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What to do |
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Assessment |
In the previous two investigations, students examined some of the physical properties of different types of rocks. They began classifying rocks according to their different characteristics. In this investigation, students think about how rocks change through time. They consider the breakdown of rock into smaller and smaller pieces through processes that collectively are known as weathering. The specific process that students examine in this investigation is abrasion, the action of rocks and sediment grinding against each other and wearing away exposed surfaces. In nature, abrasion occurs as wind and water rush over rocks, causing them to bump against one another and changing their shapes. Rocks become smoother as rough and jagged edges break off. Students run a model of this process by shaking sugar cubes and gravel together in a plastic container. The sugar cubes become smoother as their edges break off when they collide in the container.
For each student:
For instructional purposes:
This investigation is generally considered safe to do with students. Do not provide glass containers to students because the shaking of the gravel could cause them to break. Students should be reminded not to eat the sugar cubes. Also, review the investigation for your specific setting, materials, students, and conventional safety precautions.
Most probably, students recognize that in nature rocks exist in different sizes, from exposed mountain sides and plateaus to boulders to gravel to grains of sand. The processes by which rocks break down into smaller and smaller pieces, however, may be new to students. Begin the investigation by asking students about the various sizes of rocks they may have experienced or seen.
Have your students discuss these questions, first in pairs, then groups and then as a whole class. Record their answers on a flipchart that you can refer to throughout the investigation.
After the scene is set, introduce your students to the investigation question: “How do rocks break down into smaller pieces?”
Have your students discuss the question in pairs, then in groups, and then as a whole class. Record their answers on the flipchart.
Have your students brainstorm ideas about how this investigation question could be investigated.
Tell your students that they will be investigating this question and at the end of their study they will be able to provide reliable answers.
Students will probably have had some personal experiences of breaking rocks apart, such as by hitting a rock with a hammer or throwing a rock onto a hard surface. Some may be able to transfer this knowledge to a natural setting, such as rocks grinding against each other in a rockfall or landslide. Here are some initial questions that your students can discuss, in pairs, in groups, and as a whole class:
Have your students share their ideas with the class and record them as a list on the flipchart.
Have students think about what they would like to learn about the breakdown of rock into smaller and smaller pieces. Record their ideas on the flipchart as a list called “Questions we have about how rocks breakdown.” This list will provide further insights into what your students know, and also what they would like to know. By the end of the investigation, some of these questions will probably be answered.
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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:
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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.
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.
The following passage provides more detailed information related to this investigation that you may choose to explain to your students.
Physical 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 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 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 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.