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CHAPTER 11
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Deformation of the Crust
1 How Rock Deforms
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• What is isostasy?
• What are the three main types of stress?
• How are folds and faults alike and different?
What Is Isostasy?
Mountain ranges are proof that the shape of Earth’s
surface is constantly changing. These changes are caused
by deformation. Deformation is the bending and breaking of Earth’s crust.
Deformation can happen when the weight of some
part of Earth’s crust changes. If the lithosphere becomes
thicker and heavier, it sinks down into the asthenosphere.
If the lithosphere becomes thinner and lighter, it rises in
the asthenosphere.
The vertical movement of the lithosphere depends on
two opposing forces. One is the force of gravity, which
pulls the lithosphere down. The other is the buoyant
force of the asthenosphere, which pushes up on the lithosphere. When these two forces are balanced, the lithosphere and asthenosphere are in a state of isostasy.
When the weight of the lithosphere changes, the
lithosphere sinks or rises until the forces balance again.
The movement of the lithosphere puts forces on the rock
in it. These forces can cause deformation.
A
Gravitational force
B
Original
elevation
READING TOOLBOX
Summarize As you read this
section, underline sentences
that relate to the Key Idea
questions. When you finish
reading, write a short answer
to each Key Idea question
using the underlined
information.
READING CHECK
1. Explain What are the two
forces that must balance out
to maintain a state of
isostasy?
C
New elevation
Final elevation
Continental
lithosphere
LOOKING CLOSER
2. Explain Why are the
mountains shrinking from
image to image?
Buoyant force
As erosion wears away the crust, the
lithosphere becomes lighter and rises.
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TYPES OF ISOSTATIC ADJUSTMENTS
READING CHECK
3. Describe What happens
in the process of uplift?
READING CHECK
4. Explain Why does the
ocean floor rise when large
glaciers form?
An isostatic adjustment happens when Earth’s
lithosphere rises or sinks to maintain isostasy. Isostatic
adjustments are happening all the time in Earth’s crust.
For example, a mountain goes through isostatic adjustments as it erodes. Over millions of years, wind, water,
and ice wear away the rock. The mountain becomes
shorter and lighter. As the mountain shrinks, it rises in a
process called uplift.
The opposite of uplift is subsidence. During subsidence, the lithosphere becomes heavier. It sinks into the
asthenosphere. Subsidence is common in places where
large rivers flow into oceans. Large rivers generally carry
large amounts of sediment, including mud, sand, and
gravel. When the river flows into the ocean, the sediment
drops onto the ocean floor. The extra weight of the sediment makes the ocean floor sink.
Isostatic adjustments can also happen when glaciers
grow or shrink. A glacier is a huge river of ice. Glaciers
can hold huge amounts of water. Therefore, they are
very heavy. The weight of the ice makes the lithosphere
beneath the glacier sink. At the same time, the ocean
floor rises because the weight of the ocean water is less.
The water has moved onto land and has been frozen in
the glacier.
When the glacier melts, the water returns to the ocean.
The extra weight of the water causes the ocean floor to
sink. At the same time, the land that was covered with ice
rises because the weight of the crust has decreased.
Isostatic Adjustments
Cause
Effect
Wind, water, and ice
carry away rock from
mountains.
The mountains become lighter, and uplift occurs.
Rivers deposit
The ocean floor becomes heavier, and subsidence
sediment on the ocean occurs.
floor.
Glaciers form as ice
and snow build up.
The weight of the glacier causes subsidence under
the glacier. The decreased weight of ocean water
causes uplift in the oceans.
Glaciers melt as the
climate gets warmer.
The decreased weight of the glacier causes uplift on
land. The increased weight of ocean water causes
subsidence in the oceans.
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How Rock Deforms continued
What Kinds of Stress Can Act on Rocks?
As Earth’s lithosphere moves, the rock in the crust is
squeezed, stretched, and twisted. These actions put force
on the rock. Stress is the amount of force applied to a
given area of rock. Stress occurs when the lithosphere
sinks and is squeezed by the weight of rock above it.
It also occurs when the rock in the crust rises and is
stretched out. The movement of tectonic plates past one
another can also produce stress.
There are three main types of stress: compression,
tension, and shear stress. Compression is a type of stress
that squeezes rock. Compression can change the shape of
a rock or reduce the amount of space the rock takes up.
Compression is a common kind of stress in places where
tectonic plates are moving together.
Tension is the opposite of compression. Tension is
stress that stretches rock. Tension can make rock longer
and thinner. Tension is a common kind of stress in places
where tectonic plates are moving apart.
Shear stress deforms rock by pushing its different
parts in opposite directions. Sheared rock can bend,
twist, or break apart as it moves past other rocks.
Shear stress is a common kind of stress in places
where tectonic plates slide past each other.
Critical Thinking
5. Apply Concepts Which
type of stress is probably
most common in places
where new mountains are
forming?
READING CHECK
6. Identify Which kind of
stress causes rock to twist?
The pictures on the left
show how different
kinds of stress can affect
rock layers. The pictures
on the right show how
the kinds of stress can
affect a square block.
The dotted lines show
the original shape of
each block.
LOOKING CLOSER
7. Identify Label the pictures
to indicate which pictures
show compression, which
show tension, and which
show shear stress.
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THE EFFECT OF STRESS ON ROCKS
READING CHECK
8. Identify Which type of
strain can cause a rock to
stretch?
Stress can cause deformation. Deformation that is
caused by stress is called strain. Changes in shape and
size are examples of strain.
Strain is not always permanent. If the stress is applied
slowly, the rock might go back to its original shape when
the stress is removed. However, if too much stress is put
on the rock, the strain may become permanent.
There are two main types of permanent strain: brittle
and ductile. Rocks that break or fracture under stress
are brittle. Cracks and breaks in rock are types of brittle
strain. In contrast, ductile rocks respond to stress by
bending without breaking. Folds and bends in rock are
types of ductile strain.
The type of strain a rock shows depends on several
factors. These factors include temperature, pressure,
the composition of the rock, and how fast the stress is
applied.
Near Earth’s surface, where temperature and pressure
are low, brittle strain is most common. Brittle strain is
also more common when a lot of stress is applied quickly.
At higher temperature and pressure, ductile strain is
more common. Small amounts of stress applied over long
periods of time can also cause ductile strain.
LOOKING CLOSER
9. Describe Name two sets
of conditions that might have
produced the strain in the
picture.
Cracks are examples of
brittle strain.
What Features Can Strain Produce?
Different kinds of strain produce different features in
rock. Two main kinds of strain are folds and faults. A fold
occurs when a rock responds to stress in a ductile way.
A fault occurs when a rock responds to stress in a brittle
way.
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CHARACTERISTICS OF FOLDS
A fold is a bend in a rock layer. Most folds form when
rocks are compressed. As compression acts on the rock,
the rock layers wrinkle and fold over themselves. Some
folds also form because of shear stress.
Folds have different parts. They have sloping sides
called limbs. The limbs meet at the bend, or hinge, of the
rock layers.
Folds can be symmetrical or asymmetrical. If a fold
can be sliced in two identical halves, it is symmetrical.
The dividing line is called the axial plane. However, most
folds are not symmetrical.
Axial plane
LOOKING CLOSER
10. Describe Is this fold
symmetrical or asymmetrical?
How do you know?
Hinge
Limb
Folds can have many different shapes. Many folds are
bent vertically. However, some are overturned and seem
to be lying on their sides. Folds can be open or very tight.
The limbs can be even, or one can be steeper than the
other. The hinge can be a smooth bend or a sharp point.
Each fold is unique because it formed under a unique
combination of conditions.
Folds can vary in size as well as shape. Some are
smaller than your hand. Others cover thousands of square
kilometers. Some folds are so large that they form ridges
and valleys.
TYPES OF FOLDS
Geologists classify folds based on their characteristics.
There are three main kinds of folds: anticlines, synclines,
and monoclines. In an anticline, the oldest rocks are in
the middle of the fold. In a syncline, the oldest rocks are
on the outside of the fold. In a monocline, the two limbs
are horizontal or almost horizontal. The pictures on the
next page show examples of these kinds of folds.
READING CHECK
11. Identify What are the
three main kinds of folds?
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Anticline
The oldest rocks are on the
inside of an anticline. Most
anticlines are arch shaped.
Syncline
LOOKING CLOSER
12. Analyze Processes Why
would a syncline be more
likely to have a bowl shape?
The oldest rocks are on the
outside of a syncline. Most
synclines are bowl shaped.
Monocline
Monoclines form when one
part of the crust is lifted
relative to another part.
CHARACTERISTICS OF FAULTS
READING CHECK
Faults form when rock deforms in a brittle way. A
fault is a break in the body of a rock along which the
surrounding rock moves. The fault plane is the plane
along which the rock moves. If the fault plane is not vertical, the rock above the fault plane is known as the
hanging wall. The rock below the fault plane is called
the footwall.
13. Define What is the
hanging wall?
Footwall
Hanging wall
Fault plane
Like folds, faults can vary greatly in size. Small faults
may affect only a few layers of rock found in a small
region. Other faults are thousands of kilometers long.
These large faults are often composed of smaller, connected faults, rather than a single fault.
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TYPES OF FAULTS
There are two main types of faults: normal faults and
reverse faults. In a normal fault, the hanging wall moves
down relative to the footwall. In a reverse fault, the
hanging wall moves up relative to the footwall. A thrust
fault is a type of reverse fault in which the fault plane is
almost horizontal.
Footwall
Hanging wall
Most normal faults form
when rock is under tension.
Normal fault
Most reverse faults form
when rock is under
compression.
Reverse fault
LOOKING CLOSER
A thrust fault is a type of
reverse fault.
Thrust fault
In some faults, the rock does not move up and down.
Instead, it moves horizontally. The two pieces of rock
slide past one another. This type of fault is called a
strike-slip fault. The rock in a strike-slip fault moves parallel to the direction of the fault’s length. The fault plane
in a strike-slip fault may be vertical or tilted.
Strike-slip faults are most commonly found at transform boundaries, where tectonic plates grind past each
other. They may also occur at fracture zones between
segments of mid-ocean ridges.
14. Describe Label the
hanging wall and the
footwall in the pictures of the
reverse fault and the thrust
fault.
LOOKING CLOSER
15. Compare How is a
strike-slip fault different from
a normal fault?
Strike-slip faults are common in areas that have
shear stress acting on them.
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Section 1 Review
SECTION VOCABULARY
deformation the bending, tilting, and breaking
of Earth’s crust; the change in the shape of
rock in response to stress
fault a break in a body of rock along which
one block slides relative to another; a form of
brittle strain
fold a form of ductile strain in which rock layers
bend, usually as a result of compression
isostasy a condition of gravitational and buoyant
equilibrium between Earth’s lithosphere and
asthenosphere
stress the amount of force per unit area that acts
on a rock
strain any change in a rock’s shape or volume
caused by stress
1. Compare Complete the Venn diagram below to compare folds and faults.
Folds
Faults
2. Apply Concepts What would you expect a rock that has experienced tension to
look like?
3. Infer About 20,000 years ago, many areas in northern North America were cov-
ered with glaciers. Since then, the glaciers have melted. The lithosphere in many
of these areas is now experiencing uplift. Why is this?
4. Describe How can a geologist tell if a fold is a syncline or an anticline?
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