## National Geographic

To read about desert landscapes formed by weathering and erosion, go to the National Geographic Expedition on page 898.

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.

Section 1 • Weathering 169

(bl)Mark Skalny/Visuals Unlimited, (bc)Charles & Josette Lenars/CORBIS

Figure 7.8 When the same object is broken into two or more pieces, the surface area increases. The large cube has a volume of 1000 cm3. When it is broken into 1000 pieces, the volume is unchanged, but the surface area is increased one thousand times.

10 cm

Figure 7.8 When the same object is broken into two or more pieces, the surface area increases. The large cube has a volume of 1000 cm3. When it is broken into 1000 pieces, the volume is unchanged, but the surface area is increased one thousand times.

10 cm

Surface area 600 cm2

1 cmH

1 cmH

Surface area 6000 cm2

Volume constant 1000 cm3 = 1L

Surface area 6000 cm2

Volume constant 1000 cm3 = 1L

Surface area The rate of weathering also depends on the surface area that is exposed. Mechanical weathering breaks rocks into smaller pieces. As the pieces get smaller, their surface area increases, as illustrated in Figure 7.8. When this happens, there is more total surface area available for chemical weathering. The result is that weathering has more of an effect on smaller particles, as you learned in the Launch Lab.

Topography The slope of a landscape also determines the rate of weathering. Rocks on level areas are likely to remain in place over time, whereas the same rocks on slopes tend to move as a result of gravity. Steep slopes therefore promote erosion and continually expose less-weathered material.

Section 7.1 Assessment

Section Summary

I Mechanical weathering changes a rock's size and shape.

I Frost wedging and exfoliation are forms of mechanical weathering.

I Chemical weathering changes the composition of a rock.

I The rate of chemical weathering depends on the climate, rock type, surface area, and topography.

Understand Main Ideas

1. imanUdM Distinguish between the characteristics of an unweathered rock and those of a highly weathered rock.

2. Describe the factors that control the rate of chemical weathering and those that control the rate of physical weathering.

3. Compare chemical weathering to mechanical weathering.

4. Analyze the relationship between surface area and weathering.

Think Critically

5. Infer which would last longer, the engraving in a headstone made of marble, or an identical engraving in a headstone made of granite.

«MHiT.W Earth Science

6. Infer the relationship between weathering and surface area by graphing the relationship between the rate of weathering and the surface area of a material.

170 Chapter 7 • Weathering, Erosion, and Soil lino Self-Check Quiz glencoe.com

170 Chapter 7 • Weathering, Erosion, and Soil

N4BEn Erosion transports weathered materials across Earth's

Real-World Reading Link Have you ever noticed the mud that collects on sidewalks and streets after a heavy rainfall? Water carries sediment to the side-

Recall that the process of weathering breaks rock and soil into smaller pieces, but never moves it. The removal of weathered rock and soil from its original location is a process called erosion. Erosion can remove material through a number of different agents, including running water, glaciers, wind, ocean currents, and waves. These agents of erosion can carry rock and soil thousands of kilometers away from their source. After the materials are transported, they are dropped in another location in a process known as

Gravity is associated with many erosional agents because the force of gravity tends to pull all materials downslope. Without gravity, neither streams nor glaciers would flow. In the process of erosion, gravity pulls loose rock downslope. Figure 7.9 shows the effects of gravity on the landscape of Watkins Glen State Park in New York. The effects of gravity on erosion by running water can

Figure 7.9 Within about 3000 m, the stream descends 120 m at Watkins Glen State Park in New York.

Calculate the average descent of the stream per meter along the river.

Figure 7.9 Within about 3000 m, the stream descends 120 m at Watkins Glen State Park in New York.

Calculate the average descent of the stream per meter along the river.

Section 2 • Erosion and Deposition 171

John Anderson/Animals Animals

Figure 7.10 Rill erosion can occur in an agricultural field. Gully erosion often develops from rills. Infer land management practices that can slow or prevent the development of gully erosion.

Figure 7.10 Rill erosion can occur in an agricultural field. Gully erosion often develops from rills. Infer land management practices that can slow or prevent the development of gully erosion.

Erosion by Water

Moving water is perhaps the most powerful agent of erosion. Stream erosion can reshape entire landscapes. Stream erosion is greatest when a large volume of water is moving rapidly, such as during spring thaws and torrential downpours. Water flowing down steep slopes has additional erosive potential resulting from gravity, causing it to cut downward into the slopes, carving steep valleys and carrying away rock and soil. Swiftly flowing water can also carry more material over long distances. The Mississippi River, for example, carries an average of 400,000 metric tons of sediment each day from thousands of kilometers away.

^p Reading Check Predict what time of year water has the most potential for erosion.

Erosion by water can have destructive results. For example, water flowing downslope can carry away fertile agricultural soil. Rill erosion develops when running water cuts small channels into the side of a slope, as shown in Figure 7.10. When a channel becomes deep and wide, rill erosion evolves into gully erosion, also shown in Figure 7.10. The channels formed in gully erosion can transport much more water, and consequently more soil, than rills. Gullies can be more than 3 m deep and can cause major problems in farming and grazing areas.

ji/jjjjuyj v-

Model Erosion

How do rocks erode? When rocks are weathered by their surrounding environment, particles can be carried away by erosion.

Procedure ^Blt^

1. Read and complete the lab safety form.

2. Carve your name deeply into a bar of soap with a toothpick. Measure the mass of the soap.

3. Measure and record the depth of the letters carved into the soap.

4. Place the bar of soap on its edge in a catch basin.

5. Slowly pour water over the bar of soap until a change occurs in the depth of the carved letters.

6. Measure and record the depth of the carved letters. Analysis

1. Describe how the depth of the letters carved into the bar of soap changed.

2. Infer whether the shape, size, or mass of the bar of soap changed.

3. Consider what additional procedure you could follow to determine whether any soap wore away.

172 Chapter 7 • Weathering, Erosion, and Soil

(tl)William Banaszewski/Visuals Unlimited, (tcl)Inga Spence/Visuals Unlimited

Rivers and streams Each year, streams carry billions of metric tons of sediments and weathered material to coastal areas. Once a river enters the ocean, the current slows down, which reduces the potential of the stream to carry sediment. As a result, streams deposit large amounts of sediments in the region where they enter the ocean. The buildup of sediments over time forms deltas, such as the Colorado River Delta, shown in Figure 7.11. The volume of river flow and the action of tides determines the shapes of deltas, most of which contain fertile soil. The Colorado River Delta shows the classic fan shape associated with many deltas.

Wave action Erosion of materials also occurs along the ocean floor and at continental and island shorelines. The work of ocean currents, waves, and tides carves out cliffs, arches, and other features along the continents' edges. In addition, sand particles accumulate on shorelines and form dunes and beaches. The constant movement of water and the availability of accumulated weathered material result in a continuous erosional process, especially along ocean shorelines. Sand along a shoreline is repeatedly picked up, moved, and deposited by ocean currents. As a result, sandbars form from offshore sand deposits. If the sandbars continue to be built up with sediments, they can develop into barrier islands. Many barrier islands, such as the Outer Banks of North Carolina shown in Figure 7.12, have formed along both the Gulf and Atlantic Coasts of the United States.

Just as shorelines are built by the process of deposition in some areas, they are reduced by the process of coastal erosion in other areas. Changing tides and conditions associated with coastal storms can also have a great impact on coastal erosion. Human development and population growth along shorelines have led to attempts to control the erosion of sand. However, efforts to keep the sand on one beachfront disrupt the natural migration of sand along the shore, depleting sand from another area. You will learn more about ocean and shoreline features in Chapters 15 and 16.

Figure 7.11 Streams slow down when they meet the ocean. In these regions, sediments are deposited by the river, resulting in the development of a delta.

Figure 7.12 The Outer Banks of North Carolina have been built over time by deposition of sand and sediments.

Figure 7.12 The Outer Banks of North Carolina have been built over time by deposition of sand and sediments.

Section 2 • Erosion and Deposition 173

(tr)Annie Griffiths Belt/National Geographic Image Collection, (b)Larry Cameron/Photo Researcherts, Inc.

Figure 7.13 Iceberg Lake in Glacier National Park, Montana, was formed by glaciers.

Figure 7.14 A windbreak can reduce the speed of the wind for distances up to 30 times the height of the tree.

Calculate If these trees are 10 m tall, what is the distance over which they can serve as a windbreak?

### Glacial Erosion

Although glaciers currently cover less than 10 percent of Earth's surface, they have covered over 30 percent of Earth's surface in the past. Glaciers left their mark on much of the landscape, and their erosional effects are large-scale and dramatic. Glaciers scrape and gouge out large sections of Earth's landscape. Because they can move as dense, enormous rivers of slowly flowing ice, glaciers have the capacity to carry huge rocks and piles of debris over great distances and grind the rocks beneath them into flour-sized particles. Glacial movements scratch and grind surfaces. The features left in the wake of glacial movements include steep U-shaped valleys and lakes, such as the one shown in Figure 7.13.

The effects of glaciers on the landscape also include deposition. For example, soils in the northern parts of the United States are formed from material that was transported and deposited by glaciers. Although the most recent ice age ended 15,000 years ago, glaciers continue to affect erosional processes on Earth.

### Wind Erosion

Wind can be a major erosional agent, especially in arid and coastal regions. Such regions tend to have little vegetation to hold soil in place. Wind can easily pick up and move fine, dry particles. The effects of wind erosion can be both dramatic and devastating. The abrasive action of windblown particles can damage both natural features and human-made structures. Winds can blow against the force of gravity and easily move fine-grained sediments and sand uphill.

Wind barriers One farming method that can reduce the effects of wind erosion is the planting of wind barriers, also called windbreaks, shown in Figure 7.14. Windbreaks are trees or other vegetation planted perpendicular to the direction of the wind. A wind barrier might be a row of trees along the edge of a field. In addition to reducing erosion, wind barriers can trap blowing snow, conserve moisture, and protect crops from the effects of the wind.

Figure 7.14 A windbreak can reduce the speed of the wind for distances up to 30 times the height of the tree.

Calculate If these trees are 10 m tall, what is the distance over which they can serve as a windbreak?

174 Chapter 7 • Weathering, Erosion, and Soil

(tl)William Manning/CORBIS, (b)David R. Frazier/Photo Researchers, Inc.

Figure 7.15 In this construction project, the landscape was considerably altered. Analyze the results of this alteration of the landscape.

### Erosion by Living Things

Plants and animals also play a role in erosion. As plants and animals carry out their life processes, they move Earth's surface materials from one place to another. For example, Earth materials are moved when animals burrow into soil. Humans excavate large areas and move soil from one location to another, as shown in Figure 7.15. Planting a garden, developing a new athletic field, and building a highway are all examples of human activities that result in the moving of Earth materials from one place to another. You will learn more about how human activity impacts erosion in Chapter 26. ^

Figure 7.15 In this construction project, the landscape was considerably altered. Analyze the results of this alteration of the landscape.

Section 7.2 Assessment

Section Summary

I The processes of erosion and deposition have shaped Earth's landscape in many ways.

I Gravity is the driving force behind major agents of erosion.

I Agents of erosion include running water, waves, glaciers, wind, and living things.

Understand Main Ideas

1. immine<TTfla Discuss how weathering and erosion are related.

2. Describe how gravity is associated with many erosional agents.

3. Classify the type of erosion that could move sand along a shoreline.

4. Compare and contrast rill erosion and gully erosion. Think Critically

5. Generalize about which type of erosion is most significant in your area.

6. Diagram a design for a wind barrier to prevent wind erosion.

€HZ222a^Earth Science

7. Research how a development in your area has alleviated or contributed to erosion. Present your results to the class, including which type of erosion occurred, and where the eroded materials will eventually be deposited.

Self-Check Quiz glencoe.com

Section 2 • Erosion and Deposition 175

Robert Llewellyn/zefa/CORBIS

Section 7.3

Soil

Section 7.3

Objectives I Describe how soil forms. I Recognize soil horizons in a soil profile.

I Differentiate among the factors of soil formation.

Review Vocabulary organism: anything that has or once had all the characteristics of life

New Vocabulary soil residual soil transported soil soil profile soil horizon

Soil

Soil forms slowly as a result of mechanical and chemical processes.

Real-World Reading Link What color is soil? Soils can be many different colors—dark brown, light brown, red, or almost white. Soils develop through the interaction of a number of factors, which determine the color of soil.

### Soil Formation

What is soil? It is found almost everywhere on Earth's surface. Weathered rock alone is not soil. Soil is the loose covering of weathered rock particles and decaying organic matter, called humus, overlying the bedrock of Earth's surface, and serves as a medium for the growth of plants. Soil is the product of thousands of years of chemical and mechanical weathering and biological activity.

Soil development The soil-development process often begins when weathering breaks solid bedrock into smaller pieces. These pieces of rock continue to undergo weathering and break down into smaller pieces. Worms and other organisms help break down organic matter and add nutrients to the soil as well as creating passages for air and water, as shown in Figure 7.16.

As nutrients are added to the soil, its texture changes, and the soil's capacity to hold water increases. While all soil contains some organic matter in various states of decay, the amount varies widely among different types of soil. For example, as much as 5 percent of the volume of prairie soils is organic matter, while most desert soils have almost no organic matter.

Figure 7.16 Organisms in the soil change the soil's structure over time by adding nutrients and passages for air. Infer how animals also alter the soil by adding organic material.

### Soil Layers

During the process of its formation, soil develops layers. Most of the volume of soil is formed from the weathered products of a source rock, called the parent material. The parent material of a soil is often the bedrock. As the parent material weathers, the weathering products rest on top of the parent material. Over time, a layer of the smallest pieces of weathered rock develops above the parent material. Eventually, living organisms such as plants and animals become established, and use nutrients and shelter available in the material. Rainwater seeps through this top layer of materials and dissolves soluble minerals, carrying them into the lower layers of the soil.

A soil whose parent material is the local bedrock is called residual soil. Kentucky's bluegrass soil is an example of residual soil, as are the red soils in Georgia. Not all soil develops from local bedrock. Transported soil, shown in the valley in Figure 7.17, is soil that develops from parent material that has been moved far from its original location. Agents of erosion transport parent material from its place of origin to new locations. For example, glaciers have transported sediments from Canada to many parts of the United States. Streams and rivers, especially during times of flooding, also transport sediments downstream to floodplains. Winds also carry sediment to new locations. Over time, processes of soil formation transform these deposits into mature soil layers.

^p Reading Check Explain how residual soils are different from transported soils.

Figure 7.17 In a stream valley, transported soils are often found in the flood plain. Residual soils are often found in the higher, mountainous regions.

William D. Bachman/Photo Researchers, Inc.

William D. Bachman/Photo Researchers, Inc.

### CAREERS in EARTH SCIllNCl r

Landscaper A landscaper uses his or her knowledge of soils and performs tests to evaluate soils at different sites. Landscapers use the information they gather to choose plants that are appropriate to the soil conditions. To learn more about Earth science careers, visit glencoe.com.

Figure 7.17 In a stream valley, transported soils are often found in the flood plain. Residual soils are often found in the higher, mountainous regions.

Soil profiles Digging a deep hole in the ground will reveal a soil profile. A soil profile is a vertical sequence of soil layers. Some soil profiles have more distinct layers than others. Relatively new soils that have not yet developed distinct layers are called undeveloped soils, shown in Figure 7.18. It can take tens of thousands of years for distinct layers to form in a soil. Those soils are called mature. An example is shown in Figure 7.18.

^p Reading Check Explain the difference between a mature and an undeveloped soil.

Soil horizons A distinct layer within a soil profile is called a soil horizon. There are typically four major soil horizons in mature soils, O, A, B, and C. The O-horizon is the top layer of organic material, which is made of humus and leaf litter. Below that, the A-horizon is a layer of weathered rock combined with a rich concentration of dark brown organic material. The B-horizon, also called the zone of accumulation, is a red layer that has been enriched over time by clay and minerals deposited by water flowing from the layers above, or percolating upward from layers below. Usually the clay gives a blocky structure to the B-horizon. Accumulations of certain minerals can result in a hard layer called hardpan. Hardpan can be so dense that it allows little or no water to pass through it. The C-horizon contains little or no organic matter, and is often made of broken-down bedrock. The development of each horizon depends on the factors of soil formation.

178 Chapter 7 • Weathering, Erosion, and Soil

(tl)Photo courtesy of USDA Natural Resources Conservation Service, (tr)Photo courtesy of USDA Natural Resources Conservation Service

### Factors of Soil Formation

Five factors influence soil formation: climate, topography, parent material, biological organisms, and time. These factors combine to produce different types of soil, called soil orders, from region to region. Soil taxonomy (tak SAH nuh mee) is the system that scientists use to classify soils into orders and other categories. The five factors of soil formation result in 12 different soil orders.

Climate Climate is the most significant factor controlling the development of soils. Temperature, wind, and the amount of rainfall determines the type of soil that can develop.

Recall from Section 7.1 that rocks tend to weather rapidly under humid, temperate conditions, such as those found in climates along the eastern United States. Weathering results in soils that are rich in aluminum and iron oxides. Water from abundant rainfall moves downward, carrying dissolved minerals into the B-horizon. In contrast, the soils of arid regions are so dry that water from below ground moves up through evaporation, and leaves an accumulation of white calcium carbonate in the B-horizon. Tropical areas experience high temperatures and heavy rainfall. These conditions lead to the development of intensely weathered soils where all but the most insoluble minerals have been flushed out.

Topography Topography, which includes the slope and orientation of the land, affects the type of soil that forms. On steep slopes, weathered rock is carried downhill by agents of erosion. As a result, hillsides tend to have shallow soils, while valleys and flat areas develop thicker soils with more organic material. The orientation of slopes also affects soil formation. In the northern hemisphere, slopes that face south receive more sunlight than other slopes. The extra sunlight allows more vegetation to grow. Slopes without vegetation tend to lose more soil to erosion. Figure 7.19 shows how the orientation and slope of a landscape can affect the formation of soil.

North side

South side

North side

South side

Figure 7.19 The slope on the right side faces south, and the slope on the left side faces north.

Interpret why one slope has more vegetation than the other.

Figure 7.19 The slope on the right side faces south, and the slope on the left side faces north.

Interpret why one slope has more vegetation than the other.

Parent material Recall that a soil can be either residual or transported. If the soil is residual, it will have the same chemical composition as the local bedrock. For example, in regions near volcanoes, the soils form from weathered products of lava and ash. Volcanic soils tend to be rich in the minerals that were present in the lava. If the soil is transported, the minerals in the soil are likely to be different from those in the local bedrock.

Biological organisms Organisms including fungi and bacteria, as well as plants and animals, interact with soil. Microorganisms decompose dead plants and animals. Plant roots can open channels, and when they decompose, they add organic material to the soil. Different types of biological organisms in a soil can result in different soil orders. Mollisols (MAH lih sawlz), which are called prairie soils, and alfisols (AL fuh sawlz), also called woodland soils, both develop from the same climate, topography, and parent material. The different sets of organisms result in two soils with entirely different characteristics. For example, the activity of prairie organisms in mollisols produces a thick A-horizon, rich in organic matter. Some of the most fertile agricultural lands in the Great Plains region are mollisols.

^P Reading Check Describe how microorganisms affect soil formation.

Time The effects of time alone can determine the characteristics of a soil. New soils, such as entisols (EN tih sawlz), are often found along rivers, where sediment is deposited by periodic flooding. This type of soil is shown as a light blue color in Figure 7.20. These soils have had little time to weather and develop soil horizons. The effects of time on soil can be easy to recognize. After tens of thousands of years of weathering, most of the original minerals in a soil are changed or washed away. Minerals containing aluminum and iron remain, which can give older soils, such as ultisols (UL tih sawlz), a red color. Figure 7.21 shows the locations of the 12 soil orders in the United States.

Figure 7.20 Soil types vary widely from one area to the next, depending on the local climate, topography, parent material, organisms, and age of the soil. Entisols are shown in light blue and ultisols are shown in orange on this map. Infer how differences in topography have affected the types of soils in North Carolina.

Figure 7.20 Soil types vary widely from one area to the next, depending on the local climate, topography, parent material, organisms, and age of the soil. Entisols are shown in light blue and ultisols are shown in orange on this map. Infer how differences in topography have affected the types of soils in North Carolina.

180 Chapter 7 • Weathering, Erosion, and Soil

State Soil Geographic Database (STATSGO)/NRCS/USDA

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### Responses

• Adelmo
What type of erosion could move sand along a shore line?
8 years ago
• Kaarina
What is the difference between Residual soils and transported soil?
7 months ago