# StartUp Activities

LAUNCH L

How does change relate to surface area?

Surface area is a measure of the interface between an object and its environment. An object having more surface area can be affected more rapidly by its surroundings.

Types of Weathering Make this Foldable to explain the types of weathering and what affects the rate of weathering.

Procedure E*3 ILi K^

1. Read and complete the lab safety form.

2. Fill two 250-mL beakers with water at room temperature.

3. Drop a sugar cube in one beaker and 5 mL of granulated sugar in the other beaker at the same time. Record the time.

4. Slowly and continuously use a stirring rod to stir the solution in each beaker.

5. Observe the sugar in both beakers. Using a stopwatch, record the amount of time it takes for the sugar to completely dissolve in each beaker of water.

Analysis

1. Describe what happened to the sugar cube and the granulated sugar.

2. Explain why one form of sugar dissolved faster than the other.

3. Infer how you could decrease the time required for the slower-dissolving form of sugar.

STEP 1 Fold a sheet of paper in half vertically.

STEP 2 Make a 3-cm fold at the top and crease.

STEP 3 Unfold the paper and draw lines along the fold lines. Label the columns Mechanical Weathering and Chemical Weathering.

Irf.Mllli* Use this Foldable with Section 7.1.

As you read this section, explain the types of weathering and the variables in the processes.

Earth

Scienc^pnlinS

Visit glencoe.com to ^ study entire chapters online;

► explore h Mqjj^ animations:

• Interactive Time Lines

• Interactive Figures

• Interactive Tables

► review content with the Interactive Tutor and take Self-Check Quizzes.

Chapter 7 • WeatXering, Erosion, and Soil 163

STEP 1 Fold a sheet of paper in half vertically.

STEP 2 Make a 3-cm fold at the top and crease.

Chapter 7 • WeatXering, Erosion, and Soil 163

Section 7.1

Objectives

Weathering

Section 7.1

### Objectives

I Distinguish between mechanical and chemical weathering. I Describe the different factors that affect mechanical and chemical weathering. I Identify variables that affect the rate of weathering.

Review Vocabulary acid: solution that contains hydrogen ions

New Vocabulary weathering mechanical weathering frost wedging exfoliation chemical weathering oxidation

Weathering

Weathering breaks down materials on or near Earth's surface.

Real-World Reading Link You might have noticed that rust will begin to form at places on a car where the paint has chipped. In regions that are cold, rust seems to eat away at the paint of the car. This is an example of weathering.

### Mechanical Weathering

Weathering is the process in which materials on or near Earth's surface break down and change. Mechanical weathering is a type of weathering in which rocks and minerals break down into smaller pieces. This process is also called physical weathering. Mechanical weathering does not involve any change in a rock's composition, only changes in the size and shape of the rock. A variety of factors are involved in mechanical weathering, including changes in temperature and pressure.

Effect of temperature Temperature plays a role in mechanical weathering. When water freezes, it expands and increases in volume by 9 percent. You have observed this increase in volume if you have ever frozen water in an ice cube tray. In many places on Earth's surface, water collects in the cracks of rocks and rock layers. If the temperature drops to the freezing point, water freezes, expands, exerts pressure on the rocks, and can cause the cracks to widen slightly, as shown in Figure 7.1. When the temperature increases, the ice melts in the cracks of rocks and rock layers. The freeze-thaw cycles of water in the cracks of rocks is called frost wedging. Frost wedging is responsible for the formation of potholes in many roads in the northern United States where winter temperatures vary frequently between freezing and thawing.

Figure 7.1 Frost wedging begins in hairline fractures of a rock. Repeated cycles of freeze and thaw cause the crack to expand over time. Predict the results of additional frost wedging on this boulder.

Figure 7.1 Frost wedging begins in hairline fractures of a rock. Repeated cycles of freeze and thaw cause the crack to expand over time. Predict the results of additional frost wedging on this boulder.

164 Chapter 7 • Weathering, Erosion, and Soil

Larry Stepanowicz/Visuals Unlimited

Effect of pressure Another factor involved in mechanical weathering is pressure. Roots of trees and other plants can exert pressure on rocks when they wedge themselves into the cracks in rocks. As the roots grow and expand, they exert increasing amounts of pressure which often causes the rocks to split, as shown in Figure 7.2.

On a much larger scale, pressure also functions within Earth. Bedrock at great depths is under tremendous pressure from the overlying rock layers. A large mass of rock, such as a batholith, may originally form under great pressure from the weight of several kilometers of rock above it. When the overlying rock layers are removed by processes such as erosion or even mining, the pressure on the bedrock is reduced. The bedrock surface that was buried expands, and long, curved cracks can form. These cracks, also known as joints, occur parallel to the surface of the rocks. Reduction of pressure also allows existing cracks in the bedrock to widen. For example, when several layers of overlying rocks are removed from a deep mine, the sudden decrease of pressure can cause large pieces of rocks to explode off the walls of the mine tunnels.

Over time, the outer layers of rock can be stripped away in succession, similar to the way an onion's layers can be peeled. The process by which outer rock layers are stripped away is called exfoliation. Exfoliation often results in dome-shaped formations, such as Moxham Mountain in New York and Half Dome in Yosemite National Park in California, shown in Figure 7.3.

Figure 7.2 Tree roots can grow within the cracks and joints in rocks and eventually cause the rocks to split.

Incorporate information from this section into your Foldable.

Figure 7.3 The rock that makes up Half Dome in Yosemite National Park fractures along its outer surface in a process called exfoliation. Over time this has resulted in the dome shape of the outcrop.

Figure 7.3 The rock that makes up Half Dome in Yosemite National Park fractures along its outer surface in a process called exfoliation. Over time this has resulted in the dome shape of the outcrop.

Section 1 • Weathering 165

(tr)John Serrao/Visuals Unlimited, (b)Bruce Hayes/Photo Researchers, Inc.

Chemical Weathering

Chemical weathering is the process by which rocks and minerals undergo changes in their composition. Agents of chemical weathering include water, oxygen, carbon dioxide, and acid precipitation. The interaction of these agents with rock can cause some substances to dissolve, and some new minerals to form. The new minerals have properties different than those that were in the original rock. For example, iron often combines with oxygen to form iron oxide, such as in hematite.

^p Reading Check Express in your own words the effect that chemical weathering has on rocks.

The composition of a rock determines the effects that chemical weathering will have on it. Some minerals, such as calcite, which is composed of calcium carbonate, can decompose completely in acidic water. Limestone and marble are made almost entirely from calcite, and are therefore greatly affected by chemical weathering. Buildings and monuments made of these rocks usually show signs of wear as a result of chemical weathering. The statue in Figure 7.4 is made of sandstone, which also weathers relatively easily.

Temperature is another significant factor in chemical weathering because it influences the rate at which chemical interactions occur. Chemical reaction rates increase as temperature increases. With all other factors being equal, the rate of chemical weathering reactions doubles with each 10°C increase in temperature.

Effect of water Water is an important agent in chemical weathering because it can dissolve many kinds of minerals and rocks. Water also plays an active role in many reactions by serving as a medium in which the reactions can occur. Water can also react directly with minerals in a chemical reaction. In one common reaction with water, large molecules of the mineral break down into smaller molecules. This reaction decomposes and transforms many silicate minerals. For example, potassium feldspar decomposes into kaolinite, a fine-grained clay mineral common in soils.

Effect of oxygen An important element in chemical weathering is oxygen. The chemical reaction of oxygen with other substances is called oxidation. Approximately 21 percent of Earth's atmosphere is oxygen gas. Iron in rocks and minerals combines with this atmospheric oxygen to form minerals with the oxidized form of iron. A common mineral that contains the oxidized form of iron is hematite.

166 Chapter 7 • Weathering, Erosion, and Soil

Effect of carbon dioxide Another atmospheric gas that contributes to the chemical weathering process is carbon dioxide. Carbon dioxide is a gas that occurs naturally in the atmosphere as a product of living organisms. When carbon dioxide combines with water in the atmosphere, it forms a very weak acid called carbonic acid that falls to Earth's surface as precipitation.

Precipitation includes rain, snow, sleet, and fog. Natural precipitation has a pH of 5.6. The slight acidity of precipitation causes it to dissolve certain rocks, such as limestone.

Decaying organic matter and respiration produce high levels of carbon dioxide. When slightly acidic water from precipitation seeps into the ground and combines with carbon dioxide in the soil, carbonic acid becomes an agent in the chemical weathering process. Carbonic acid slowly reacts with minerals such as calcite in limestone and marble to dissolve rocks. After many years, limestone caverns can form where the carbonic acid flowed through cracks in limestone rocks and reacted with calcite.

Oft Effect of acid precipitation Another agent of chemical weathering is acid precipitation, which is caused by sulfur dioxide and nitrogen oxides that are released into the atmosphere, in large part by human activities. Sulfur dioxide is primarily the product of industrial burning of fossil fuels. Motor-vehicle exhausts also contribute to the emissions of nitrogen oxides. These two gases combine with oxygen and water in the atmosphere, forming sulfuric and nitric acids, which are strong acids.

The acidity of a solution is described using the pH scale, as you learned in Chapter 3. Acid precipitation is precipitation that has a pH value below 5.6—the pH of normal rainfall. Because strong acids can be harmful to many organisms and destructive to humanmade structures, acid precipitation often creates problems. Many plant and animal populations cannot survive even slight changes in acidity. Acid precipitation is a serious issue in New York, as shown in Figure 7.5, and in West Virginia and much of Pennsylvania.

Vocabulary

Process a natural phenomenon marked by gradual changes that lead toward a particular result The process of growth changes a seedling into a tree

Figure 7.5 The forests of the Adirondack Mountains have been damaged by the effects of acid precipitation. Acid precipitation can make forests more vulnerable to disease and damage by insects.

Section 1 • Weathering 167

Rob & Ann Simpson/Visuals Unlimited ffiiM'v mm-M

Figure 7.6 The impact of chemical weathering is related to a region's climate. Warm, lush areas such as the tropics experience the fastest chemical weathering. Infer what parts of the world experience less chemical weathering.

### Rate of Weathering

The natural weathering of Earth materials occurs slowly. For example, it can take 2000 years to weather 1 cm of limestone, and most rocks weather at even slower rates. Certain conditions and interactions can accelerate or slow the weathering process, as demonstrated in the GeoLab at the end of this chapter.

Effects of climate on weathering Climate is the major influence on the rate of weathering of Earth materials. Precipitation, temperature, and evaporation are factors that determine climate. The interaction between temperature and precipitation in a given climate determines the rate of weathering in a region.

^ Reading Check Explain why different climates have different rates of weathering.

Rates of chemical weathering Chemical weathering is rapid in climates with warm temperatures, abundant rainfall, and lush vegetation. These climatic conditions produce soils that are rich in organic matter. Water from heavy rainfalls combines with the carbon dioxide in soil organic matter and produces high levels of carbonic acid. The resulting carbonic acid accelerates the weathering process. Chemical weathering has the greatest effects along the equator, where rainfall is plentiful and the temperature tends to be high, as shown in Figure 7.6.

Figure 7.6 The impact of chemical weathering is related to a region's climate. Warm, lush areas such as the tropics experience the fastest chemical weathering. Infer what parts of the world experience less chemical weathering.

Rates of physical weathering Conversely, physical weathering can break down rocks more rapidly in cool climates. Physical weathering rates are highest in areas where water in cracks within the rocks undergoes repeated freezing and thawing. Conditions in such climates do not favor chemical weathering because cool temperatures slow or inhibit chemical reactions. Little or no chemical weathering occurs in areas that are frigid year-round.

The different rates of weathering caused by different climatic conditions can be emphasized by a comparison of Asheville, North Carolina, and Phoenix, Arizona. Phoenix has dry, warm, conditions; temperatures do not drop below the freezing point of water, and humidity is low. In Asheville, temperatures frequently drop below freezing during the winter months, and Asheville has more monthly rainfall and higher levels of humidity than Phoenix. Because of these differences in their climates, rocks and man-made structures in Asheville experience higher rates of mechanical and chemical weathering than those in Phoenix.

Figure 7.7 shows how rates of weathering are dependent on climate. Both Egyptian obelisks were carved from granite more than one thousand years ago. For more than a thousand years, they stood in Egypt's dry climate, showing few effects of weathering. In 1881, Cleopatra's Needle was transported from Egypt to New York City. In the time that has passed since then, the acid precipitation and the repeated cycles of freezing and thawing in New York City accelerated the processes of chemical and physical weathering. In comparison, the obelisk that remains in Egypt appears unchanged.

Rock type and composition. Not all the rocks in the same climate weather at the same rate. The effects of climate on the weathering of rock also depends on the rock type and composition For example, rocks containing mostly calcite, such as limestone and marble, are more easily weathered than rocks containing mostly quartz, such as granite and quartzite.

Figure 7.7 The climate of New York City caused the obelisk on the left to weather rapidly. The obelisk on the right has been preserved by Egypt's dry, warm climate.

+2 0

### Responses

• Mattia
How does global warming affect weathering of rocks?
6 years ago
• omar asmara
Why do rocks in ashville, NC experience a higher rate of weathering than Phoenix ,AZ?
8 years ago
• stefan
How do humans affect weathering images?
9 years ago
• david
Which part of europe experiences the highest level of acid rain?
10 years ago
• nebyat
What is mechanical weatheringice wedging?
10 years ago
• amira
What parts of the world experience more physical weathering?
10 years ago
• aila saarij
How does ice wedging causes mechanical weathering?
10 years ago