EARTH SCIENCE geology, the environment, and the universe 2008 (9)

Mass Movements, Wind, and Glaciers BIG Idea Movements due to gravity, winds, and glaciers shape and change Earth’s surface.. MAIN Idea Mass movements alter Earth’s surface over time

Mass Movements, Wind, and Glaciers

BIG Idea Movements due

to gravity, winds, and glaciers

shape and change Earth’s

surface.

MAIN Idea Mass movements

alter Earth’s surface over time

due to gravity moving sediment

and rocks downslope.

8.2 Wind

MAIN Idea Wind modifies

landscapes in all areas of the

world by transporting sediment.

8.3 Glaciers

MAIN Idea Glaciers modify

landscapes by eroding and

depositing rocks.

GeoFacts

• More than 100,000 glaciers

exist in Alaska, but ice covers

only 5 percent of the state.

• Glaciers form when more snow

falls in an area than melts in

the same area.

• Layers of snow on the glacier

create pressure that changes

the snow underneath to ice.

Glacial till

Calving glacier

(t)Steve McCutcheon/Visuals Unlimited, (b)Bernhard Edmaier/Science Photo Library , (bkgd)Gregory Dimijian/Photo Researchers

Section 1 • XXXXXXXXXXXXXXXXXX 193

Start-Up Activities

Chapter 8 • Mass Movements, Wind, and Glaciers 193

How does water affect

sediments on slopes?

Water has a significant effect on sediments on slopes

In this activity, you will demonstrate how the addition

of water affects how sediments are held together.

Procedure

1 Read and complete the lab safety form.

2 Place 225 mL of sand in each of three

separate containers, such as aluminum pie plates.

3 Add 20 mL of water to the first container of

sand, and mix well Add 100 mL of water to the second container of sand, and mix well

Add 200 mL of water to the third container

of sand, and mix well.

4 Empty the three mixtures of sand and water

onto a tray or piece of cardboard Keep each mixture separate.

5 Test each mixture for its ability to be molded

and retain its shape Compare your results for the three samples.

Analysis

1 Describe how the addition of water affected

the sand’s ability to be molded in the three samples.

2 Explain why one mixture was better able to

maintain its shape than the others.

3 Infer how water affects sediment on slopes.

L

Earth Make this Foldable to

explain different processes that shape Earth’s surface

STEP 1 Fold the tom of a horizontal sheet

bot-of paper up about 3 cm.

STEP 2 Fold in thirds.

STEP 3 Unfold and dot with glue or staple to make three pockets Label

as shown.

F OLDABLES Use this Foldable with Sections 8.1, 8.2, and 8.3 As you read, use index cards to summarize information in your own words and place them in the appropriate pockets

Glaciers

Mass Movements Wind

Visit glencoe.com to study entire chapters online;

• Interactive Time Lines

• Interactive Figures

• Interactive Tables access Web Links for more information, projects, and activities;

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

194 Chapter 8 • Mass Movements, Wind, and Glaciers

Objectives

◗ Analyze the relationship between

gravity and mass movements.

◗ Identify factors that affect mass

gravity: the force every object exerts

on every other object due to their

MAIN Idea Mass movements alter Earth’s surface over time due

to gravity moving sediment and rocks downslope.

Real-World Reading Link How fast can you travel on a waterslide? A ber of factors might come into play, including the angle of the slide, the amount

num-of water on the slide, the material num-of the slide, friction, and your own mass

These factors also affect mass movements on Earth’s surface.

Mass Movements

How do landforms, such as mountains, hills, and plateaus, wear down and change? Landforms can change through processes involv-ing wind, ice, and water, and sometimes through the force of gravity alone The downslope movement of soil and weathered rock result-

ing from the force of gravity is called mass movement Recall from

Chapter 7 that weathering processes weaken and break rock into smaller pieces Mass movements often carry the weathered debris downslope Because climate has a major effect on the weathering activities that occur in a particular area, climatic conditions deter-mine the extent of mass movement

All mass movements, such as the one shown in Figure 8.1,

occur on slopes Because few places on Earth are completely flat, almost all of Earth’s surface undergoes mass movement Mass movements range from motions that are barely detectable to sud-den slides, falls, and flows The Earth materials that are moved range in size from fine-grained mud to large boulders

Reading Check Describe how gravity causes a mass movement.

■ Figure 8.1 Mass movements can cause

tree trunks to curve in order to continue

grow-ing opposite the pull of gravity, which is

toward the center of Earth.

Dr Marli Miller/Visuals Unlimited

Tilted fence posts, trees, and poles

Section 1 • Mass Movements 195

Factors that Influence

Mass Movements

Several factors influence the mass movements of Earth’s material

One factor is the material’s weight, which works to pull the material

downslope A second factor is the material’s resistance to sliding or

flowing, which depends on the amount of friction, how cohesive the

material is, and whether it is anchored to the bedrock A third factor

is a trigger, such as an earthquake, that shakes material loose Mass

movement occurs when the forces pulling material downslope are

stronger than the material’s resistance to sliding, flowing, or falling

Water is a fourth variable that influences mass movements The

landslide shown in Figure 8.2 occurred after days of heavy rains

Sat-uration by water greatly increases the weight of soils and sediments In

addition, as the water fills the tiny open spaces between grains, it acts

as a lubricant between the grains, reducing the friction between them

Types of Mass Movements

Mass movements are classified as creep, flows, slides, and rockfalls

Mass movements move different types of materials in various ways

Creep The slow, steady, downhill flow of loose, weathered Earth

materials, especially soils, is called creep Because movement might be

as little as a few centimeters per year, the effects of creep are usually

noticeable only over long periods of time One way to tell whether

creep has occurred is to observe the positions of structures and

objects As illustrated in Figure 8.3, creep can cause once-vertical

utility poles and fences to tilt, and trees and walls to break Loose

materials on almost all slopes undergo creep

One type of creep that usually occurs in regions of permafrost, or

permanently frozen soil, is called solifluction (SOH luh fluk shun)

The material moved in solifluction is a mudlike liquid that is produced

when water is released from melting permafrost during the warm

sea-son The water saturates the surface layer of soil and is unable to move

downward As a result, the surface layer can slide slowly downslope

■ Figure 8.3 All slopes undergo creep of some kind.

■ Figure 8.2 Mass movements like the one shown here can significantly alter landscapes

Summarize the factors that might have been involved in the mass movement.

(t)David McNew/Getty Images, (b)Ralph Lee Hopkins/Photo Researchers

196 Chapter 8 • Mass Movements, Wind, and Glaciers

Flows In some mass movements, Earth materials flow as if they were a thick liquid

The materials might move as slowly as a few centimeters per year or as rapidly as hundreds

of kilometers per hour Earth flows are erately slow movements of soils, whereas

mud and water Mudflows can be triggered

by earthquakes or similar vibrations and are common in volcanic regions where the heat from a volcano melts snow on nearby slopes that have fine sediment and little vegetation

The meltwater fills the spaces between the small particles of sediment and allows them

to slide readily over one another and move down slope

A lahar (LAH har) is a type of mudflow that occurs after a volcanic eruption Often a lahar results when a snow-topped volcanic mountain erupts and melts the snow on top of a moun-tain The melted snow mixes with ash and flows downslope Figure 8.4 shows how a lahar that originated from Nevado del Ruiz, one of the volcanic mountains in the Andes, devastated a town The Nevado del Ruiz is 5389

m high and covered with 25 km2 of snow and ice, which melted when it erupted Four hours after Nevado del Ruiz erupted, lahars had trav-eled more than 100 km downslope As a result

of these lahars, which occurred in 1985, approximately 23,000 people were killed, 5000 were injured, and 5000 homes were destroyed

Reading Check Determine what triggers

a lahar.

Mudflows are also common in sloped, arid regions that experience intense, short-lived rainstorms The Los Angeles Basin in Southern California is an example of an area where mud-flows are common In such areas, periods of drought and forest fires leave the slopes with lit-tle protective vegetation When heavy rains eventually fall in these areas, they can cause massive, destructive mudflows because there is little vegetation to anchor the soil Mudflows are especially destructive in areas where urban development has spread to the bases of moun-tainous areas These mudflows can burry homes, as shown in Figure 8.5.

semi-■ Figure 8.4 The city of Armero, in Colombia, was covered in

mud and debris by a lahar that contained snowmelt and volcanic

material.

Describe the effect of the lahar on the city shown above

■ Figure 8.5 Mudflows can be extremely destructive and can

result in severe property damage, road closures, and power outages

(t)Steve Raymer/National Geographic Image Collection , (b)Gene Blevins/LA Daily News/CORBIS

Section 1 • Mass Movements 197

■ Figure 8.6 Landslides in the Philippines devastated the town of San Ricardo in December 2003.

Slides A rapid, downslope movement of Earth materials that

occurs when a relatively thin block of soil, rock, and debris

sepa-rates from the underlying bedrock is called a landslide, shown in

Figure 8.6. The material rapidly slides downslope as one block,

with little internal mixing A landslide mass eventually stops and

becomes a pile of debris at the bottom of a slope, sometimes

dam-ming rivers and causing flooding Landslides are common on steep

slopes, especially when soils and weathered bedrock are fully

satu-rated by water This destructive form of mass movement causes

damage costing almost 2 billion dollars and 25 to 50 associated

deaths per year in the United States alone You will explore the

movement of a landslide in the GeoLab at the end of this chapter

A rockslide is a type of landslide that occurs when a sheet of

rock moves downhill on a sliding surface During a rockslide, some

blocks of rock are broken into smaller blocks as they move

downslope, as shown in Figure 8.7. Often triggered by

earth-quakes, rockslides can move large amounts of material

■ Figure 8.7 During this rockslide, blocks of rock were broken into smaller blocks as they moved downslope.

Interactive Figure To see an animation

(t)Handout/Malacanang/Reuters/CORBIS , (b)Lloyd Cluff/CORBIS

198 Chapter 8 • Mass Movements, Wind, and Glaciers

Slumps When the mass of material in a landslide moves along

a curved surface, a slump results Material at the top of the slump

moves downhill, and slightly inward, while the material at the tom of the slump moves outward Slumps can occur in areas that have thick soils on moderate-to-steep slopes Sometimes, slumps occur along highways where the slopes of soils are extremely steep

bot-Slumps are common after rains, when water reduces the frictional contact between grains of soil and acts as a lubricant between sur-face materials and underlying layers The weight of the additional water pulls material downhill As with other types of mass move-ment, slumps can be triggered by earthquakes Slumps leave cres-cent-shaped scars on slopes, as shown in Figure 8.8.

Reading Check Describe what conditions can cause a slump.

Avalanches Landslides that occur in mountainous areas with

thick accumulations of snow are called avalanches About

10,000 avalanches occur each year in the mountains of the western United States Radiation from the Sun can melt surface snow, which then refreezes at night into an icy crust Snow that falls on top of this crust can eventually build up, become heavy, slip off, and slide downslope as an avalanche Avalanches can happen in early winter when snow accumulates on the warm ground The snow in contact with the warm ground melts, then refreezes into

a layer of jagged, slippery snow crystals

Avalanches of dangerous size, like the one shown in Figure 8.9,

occur on slope angles between 30° and 45° When the angle of a slope

is greater than 45°, enough snow cannot accumulate to create a large avalanche At angles less than 30°, the slope is not steep enough for snow to begin sliding A vibrating trigger, even from a single skier, can send this unstable layer sliding down a mountainside Avalanches pose significant risks in places such as Switzerland, where more than

50 percent of the population lives in avalanche terrain

■ Figure 8.9 Vibrations from a single

skier can trigger an avalanche.

Identify the conditions that make a

landscape more vulnerable to avalanches.

■ Figure 8.8 Slumps leave distinct

crescent-shaped scars on hillsides as the

soil rotates downward.

(t)Dr Marli Miller/Visuals Unlimited , (b)Mauritius/SuperStock

Section 1 • Mass Movements 199

Rockfalls On high cliffs, rocks are loosened by physical

weather-ing processes, such as freezweather-ing and thawweather-ing, and by plant growth As

rocks break up and fall directly downward, they can bounce and roll,

ultimately producing a cone-shaped pile of coarse debris, called

talus, at the base of the slope Rockfalls, such as the one shown in

Figure 8.10, commonly occur at high elevations, in steep road cuts,

and on rocky shorelines Rockfalls are less likely to occur in humid

regions where the rock is typically covered by a thick layer of soil,

vegetation, and loose materials On human-made rock walls, such as

road cuts, rockfalls are particularly common

Mass Movements Affect People

While mass movements are natural processes, human activities

often contribute to the factors that cause mass movements Activities

such as the construction of buildings, roads, and other structures can

make slopes unstable In addition, poor maintenance of septic

sys-tems, which often leak, can trigger slides In the Philippines,

mud-slides, shown in Figure 8.11, were triggered after ten days of

torrential rains delivered 200 cm of precipitation A village estimated

to have 3000 residents was totally destroyed

■ Figure 8.10 This rockfall in Topanga Canyon, California, was unusual in that it involved mainly one large rock.

■ Figure 8.11 The mudflow on the island of Luzon occurred after days of rain.

(t)Ted Soqui/CORBIS , (b)Yann Arthus-Bertrand/CORBIS

Self-Check Quiz glencoe.com

200 Chapter 8 • Mass Movements, Wind, and Glaciers

Reducing the risks Catastrophic mass ments are most common on slopes greater than 25°

move-that experience annual rainfall of over 90 cm Risk increases if that rainfall tends to occur in a short period of time Humans can minimize the destruc-tion caused by mass movements by not building structures on or near the base of steep and unstable slopes

Although preventing mass-movement disasters

is not easy, some actions can help reduce the risks

For example, a series of trenches can be dug to divert running water around a slope and control its drainage Landslides can be controlled by covering steep slopes with materials such as steel nets, shown in Figure 8.12, and constructing fences along highways in areas where rockslides are com-mon Other approaches involve the installation of retaining walls to support the bases of weakened slopes and prevent them from falling Most of these efforts at slope stabilization and mass-move-ment prevention are only temporarily successful

The best way to reduce the number of disasters related to mass movements is to educate people about the problems of building on steep slopes

For example, The United States Geological Survey (USGS) collects data about landslides in an effort to learn more about where and when landslides will occur This information helps people decide where they can safely build homes or businesses

■ Figure 8.12 Covering hillsides with steel nets can

reduce risks of mass movements and harm to humans

Identify the type of mass movement that these steel

nets help prevent

Section Summary

◗◗ Mass movements are classified in

part by how rapidly they occur.

◗

◗ Factors involved in the mass

move-ment of Earth materials include the

material’s weight, its resistance to

sliding, the trigger, and the presence

of water.

◗

◗ Mass movements are natural

pro-cesses that can affect human life and

activities.

◗

◗ Human activities can increase the

potential for the occurrence of mass

movements.

Understand Main Ideas

increasing speed: slides, creep, flows, and rockfalls.

2 Identify the underlying force behind all forms of mass movement.

3 Analyze how water affects mass movements by using two examples of mass movement.

4 Appraise the effects of one type of mass movement on humans.

Think Critically

5 Generalize in which regions of the world mudflows are more common.

6 Evaluate how one particular human activity can increase the risk of mass ment and suggest a solution to the problem.

Wind Erosion in the United States

Areas of wind erosion

Section 2 • Wind 201

Objectives

◗ Describe conditions that contribute

to the likelihood that an area will

experience wind erosion.

◗ Identify wind-formed landscape

features.

◗ Describe how dunes form and

migrate.

Review Vocabulary

velocity: the speed of an object and

its direction of motion

Wind Erosion and Transport

A current of rapidly moving air can pick up and carry sediment

in the same way that water does However, except for the extreme winds of hurricanes, tornadoes, and other strong storms, winds cannot generally carry particles as large as those transported by moving water Regardless, wind is a powerful agent of erosion

Winds transport materials by causing their particles to move in different ways For example, wind can move sand on the ground in

a rolling motion A method of transport by which strong winds cause small particles to stay airborne for long distances is called suspension Another method of wind transport, called saltation, causes a bouncing motion of larger particles Saltation accounts for most sand transport by wind Limited precipitation leads to an increase in the amount of wind erosion because precipitation holds down sediments and allows plants to grow Thus, wind transport and erosion primarily occur in areas with little vegetative cover, such as deserts, semiarid areas, seashores, and some lakeshores

Wind erosion is a problem in many parts of the United States, as shown in Figure 8.13.

■ Figure 8.13 Wind erosion does not

affect all areas of the United States equally.

Observe which areas are subject to

wind erosion.

202 Chapter 8 • Mass Movements, Wind, and Glaciers

Deflation The lowering of the land surface that results from the

wind’s removal of surface particles is called deflation During the

1930s, portions of the Great Plains region, which stretches from Montana to Texas, experienced severe drought The area was already suffering from the effects of poor agricultural practices, in which large areas of natural vegetation were removed to clear the land for farming Strong winds readily picked up the dry surface particles, which lacked any protective vegetation Severe dust storms resulted in daytime skies that were often darkened, and the region became known as the Dust Bowl

Today, the Great Plains are characterized by thousands of shallow depressions known as deflation blowouts Many are the result of the removal of surface sediment by wind erosion during the 1930s The depressions range in size from a few meters to hundreds of meters in diameter Deflation blowouts are also found in other areas that have sandy soil, as shown in Figure 8.14. Wind erosion continues today throughout the world, as shown by the sandstorm in Figure 8.15.

Reading Check Explain how deflation removes surface particles.

■ Figure 8.15 A sandstorm in a desert

region fills the air with dust

■ Figure 8.14 Through deflation, the

wind can create a bowl-shaped blowout.

(t)Jerome Wyckoff/Animals Animals , (b)Remi Benali/CORBIS

Arch Pillar

Section 2 • Wind 203

Deflation is a major problem in many agricultural areas of the

world as well as in deserts, where wind has been consistently strong

for thousands of years In areas of intense wind erosion, coarse

gravel and pebbles are usually left behind as the finer surface

mate-rial is removed by winds The coarse surface left behind is called

desert pavement

Abrasion Another process of erosion, called abrasion, occurs

when particles such as sand rub against the surface of rocks or

other materials Abrasion occurs as part of the erosional activities

of winds, streams, and glaciers In wind abrasion, wind picks up

materials such as sand particles and blows them against anything

in their path Because sand is often made of quartz, a hard mineral,

wind abrasion can be an effective agent of erosion — windblown

sand particles eventually wear away rocks Structures, such as

tele-phone poles, can also be worn away or undermined by wind

abra-sion, and paint and glass on homes and vehicles can be damaged

by windblown sand

Materials that are exposed to wind abrasion show unique

char-acteristics For example, windblown sand causes rocks to become

pitted and grooved With continued abrasion, rocks become

pol-ished on the windward side and develop smooth surfaces with

sharp edges In areas of shifting winds, abrasion patterns

corre-spond to wind shifts, and different sides of rocks become polished

and smooth Rocks shaped by windblown sediments, such as those

shown in Figure 8.16, are called ventifacts Ventifacts are found

in various shapes and sizes, and include arches and pillars

Reading Check Identify the unique characteristics of materials

shaped by abrasion

■ Figure 8.16 Ventifacts form in different types of environments but most commonly

in arid climates where wind can be a dominant erosional force.

To read about how wind has shaped desert

Expedition on page 898.

(l)Robert Barber/Visuals Unlimited , (r)David Nunuk/Photo Researchers

Wind direction Windward

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 8.17 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.

■ Figure 8.18 Dune migration is

caused by wind.

VOCABULARY

A CADEMIC VOCABULARY

migrate

to move from one location to another

Dunes migrate as wind blows over

sand.

Interactive Figure To see an animation

Section 2 • Wind 205

Example of Dune Description

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

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 Dunes

• form U-shapes

• 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

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

Interactive Table To explore more about sand dunes, visit

glencoe.com.

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

Self-Check Quiz glencoe.com

Loess deposits

Sandy areas where dunes are found

Distribution of Loess Deposits in the United States

206 Chapter 8 • Mass Movements, Wind, and Glaciers

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, wind-

blown 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

■ Figure 8.19 This map shows the

loca-tion of loess deposits in the continental

◗ Wind can transport sediment in

sev-eral ways, including suspension and

saltation.

◗

◗ Dunes form when wind velocity

slows down and windblown sand is

deposited.

◗

◗ Dunes migrate as long as winds

con-tinue to blow.

Understand Main Ideas

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.