Chapter 7
Chapter 7 Weathering and Soil Formation
Table of Contents
Section 1 Weathering
Section 2 Rates of Weathering
Section 3 From Bedrock to Soil
Section 4 Soil Conservation
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Chapter 7 Section 1 Weathering
Bellringer
How do you think potholes form in paved roads?
Write a few sentences that describe how water contributes to the formation of potholes. Illustrate how cycles of freezing and thawing cause potholes to grow.
Record your answers in your science journal .
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Chapter 7 Section 1 Weathering
Objectives
• Explain how mechanical weathering and chemical weathering shape and reshape Earth’s surface.
• Describe how ice, water, wind, gravity, plants, and animals cause mechanical weathering.
• Describe how plants and animals can shape
Earth’s landscape.
• Describe how water, acids, plants, and air cause chemical weathering of rocks.
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Chapter 7 Section 1 Weathering
Mechanical Weathering
• Small rocks often are pieces of larger rocks. The process by which rock materials are broken down by the action of physical and chemical processes is known as weathering .
• Mechanical weathering is the breakdown of rock into smaller pieces by physical means, such as ice, wind, water, gravity, plants, or animals.
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Chapter 7 Section 1 Weathering
Mechanical Weathering, continued
• Ice The alternate freezing and thawing of soil and rock, called frost action , is a form of mechanical weathering. One type of frost action is ice wedging.
• Ice wedging starts when water seeps into cracks during warm weather. When temperatures drop, the water freezes and expands. The ice then pushes against the sides of the crack, causing the crack to widen.
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Chapter 7
Ice Wedging
Section 1 Weathering
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Chapter 7 Section 1 Weathering
Mechanical Weathering, continued
• Abrasion is the grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles.
• Wind, Water, and Gravity are three forms of abrasion. Wind blows sand and silt against rock.
Rocks scrape against each other as they roll along the bottom of swiftly flowing rivers. Abrasion also occurs when rocks fall on one another.
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Chapter 7 Section 1 Weathering
Mechanical Weathering, continued
• Plants often send roots into cracks in rocks. As the plant grows, the expanding root widens the crack and can eventually split the rock apart.
• Animals cause weathering by burrowing in the soil and loosening rocks to be exposed to weathering.
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Chapter 7 Section 1 Weathering
Chemical Weathering
• The process by which rocks break down through chemical reactions is called chemical weathering .
• Common agents of chemical weathering are water, weak acids, and air.
• Water can break down even hard rock, such as granite. But it may take thousands of years.
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Chapter 7 Section 1 Weathering
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Chapter 7 Section 1 Weathering
Chemical Weathering, continued
• Acid precipitation is rain, sleet, or snow with a high concentration of acids. High levels of acidity can cause rapid weathering of rock.
• Sulfuric and nitric acids from natural sources, such as volcanoes, can make precipitation acidic. Also, acid precipitation can be caused by air pollution from the burning of fossil fuels.
• If the acidity is too high, acid precipitation can be harmful to plants and animals.
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Chapter 7 Section 1 Weathering
Chemical Weathering, continued
• Acids in Groundwater In certain places groundwater contains weak acids, such as carbonic or sulfuric acid. These acids react with the rocks in the ground.
• Acids in Living Things Living organisms such as lichens, which consist of fungi and algae, produce acids that can slowly break down rock.
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Chapter 7 Section 1 Weathering
Chemical Weathering, continued
• Air The oxygen in air reacts with metals causing rust in a process called oxidation.
• Oxidation is a chemical reaction that occurs when an element, such as iron, combines with oxygen to form an oxide. Water speeds up the process.
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