Chapter 7

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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.
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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
Section 1 Weathering
Ice Wedging
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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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