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.

8.1 Mass Movements
MAIN Idea Mass movements
alter Earth’s surface over time
due to gravity moving sediment
and rocks downslope.

8.2 Wind

Glacial till

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.

Calving glacier

192
(t)Steve McCutcheon/Visuals Unlimited, (b)Bernhard Edmaier/Science Photo Library, (bkgd)Gregory Dimijian/Photo Researchers


Start-Up Activities
External Processes that Shape
Earth Make this Foldable to
explain different processes that
shape Earth’s surface.

LAUNCH Lab
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.

STEP 1 Fold the bottom of a horizontal sheet
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.

Mmasesnts
Move

Wind Gla
ciers

FOLDABLES 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.

Visit glencoe.com to
study entire chapters online;
explore
•

Interactive Time Lines

•

Interactive Figures

•

Interactive Tables

animations:

access Web Links for more information, projects,
and activities;
review content with the Interactive
Tutor and take Self-Check Quizzes.

Chapter 8 •Section
Mass Movements,
1 • XXXXXXXXXXXXXXXXXX
Wind, and Glaciers 193


Section 8 .1
Objectives
◗ Analyze the relationship between
gravity and mass movements.
◗ Identify factors that affect mass
movements.
◗ Distinguish between types of
mass movements.
◗ Relate how mass movements affect
people.

Mass Movements
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 num-

ber of factors might come into play, including the angle of the slide, the amount
of water on the slide, the material of the slide, friction, and your own mass.

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

Review Vocabulary
gravity: the force every object exerts
on every other object due to their
masses

New Vocabulary
mass movement
creep
mudflow
landslide
slump
avalanche

Mass Movements
How do landforms, such as mountains, hills, and plateaus, wear
down and change? Landforms can change through processes involving wind, ice, and water, and sometimes through the force of gravity
alone. The downslope movement of soil and weathered rock resulting 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 determine 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 sudden 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 growing opposite the pull of gravity, which is
toward the center of Earth.

194 Chapter 8 • Mass Movements, Wind, and Glaciers
Dr. Marli Miller/Visuals Unlimited


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. Saturation 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.

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.
■

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 season. 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.

Tilted fence posts,
trees, and poles

Section 1 • Mass Movements 195
(t)David McNew/Getty Images, (b)Ralph Lee Hopkins/Photo Researchers


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.

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 moderately slow movements of soils, whereas
mudflows are swiftly moving mixtures of
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
downslope.
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 mountain. 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 traveled 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, semiarid regions that experience intense, short-lived
rainstorms. The Los Angeles Basin in Southern
California is an example of an area where mudflows are common. In such areas, periods of
drought and forest fires leave the slopes with little 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 mountainous areas. These mudflows can burry
homes, as shown in Figure 8.5.
196 Chapter 8 • Mass Movements, Wind, and Glaciers
(t)Steve Raymer/National Geographic Image Collection, (b)Gene Blevins/LA Daily News/CORBIS


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 separates 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 damming rivers and causing flooding. Landslides are common on steep
slopes, especially when soils and weathered bedrock are fully saturated 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 earthquakes, 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
of a rockslide, visit glencoe.com.

Section 1 • Mass Movements 197
(t)Handout/Malacanang/Reuters/CORBIS, (b)Lloyd Cluff/CORBIS


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 bottom 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.
Slumps are common after rains, when water reduces the frictional
contact between grains of soil and acts as a lubricant between surface materials and underlying layers. The weight of the additional
water pulls material downhill. As with other types of mass movement, slumps can be triggered by earthquakes. Slumps leave crescent-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
■

Figure 8.8 Slumps leave distinct

crescent-shaped scars on hillsides as the
soil rotates downward.

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.

198

Chapter 8 • Mass Movements, Wind, and Glaciers

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


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

Rockfalls On high cliffs, rocks are loosened by physical weathering processes, such as freezing and thawing, 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 systems, which often leak, can trigger slides. In the Philippines, mudslides, 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 8.11 The mudflow on the
island of Luzon occurred after days of rain.
■

Section 1 • Mass Movements 199
(t)Ted Soqui/CORBIS, (b)Yann Arthus-Bertrand/CORBIS


■ 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 8 .1

Assessment

Section Summary

Understand Main Ideas

◗ Mass movements are classified in
part by how rapidly they occur.

1.

◗ Factors involved in the mass movement of Earth materials include the
material’s weight, its resistance to
sliding, the trigger, and the presence
of water.

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

◗ Mass movements are natural processes that can affect human life and
activities.

Think Critically

◗ Human activities can increase the
potential for the occurrence of mass
movements.

MAIN Idea Organize the following types of mass movements in order of
increasing speed: slides, creep, flows, and rockfalls.

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.
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 movement and suggest a solution to the problem.

Earth Science
7. Make a poster that compares and contrasts solifluction and a slump. Consider the

way soil moves and the role of water.

200 Chapter 8 • Mass Movements, Wind, and Glaciers

Self-Check Quiz glencoe.com

Michael Habicht/Animals Animals

Reducing the risks Catastrophic mass movements are most common on slopes greater than 25°
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 destruction 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 common. 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-movement 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.


Section 8.
8.2
2
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.

Wind
MAIN Idea Wind modifies landscapes in all areas of the world by
transporting sediment.
Real-World Reading Link If you have ever been on a beach on a windy day,

you might have felt the stinging of sand on your face. Sand travels in the wind if
the wind is fast enough.

Review Vocabulary
velocity: the speed of an object and
its direction of motion

New Vocabulary

deflation
abrasion
ventifact
dune
loess

■ Figure 8.13 Wind erosion does not
affect all areas of the United States equally.
Observe which areas are subject to
wind erosion.

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.

Wind Erosion in the United States

Areas of wind erosion

Section 2 • Wind

201


■ Figure 8.14 Through deflation, the
wind can create a bowl-shaped blowout.

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.

202

Chapter 8 • Mass Movements, Wind, and Glaciers

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


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 material 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 telephone poles, can also be worn away or undermined by wind abrasion, and paint and glass on homes and vehicles can be damaged
by windblown sand.
Materials that are exposed to wind abrasion show unique characteristics. For example, windblown sand causes rocks to become
pitted and grooved. With continued abrasion, rocks become polished on the windward side and develop smooth surfaces with
sharp edges. In areas of shifting winds, abrasion patterns correspond 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.

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

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.

Arch

Pillar
Section 2 • Wind 203
(l)Robert Barber/Visuals Unlimited, (r)David Nunuk/Photo Researchers


Thane/Animals Animals

■ 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.

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.

VOCABULARY
ACADEMIC VOCABULARY
migrate
to move from one location to another
Dunes migrate as wind blows over
sand.

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

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.
Wind direction

Figure 8.18 Dune migration is

caused by wind.

204

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.

Leeward

Chapter 8 • Mass Movements, Wind, and Glaciers

Windward


Table 8.1

Types of Dunes
Example of Dune

Interactive Table To explore
more about sand dunes, visit
glencoe.com.

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

Section 2 • Wind

205

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


Figure 8.19 This map shows the location of loess deposits in the continental
United States.

■

Distribution of Loess Deposits in the United States

Sandy areas where
dunes are found
Loess deposits

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

Understand Main Ideas

◗ Wind is a powerful agent of erosion.

1.

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

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

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

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

◗ Dunes migrate as long as winds continue to blow.

MAIN Idea Evaluate the various types of landforms formed by wind and how
these landforms are created.

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

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.

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


Section 8 . 3
Objectives

Glaciers

◗ Explain how glaciers form.
◗ Compare and contrast the conditions that produce valley glaciers
with those that produce continental
glaciers.
◗ Describe how glaciers modify

landscapes.
◗ Recognize glacial features.

MAIN Idea Glaciers modify landscapes by eroding and depositing
rocks.
Real-World Reading Link Have you ever wondered what formed the land-

scape around you? Glaciers might have left deposits of sediment as well as
carved features in rock that you see every day.

Review Vocabulary

Moving Masses of Ice

latitude: distance in degrees north
and south of the equator

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.

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

■

Figure 8.20
Greenland

Glaciers around the world
have changed in distribution throughout geologic
time.
Infer what changes
have occurred in the
distribution of glaciers
around the world.

North
America

Europe

Asia

Africa
South
America

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

Glaciers
(present)

Australia

Glaciers
(18,000
years ago)

Antarctica

Section 3 • Glaciers 207


FOLDABLES
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.
Reading Check Describe how V-shaped valleys become U-shaped.

Continental glaciers Glaciers that cover broad, continentsized 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.

Data 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

Amount of radioactivity

Radioactivity in Ice Cores
Pre-test ban
(1964–65)

High

Start of atomicbomb testing
(mid-1950s)

Chernobyl
(1987–88)

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.

Low
0

100

200

300

400

500

Depth (cm)

208 Chapter 8 • Mass Movements, Wind, and Glaciers

600

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


(l)Dr. Marli Miller/Visuals Unlimited, (c)Karl Weatherly/CORBIS, (r)Adam Jones/Visuals Unlimited

Cirque

Horn

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.

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

Careers In Earth Science

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.

Section 3 • Glaciers 209


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.

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.
■

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 drumlin’s 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
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?

210

Chapter 8 • Mass Movements, Wind, and Glaciers

E. R. Degginger/Photo Researchers

Glacial Deposition


Visualizing Continental
Glacial Features
Figure 8.23 Continental glaciers carve out vast regions of landscape, leaving behind distinctive features
such as kames, eskers, drumlins, and moraines.

Retreating
glacier

Kame
Moraine

k

roc

d
Be

Drumlins
Esker
uth

So

Kames are short cone-shaped mounds of
sorted deposits. They are shaped from outwash left as glaciers recede.

Eskers are long ridges of sorted deposits.
They are shaped from outwash left as glaciers recede.

Drumlins are shaped as the glacier moves
over old moraines. They are unsorted.

To explore more about glacial
features, visit glencoe.com.

Section 3 • Glaciers 211
(l)R.B. Colton/USGS, (c)Tom Bean/CORBIS, (r)Gustav Verderber/Visuals Unlimited


Thomas & Pat Leeson/Photo Researchers

■ Figure 8.24 These kettle lakes in
North Dakota are a result of glacial retreat.
Describe how you might be able to
locate kettles on a topographic map.

VOCABULARY

SCIENCE USAGE V. COMMON USAGE
Kettle
Science usage: a steep-sided depression formed by a glacier
Common usage: a metallic pot used
for cooking

Section 8 . 3

Glacial lakes Sometimes, a large block of ice breaks off a continental glacier and the surrounding area is covered by sediment. When the
ice block melts, it leaves behind a depression called a kettle hole. After
the ice block melts, the kettle hole fills with water from precipitation
and runoff to form a kettle lake. Kettles or kettle lakes, such as those
shown in Figure 8.24, are common in New England, New York, and
Wisconsin. With valley glaciers, cirques can also fill with water, and
they become cirque lakes. When a terminal moraine blocks off a valley, the valley fills with water to form a lake. Moraine-dammed lakes
include the Great Lakes and the Finger Lakes of northern New York,
which are long and narrow.
Mass movements, wind, and glaciers all contribute to the changing
of Earth’s surface. These processes erode landforms constantly, and in
many ways, they also impact human populations and activities.

Assessment

Section Summary

Understand Main Ideas

◗ Glaciers are large moving masses of
ice that form near Earth’s poles and
in mountain areas.

1.

◗ Glaciers can be classified as valley
glaciers or continental glaciers.
◗ Glaciers modify the landscape by
erosion and deposition.
◗ Features formed by glaciers include
U-shaped valleys, hanging valleys,
moraines, drumlins, and kettles.

MAIN Idea

Describe two examples of how glaciers modify landscapes.

2. Explain how glaciers form.
3. Compare and contrast the characteristics of valley glaciers and continental
glaciers.
4. Differentiate among different glacial depositional features.

Think Critically
5. Evaluate the evidence of past glaciers that can be found on Earth today.
6. Infer whether valley glaciers or continental glaciers have shaped more of the
landscape of the United States.

Earth Science
7. Deduce how you might distinguish a lake formed in a cirque and a lake formed
in a kettle.

212

Chapter 8 • Mass Movements, Wind, and Glaciers

Self-Check Quiz glencoe.com


Slipping Away
On the morning of January 10, 2005, the residents of La Conchita, California, awoke to find
the highway out of town closed in both directions, due to landslides. Around 12:30 P.M. many
residents heard an ominous roar as the bluff
above the town unleashed 600,000 metric tons
of dirt and mud, covering four blocks in 10 m of
debris. Scientists went to the scene to discover
exactly what had caused this enormous landslide
and whether one could happen again.

The setting La Conchita is built on a narrow
swatch of land between the highway and a
huge bluff. The bluff is held together weakly,
so it is susceptible to being loosened by heavy
water content, such as a prolonged, heavy
rain. The slope is further weakened from the
effects of regular landslides, as well as being
on a fault line.
In the two weeks prior to the landslide, the area
had received a record amount of rain—about
35 cm—the amount it normally receives in a
year! The excess water caused the earth to literally slide off the face of the mountain.

A history of landslides This event was

not, however, the first landslide to hit the
area. In fact, the mountain bluff is scarred with
the evidence of many landslides. Ten years earlier, in March of 1995, two devastating landslides hit the area in the span of a week. These
landslides were also caused by a large amount
of rain, but the movement of the earth was
relatively slow, so residents were able to get
away. The 2005 landslide was a continuation of
the 1995 slide — the soil that was deposited by
the earlier slide was loosened by the rainwater
and slipped down the slope. After the 1995
slide, the state government erected a retaining
wall to keep the landslides at bay. However,
soil, mud, and debris from the 2005 disaster
passed right over parts of the wall.

The mass movement at La Conchita, California, in 2005 killed ten people.

The debate Could the 2005 landslide have
been detected and the people warned in time
to prevent loss of life? Most likely, yes. In fact,
some of the residents of the town are suing
the government for failure to protect their citizens, as well as failure to adequately notify
them of the impending danger.
Are governments responsible for providing warnings and protection to citizens who move into
areas that are prone to natural disasters? Or,
does the responsibility lie with the citizens that
might not have understood the dangers of living
in the area? These questions and more are sure
to be considered by the residents and government of La Conchita, as well as cities and local
governments of disaster- prone areas throughout

the United States for years to come.

Earth Science
Debate Research information about a natural disaster that has occurred near your
location. Hold a classroom debate on the
topic of why people should, or should not,
live in an area where natural disasters have
occurred. To learn more about natural
disasters, visit glencoe.com.
Earth Science & Society

213

David McNew/Getty Images


USGS

MAPPING: MAP A LANDSLIDE

This image shows the Tully Valley landslide three days after it occurred. The Tully Farms Road is covered up to 5 m deep with clay.

Background: Around midday on April 27, 1993, in a
normally quiet, rural area of New York, the landscape
dramatically changed. Unexpectedly, almost
1 million m3 of earth debris slid 300 m down the lower
slope of Bare Mountain and into Tully Valley. The debris
flowed over the road and buried nearby homes. The
people who lived there had no knowledge of any prior
landslides occurring in the area, yet this landslide was

the largest to occur in New York in more than 75 years.
Question: How can you use a drawing based on a
topographic map to infer how the Tully Valley Landslide
occurred?

Materials
metric ruler

3. Measure the length and width of the Tully Valley in
kilometers. Double-check your results.

Analyze and Conclude
1. Interpret Data What does the shape of the valley tell
you about how it formed?
2. Determine In what direction did the landslide flow?
3. Determine In what direction does the Onondaga
Creek flow?
4. Infer from the map which side of Tully Valley has the
steepest valley walls.
5. Deduce What conditions must have been present for
the landslide to occur?
6. Infer At the time of the Tully Valley Landslide, the
trees were bare. How could this have affected the conditions that caused the landslide?

Procedure
Imagine that you work for the United States Geological
Survey (USGS) specializing in mass movements. You have
just been asked to evaluate the Tully Valley Landslide.
1. Read and complete the lab safety form.
2. Check the map’s scale.

214

GeoLab

Earth Science
Explain why the mass movement event you examined in
this GeoLab is classified as a landslide. Differentiate a
landslide from a creep, slump, flow, avalanche, and
rockfall.


Syracuse
U.S
. Ro

ute

20

Landslide
area

reek
aga C
o nd
On

Previous
landslides

New
York

Onondaga
County

0
0

.5

1.0

.5

1

1.5 miles
2 kilometers

E
W

r
Ba

1993
landslide
nt

ou

Tully Farms Road

eM
ain

Rattle
sna
ke

N

R

ow
ainb

ek
Cre

Gulf

Otisco Road

KEY
W

E Line of landslide section

Onondaga C

Valley floor
Valley walls

reek

Edge of valley floor
Stream channel
(arrow shows direction
of stream flow)

New York Route 11-A

Onondaga Creek

Brine field

GeoLab 215


Download quizzes, key
terms, and flash cards
from glencoe.com.

BIG Idea Movements due to gravity, winds, and glaciers shape and change
Earth’s surface.
Vocabulary

Key Concepts

Section 8.1 Mass Movements
• avalanche (p. 198)
• creep (p. 195)
• landslide (p. 197)
• mass movement (p. 194)
• mudflow (p. 196)
• slump (p. 198)

Mass movements alter Earth’s surface over time due to gravity
moving sediment and rocks downslope.
Mass movements are classified in part by how rapidly they occur.
Factors involved in the mass movement of Earth materials include the
material’s weight, its resistance to sliding, the trigger, and the presence
of water.
Mass movements are natural processes that can affect human life and
activities.
Human activities can increase the potential for the occurrence of mass
movements.

MAIN Idea

•
•

•
•

Section 8.2 Wind

• abrasion (p. 203)
• deflation (p. 202)
• dune (p. 204)
• loess (p. 206)
• ventifact (p. 203)

Wind modifies landscapes in all areas of the world by transporting sediment.
Wind is a powerful agent of erosion.
Wind can transport sediment in several ways, including suspension and
saltation.
Dunes form when wind velocity slows down and windblown sand is
deposited.
Dunes migrate as long as winds continue to blow.

MAIN Idea

•
•
•
•

Section 8.3 Glaciers
• cirque (p. 209)
• continental glacier (p. 208)
• drumlin (p. 210)
• esker (p. 210)
• glacier (p. 207)
• kame (p. 210)
• kettle (p. 212)
• moraine (p. 210)

• outwash plain (p. 210)
• valley glacier (p. 208)

216

Chapter 8
X • Study Guide

MAIN Idea

Glaciers modify landscapes by eroding and depositing rocks.

• Glaciers are large moving masses of ice that form near Earth’s poles and

in mountain areas.
• Glaciers can be classified as valley glaciers or continental glaciers.
• Glaciers modify the landscape by erosion and deposition.
• Features formed by glaciers include U-shaped valleys, hanging valleys,

moraines, drumlins, and kettles.

Vocabulary
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