Science at key stage 3 (Year 8)
Unit 8G: Rocks and weathering
Section 3: How do changes in temperature cause rocks to weather?
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Objectives |
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- that rocks are broken down by forces that result from stresses generated when water in cracks and fissures expands on freezing
- that rocks at the Earth's surface are broken down by forces that result from stresses generated when rocks expand and contract on heating and cooling
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Activities |
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Outcomes |
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Children: |
- Show pupils pictures or a video clip as a stimulus and ask them to suggest why mountaineers climbing in high mountains,
eg the Alps or Himalayas, start early in the morning and try to complete their climbing on mountain faces before midday.
- Demonstrate the magnitude of the forces arising from expansion or contraction of a solid,
eg by repeatedly heating a corner of a chip of granite to red heat then quenching it in cold water or by using a breaking-bar experiment. Establish with pupils that these forces are large enough to cause pieces of rock to break off and are most significant where there are large temperature ranges.
- Present pupils with a rock sample containing cracks and soak in water. Explain that this is to be used to model what happens when water freezes and thaws. Ask them to suggest how this might be done and how to record the results. Use a digital camera to create a time-lapse sequence showing the number of freeze-thaw cycles on the rock sample and ask pupils to examine the changes, particularly the width of cracks and the shape and size of fragments, using a computer-generated slide show. Discuss how the angular fragments are formed. Ask pupils to re-evaluate their suggestions about mountaineers and explain why rockfalls can be a major hazard to climbing.
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- explain how water absorbed by rocks expands on freezing and fragments the rock
- describe how changes in temperature can result in rock fragmentation
- describe conditions when fragmentation is likely to occur
- explain that the forces arising from expansion and contraction are great enough to break off pieces of rock
- relate expansion and contraction to the particle model of matter
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Points to note |
- If pupils do not know that water expands when it freezes, a demonstration using a plastic bottle filled with water frozen in a freezer would be helpful.
- Extension: ask pupils to use a hand lens to look at a sample of highly porous rock that has been soaked in a saturated salt solution,
eg sodium sulphate solution, and then dried, and to note the presence of crystals occupying the pore spaces. Discuss how the salt can be leached in solution from the rock and then crystallised on nearing the surface of the rock.
- Extension: use a digital camera to create a time-lapse sequence showing a number of saturation-drying cycles on the rock sample. Ask pupils to examine the changes using a computer-generated slide show and to record the size and shape of the fragments that are formed. Ask pupils to explain how the growth of salt crystals breaks down the rock.
Safety
- eye protection should be worn when heating granite
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This unit is divided into sections. Each section contains a sequence of
activities with related objectives and outcomes. You can view this unit by
moving through the sections or print/download the whole unit.
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