Download Section 4 Soil Conservation Chapter 9

Chapter 9
Weathering and Soil Formation
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Section 1 Weathering
Section 2 Rates of Weathering
Section 3 From Bedrock to Soil
Section 4 Soil Conservation
Concept Map
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Chapter 9
Section 1 Weathering
Bellringer
1. How do potholes form in paved roads?
2. How does water flowing downhill contribute to the
formation of potholes?
3. How do cycles of freezing and thawing cause
potholes to grow?
Write your answers in your science journal.
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Chapter 9
Section 1 Weathering
What You Will Learn
• Ice, water, wind, gravity, plants, and animals can
cause mechanical weathering by breaking rock into
pieces.
• Water, acids, and air can cause chemical weathering
of rocks.
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Chapter 9
Section 1 Weathering
Mechanical Weathering
• Mechanical weathering is the breakdown of rock
into smaller pieces by physical means.
• Agents of mechanical weathering include ice, wind,
water, gravity, plants, and even animals.
• One cause of mechanical weathering is frost action.
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Chapter 9
Section 1 Weathering
Mechanical Weathering, continued
• Frost action is the alternate freezing and thawing of
soil and rock.
• Ice wedging is one form of frost action.
• Ice wedging starts when water seeps into rocks
during warm weather.
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Chapter 9
Section 1 Weathering
Mechanical Weathering, continued
• When temperatures drop, the water freezes and
expands.
• The ice pushes on the sides of the cracks, causing
the cracks to widen.
• As the cycle repeats, the rock eventually breaks
apart.
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Chapter 9
Weathering and Soil Formation
Ice Wedging
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Chapter 9
Section 1 Weathering
Mechanical Weathering, continued
• The roots of plants commonly grow into existing
cracks in rocks.
• As the plant grows, the force of its expanding roots
widens the cracks in rock.
• In time, the whole rock can split apart.
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Chapter 9
Section 1 Weathering
Mechanical Weathering, continued
• Earthworms cause a lot of weathering.
• Earthworms tunnel through soil and move soil
particles around. This motion can also break down
soil into smaller particles.
• Earthworms also expose fresh surfaces to
weathering.
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Chapter 9
Section 1 Weathering
Mechanical Weathering, continued
• Ants, worms, mice, coyotes, rabbits, and other
animals that burrow cause mechanical weathering.
• The mixing and digging that animals do also
contributes to chemical weathering.
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Chapter 9
Section 1 Weathering
Chemical Weathering
• The process by which rocks break down as a result
of chemical reactions is called chemical weathering.
• Common agents of chemical weathering are water,
acids, and air.
• These agents cause reactions that weaken or destroy
bonds between elements in the rock.
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Chapter 9
Section 1 Weathering
Chemical Weathering, continued
• If you drop a sugar cube into water, it dissolves.
• In a similar way, water dissolves some of the
chemicals that make up rock.
• Water may take thousands of years to break up hard
rock, such as granite.
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Chapter 9
Section 1 Weathering
Chemical Weathering, continued
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Chapter 9
Section 1 Weathering
Chemical Weathering, continued
• Precipitation is naturally acidic.
• However, rain, sleet, or snow that contains a higher
concentration of acid than normal is called acid
precipitation.
• Sulfuric and nitric acids can combine with water in the
atmosphere to form acid precipitation.
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Chapter 9
Section 1 Weathering
Chemical Weathering, continued
• Natural sources, such as volcanoes, can produce
sulfuric and nitric acids.
• Air pollution from the burning of fossil fuels also
produces these chemical compounds.
• Acid precipitation can cause rapid weathering of rock.
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Chapter 9
Section 1 Weathering
Chemical Weathering, continued
How Acids in Groundwater Weather Rock
• In certain places, water flows through rock
underground.
• This water, called groundwater, commonly contains
dilute acids, such as carbonic or sulfuric acid.
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Chapter 9
Section 1 Weathering
Chemical Weathering, continued
• When this groundwater comes into contact with
limestone, a chemical reaction takes place.
• This chemical reaction dissolves rock.
• Over a long period of time, the dissolving of
limestone may form caverns.
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Chapter 9
Section 1 Weathering
Chemical Weathering, continued
How Acids in Living Things Weather Rock
• All living things make dilute acids in their bodies.
• When living things come into contact with rock, some
of the acids are transferred to the rock’s surface.
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Chapter 9
Section 1 Weathering
Chemical Weathering, continued
• The acids react with chemicals in rock and produce
areas of weakness in the rock.
• The weakened areas are more easily weathered.
• The rock may also crack in these areas, exposing
more rock to weathering agents.
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Chapter 9
Section 1 Weathering
Chemical Weathering, continued
How Air Chemically Weathers Rock
• Some rocks change color as iron and other elements
in the rocks react with oxygen in the air.
• Oxidation is a chemical reaction in which an element
combines with oxygen to form an oxide.
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Chapter 9
Section 1 Weathering
Chemical Weathering, continued
• Oxidation causes rust.
• Cars, bicycles, and other metal objects can
experience oxidation if they are exposed to air and
rain for a long time.
• Exposure to water speeds up the process of
oxidation.
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Chapter 9
Section 2 Rates of Weathering
Bellringer
Describe two different ways to do the same chore.
Explain each process and the amount of time it would
take. Why might two different ways of doing the same
chore take different amounts of time?
Write your answers in your science journal.
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Chapter 9
Section 2 Rates of Weathering
What You Will Learn
• Differential weathering is the process by which softer
rocks weather more rapidly than harder rocks do.
• Surface area, climate, and elevation are factors that
affect the rate at which rock weathers.
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Chapter 9
Section 2 Rates of Weathering
Differential Weathering
• Hard rocks, such as granite, weather more slowly
than softer rocks, such as limestone.
• Differential weathering is the process by which softer,
less weather-resistant rocks wear away and leave
harder, more weather-resistant rocks behind.
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Chapter 9
Section 2 Rates of Weathering
The Surface Area of Rocks
• The greater the proportion of a rock that is exposed
to weathering, the faster the rock will wear down.
• A large rock has a large surface area that is exposed
to weathering.
• But a large rock also has a large volume.
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Chapter 9
Section 2 Rates of Weathering
The Surface Area of Rocks, continued
• Most of this volume is in the interior of the rock,
protected from weathering agents.
• If the large rock is broken into smaller pieces, the rate
of weathering increases.
• This is because the surface-to-volume ratio
increases, and more surface area is exposed to
weathering agents.
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Chapter 9
Section 2 Rates of Weathering
The Surface Area of Rocks, continued
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Chapter 9
Section 2 Rates of Weathering
Weathering and Climate
• Climate is the average weather conditions of an area
over a long period of time.
• Temperature and moisture affect rates of chemical
and mechanical weathering.
• More chemical reactions occur in climates that have
higher temperatures.
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Chapter 9
Section 2 Rates of Weathering
Weathering and Climate, continued
• Climatic regions that experience many thaws and
freezes have a greater rate of mechanical
weathering.
• The rate of chemical reactions, such as oxidation,
may increase in wet climates.
• Ice wedging and abrasion are also more common in
wet climates.
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Chapter 9
Section 2 Rates of Weathering
Other Factors that Affect Weathering
Elevation
• Rocks at higher elevations are exposed to high winds
and temperature extremes.
• They may also be exposed to large amounts of
precipitation.
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Chapter 9
Section 2 Rates of Weathering
Other Factors that Affect Weathering,
continued
• These processes at high elevation can cause rapid
weathering.
• However, rocks at sea level may also experience
rapid weathering, through wave action.
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Chapter 9
Section 2 Rates of Weathering
Other Factors that Affect Weathering,
continued
Slope
• The steep sides of mountains and hills increase the
speed of water running downhill.
• Water that flows more rapidly has more energy to
break down rock.
• Thus, rocks on steeper slopes experience increased
weathering.
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Chapter 9
Section 2 Rates of Weathering
Other Factors that Affect Weathering,
continued
Biological Factors
• Organisms in the soil, such as bacteria or fungi, can
produce acids that can speed chemical weathering.
• The activities of burrowing animals and plant roots
also speed mechanical weathering.
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Chapter 9
Weathering and Soil Formation
Rates of Weathering
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Chapter 9
Section 3 From Bedrock to Soil
Bellringer
Why is soil important in a land ecosystem?
Write your answers in your science journal.
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Chapter 9
Section 3 From Bedrock to Soil
What You Will Learn
• Soil is a mixture of weathered rock, organic material,
water, and air.
• Soil composition, texture, fertility, and pH affect plant
growth.
• Climate affects the types of soil that are found in
different places.
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Chapter 9
Section 3 From Bedrock to Soil
The Source of Soil
• Soil is a loose mixture of small mineral fragments,
organic material, water, and air that can support the
growth of vegetation.
• The rock that breaks down to form a soil is called
parent rock.
• Different parent rocks have different chemical
compositions. Therefore, soils also differ in chemical
composition.
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Chapter 9
Section 3 From Bedrock to Soil
The Source of Soil, continued
• Bedrock is the layer of rock beneath soil.
• In some cases, the soil remains above the bedrock
that weathered to form the soil.
• Soil that remains above its parent rock material is
called residual soil.
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Chapter 9
Section 3 From Bedrock to Soil
The Source of Soil, continued
• In other cases, the bedrock below the soil is not the
soil’s parent rock.
• Soil can be carried away from parent rock by wind,
water, ice, or gravity.
• This soil is called transported soil.
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties
Soil Composition
• Soil composition describe the kinds and relative
amounts of materials that the soil contains.
• Soil is made of different percentages of mineral
fragments, organic material, water, and air.
• For example, black soil is often rich in organic
material.
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties, continued
Soil Texture
• Soil is made of particles of different sizes.
• Some particles are as large as sand, others cannot
be seen without a microscope.
• Soil texture describes the relative amounts of soil
particles of different sizes.
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties, continued
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties, continued
• Soil texture affects the consistency of soil.
• Consistency describes a soil’s ability to be broken up
for farming.
• Soil texture also influences infiltration, or the ability of
water to move through soil.
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties, continued
Soil Fertility
• The ability of soil to hold nutrients and supply
nutrients to plants is called soil fertility.
• Nutrients in soil may come from the parent rock, or
from humus, the organic material that forms in soil
from the decayed remains of plants and animals.
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties, continued
Soil Horizons
• Due to the way it forms, soil often ends up in a series
of layers.
• Because the layers are horizontal, soil scientists call
these layers horizons.
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties, continued
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties, continued
• The top layer of soil is commonly called topsoil.
• Topsoil has more humus than lower layers of soil do.
• The humus is rich in the nutrients that plants need to
be healthy.
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties, continued
Soil pH
• Soils can be acidic, neutral, or basic.
• The pH scale is used to measure how acidic or basic
something is.
• The scale ranges from very acidic (0), to neutral (7),
to very basic (14).
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties, continued
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Chapter 9
Section 3 From Bedrock to Soil
Soil Properties, continued
• The pH of a soil influences how nutrients dissolve in
the soil.
• Soils that are too basic or acidic may restrict the
uptake of nutrients by plants.
• Soil acidity is influenced by the parent rock, acidity of
rainwater, use of fertilizers, and the extent of
chemical weathering.
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Chapter 9
Section 3 From Bedrock to Soil
Soil and Climate
Tropical Climates
• Tropical rain forests receive a lot of direct sunlight
and a large amount of rain.
• In a tropical rain forest, the fast decay of organic
matter provides rich humus to the soil.
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Chapter 9
Section 3 From Bedrock to Soil
Soil and Climate, continued
• However, constant rain can leach, or remove,
nutrients from this soil.
• As a result, many tropical soils are nutrient-poor.
• Another reason that the soils in tropical areas are
nutrient-poor is that the lush vegetation quickly uses
up most of the nutrients in the soil.
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Chapter 9
Weathering and Soil Formation
Leaching
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Chapter 9
Section 3 From Bedrock to Soil
Soil and Climate, continued
Desert and Arctic Climates
• Desert and arctic climates receive little rainfall.
• Thus, little leaching of nutrients happens in soils in
these climates.
• However, lack of rain also leads to low rates of
chemical weathering, and little plant and animal life.
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Chapter 9
Section 3 From Bedrock to Soil
Soil and Climate, continued
• As a result, soil forms slowly.
• Because of the lack of organisms, little humus
develops.
• Arctic soil has more humus than desert soil does
because the cold climate slows the breakdown of
organic materials.
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Chapter 9
Section 3 From Bedrock to Soil
Soil and Climate, continued
• Desert soils can develop high concentrations of salt.
• Groundwater in which mineral salts are dissolved
seeps into desert soil.
• When the water evaporates, the salts are left in the
soil. These salts prevent plants from absorbing water
from the soil.
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Chapter 9
Section 3 From Bedrock to Soil
Soil and Climate, continued
Temperate Forest and Grassland Climates
• Much of the continental United States has a
temperate climate.
• High rates of mechanical weathering cause thick soil
layers to form.
• Moderate amounts of rain increase chemical
weathering without leaching away nutrients.
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Chapter 9
Section 3 From Bedrock to Soil
Soil and Climate, continued
• Temperate soils are some of the most productive
soils in the world.
• The midwestern area of the United States has earned
the nickname “breadbasket” for the many crops that
the soil in this region supports.
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Chapter 9
Section 4 Soil Conservation
Bellringer
President Franklin D. Roosevelt once said, “The nation
that destroys its soil destroys itself.”
What do you think President Roosevelt meant when he
said this?
Write your thoughts in your science journal.
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Chapter 9
Section 4 Soil Conservation
What You Will Learn
• Soil is important for plants and animals and for the
storage of water.
• Farmers use a variety of methods to prevent soil
damage and loss.
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Chapter 9
Section 4 Soil Conservation
The Importance of Soil
• Soil conservation is a method to maintain the
fertility of the soil by protecting the soil from erosion
and nutrient loss.
• Soil provides minerals and other nutrients to plants.
• If soil loses these nutrients, plants will not be able to
grow.
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Chapter 9
Section 4 Soil Conservation
The Importance of Soil, continued
• Many land animals get their energy from plants.
• These animals either eat plants, or eat other animals
that eat plants.
• If plants can’t get enough nutrients from soil, then
animals can’t get enough nutrients from plants.
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Chapter 9
Section 4 Soil Conservation
The Importance of Soil, continued
• Soil also provides habitat, or places for animals to
live.
• Earthworms, spiders, ants, moles, and many other
animals live in soil.
• If the soil disappears, so does the habitat for these
animals.
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Chapter 9
Section 4 Soil Conservation
The Importance of Soil, continued
• Soil is also very important for water storage.
• Without soil to hold water, plants would not get the
moisture or the nutrients that they need.
• In addition, soil keeps water from running off, flowing
elsewhere, and causing flooding.
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Chapter 9
Section 4 Soil Conservation
Soil Damage and Loss
• Soil loss is a major problem around the world.
• One cause of soil loss is soil damage.
• Soil can be damaged from overuse by poor farming
techniques or by overgrazing.
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Chapter 9
Section 4 Soil Conservation
Soil Damage and Loss, continued
• Overused soil can lose its nutrients and become
infertile. Plants can’t grow in infertile soil.
• Without plants to hold and help cycle water in an
area, the area becomes a desert.
• This process is called desertification.
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Chapter 9
Section 4 Soil Conservation
Soil Damage and Loss, continued
• When soil is not protected, it can be exposed to
erosion.
• Erosion is the process by which wind, water, or
gravity transports soil and sediment from one location
to another.
• Roots of plants and trees can help hold soil in place
to prevent erosion.
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Chapter 9
Section 4 Soil Conservation
Soil Conservation on Farmland
• Farmers can use a variety of methods to prevent
erosion.
• In contour plowing, a farmer plows across the slope
of the hills.
• In this method the rows act as a series of dams
instead of a series of rivers.
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Chapter 9
Section 4 Soil Conservation
Soil Conservation on Farmland, continued
• If the hills are very steep, a farmer can use terracing.
• Terracing changes one steep field into a series of
small, flat fields.
• No-till farming, the practice of leaving old stalks,
provides cover that reduces water runoff and slows
soil erosion.
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Chapter 9
Section 4 Soil Conservation
Soil Conservation on Farmland, continued
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Chapter 9
Section 4 Soil Conservation
Soil Conservation on Farmland, continued
• Soybeans, peanuts, and some other plants help
return important nutrients to the soil.
• These plants are called cover crops.
• Cover crops are crops that are planted between
harvests to replace certain nutrients and to prevent
erosion.
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Chapter 9
Section 4 Soil Conservation
Soil Conservation on Farmland, continued
• Another way to slow nutrient depletion is through
crop rotation.
• Different plants use different nutrients from the soil.
• By planting a different crop each year, a farmer can
slow the process of nutrient depletion.
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Chapter 9
Weathering and Soil Formation
Concept Map
Use the terms below to complete the concept map
on the next slide.
crop rotation
soil erosion
contour plowing
terracing
soil conservation
cover crops
no-till farming
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Chapter 9
Weathering and Soil Formation
Concept Map
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Chapter 9
Weathering and Soil Formation
Concept Map
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