CHAPTER 14 SECTION Weathering and Erosion 4 Erosion KEY IDEAS As you read this section, keep these questions in mind: What is erosion, and what are four agents of erosion? What are four farming methods that conserve soil? How does gravity contribute to erosion? What are the three major landforms shaped by weathering and erosion? What Is Erosion? Weathering causes rock particles to break away from rock. However, the rock particles do not always stay near the parent rock. Forces in the environment may move the particles to other places. This process, in which Earth materials are moved from one place to another, is called erosion. The most common agents of erosion are gravity wind glaciers water How Does Erosion Affect Soil? Tiny particles of rock that form during weathering mix with water, air, and humus to form soil. Soil erodes constantly, but soil erosion is normally a slow process. New soil forms fast enough to replace eroding soil. However, living and nonliving factors can upset this natural balance. When this happens, soil erodes quickly before new soil can replace it. Some scientists think that soil erosion is the greatest environmental problem today. Erosion removes fertile topsoil and makes it harder to grow crops. Certain farming techniques can accelerate, or speed up, soil erosion. For example, some farmers plow long, narrow rows called furrows. Furrows on slopes channel water from rainfall, washing away the soil. Over time, the furrows become larger. They form deep ditches, or gullies. This type of soil erosion is called gullying. Gullying can ruin farmland. READING TOOLBOX Determine Cause and Effect As you read, make a chart showing the causes and effects of erosion. When you are finished, compare your chart with a partner s. Critical Thinking 1. Identify Relationships What is the relationship between weathering and erosion? Critical Thinking 2. Make Connections What is the relationship between soil erosion and hunger? Holt McDougal Earth Science 215 Weathering and Erosion
3. Describe What kind of conditions can lead to sheet erosion? SHEET EROSION In the process of sheet erosion, wind and water remove layers of topsoil. Over time, erosion may expose the surface of the subsoil. Sheet erosion may happen in places where heavy rains wash away layers of topsoil. During dry periods, wind can also cause sheet erosion. Wind carries the loose, dry soil away in clouds of dust and sand. These soil particles may form large dust storms. RESULTS OF SOIL EROSION Constant erosion makes it harder for soil to support life. The flowchart below shows how erosion affects the fertility of soil. Erosion removes the A horizon, which contains fertile humus. LOOKING CLOSER 4. Describe What makes the B horizon at risk for erosion? The B horizon is less fertile because it does not have much humus. The B horizon is difficult to farm. The B horizon has no plant life to protect it from more erosion. Erosion continues to remove layers of soil. Over time, erosion may remove all the soil. EFFECTS OF HUMAN ACTIVITIES ON EROSION RATES Human activities affect how quickly soil erodes. Farming techniques and construction projects can increase the rate of erosion. For example, people remove plants to build houses and roads. Without plants for protection, the topsoil erodes more quickly. New soil cannot form fast enough. In some areas, it may take thousands of years for new topsoil to form. Human actions can also reduce or prevent soil erosion. Soil conservation involves taking steps to prevent soil erosion. Holt McDougal Earth Science 216 Weathering and Erosion
What Are Some Methods of Soil Conservation? People have begun to understand how human activities affect the environment. Now, many people are trying to protect soil through soil conservation. For example, some land development projects are leaving trees and plants in place when possible. Other projects are adding plants to the soil to prevent erosion. Farmers also use soil conservation methods to prevent erosion. Some of these methods are contour plowing strip-cropping terracing crop rotation CONTOUR PLOWING Farmers around the world use different planting methods to reduce soil erosion. In the contour plowing method, farmers plow in curved bands. These bands follow the contour, or shape, of the land. Contour plowing prevents water from flowing straight down slopes. Therefore, this method helps prevents gullying. STRIP-CROPPING Farmers may also use strip-cropping to prevent erosion. In this method, farmers plant crops in alternating bands, as shown below. For example, a farmer may plant rows of corn in one band. Then the farmer plants a cover crop, such as alfalfa, next to the corn. A cover crop fully covers the surface of the land. This cover crop slows the runoff of rainwater and protects the soil. Talk About It Discuss With a partner, talk about land use in your community. What activities may be damaging the soil? What activities may be helping the soil? 5. Describe How does contour plowing help conserve soil? 6. Identify Give an example of a cover crop. Contour Plowing Strip-Cropping Holt McDougal Earth Science 217 Weathering and Erosion
TERRACING Normally, the soil on hillsides is thin because of erosion. However, farmers can use terracing to farm on slopes. In this method, farmers build steplike ridges, or terraces, that follow the shape of a sloped field. Terraces slow the movement of water down the slope. Slowing the water prevents soil from washing away quickly. Many farmers in Asia use terracing to grow rice. 7. Explain What is one advantage of terracing? Terraced fields such as these help slow runoff and prevent rapid soil erosion. 8. Identify What are two functions of cover crops? CROP ROTATION In crop rotation, farmers change the type of crop they plant each year. One year, a farmer may plant a crop that will be harvested, or picked. The next year, the farmer may plant a cover crop. The cover crop is not harvested. Instead, it helps to slow runoff and hold the soil in place. Other types of crop rotation help keep the soil fertile. What Is the Role of Gravity in Erosion? Gravity causes rock fragments to move down a slope. This movement of fragments down a slope is called mass movement. Some mass movements happen quickly. Others happen very slowly. The most destructive mass movements happen quickly. Types of Mass Movement Fast Mass Movements rockfalls landslides mudflows slumps Slow Mass Movements solifluction creep Holt McDougal Earth Science 218 Weathering and Erosion
ROCKFALLS AND LANDSLIDES The fastest kind of mass movement is a rockfall. In a rockfall, rock falls from a steep cliff. The rocks in rockfalls can be tiny fragments or giant boulders. In a landslide, masses of loose rock and soil suddenly fall down a slope. In large landslides, loose blocks of bedrock fall. This happens on very steep slopes. Small landslides often happen on the steep hills next to highways. Heavy rainfall, volcanic eruptions, and earthquakes can cause landslides. Critical Thinking 9. Compare What is the difference between a rockfall and a landslide? An earthquake in El Salvador caused this landslide. MUDFLOWS AND SLUMPS The quick movement of a large amount of mud forms a mudflow. Mudflows happen in mountain regions because of volcanic eruptions or heavy rainfall. Mudflows move down slopes and valleys. The mud typically spreads out in a fan shape at the base of the slope. Sometimes, a large block of soil and rock falls downhill in one piece. The block then slides along the slope of the surface. This movement is a slump. Slumping happens on very steep slopes. Water and loss of friction cause the soil and rock to slip downhill. SOLIFLUCTION Many slopes do not seem to change. However, such slopes often experience slow mass movements. Slow mass movements actually move more rock material overall than landslides. One form of slow mass movement is solifluction. In solifluction, soil that is soaked with water flows over hard or frozen layers of soil. Solifluction happens in arctic and mountain climates, where the subsoil is always frozen. Water from the topsoil cannot flow into the frozen subsoil. As a result, the surface layer becomes muddy and flows slowly downhill. Critical Thinking 10. Infer Solifluction can also happen in warmer areas where the subsoil is made of hard clay. Why would subsoil with hard clay likely lead to solifluction? Holt McDougal Earth Science 219 Weathering and Erosion
CREEP Another form of slow mass movement is creep. Creep is the slow downhill movement of weathered rock material. Soil creep moves the most soil of all types of mass movements. Factors such as water between rock particles, growing plants, burrowing animals, and freezing and thawing cause soil creep. All of these factors help loosen particles, which allows gravity to pull the particles downhill. The rock fragments form piles as they collect at the base of a slope. These piles are called talus. Talus weathers into smaller fragments that move farther down the slope. The fragments wash into gullies and streams. Eventually, the gullies and streams carry the particles into rivers. 11. Define What is talus? The movement of rock fragments downhill caused these talus cones to form at the base of the Canadian Rockies. Talk About It Discuss Use an encyclopedia or the Internet to find images of each major landform. Describe each landform to a partner. Work with your partner to locate an example of each landform on a map. 12. Identify What are two forces that affect mountains? How Does Erosion Affect Landforms? The physical features on Earth s surface are called landforms. Weathering and erosion help shape three major landforms: mountains, plains, and plateaus. They also shape minor landforms such as hills, valleys, and dunes. EROSION OF MOUNTAINS Mountains go through different stages over time. First, tectonic forces form mountains by lifting Earth s crust. The forces cause mountains to rise. The mountains rise faster than they are eroded. The mountains are generally rugged and have sharp peaks. Over time, forces stop uplifting the mountains. Weathering and erosion wear down the rugged peaks. The mountains become rounded and have gentle slopes. Eventually mountains become low hills. Such features are called peneplains, which means almost flat. Holt McDougal Earth Science 220 Weathering and Erosion
These mountains in the Andes are still being uplifted. LOOKING CLOSER 13. Apply Reasoning Which of these mountain ranges is probably older the Andes or the Appalachians? Explain your reasoning. The Appalachian Mountains have been eroded over millions of years. EROSION OF PLAINS AND PLATEAUS A plain is a flat landform near sea level. A plateau is a wide, flat landform that has a high elevation. A plateau experiences much more erosion than a plain. Young plateaus typically have deep valleys that separate the wide, flat regions. Plateaus weather in different ways. In dry climates, rocks that are resistant to weathering form plateaus with flat tops. Over time, erosion may split the plateaus into smaller areas called mesas. Mesas erode into even smaller formations called buttes. In dry areas, buttes and mesas have steep walls and flat tops. In wet climates, humidity and precipitation weather landforms into round shapes. 14. Sequence Which landform develops first a mesa or a butte? Explain. Ancient rivers carved plateaus into mesas. The mesas eventually eroded into the buttes of Monument Valley in Arizona. Holt McDougal Earth Science 221 Weathering and Erosion
Section 4 Review SECTION VOCABULARY creep the slow downhill movement of weathered rock material erosion a process in which the materials of Earth s surface are loosened, dissolved, or worn away and transported from one place to another by a natural agent, such as wind, water, ice, or gravity landform a physical feature of Earth s surface mass movement the movement of a large mass of sediment or a section of land down a slope sheet erosion the process by which water flows over a layer of soil and removes the topsoil solifluction the slow, downslope flow of soil saturated with water in areas surrounding glaciers at high elevations 1. Define In your own words, define erosion. 2. Organize Complete the spider map below to identify and describe four methods of soil conservation. Be sure to include how each method protects soil. Soil conservation methods 3. Identify What are three major landforms shaped by weathering and erosion? 4. Explain How does gravity cause erosion? Give two examples. 5. Apply Ideas Imagine that you wanted to grow grapes on a hillside. What farming method or methods would you use? Explain your answer. Holt McDougal Earth Science 222 Weathering and Erosion