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Assessment
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Show the Images of Weathering pictures to your students. Have your students answer the following questions as related to the pictures:
You could also have students conduct another model of abrasion. But, instead of using sugar cubes, they use small pieces of broken limestone, all about the same size. They put the pieces into a plastic container filled with water. They shake the container for a total of 700 shakes. After each 100 shakes, they observe how the shapes of the rocks change. Have students predict how the rocks will change before they begin the activity to see if they are able to apply what they learned in the sugar cube activity to this new situation.
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Complete this investigation by asking your students to reflect on this question and how their answers may have changed as a result of the investigation. For example, they should note that rocks breakdown into smaller and smaller pieces as they bounce, collide, and grind against each other. They also change shape, becoming rounder and more smooth.
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The following passage provides more detailed information related to this investigation that you may choose to explain to your students. Physical WeatheringPhysical weathering, sometimes called mechanical weathering, includes all the processes which break rocks apart without changing their chemical composition. Following are examples of physical weathering processes. Rock AbrasionRock abrasion occurs when rocks collide with one another or rub against one another. Collisions, if they are strong enough, can cause pieces of rock to break into two or more pieces, or cause small chips to be broken off a large piece. When two pieces of rock are rubbed together, the mineral grains in the rocks can be broken away from the rock surface. Rock abrasion occurs commonly in landslides where pieces of rock slide past one another as the mass moves downhill. It also occurs at the base of a glacier where pieces of rock that are frozen into the ice are dragged along beneath the glacier. In fast-moving streams and rivers, pieces of rock that are being moved by the flow rub against one another and against other pieces resting on the river bed. Ice wedgingIce wedging refers to the repeated freezing and melting of water within small cracks in rocks near the surface. The water in the cracks freezes as the temperature drops below freezing. As the water freezes, it expands. This expansion exerts tremendous pressure on the surrounding rock and acts like a wedge, making cracks wider. After repeated freezing and thawing of water, the rock breaks apart. Plant rootsPlant roots can grow in cracks. As the plant grows, the root becomes larger. The pressure of a confined growing root can be substantial. These pressures make cracks in the rocks larger, and, as roots grow, they can break rocks apart. |
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