the brown rust limonite is formed. With oxygen alone, the red rust hematite is formed. How does the speed of rusting in high humidity compare with its speed in the presence of water? Humidity is the amount of water vapor (water in the gas phase) contained in air. Use two identical containers, one for high humidity and the other for low humidity, such as plastic see-through boxes with lids. Cover the bottom of one with a thin layer of water and the bottom of the other with borax, which makes the air dry. Place a bowl in each box and relatively equal size balls of dry steel wool in each bowl (see Figure 14.2). Observe for signs of rust for seven or more days.

Get the Facts

1. After rocks on a hill are weathered, the particles are pulled downhill by gravity. The particles fall or slide and come to rest at the steepest angle at which they can remain stable. This is called the angle of rest, or angle of repose. Find out more about gravity as an agent of erosion. How does friction affect the angle of rest? See John Farndon, How the Earth Works (New York: Reader's Digest Association, 1992), p. 108.
2. Ocean waves erode the rocky projections of land that extend into deep water, called headlands. Find out more about the erosion of headlands by ocean waves. What is the difference between sea cliffs, sea caves, sea arches, and sea stacks? For information about the erosion of shorelines by wave action, see Janice VanCleave's Oceans for Every Kid (New York: Wiley, 1996), pp. 87–94.
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