unit 1 lesson 2 weathering copyright © houghton mifflin harcourt publishing company
TRANSCRIPT
Break It Down
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What is weathering?
• Weathering is the breakdown of rock material by physical and chemical processes.
• Two kinds of weathering are physical weathering and chemical weathering.
Unit 1 Lesson 2 Weathering
What causes physical weathering?
• Physical weathering is the process by which rock is broken down into smaller pieces by physical changes.
• The composition of the material does not change during physical weathering.
• Agents of physical weathering include temperature changes, pressure changes, plant and animal actions, water, wind, and gravity.
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Unit 1 Lesson 2 Weathering
What causes physical weathering?
• Changes in temperature can cause a rock to break apart by weakening the structure of the rock.
• Heat causes the rock to expand; cold causes it to contract.
• Ice wedging, or frost wedging, causes cracks in rocks to widen with repeated cycles of freezing and thawing.
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Unit 1 Lesson 2 Weathering
What causes physical weathering?
• How can a small crack in a rock eventually split the rock into two or more pieces?
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Unit 1 Lesson 2 Weathering
What causes physical weathering?• Rocks that formed under
pressure deep within Earth can be exposed to the surface.
• As material is removed above the rock, the pressure decreases and the rock expands.
• Exfoliation is the process by which the outer layers of rock slowly peel away due to pressure changes.
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Unit 1 Lesson 2 Weathering
What causes physical weathering?• Animals such as prairie dogs,
can cause physical weathering by digging burrows.
• New rocks, soils, and other materials become exposed at the surface as a result of animal actions.
• Materials exposed at the surface are more likely to undergo weathering than those below.
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Unit 1 Lesson 2 Weathering
What causes physical weathering?
• Abrasion is the breaking down and wearing away of rock material by the mechanical action of other rocks.
• Three agents of physical weathering that can cause abrasion are moving water, wind, and gravity.
• Rocks suspended in a glacier can also cause abrasion of other rocks on Earth’s surface.
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Unit 1 Lesson 2 Weathering
What causes physical weathering?
• Roots of plants start out as tiny strands that may grow in small cracks in rocks.
• As the roots grow, they put more pressure on the rock, causing the rock to expand and eventually break apart.
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Unit 1 Lesson 2 Weathering
Reaction
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What causes chemical weathering?
• Chemical weathering is the breakdown of rocks by chemical reactions.
• Chemical weathering changes both the composition and appearance of rocks.
• Agents of chemical weathering include oxygen in the air and acids.
Unit 1 Lesson 2 Weathering
What causes chemical weathering?
• Oxidation is the process by which chemicals in rock combine with oxygen in the air or in water.
• Rock surfaces, especially those containing iron, sometimes change color, indicating that a chemical reaction may have occurred due to rust production.
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Unit 1 Lesson 2 Weathering
What causes chemical weathering?
• Acids can cause chemical weathering by breaking down minerals faster than water alone.
• Acids in the atmosphere are created when chemicals (due to the burning of fossil fuels) combine with water in the air.
• Acid precipitation occurs when strong acids fall to Earth as rain, sleet, or snow.
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Unit 1 Lesson 2 Weathering
What causes chemical weathering?
• Acids in groundwater can cause rock to dissolve.
• A small crack in the rock can result in the formation of extensive cave systems carved out over time.
• Rock material dissolved in groundwater can be carried and deposited in new locations over time.
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Unit 1 Lesson 2 Weathering
What causes chemical weathering?
• Acids produced by living things such as lichens and mosses, can cause chemical weathering.
• Chemical reactions occur as these acids move through tiny spaces in rock material.
• As the acids seep deeper, cracks form. Eventually, the rock can break apart.
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Unit 1 Lesson 2 Weathering