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Chapter 9 Weathering and Soil Formation

Preview

Section 1 Weathering

Section 2 Rates of Weathering

Section 3 From Bedrock to Soil

Section 4 Soil Conservation

Concept Map


Section 1 WeatheringChapter 9

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.



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.



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.



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.




• 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.


Weathering and Soil Formation

Ice Wedging

Chapter 9




• 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.




• 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.




• 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.



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.



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.







• 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.




• 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.




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.




• 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.




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.




• 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.




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.




• 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.


Section 2 Rates of WeatheringChapter 9

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?




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.



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.



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.



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.



The Surface Area of Rocks, continued



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.



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.



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.



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.



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.

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.

Other Factors that Affect Weathering, continued



Rates of Weathering

Chapter 9


Section 3 From Bedrock to SoilChapter 9

Bellringer

Why is soil important in a land ecosystem?




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.



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.



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.



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.



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.



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.







• 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.




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.




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.







• 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.




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).







• 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.



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.



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.



Leaching

Chapter 9




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.




• 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.




• 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.




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.




• 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.


Section 4 Soil ConservationChapter 9

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.



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.



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.



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.




• 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.




• 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.



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.



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.



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.



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.



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.






• 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.




• 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.



Chapter 9 Weathering and Soil Formation

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

Concept Map


Weathering and Soil FormationChapter 9

Concept Map


Weathering and Soil FormationChapter 9

Concept Map

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