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Interactive Textbook 173 Weathering and Soil Formation

SECTION

2 Rates of WeatheringWeathering and Soil Formation

Name Class Date

CHAPTER 10

After you read this section, you should be able to answer these questions:

• What is differential weathering?

• What factors affect how fast rock weathers?

What Is Differential Weathering?Hard rocks, such as granite, weather more slowly than

softer rocks, such as limestone. Differential weathering

happens when softer rock weathers away and leaves harder, more resistant rock behind. The figures below show an example of how differential weathering can shape the landscape.

BEFORE YOU READ National Science Education StandardsES 1c, 1d

STUDY TIP

Apply Concepts After you read this section, think about how the different factors that control weathering can affect objects that you see every day. Discuss your ideas in a small group.

Say ItDiscuss In a small group, share your ideas about how some different landscape features formed because of differential weathering.

TAKE A LOOK 1. Infer Imagine that the rock on the outside of the mountain was much harder than the cooled magma. How might Devils Tower look today? Explain your answer.

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Interactive Textbook 174 Weathering and Soil Formation

SECTION 2

Name Class Date

Rates of Weathering continued

How Does a Rock’s Surface Area Affect Weathering?

Most chemical weathering takes place only on the outer surface of a rock. Therefore, rocks with a lot of surface area weather faster than rocks with little surface area. However, if a rock has a large volume as well as a large surface area, it takes longer for the rock to wear down. The figure below shows how the surface area and volume of a rock affect how fast it wears down.

Imagine a rock in the shape of a cube. Each side of the rock is 4 m long. The volume of the cube is 4 m � 4 m � 4 m � 64 m3. Each face of the rock has an area of 4 m � 4 m � 16 m2.Because there are six faces on the rock, it has a surface area of

6 � 16 m2 � m2.

Cube with 4 m sides: volume � 64 m3,surface area � m2

Now imagine eight smaller rocks that are shaped like cubes. Each small rock’s side is 2 m long. The volume of each small rock is 2 m � 2 m � 2 m � 8 m3. The total volume of all 8 rocks is 8 � 8 m3 � 64 m3,the same as the large rock. Each face of each small rock has an area of 2 m � 2 m � 4 m2. Each small rock has six faces, and there are eight rocks total. Therefore, the total surface area of all eight small rocks

is 8 � 6 � 4 m2 � m2. This is twice as big as the surface area of the large rock.

2 m

2 m 2 m

Eight cubes with 2 m sides: volume �

64 m3, surface area � m2

Over time, the rock weathers. Its volume and surface area get smaller.

In the same amount of time, the smaller rocks weather more and become much smaller. They lose a larger fraction of their volume than the larger rock.

More time passes. The large rock is weathered even more. It is now much smaller than it was before it was weathered.

In the same amount of time, the small rocks have completely worn away. They took much less time to wear away than the large rock. Even though their volume was the same, they had more surface area than the large rock. The large surface area allowed them to wear away more quickly.

Critical Thinking2. Explain Why does most chemical weathering happen only on the outer surface of a rock?

Math Focus3. Calculate Determine the surface areas of the large cube and the eight smaller cubes. Write these values in the blank lines on the fi gure.

TAKE A LOOK 4. Apply Concepts Why do the edges and corners of the cubes weather faster than the faces? (Hint: remember that objects with large sur-face areas weather quickly.)

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Interactive Textbook 175 Weathering and Soil Formation

SECTION 2

Name Class Date

Rates of Weathering continued

How Does Climate Affect Weathering?The rate of weathering in an area is affected by the cli-

mate of that area. Climate is the average weather condi-tions of an area over a long period of time. Some features of climate that affect weathering are temperature, mois-ture, elevation, and slope.

TEMPERATURETemperature is a major factor in both chemical and

mechanical weathering. Cycles of freezing and thawing increase the chance that ice wedging will take place. Areas in the world that have many freezes and thaws have faster mechanical weathering than other regions do.

High temperatures can also speed up weathering. Many chemical reactions happen faster at higher tem-peratures. These reactions can break down rock quickly in warm areas.

MOISTURE Water can interact with rock as precipitation, as run-

ning water, or as water vapor in the air. Water can speed up many chemical reactions. For example, oxidation can happen faster when water is present.

Water is also important in many kinds of mechanical weathering. For example, ice wedging cannot happen without water. Abrasion also happens faster when water is present.

ELEVATION AND SLOPEElevation and slope can affect how fast weathering

occurs. Elevation is a measure of how high a place is above sea level. Slope is a measure of how steep the sides of a mountain or hill are. The table below shows how elevation and slope affect weathering.

Factor How the factor affects weathering

Elevation Rocks at high elevations are exposed to low temperatures and high winds. They can weather very quickly. Rocks at sea level can be weathered by ocean waves.

Slope The steep sides of mountains and hills make water fl ow down them faster. Fast-moving water has more energy to break down rock than slow-moving water. Therefore, rocks on steep slopes can weather faster than rocks on level ground.

READING CHECK

5. Identify What is one way that temperature affects mechanical weathering?

TAKE A LOOK 6. Explain Why do rocks on the sides of mountains weather faster than rocks on level ground?

Copyright © by Holt, Rinehart and Winston. All rights reserved.

Interactive Textbook 176 Weathering and Soil Formation

Name Class Date

Section 2 Review NSES ES 1c, 1d

SECTION VOCABULARY

differential weathering the process by which softer, less weather resistant rocks wear away at a faster rate than harder, more weather resistant rocks do

1. Identify What are four factors that affect how fast weathering happens?

2. Explain Why does it take less time for small rocks to wear away than it does for large rocks to wear away?

3. Describe Imagine a rock on a beach and a rock on the side of a mountain. How would the factors that control weathering be different for these two rocks?

4. Apply Concepts Two rivers run into the ocean. One river is very long. The other is very short. Which river probably drops the smallest rock pieces near the ocean? Explain your answer.

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Interactive Textbook Answer Key 46 Earth Science

Earth Science Answer Key continued

4. Both happen when non-rock material moves into cracks in a rock, expands, and causes the cracks to widen.

SECTION 2 RATES OF WEATHERING 1. Devils Tower would probably be much

larger because the outer rock would not have worn away as much.

2. Chemical weathering happens when acid, water, or air react with rock. Only the outside of a rock is exposed to these agents, so only the outside experiences chemical weathering.

3. 96; 96; 192; 192

4. The edges and corners are where the most surface area is found.

5. Freezing and thawing are dependent on tem-perature.

6. Water on slopes has more energy to weather rock than water that isn’t moving.

Review 1. temperature, moisture, elevation, slope

2. Large rocks have less surface area for their volume than small rocks. Therefore, a smaller portion of the rock is exposed to weathering. It takes longer for the rock to wear away. The small rock has more surface area for its vol-ume, so it weathers away faster.

3. The rock on the beach would be affected by waves and a lot of water. The rock on the side of the mountain would be affected by fast-moving water and possibly by high winds. It probably would also be exposed to colder temperatures than the rock on the beach.

4. The longer river probably carries the small-est sediment. The rocks in the longer river travel a longer distance before they drop. Therefore, they have more chance to weather. The particles will tend to be smaller than those in the shorter river.

SECTION 3 FROM BEDROCK TO SOIL 1. The rocks from which they form have

different compositions.

2. (from left to right) residual soil, transported soil

3. the amounts of different-sized particles in a soil

4. about 25 times

5. organic material that comes from decayed organisms

6. O, A, and B

7. lemon juice

8. parent rock, acid precipitation, and fertilizers

9. Rainwater leaches nutrients from the soil.

10. Groundwater doesn’t evaporate as fast in cold climates.

11. Type of climate Description of climate

Features of the soil in this climate

Tropical climates warm temperatures, a lot of rain, many living things

has a lot of humus, but may be poor in nutrients

Deserts and arctic climates

very little rain, few living things

has little humus, poor in nutrients

Temperate forests and grasslands

mediumamount of rain, temperaturechanges often

has a lot of humus, rich in nutrients

Review 1. structure, texture, fertility

2. Neither arctic climates nor desert climates get a lot of rain.

3. Flowing water can leach nutrients out of soils. This makes the soils less fertile.

4. Most plants grow best in soil that is slightly acidic or neutral. If the pH is too low or too high, plants cannot grow.

5. the relative amounts of sand, silt, and clay in the soil

6. mineral fragments, organic material, water, air

SECTION 4 SOIL CONSERVATION 1. Animals get energy from plants, which need

soil to grow. Some animals live in soil.

2. What does soil provide?

Why is it important?

Nutrients Nutrients help to keep plants healthy. Animals get energy from plants.

Habitat Animals and other organisms need places to live.

Water storage Soil provides water to plants and helps prevent fl ooding.

3. Plant roots hold soil in place. They help to keep wind and water from carrying the soil away.

4. Slow-moving water carries away less soil.

5. Terraces can be difficult and expensive to build. They are only economical on very steep slopes where other methods don’t work.