Name
CHAPTER 14
Class
Date
Weathering and Erosion
2 Rates of Weathering
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• How does rock composition affect weathering rate?
• How does surface area affect weathering rate?
• How do climate and topography affect weathering
rate?
What Affects the Rate of Weathering?
In general, mechanical and chemical weathering
processes happen very slowly. For example, a layer of
limestone that is 150 m thick could take 30 million years
to dissolve through carbonation.
However, not all rocks weather at the same rate. The
rate of weathering depends on many factors, such as
•
•
•
•
rock composition
READING TOOLBOX
Graphic Organizer As you
read, create a spider map.
Label the center oval “Factors
that affect the rate of weathering.” Fill in the map with
the factors explained in this
section.
amount of exposure
climate
topography
READING CHECK
How Does Rock Composition Affect the Rate
of Weathering?
The materials that make up a rock affect the rate of
weathering. Some rocks are softer and less resistant
to weathering. That is, they weather more easily. Other
rocks are harder and more resistant to weathering. They
weather much more slowly.
In many cases, a single rock mass may be made up of
several different kinds of rock. The different rocks may
weather at different rates. In the process of differential
weathering, softer rock wears away and leaves harder
rock behind. For example, quartz is a hard mineral that
resists weathering. For this reason, rock that contains
quartz will not weather as quickly as rock without quartz.
How easily a rock weathers depends mostly on the
material that holds the rock particles together. For
example, the particles in some sandstones are held
together with strong silicate minerals. These sandstones
resist weathering longer than sandstones without silicates.
1. List Name three factors
that affect the rate of weathering.
READING CHECK
2. Identify How does quartz
affect a rock’s resistance to
weathering?
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Name
SECTION 2
Class
Date
Rates of Weathering continued
How Does Exposure Affect the Rate of
Weathering?
READING CHECK
3. Identify What are
two exposure factors
that determine rate of
weathering?
Math Skills
4. Apply Concepts Suppose
the surface area of a cube
is 24 mm2. If you break that
cube into 8 smaller cubes,
what is the total surface
area?
The amount a rock is exposed to weathering agents
helps determine the rate of weathering. Rocks weather
faster when they are more exposed to weathering agents.
Surface area and openings in the rock determine how
much a rock is exposed.
SURFACE AREA
A rock’s surface area is the part of the rock that is
exposed to air, water, and other weathering agents.
Surface area increases as a rock breaks into smaller
pieces.
For example, imagine a rock is a cube with six sides
exposed, as shown below. The areas of the six sides add
up to the total surface area of the cube.
Now imagine the rock breaks into eight smaller cubes.
The amount of material stays the same. However, the
rock’s total surface area doubles. The rock is exposed in
more places, so it will weather faster.
To find the surface
area of a cube, add
up the surface areas
of the six faces, or
sides.
LOOKING CLOSER
5. Explain Suppose the
cubes represent rock. Which
sample of rock would
weather faster? Explain.
If the cube breaks into smaller cubes, more total faces
will be exposed. Total surface area has increased.
FRACTURES AND JOINTS
Recall that joints are areas of weakness in a rock,
and fractures are breaks in a rock. Fractures and joints
increase the surface area of the rock. The increased
surface area speeds up the weathering process.
Fractures and joints also allow water to flow into the
rock. The water may break the rock through ice wedging.
Water also chemically weathers the rock exposed in the
fracture or joint. The chemical weathering removes rock
material and makes the opening in the rock weaker.
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Weathering and Erosion
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SECTION 2
Class
Date
Rates of Weathering continued
How Does Climate Affect the Rate of
Weathering?
Climate also affects how fast rock weathers. Rates of
weathering are highest in climates that have periods of
hot and cold weather. Regular freezing and thawing can
cause ice wedging. Chemical weathering can then act
quickly on the fractured rock. When temperature rises,
the chemical reactions accelerate, or speed up. Rates of
weathering are also high in warm, humid climates. The
moisture in the air easily weathers exposed surfaces.
The lowest rates of weathering occur in hot, dry
climates. The lack of water limits many weathering
processes, such as carbonation and ice wedging.
Weathering is also slow in very cold climates.
How Does Topography Affect the Rate of
Weathering?
Topography is the elevation and shape of the surface
of land. Temperatures at high elevations are generally
low. For this reason, ice wedging is more common at high
elevations than at low elevations.
Weathering may happen more quickly on steep slopes,
such as mountains. Gravity and heavy rain cause rocks to
slide down the mountains. The rocks hit each other and
break apart. As rocks fall away, new surfaces are exposed
to weathering.
Talk About It
Make Connections With a
partner, discuss the climate
you live in. What do you
think the rate of weathering
is like in your climate?
Critical Thinking
6. Infer Would ice wedging
be more common in the
mountains or at sea level?
Explain your answer.
How Do Plants and Animals Affect the Rate
of Weathering?
Plants and animals can affect the rate of weathering.
The table below gives examples of activities that disturb
or break rocks.
Activities that Affect the Rate of Weathering
Humans
Other animals
Plants
Activity
Effect
construction
exposes new rock surfaces
mining
exposes rock to strong acids
recreational activities
exposes new rock surfaces
biological waste
causes chemical weathering
digging
exposes new rock surfaces
growth of roots
breaks apart rock
LOOKING CLOSER
7. Identify Name two
human activities that affect
the rate of weathering.
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Weathering and Erosion
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Section 2 Review
SECTION VOCABULARY
differential weathering the process by which
softer, less weather-resistant rocks wear away
at a faster rate than harder, more weatherresistant rocks
1. Analyze Conclusions A geologist finds a sharp, jagged rock formation. She
concludes that the formation is a result of differential weathering. What does
the geologist mean?
2. Identify In which climates is the rate of weathering the fastest? Why?
3. Describe What are two ways that topography can affect the rate of weathering?
4. Explain How does surface area affect the rate of weathering?
5. Analyze Processes Imagine that you want to protect an important stone
monument from weathering by moving it to an area with a different climate.
What type of climate would you choose? Explain your answer.
6. Apply Concepts Think of some outdoor activities that you do for fun. Describe
how one of these activities might affect the rocks around you.
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Weathering and Erosion