National Geographic

To read about how wind has shaped desert landscapes, read the National Geographic Expedition on page 898.

Figure 8.17 Great Sand Dunes National Monument, in southern Colorado, contains North America's highest sand dunes of more than 228.6 m. Identify the dominant direction of wind in the figure.

Vocabulary

Academic vocabulary migrate to move from one location to another Dunes migrate as wind blows over sand

Figure 8.17 Great Sand Dunes National Monument, in southern Colorado, contains North America's highest sand dunes of more than 228.6 m. Identify the dominant direction of wind in the figure.

Deposition National Geographic

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Interactive Figure To see an animation of dune migration, visit glencoe.com.

Wind Deposition

Wind deposition occurs in areas where wind velocity decreases. As the wind velocity slows down, some of the windblown sand and other materials cannot stay airborne, and they drop out of the air stream to form a deposit on the ground.

Dunes In windblown environments, sand particles tend to accumulate where an object, such as a rock, landform, or piece of vegetation, blocks the forward movement of the particles. Sand continues to be deposited as long as winds blow in one general direction. Over time, the pile of windblown sand develops into a dune, as shown in Figure 8.17. All dunes have a characteristic profile. The gentler slope of a dune, located on the side from which the wind blows, is called the windward side. The steeper slope, on the side protected from the wind, is called the leeward side. The conditions under which a dune forms determine its shape. These conditions include the availability of sand, wind velocity, wind direction, and the amount of vegetation present. The different types of dunes are shown in Table 8.1.

Dune migration As long as winds continue to blow, dunes will migrate. As shown in Figure 8.18, dune migration is caused when prevailing winds continue to move sand from the windward side of a dune to its leeward side, causing the dune to move slowly over time.

Figure 8.18 Dune migration is caused by wind.

Wind direction

Leeward

Windward

Wind direction

Leeward

Windward

Figure 8.18 Dune migration is caused by wind.

Windward Face And Leeward Face Dune
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Table 8.1

Interactive Table To explore

Tvpes of Dunes more about sand dunes, visit glencoe.com.

Example of Dune

Description

Parabolic Dune

Barchan Dunes

• form solitary, crescent shapes

• form from a small amount of sand

• covered by minimal or no vegetation

• form in flat areas of constant wind direction

• crests point downwind

• reach maximum size of 30 m

Large Transverse Dunes

Transverse Dunes

• form series of ridge shapes

• form from a large amount of sand

• covered by minimal or no vegetation

• form in ridges that are perpendicular to the direction of the strong wind

• reach maximum size of 25 m

Parabolic Dune

Parabolic Dunes

• form from a large amount of sand

• covered by minimal vegetation

• form in humid areas with moderate winds

• crests point upwind

• reach maximum size of 30 m

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Longitudinal Dunes

• form series of ridge shapes

• form from small or large amounts of sand

• covered by minimal or no vegetation

• form parallel to variable wind direction

• reach maximum height of 300 m

(t to b)George Steinmetz/CORBIS, (2)George Steinmetz/CORBIS, (3)George Steinmetz/CORBIS, (4)ABPL Library/Photo Researchers

George Steinmetz Corbis

Loess Wind can carry fine, lightweight particles such as silt and clay in great quantities and for long distances. Many parts of Earth's surface are covered by thick layers of windblown silt, which are thought to have accumulated as a result of thousands of years of dust storms. The source of these silt deposits might have been the fine sediments that were exposed when glaciers melted after the last ice age, more than 10,000 years ago. These thick, windblown silt deposits are known as loess (LESS). Figure 8.19 shows loess deposits in Illinois, Wisconsin, Iowa, Missouri, Nebraska, Kansas, and Idaho. Loess soils are some of the most fertile soils because they contain abundant minerals and nutrients.

Section 8.2 Assessment

Section Summary

I Wind is a powerful agent of erosion.

I Wind can transport sediment in several ways, including suspension and saltation.

I Dunes form when wind velocity slows down and windblown sand is deposited.

I Dunes migrate as long as winds continue to blow.

Understand Main Ideas

1. manddflS Evaluate the various types of landforms formed by wind and how these landforms are created.

2. Identify conditions that can contribute to an increase in wind erosion.

3. Examine why loess can travel much greater distances than sand.

4. Classify the four types of dunes as they are related to wind, vegetation, and amount of sand available.

Think Critically

5. Infer how the movement of sand grains by saltation affects the overall movement of dunes.

6. Evaluate why wind erosion is an effective agent of erosion.

CZHZ^^Earth Science

7. Predict how human activities directly affect wind erosion on coastlines.

206 Chapter 8 • Mass Movements, Wind, and Glaciers

Self-Check Quiz glencoe.com

206 Chapter 8 • Mass Movements, Wind, and Glaciers

Self-Check Quiz glencoe.com

Section 8.3

Objectives

I Explain how glaciers form.

I Compare and contrast the conditions that produce valley glaciers with those that produce continental glaciers.

I Describe how glaciers modify landscapes.

I Recognize glacial features.

Review Vocabulary latitude: distance in degrees north and south of the equator

New Vocabulary glacier valley glacier continental glacier cirque moraine outwash plain drumlin esker kame kettle

Figure 8.20

Glaciers around the world have changed in distribution throughout geologic time.

Infer what changes have occurred in the distribution of glaciers around the world.

Figure 8.20

Glaciers around the world have changed in distribution throughout geologic time.

Infer what changes have occurred in the distribution of glaciers around the world.

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Interactive Figure To see an animation of glacier formation, visit glencoe.com.

Glaciers

IMAN<nea Glaciers modify landscapes by eroding and depositing rocks.

Real-World Reading Link Have you ever wondered what formed the landscape around you? Glaciers might have left deposits of sediment as well as carved features in rock that you see every day.

Moving Masses of Ice

A large, moving mass of ice is called a glacier. Glaciers form near Earth's poles and in mountainous areas at high elevations. They currently cover about 10 percent of Earth's surface, as shown in Figure 8.20. In the past, glaciers were more widespread than they are today. During the last ice age, which began about 1.6 mya and ended more than 10,000 years ago, ice covered about 30 percent of Earth.

Areas at extreme northern and southern latitude, such as Greenland and Antarctica, and areas of high elevations, such as the Alps, have temperatures near 0°C year-round. Cold temperatures keep fallen snow from completely melting, and each year the snow that has not melted accumulates in an area called a snowfield. Thus, the total thickness of the snow layer increases as the years pass. The accumulated snow develops into a glacier. The weight of the top layers of snow eventually exerts enough downward pressure to force the accumulated snow below to recrystallize into ice. A glacier can develop in any location that provides the necessary conditions. Glaciers can be classified as one of two types—valley glaciers or continental glaciers.

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Glaciers (present) — Glaciers (18,000 years ago)

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Antarctica Glacier Global Warming
Antarctica

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Incorporate information from this section into your Foldable.

Valley glaciers Glaciers that form in valleys in high, mountainous areas are called valley glaciers. The movement of a valley glacier occurs when the growing ice mass becomes so heavy that the ice maintains its rigid shape and begins to flow, much like toothpaste. For most valley glaciers, flow begins when the accumulation of snow and ice exceeds 20 m in thickness. As a valley glacier moves, deep cracks in the surface of the ice, called crevasses, can form.

The speed of a valley glacier's movement is affected by the slope of the valley floor, the temperature and thickness of the ice, and the shape of the valley walls. The sides and bottom of a valley glacier move more slowly than the middle because friction slows down the sides and bottom where the glacier comes in contact with the ground. Movement downslope is usually slow—less than a few millimeters per day. Over time, as valley glaciers flow downslope, their powerful carving action transitions V-shaped stream valleys into U-shaped glacial valleys.

^p Reading Check Describe how V-shaped valleys become U-shaped.

Continental glaciers Glaciers that cover broad, continent-sized areas are called continental glaciers. These glaciers form in cold climates where snow accumulates over many years. A continental glacier is thickest at its center. The weight of the center forces the rest of the glacier to flatten in all directions. In the past, when Earth experienced colder average temperatures than it does today, continental glaciers covered huge portions of Earth's surface. Today, they are confined to Greenland and Antarctica.

»ATA ANALYSIS LAB

Based on Real Data*

Interpret the Data

How much radioactivity is in ice cores?

Glaciologists have found that ice cores taken from the arctic region contain preserved radioactive fallout. Data collected from the study of these ice cores have been plotted on the graph.

Data and Observations

Radioactivity in Ice Cores

Radioactivity in Ice Cores

I High

100 200 300 400 Depth (cm)

100 200 300 400 Depth (cm)

Think Critically

1. Determine the depth in the ice cores where the highest and lowest amounts of radioactivity were found.

2. Describe what happened to the amount of radioactivity in the ice cores between the pretest ban and Chernobyl.

3. Infer what happened to the amount of radioactivity in the ice cores after Chernobyl.

4. Explain what information or material other than radioactive fallout you think ice cores might preserve within them.

*Data obtained from: Mayewski, et al. 1990. Beta radiation from snow. Nature 345:25.

Hanging Valley Geography

Figure 8.21 Glacial erosion by valley glaciers creates features such as cirques, horns, and hanging valleys.

Glacial movement Both valley glaciers and continental glaciers move outward when snow gathers at the zone of accumulation, a location in which more snow falls than melts, evaporates, or sublimates. For valley glaciers, the zone of accumulation is at the top of mountains, while for continental glaciers, the zone of accumulation is the center of the ice sheet. Both types of glaciers recede when the ends melt faster than the zone of accumulation builds up snow and ice.

Glacial Erosion

Of all the erosional agents, glaciers are the most powerful because of their great size, weight, and density. When a valley glacier moves, it breaks off pieces of rock through a process called plucking. When glaciers with embedded rocks move over bedrock, they act like the grains on a piece of sandpaper, grinding parallel scratches into the bedrock. Small scratches are called striations, and larger ones are called grooves. Scratches and grooves provide evidence of a glacier's history and indicate its direction of movement.

Glacial erosion by valley glaciers can create features like those shown in Figure 8.21. At the high elevations where snow accumulates, valley glaciers also scoop out deep depressions, called cirques. Where two cirques on opposite sides of a valley meet, they form a sharp, steep ridge called an arête. When there are glaciers on three or more sides of a mountaintop, the carving action creates a steep, pyramid-shaped peak. This is known as a horn. The most famous example of this feature is Switzerland's Matterhorn.

Valley glaciers can also leave hanging valleys in the glaciated landscape. Hanging valleys are formed when tributary glaciers converge with the primary glaciers and later retreat. The primary glacier is so thick that it meets the height of the smaller tributary glacier. When the glaciers melt, the valley is left hanging high above what is now a river in the primary valley floor. Hanging valleys today are often characterized by waterfalls where the tributary glacier used to be.

Figure 8.21 Glacial erosion by valley glaciers creates features such as cirques, horns, and hanging valleys.

CAREERS in EARTH SCIENC1

Glaciologist Glaciologists study snow and ice in the environment. They often conduct field research in remote locations on glaciers, ice sheets, and frozen tundra. To learn more about Earth science careers, visit glencoe.com.

How Identify Glaciers Maps
Figure 8.22 Elongated landforms called drumlins can be grouped together as a drumlin field in areas once covered by continental glaciers. Describe how you could identify a drumlin on a topographic map.

Glacial Deposition

Glacial till is the unsorted rock, gravel, sand, and clay that glaciers carry embedded in their ice and on their tops, sides, and front edges. Glacial till is formed from the grinding action of the glacier on underlying rock. Glaciers deposit unsorted ridges of till called moraines when the glacier melts. Moraines can be terminal or lateral. Terminal moraines are found where the glacier melts, and lateral moraines are located along the direction of glacier flow.

Outwash When the farthest ends of a glacier melt and the glacier begins to recede, meltwater floods the valley below. Meltwater contains gravel, sand, and fine silt. When this sediment is deposited by meltwater carried away from the glacier, it is called outwash. Because of the way water transports sediment, outwash is always sorted by particle size. The area at the leading edge of the glacier where the meltwater flows and deposits outwash is called an outwash plain.

Drumlins, eskers, and kames Continental glaciers that move over older moraines form the material into elongated landforms called drumlins, shown in Figure 8.22. A drumlins steeper slope faces the direction from which the glacier came. Streams flowing under melting glaciers leave long, winding ridges of layered sediments called eskers, shown in Figure 8.23. A kame is a mound of layered sediment deposited at the retreating glacier face and is conical in shape. Kames are also shown in Figure 8.23.

Model Glacial Deposition

How do glaciers deposit different types of rocks and sediments? Glaciers are powerful forces of erosion. As they move across the land, they pick up rocks and sediments, and carry them to new locations. When a glacier melts, these materials are left behind and deposits form in different shapes.

Procedure B'3

1. Read and complete the lab safety form.

2. Work with a group of 2 to 3 other students. One student should obtain four glaciers from your teacher.

3. Place the glaciers on a baking pan. In front of each glacier, place a popsicle stick (to prevent the glacier from sliding down the pan).

4. Place a textbook under one end of the baking pan (your glaciers should be toward the elevated end of the pan).

5. Observe what happens as the glaciers melt. Record your observations in your science journal.

6. Dispose of your materials as your teacher instructs. Analysis

1. Discuss Did the materials differ in the way they were deposited by the melting ice cubes? Were your results similar to those of your classmates? Explain.

2. Explain how this activity modeled the formation of meltwater.

3. Apply Which materials in this activity modeled glacial till?

4. Apply How did this activity model glacial deposition and the formation of a moraine?

□ NATIONAL GEOGRAPHIC

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Responses

  • jani wideroos
    Is the soil in a kame sorted or unsorted?
    8 years ago
  • antonino
    Can loess travel much greater distances than sand ?
    8 years ago
  • stella
    Why can loess trael.much greater distances than sand?
    2 years ago

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