23.2 Weathering and Mass Movement

23.2 Weathering and
Mass Movement
Key Concepts
What are the agents
of erosion?
What causes mechanical
and chemical weathering?
What factors affect the rate
at which rocks weather?
What force causes mass
movement?
Vocabulary
◆ erosion
◆ weathering
◆ mechanical
weathering
◆ abrasion
◆ chemical
weathering
◆ mass movement
Reading Strategy
Concept Map As you read, draw a concept
map showing the key factors that affect the
rate of weathering.
Rate of
weathering
Imagine walking through a pine forest in southern Utah. Suddenly
your trail twists, leading you to the edge of a canyon. The curved end
of the canyon is shaped like a huge football stadium. But instead of
cheering sports fans, this stadium is full of strange columns of rock in
yellow, brilliant orange, and rusty red. You are looking at Bryce Canyon,
which is famous for its fantastic rock formations, called “hoodoos.” The
hoodoos are made of different types of sedimentary rock.
Millions of years ago, what is now Bryce Canyon was solid layers of
rock. Very slowly, much of this rock was broken down and worn away
until only the hoodoos were left standing.
Erosion
Bryce Canyon formed by erosion, the process that wears down and
carries away rock and soil. Erosion cuts canyons through solid rock
and wears away the cliffs above beaches.
Erosion is the destructive process that has shaped Earth’s surface over
hundreds of millions of years. Erosion acts through weathering,
the force of gravity, and through the movement of streams,
groundwater, glaciers, wind, and waves. Over time, erosion breaks
down even the tallest mountains. The end product of erosion is sediment.
Recall that sediment is composed of particles of rock and soil
and the remains of living things.
Section Resources
Print
• Reading and Study Workbook With
Math Support, Section 23.2
• Transparencies, Section 23.2
depends
on
temperature
a. ?
b. ?
Figure 7 Incredible rock
formations called hoodoos
can be seen at Bryce Canyon
National Park in Utah.
Earth’s Surface 709
Technology
• Interactive Textbook, Section 23.2
• Presentation Pro CD-ROM, Section 23.2
• Go Online, NSTA SciLinks, Weathering
Section 23.2
1
FOCUS
Objectives
23.2.1 Describe the processes by
which erosion wears down and
carries away rock.
23.2.2 Distinguish between chemical
and mechanical weathering
and describe the factors that
affect the rate of weathering.
23.2.3 Explain how the force of
gravity contributes to erosion
by mass movement.
Build Vocabulary
Venn Diagram Have students use
a Venn diagram to compare chemical
and mechanical weathering. Instruct
students to create the diagram while
they read Mechanical Weathering and
Chemical Weathering, pp. 710–711.
After they have completed the diagrams,
have students work in groups to
derive concise definitions of mechanical
weathering and chemical weathering.
Reading Strategy
a. Availability of water b. Type of rock
2
Reading Focus
INSTRUCT
Erosion
FYI
Although erosion is often narrowly
defined as the transportation of
weathered rock to distant areas, this
text defines erosion more broadly as
the process that wears down and carries
away rock and soil. This conforms with
the definition of erosion in the Dictionary
of the American Geological Institute.
Build Reading Literacy
L2
L2
L1
Predict Refer to page 66D in
Chapter 3, which provides the
guidelines for predicting.
Before students begin reading Erosion,
tell them to examine the picture of
Bryce Canyon in Figure 7. Ask students
to make a written prediction about how
this landscape was formed. After they
read Erosion, have students write a
response to their prediction, correcting
any misconceptions and adding details.
Visual, Verbal
Earth’s Surface 709

Section 23.2 (continued)
Weathering
Chemical Weathering
Purpose Students observe how rock
can be broken down by a chemical
reaction.
Materials calcite, 1M HCl, dropper
Procedure Tell students that calcite is
particularly susceptible to chemical
weathering by acidic solutions such as
rain. Explain that rain contains sulfuric
and nitric acids, and can be strongly
acidic in areas of heavy pollution.
Buildings and other structures made of
marble and other calcite-containing
stone exhibit severe chemical
weathering over a period of years in
regions where acid rain is a common
problem. Ask, What do you think will
happen when I drip acid on the rock?
(The calcite crystals will react with the
acid.) Drip a few drops on the rock,
being sure to hit calcite crystals.
Expected Outcome The calcite
crystals will fizz and bubble.
Visual
Download a worksheet on
weathering for students to
complete, and find additional
teacher support from NSTA SciLinks.
710 Chapter 23
L2
Figure 8 When water expands to
form ice, it can pry open cracks in
rock. This is called frost wedging.
Classifying Is frost wedging a
form of mechanical weathering
or chemical weathering?
For: Links on weathering
Visit: www.SciLinks.org
Web Code: ccn-3232
710 Chapter 23
Customize for Inclusion Students
Gifted
Have students collect rain samples from
around their homes or the school, and use
litmus paper or a pH meter to measure the pH
of the rain in your area. Tell students to make
sure they place their collection containers
Weathering
Erosion starts with weathering. Weathering is the
process by which rocks are chemically altered or
physically broken down into fragments at or near
Earth’s surface. There are two forms of
weathering: mechanical and chemical. They
cause rocks to disintegrate or decompose.
Mechanical Weathering Under the right
conditions, even the hardest rock can be broken
into smaller pieces. Mechanical weathering is the
process of physically breaking rock into smaller
fragments. Mechanical weathering occurs through
frost wedging, abrasion, growth of plant roots,
and other processes.
The most common form of mechanical weathering
is frost wedging, shown in Figure 8. When
water freezes to form ice, the water expands. This expansion pries open
any cracks that the water has seeped into. Frost wedging is an important
cause of weathering in mountains, where water often freezes at night
and melts during the day. Frost wedging pries rocks apart a little bit
every night. If you live in a region that has cold winters, you may see
how frost wedging damages roads. Many of the potholes that form in
roads during winter are the result in part of frost wedging.
Another form of mechanical weathering, called abrasion, occurs
when rocks scrape or grind against one another. For example, abrasion
occurs when sand carried by water or wind causes bits of rock to
break off as if they were being sandblasted.
Mechanical weathering also occurs in other ways. Plant roots can
grow into cracks in a rock. Roots exert a powerful force that can
slowly pry the rock apart. In another form of weathering, erosion
removes material from the surface of a mass of rock, reducing pressure
on the rock that remains. This release of pressure allows the rock
to expand, causing the outside of the rock to crack and flake off like
layers of pastry.
Chemical Weathering Chemical weathering is a process in
which rock is broken down by chemical reactions. Chemical weathering
involves chemical reactions that dissolve the minerals making up
rock or change them into new minerals. Over time, the effects of chemical
weathering can be dramatic. Pits or holes may form in the rock
surface. Eventually, the rock crumbles and disintegrates.
Water is the main agent of chemical weathering. Water is an effective
solvent. All minerals dissolve in water, though most do so very slowly.
away from buildings, trees, or other structures
that the rain might run off of before entering
the container. While you are covering chemical
weathering, have students report to the class
on the pH of the rain in your area.

Chemical weathering also occurs because
rain is slightly acidic. Water and carbon dioxide
naturally combine to form carbonic acid.
The carbonic acid in rainwater dissolves many
minerals, such as calcite. The statue of the lion
in Figure 9 has been weathered by carbonic
acid in rain. Carbonic acid and water react
with other minerals, such as the feldspar in
granite, to produce clay minerals.
Another type of chemical weathering, the
rusting of minerals that are rich in iron,
involves oxidation. This results in the formation
of new minerals that often have a red or
brown color, such as the rusted bicycle in
Figure 9.
What is the main agent of chemical weathering?
Rates of Weathering
The rate at which mechanical and chemical weathering take place
depends on three main factors: temperature, the availability of water,
and the type of rock. Chemical weathering occurs more rapidly in
places with high temperatures and abundant rainfall. These conditions
generally speed up chemical reactions. For example, granite weathers
slowly in cool regions, but more rapidly in regions that are hot and wet.
Mechanical weathering occurs faster in places where temperature conditions
frequently alternate between freezing and thawing.
The rate of weathering also depends upon the kind of rock that is
exposed at the surface. Some rocks, such as limestone and marble,
undergo rapid chemical weathering. These rocks are composed primarily
of calcite, a mineral that reacts readily with carbonic acid. One
of the tombstones in Figure 10 shows the effects of chemical weathering
on marble. In contrast, the other tombstone is made of slate, which
is composed of minerals, such as quartz and mica, that are much more
resistant to chemical weathering than calcite.
Facts and Figures
Humans as Agents of Erosion Many
human activities cause or accelerate
weathering. Industrial activity may cause
highly acidic rain that results in more rapid
chemical weathering. Humans also carry out
mechanical weathering by leveling ground
for roads and cities and by strip mining for
mineral resources. The paving of large tracts
of land causes more severe erosion of
A
Figure 10 Different rocks weather at different
rates. Even though the slate tombstone (right) is
older, it is much less weathered than the marble
one (left). Inferring What type of weathering was
likely responsible for wearing away the lettering
on the marble tombstone?
B
Figure 9 In chemical weathering,
rock is broken down by chemical
reactions. A One form of chemical
weathering occurs through
oxidation. Iron-rich minerals
become rusted like this old bicycle.
B Chemical weathering also
occurs when rainwater dissolves
or reacts with the minerals in
rocks, as with this statue of a lion.
Earth’s Surface 711
adjacent unpaved areas due to the sheeting
action of water. Soil erosion is a major
problem in agricultural areas, as the U.S.
loses billions of tons of topsoil to erosion
each year. It was agricultural practices that
led to the great Dust Bowl of the 1930s.
Deforestation also contributes to erosion,
including devastating mass movements such
as landslides.
Rates of Weathering
A common misconception is that soil has
always existed in its present form, rather
than being a product of weathering
rocks and decaying organic matter. The
weathering of rocks produces sediment,
one of the main components of soil.
Soil-producing sediment is often created
right underneath people’s feet as rain
percolates down into the ground and
bedrock undergoes chemical weathering.
In addition to sediment, soil also
contains both living and decomposing
organisms. Encourage students to look
for examples of weathering rock and
soil formation in their outdoor activities,
or bring a heavily weathered rock to
class for students to examine. Ask, Why
does soil often contain tiny stone
fragments and quartz crystals (sand)?
(As rocks undergo weathering, more
reactive minerals break down first, and
the more stable minerals, such as quartz,
are left behind.) Why is soil different
in different regions? (Both the bedrock
and organic material that produce soil are
different in different regions.)
Logical
Build Reading Literacy
Identify Main Ideas/Details Refer
to page 702D in this chapter, which
provides the guidelines for identifying
main ideas and details.
Have students read the main topic
sentence (in bold) of Rates of
Weathering. Then, ask, Given this
topic sentence, what are the main
ideas that you should look for in this
selection? (How temperature, availability
of water, and the type of rock affect the
rate of chemical and mechanical weathering)
After reading Rates of Weathering,
have students work in groups to create
index cards with brief explanations of
how these factors affect the rate of
weathering.
Verbal, Group
Answer to . . .
Figure 8 Mechanical weathering
Figure 10 Chemical weathering from
acidic precipitation
Water
L2
L1
Earth’s Surface 711

Section 23.2 (continued)
Mass Movement
Build Science Skills
Relating Cause and Effect Have
students work in groups to create a
poster showing the cause and effect of
each type of mass movement described
on p. 712. Posters should include a
flowchart that describes the conditions
during which the events occur. Portfolio
Use Visuals
Figure 11 Ask, In the top diagram,
what evidence can you see that creep
is occurring? (Displacement of a fence,
road, and electric lines) Visual
3
ASSESS
Evaluate
Understanding
Have students create flowcharts
describing the processes that produced
the formations in Figure 7.
Reteach
Use Figures 8 and 9 to review the
differences between chemical and
mechanical weathering.
Cracked sidewalk: physical change; rusty
nail: chemical change; hard-to-read
marble tombstone: chemical change
If your class subscribes
to the Interactive Textbook, use it to
review key concepts in Section 23.2.
Answer to . . .
Figure 11 The greater the slope of
the hillside, the greater the amount of
creep or slumping is.
Section 23.2 Assessment
L2
L1
L2
L2
1. Erosion is the process that wears away rock
and soil. Agents of erosion include
weathering, the force of gravity, and the
movement of streams, groundwater, glaciers,
wind, and waves.
2. Both mechanical and chemical weathering
are forms of erosion that cause rocks to
disintegrate. Mechanical weathering breaks
rocks into smaller pieces through physical
processes like frost wedging and abrasion.
712 Chapter 23
Figure 11 Creep can be caused
by the alternate freezing and
thawing of water in soil. Slumping
often occurs when soil that is rich
in clay is soaked by water.
Predicting How would the
steepness of the slope of the
hillside affect the amount of
creep or slumping?
Creep
Slumping
Section 23.2 Assessment
Reviewing Concepts
1. What is erosion? List five agents of erosion.
2. How are mechanical weathering and
chemical weathering similar? How do
they differ?
3. What three factors are most significant in
determining the rate at which rocks weather?
4. How does gravity cause erosion?
5. Identify three processes that cause
mechanical weathering.
6. What is the role of rainwater in
chemical weathering?
712 Chapter 23
Chemical weathering occurs through
chemical reactions that dissolve rock or
convert existing minerals into new minerals.
3. Temperature, the availability of water, and
the type of rock involved
4. Gravity causes erosion through mass
movement, which is the movement of rock
and soil down a slope.
5. Examples include frost wedging, abrasion,
plant roots that pry a rock apart, and release
of pressure from erosion of surface material.
Mass Movement
Once weathering loosens particles of rock, the particles, or sediment,
do not stay in the same place. Mass movement is the downward movement
of rock and soil due to gravity. Through the process of mass
movement, gravity moves loose material down a slope. There are several
types of mass movement.
Landslides The rapid movement of large amounts of rock and soil
is a landslide. Landslides often occur after heavy rains or after earthquakes
loosen materials on a steep slope. Landslides that occur on the
sides of mountains can destroy entire towns.
Mudflows Rapid mass movements of soil and other sediment
mixed with water are called mudflows. Mudflows tend to occur in areas
where fine sediment has collected in thick layers. When it rains, water
loosens the sediment and increases its weight. After a heavy rainfall,
the sediment mixed with water washes down the slope.
Creep Creep occurs when soil gradually moves down a slope. Creep
often occurs because of the formation of ice. Each time the ground
freezes, the soil expands outward. When the ground thaws, the soil moves
a small distance downhill. Creep, shown in Figure 11, happens so slowly
that it’s hard to notice. But over time the results of creep become evident.
Slumping Slumping occurs when weak layers of soil or rock suddenly
move downslope as a single unit. Gravity acting on water-saturated soil
and rock causes slumping. Slumping often leaves a curved scar.
Critical Thinking
7. Predicting Would you expect chemical
weathering to occur more rapidly in a desert
or in a rainforest? Explain.
8. Relating Cause and Effect What conditions
can cause landslides and mudflows?
Chemical and Physical Changes Use
what you learned in Chapter 2 to classify
each of the following as a chemical or
physical change: a cracked sidewalk, a rusty
nail, a hard-to-read marble tombstone.
6. Rainwater is acidic and dissolves many
minerals. Rainwater also reacts with some
minerals to change them into different
minerals.
7. In a rainforest. The greater availability of
water in a rainforest would cause higher rates
of chemical weathering.
8. Heavy rains can loosen sediment and
increase its weight, increasing the likelihood
of a landslide or mudflow. An earthquake can
also cause these types of mass movement.