Download 4 Deforming the Earth`s Crust

Name
CHAPTER 4
Class
Date
Plate Tectonics
4 Deforming the Earth’s Crust
SECTION
National Science
Education Standards
BEFORE YOU READ
After you read this section, you should be able to answer
these questions:
ES 1b, 2a
• What happens when rock is placed under stress?
• What are three kinds of faults?
• How do mountains form?
What Is Deformation?
In the left-hand figure below, the girl is bending the
spaghetti slowly and gently. The spaghetti bends, but it
doesn’t break. In the right-hand figure, the girl is bending
the spaghetti quickly and with a lot of force. Some of the
pieces of spaghetti have broken.
STUDY TIP
Learn New Words As you
read, underline words that
you don’t understand. When
you learn what they mean,
write the words and their
definitions in your notebook.
TAKE A LOOK
1. Describe Circle the
picture in which the girl is
putting the most force on the
spaghetti.
How can the same material bend in one situation but
break in another? The answer is that the stress on the
material is different in each case. Stress is the amount of
force per unit area that is placed on an object.
READING CHECK
2. Define What is stress?
DEFORMATION
Like the spaghetti, rocks can bend or break under
stress. When a rock is placed under stress, it deforms, or
changes shape. When a small amount of stress is put on a
rock slowly, the rock can bend. However, if the stress is
very large or is applied quickly, the rock can break.
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Date
Deforming the Earth’s Crust continued
What Happens When Rock Layers Bend?
READING CHECK
3. Explain How do folds
indicate that deformation has
happened?
Folding happens when rock layers bend under stress.
Folding causes rock layers to look bent or buckled. The
bends are called folds.
Most rock layers start out as horizontal layers.
Therefore, when scientists see a fold, they know that
deformation has happened.
TYPES OF FOLDS
Three of the most common types of folds are synclines, anticlines, and monoclines. In a syncline, the
oldest rocks are found on the outside of the fold. Most
synclines are U-shaped. In an anticline, the youngest
rocks are found on the outside of the fold. Most anticlines are -shaped. In a monocline, rock layers are
folded so that both ends of the fold are horizontal. The
figure below shows these kinds of folds.
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In a syncline, the youngest
rocks are on the inside of
the fold.
In an anticline, the oldest
rocks are on the inside of
the fold.
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TAKE A LOOK
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4. Identify Color the
oldest rock layers in the
figure blue. Color the
youngest rock layers red.
In a monocline, both sides of
the fold are horizontal.
What Happens When Rock Layers Break?
When rock is put under so much stress that it can no
longer bend, it may break. The crack that forms when
rocks break and move past each other is called a fault.
The blocks of rock that are on either side of the fault are
called fault ������������������������������
blocks. When fault blocks move suddenly,
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they can cause
earthquakes.
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SECTION 4
Class
Date
Deforming the Earth’s Crust continued
HANGING WALL AND FOOTWALL
When a fault forms at an angle, one fault block is
called the hanging wall and the other is called the
footwall. The figure below shows the difference between
the hanging wall and the footwall.
Fault
Hanging wall
TAKE A LOOK
5. Compare How is the
hanging wall different from
the footwall?
Footwall
The footwall is the fault block that is below the fault. The hanging wall is
the fault block that is above the fault.
Scientists classify faults by how the fault blocks have
moved along the fault. There are three main kinds of
faults: normal faults, reverse faults, and strike-slip faults.
NORMAL FAULTS
In a normal fault, the hanging wall moves down, or
the footwall moves up, or both. Normal faults form when
rock is under tension. Tension is stress that pulls rock
apart. Therefore, normal faults are common along
divergent boundaries, where Earth’s crust stretches.
READING CHECK
6. Explain Why are normal
faults common along
divergent boundaries?
Normal faults
form when rocks
are pulled apart.
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Deforming the Earth’s Crust continued
REVERSE FAULTS
In a reverse fault, the hanging wall moves up, or the
footwall moves down, or both. Reverse faults form when
rock is under compression. Compression is stress that
pushes rock together. Therefore, reverse faults are common at convergent boundaries, where plates collide.
TAKE A LOOK
7. Identify Label the
hanging walls and the
footwalls on the normal
and reverse faults.
Reverse faults form
when rocks are
pushed together.
STRIKE-SLIP FAULTS
In a strike-slip fault, the fault blocks move past each
other horizontally. Strike-slip faults form when rock is
under shear stress. Shear stress is stress that pushes different parts of the rock in different directions. Therefore,
strike-slip faults are common along transform boundaries, where tectonic plates slide past each other.
ea07ci_tec000306aa
2nd
pass
Strike-slip faults form
1/17/6
when rocks slide past
cmurphy
each other horizontally.
TAKE A LOOK
8. Describe How do
strike-slip faults form?
It can be easy to tell the difference between faults in
a diagram. However, faults in real rocks can be harder to
tell apart. The figure on the top of the next page shows an
ea07c1_tec000307a
example of a fault.
You can probably see where the fault is.
1st Pass
How can you figure
out what kind of fault it is? One way is
Mott
to look at the rock
layers around the fault. The dark rock
01-13-06
layer in the hanging wall is lower than the same layer in
the footwall. Therefore, this is a normal fault.
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Deforming the Earth’s Crust continued
Hanging wall
Footwall
TAKE A LOOK
9. Explain How can you tell
that this is a normal fault?
In these rocks, the hanging wall has moved down compared
to the footwall. Therefore, this is a normal fault.
How Do Mountains Form?
As tectonic plates move over Earth’s surface, the edges
of the plates grind against each other. This produces a lot
of stress in Earth’s lithosphere. Over very long periods of
time, the movements of the plates can form mountains.
Mountains can form in three main ways: through folding,
faulting, or volcanism.
FOLDED MOUNTAINS
Critical Thinking
10. Apply Concepts Why
does it take a very long time
for most mountains to form?
Folded mountains form when rock layers are squeezed
together and pushed upward. Folded mountains usually
form at convergent boundaries, where continents collide.
For example, the Appalachian Mountains formed hundreds
of millions of years ago when North America collided with
Europe and Africa.
FAULT-BLOCK MOUNTAINS
Fault-block mountains form when tension makes the
lithosphere break into many normal faults. Along these
faults, pieces of the lithosphere drop down compared with
other pieces. This produces fault-block mountains.
READING CHECK
11. Identify What kind
of stress forms fault-block
mountains?
Fault-block mountains form when tension causes the crust to
break into normal faults.
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Deforming the Earth’s Crust continued
VOLCANIC MOUNTAINS
Say It
Investigate Find out more
about a volcanic mountain
chain, such as the Andes, the
Hawaiian islands, or Japan.
Share what you learn with a
small group.
Volcanic mountains form when melted rock erupts
onto Earth’s surface. Most major volcanic mountains are
found at convergent boundaries.
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TAKE A LOOK
The Andes mountains
are examples of volcanic
mountains. The Andes
have formed where the
Nazca plate is subducting
beneath the South American plate.
12. Identify What kind
of convergent boundary
have the Andes mountains
formed on?
Volcanic mountains can form on land or on the ocean
floor. Volcanoes on the ocean floor can grow so tall that
they rise above the surface of the ocean. These volcanoes
form islands, such as the Hawaiian Islands.
Most of Earth’s active volcanoes are concentrated
around the edge of the Pacific Ocean. This area is known
as the Ring of Fire.
Type of Mountain Description
TAKE A LOOK
Folded
13. Describe Fill in the table
with the features of each
kind of mountain. Include
where the mountains form
and what they are made of.
Fault-block
Volcanic
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How Can Rocks Move Vertically?
���������� There are two types of vertical movements in the crust:
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uplift and subsidence. Uplift happens when parts of Earth’s
�������crust rise to higher elevations. Rocks that are uplifted may
or may not be deformed. Subsidence happens when parts
of the crust sink to lower elevations. Unlike some uplifted
rocks, rocks that subside do not deform.
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Deforming the Earth’s Crust continued
CAUSES OF SUBSIDENCE AND UPLIFT
Temperature changes can cause uplift and subsidence.
Hot rocks are less dense than cold rocks with the same
composition. Therefore, as hot rocks cool, they may sink. If
cold rocks are heated, they may rise. For example, the crust
at mid-ocean ridges is hot. As it moves away from the ridge,
it cools and subsides. Old, cold crust far from a ridge has a
lower elevation than young, hot crust at the ridge.
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TAKE A LOOK
14. Explain Why does ocean
crust far from a mid-ocean
ridge subside?
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Changes in the weight on the crust can also cause
uplift or subsidence. For example, glaciers are huge,
heavy bodies of ice. When they form on the crust, they
can push the crust down and cause subsidence. If the glaciers melt, the weight on the crust decreases. The crust
slowly rises back to its original elevation in a process
called rebound.
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TAKE A LOOK
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15. Identify What force
caused the crust to subside
in the left-hand figure?
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Fault-block mountains are an example of a third way
subsidence can happen. When the crust is under tension,
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rocks are stretched. They can break and form normal
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������������������� faults. The crust can sink along these faults, causing
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subsidence. This kind of subsidence is common in rift
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zones. A rift zone is a set of deep cracks that forms at a
divergent boundary.
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Section 4 Review
NSES
ES 1b, 2a
SECTION VOCABULARY
compression stress that occurs when forces act
to squeeze an object
fault a break in a body of rock along which one
block slides relative to another
folding the bending of rock layers due to stress
subsidence the sinking of regions of the Earth’s
crust to lower elevations
tension stress that occurs when forces act to
stretch an object
uplift the rising of regions of the Earth’s crust to
higher elevations
1. Compare How are folding and faulting similar? How are they different?
2. Describe Fill in the spaces in the table to describe the three main kinds of faults.
Kind of fault
Description
Kind of stress that
produces it
Normal
Hanging wall moves
up; footwall moves
down.
shear stress
3. Explain Why are strike-slip faults common at transform boundaries?
4. Infer Why are fault-block mountains probably uncommon at transform boundaries?
5. Define What is the Ring of Fire?
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Plate Tectonics
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Inside the Restless Earth Answer Key continued
SECTION 4 DEFORMING THE EARTH’S
CRUST
2. Magma rises toward the surface at mid-ocean
ridges. As the tectonic plates move away from
each other, the sea floor spreads apart, and
magma fills the gap. Then the magma solidifies.
3. younger
4. When oceanic lithosphere forms, magnetic minerals in the magma align with Earth’s magnetic
field. They are frozen in place when the magma
cools and hardens. As sea-floor spreading
continues, that part of the lithosphere moves
away from the ridge. If Earth’s magnetic field
reverses, then minerals will align in the opposite direction in the new lithosphere forming
at the ridge. Since lithosphere is produced on
both sides of the ridge, the normal and reversed
magnetic stripes form parallel patterns.
5. The Earth is not getting bigger, new sea
floor is being created, and the oldest sea
floor is only 180 million years old. If those
three things are true, then oceanic crust
must be being destroyed somewhere on
Earth at the same rate that it is being produced.
1. The image on the right should be circled.
2. the amount of force per unit area on an object
3. Most rock layers are horizontal when they
4.
5.
6.
7.
8.
9.
10.
SECTION 3 THE THEORY OF PLATE
TECTONICS
11.
12.
13.
1. using GPS equipment
2. mid-ocean ridges
3. The lithosphere on each plate is very thick.
When the plates collide, the lithosphere is
piled up to form tall mountains.
4. mountains, volcanoes
5. when matter carries heat from place to
place
6. In slab pull, the driving force comes from subducting slabs. In ridge push, the driving force
comes from the formation of new sea floor.
6.
Type of mountain
Description
Folded
formed at convergent
boundaries; made of folded
and crumpled rock
Fault-block
formed where the crust is under
tension; made of faulted rock
Volcanic
formed where volcanoes erupt
above Earth’s surface; made of
igneous rock
14. As it cools, it becomes denser and sinks.
15. the weight of the glacier
Review
1. a theory that describes how tectonic plates
2.
3.
4.
5.
form. They can show folded shapes only if
they have been deformed.
The oldest rock layers should be blue, and
the youngest should be red.
The hanging wall is found above the fault.
A lot of tension is produced at divergent
boundaries.
The wall above the fault plane should be
labeled “hanging wall”; the wall below the
fault plane should be labeled “footwall.”
when fault blocks slide past each other
horizontally
The hanging wall has moved down relative
to the footwall.
Most mountains are the result of plate
movements. Since plates move very slowly,
mountains form very slowly.
tension
continent-ocean
Review
1. Folding and faulting are both responses to
move and change shape as part of Earth’s
outermost layer
convergent, divergent, transform
slab pull, ridge push, convection
a few centimeters per year
Mid-ocean ridges and ocean trenches are
found only at plate boundaries. Earthquakes
and volcanoes are found mainly at plate
boundaries.
Oceanic lithosphere is colder and denser
than continental lithosphere.
stress. During folding, rocks bend under stress.
During faulting, rocks break under stress.
2.
Kind of fault
Description
Kind of stress
that produces it
Normal
Hanging wall
moves down;
footwall moves
up.
tension
Reverse
Hanging wall
moves up;
footwall moves
down.
compression
Strike-slip
Fault blocks
move past
each other
horizontally.
shear stress
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Inside the Restless Earth
F
Inside the Restless Earth Answer Key continued
3. At transform boundaries, tectonic plates move
3. Possible answers: Earthquakes happen when
past each other and produce shear stress.
4. Fault-block mountains form because of tensional stress. There is little tensional stress
at transform boundaries.
5. the edge of the Pacific Ocean, where many
volcanoes are found
pressure builds up on a rock and it breaks;
elastic rebound of rocks causes earthquakes.
4. Body waves travel through the Earth’s interior,
but surface waves travel only on its surface.
5. The strength of an earthquake is directly related
to the amount of pressure that builds up on the
rock before it breaks. Some rocks are stronger
than others, so more pressure builds up before
the rock breaks. When a lot of pressure builds
up, larger earthquakes happen.
6. Almost all earthquakes happen when rock
breaks. The rock in the crust is brittle and
breaks in response to stress. Rock in the
mantle flows in response to stress.
Chapter 5 Earthquakes
SECTION 1 WHAT ARE EARTHQUAKES?
1. A fault is a break in the crust that rocks
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
slide along.
The plate boundaries should be drawn
along the areas with large numbers of
earthquakes.
The rocks are folded.
The movement of the plates causes pressure
to build up. When the pressure is released,
an earthquake occurs.
a place where two tectonic plates are
moving apart
The “fault” labels should be located at the
places where the blocks slide past each
other. Star should be somewhere along one
of the faults.
A lot of pressure builds up before the rock
breaks.
Arrows should be perpendicular to the plate
boundary, pointing toward each other.
Arrows should be parallel to the fault,
pointing in opposite directions; arrows
should show the direction of displacement
of each fault block.
Most earthquakes happen at plate boundaries.
body waves and surface waves
surface waves
P waves move particles back and forth, and
S waves move them side to side.
Possible answer: All of their energy is
released at the surface.
SECTION 2 EARTHQUAKE MEASUREMENT
1. The focus is the place where the rock
2.
3.
4.
5.
6.
7.
breaks, and the epicenter is the point on the
surface directly above the focus.
The epicenter should be marked directly
above the focus, on the surface.
about 15 s
about 8,500 km
There should be a star at the place where all
three circles intersect.
Possible answer: Seismologists look at the
relative sizes of the different waves.
A magnitude 5.0 earthquake is 10 times
stronger than a magnitude 4.0. A magnitude
6.0 is 10 times stronger than a 5.0. So,
10 times 10 makes it 100 times stronger.
Review
1. A seismograph is an instrument that measures
ground movements. A seismogram is a tracing
of ground movements, which is produced by a
seismograph.
2. San Francisco is more likely to be an epicenter
because it is near a plate boundary.
3. A time vs. distance graph can be used to
measure how far away the epicenter is
from different seismograph stations. Those
distances can be used to make circles that
overlap at the epicenter.
4. One seismogram tells you how far away the
epicenter was and how strong the earthquake
was. It doesn’t tell you which direction the
waves came from.
Review
1. Possible answers: An earthquake is a
shaking or movement of the Earth, and a
fault is a crack in the crust that rocks can
slide on; earthquakes happen along faults.
2. Most earthquakes happen at tectonic plate
boundaries.
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Inside the Restless Earth