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KEY CONCEPT
Movement of rock
builds mountains.
BEFORE, you learned
NOW, you will learn
• Major geologic events occur at
tectonic plate boundaries
• Most faults are located along
plate boundaries
• How the folding of rock can
form mountains
• How movement along faults
can form mountains
VOCABULARY
EXPLORE Folding
folded mountain p. 80
fault-block mountain
p. 82
How does rock fold?
PROCEDURE
1
MATERIALS
Make three flat layers of clay on top
of a sheet of newspaper. Put a block at
either end of the clay.
2 Hold one block still. Push on the other block
• 2 or 3 colors of
modeling clay
• ruler
• 2 blocks
• newspaper
to slowly bring the blocks closer together.
WHAT DO YOU THINK?
• What happened to the clay when you
pushed on the block?
• What shape did the middle layer
of clay form?
• If a large block of rock reacted to
pressure in a similar way, what kind
of landform would result?
Most mountains form along
plate boundaries.
A shallow sea once covered the area that is now Mount Everest, Earth’s
tallest mountain. If you were to climb Mount Everest, you would be
standing on rocks containing the remains of ocean animals. Mount
Everest also contains rocks that formed far away at a spreading center
on the sea floor. How can rocks from the sea floor be on top of a
mountain on a continent? Plate tectonics provides the answer.
Recall that an oceanic plate sinks when it collides with a continental
plate. Some sea-floor material scrapes off the sinking plate and onto
the continent. As continental mountains form, material once at the
bottom of an ocean can be pushed many kilometers high.
Chapter 3: Mountains and Volcanoes 77
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Mountain Ranges and Belts
A mountain is an area of land that rises steeply from the land around
it. A single mountain is rare. Most mountains belong to ranges—long
lines of mountains that were formed at about the same time and by
the same processes. Ranges that are close together make up mountain
belts. For example, the Rocky Mountain belt in western North America
contains about 100 ranges.
Mount
Cumulus
Mount
Cumulus
Rocky
Mountain
Belt
Never Summer
Mountain Range
1
2
3
Mountains rise high
above the land around
them.
Most mountains are in
groups called mountain
ranges.
Closely spaced mountain
ranges make up mountain belts.
Most of the world’s major mountain belts are located along
tectonic plate boundaries. But mountain belts like the Appalachians
(AP-uh-LAY-chee-uhnz) in eastern North America are in the interior
of plates. Mountains such as these were formed by ancient plate
collisions that assembled the present-day continents.
Major Mountain Belts
Major mountain belts mark the locations of present
or past plate boundaries.
Caledonian
Belt
RO
CK
Y
North
American
Cordillera
Tasman
Belt
T
BEL
AIN
NT
OU
PACIFIC
OCEAN
Urals
M
Alps
ATLANTIC OCEAN
Appalachians
INDIAN
OCEAN
Andes
ATLANTIC OCEAN
PACIFIC
OCEAN
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78 Unit: The Changing Earth
Himalayas
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Mountains, Rocks, and Sediment
At the same time that some processes push mountains up, other
processes wear them down. At Earth’s surface, water and wind break
rocks apart and move the pieces away. As long as mountains are
pushed up faster than they wear down, they grow taller. For this
reason, young mountains tend to be tall and steep. But eventually
mountain-building processes slow, then end. Water and wind take
over. Given enough time, all mountains become rounded hills, and
then they are gone. Countless mountains have formed and worn away
throughout Earth’s long history.
Rocks break down into loose pieces that can be carried by water
or wind. These pieces are called sediments. For example, sand on a
beach is sediment. Thick layers of sediments can build up in low-lying
areas, such as valleys, lakes, or the ocean. Pieces of sediments form
sedimentary rock as they are pressed together or joined by natural
cement.
The land becomes flatter as mountains wear down and valleys fill
with sediments. If tectonic plates were to stop moving, eventually the
surfaces of all the continents would be completely flat.
Mountains Wear Down
Mountains wear down as water and wind break their
rocks into sediments and carry them away.
Young Mountains
Most young mountains are
rugged. But even as they form,
their rocks are being broken
apart.
Old Mountains
How do mountains wear away?
Most old mountains are rounded.
Lower areas around them contain thick layers of sediments.
Chapter 3: Mountains and Volcanoes 79
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Mountains can form as rocks fold.
Though people usually do not think of rocks as being able to
bend and fold, they can. Think of a wax candle. If you bend
a candle quickly, it will break. If you leave a candle propped
up at an angle, over many days it will bend. If the candle is
in a warm area, it will bend more quickly. Rocks also bend
when stress is applied slowly. Rocks deep in the crust are at
high temperatures and pressures. They are particularly likely
to bend rather than break.
check your reading
VOCABULARY
Make a word triangle
for folded mountain
in your notebook.
Under what conditions are rocks likely to bend and fold?
Remember that tectonic plates move only a few centimeters
each year. The edge of a continent along a convergent boundary
is subjected to stress for a very long time as another plate pushes
against it. Some of the continent’s rocks break, and others fold.
As folding continues, mountains are pushed up. A folded mountain
is a mountain that forms as continental crust crumples and bends
into folds.
Folded mountains form as an oceanic plate sinks under the edge
of a continent or as continents collide. One example is the Himalaya
(HIHM-uh-LAY-uh) belt, which formed by a collision between India
and Eurasia. Its formation is illustrated on page 81.
reading tip
At one time an ocean separated
India and Eurasia. As India moved northward, oceanic lithosphere
sank in a newly formed subduction zone along the Eurasian Plate.
Along the edge of Eurasia, folded mountains formed. Volcanoes
also formed as magma rose from the subduction zone to the surface.
1
Convergent Boundary Develops
2
Continental Collision Begins
3
Collision Continues
Eurasia is the landmass
consisting of Europe
and Asia.
Eventually the sea floor was
completely destroyed, and India and Eurasia collided. Subduction
ended. The volcanoes stopped erupting because they were no longer
supplied with magma. Sea-floor material that had been added to
the edge of Eurasia became part of the mountains pushed up by
the collision.
India and Eurasia continue to push together.
Their collision has formed the Himalayas, the world’s tallest
mountains. They grow even higher as rock is folded and pushed up
for hundreds of kilometers on either side of the collision boundary.
Earthquakes can also be important to the upward growth of folded
mountains. A great deal of rock in the Himalaya belt has been pushed
up along reverse faults, which are common at convergent boundaries.
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80 Unit: The Changing Earth
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Formation of Himalayas
The Himalayas are being pushed higher by an ongoing
continental collision.
1
H
Convergent Boundary Develops
As India began moving toward Eurasia 200 million years ago, a
convergent boundary developed along the edge of Eurasia. The
oceanic lithosphere between the two continents sank into a
subduction zone.
India
Eurasia
im
ala
yas
Folded mountains formed as
oceanic and
continental
plates pushed
together.
Volcanoes formed as magma
rose from the subduction
zone to the surface.
2
Continental Collision Begins
The sea floor was completely destroyed about 50 million years
ago, and India and Eurasia collided.
Crust along the
edges of both
continents was
crumpled and
folded into
mountains.
Subduction stopped after
the continents collided. No
more magma formed.
3
The Collision Continues
Currently, the Himalayas are growing more than one centimeter
higher each year.
Himalayas
As the collision
continues, the
crust keeps
folding. Also,
earthquakes
are common.
A remnant of sea floor crust
remains deep under the
mountains.
In each illustration, where is the boundary between India and Eurasia?
Chapter 3: Mountains and Volcanoes 81
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Page 6 of 8
Mountains can form as rocks move
along faults.
In the southwestern United States and northwestern Mexico, hundreds
of mountain ranges line up in rows. The ranges, as well as the valleys
between them, formed along nearly parallel normal faults. Mountains
that form as blocks of rock move up or down along normal faults are
called fault-block mountains.
CONTENT FRAME
Add information about
fault-block mountains to
your content frame.
check your reading
How can the movement of rocks along faults lead to the
formation of mountains?
Fault-block mountains form as the lithosphere is stretched and
pulled apart by forces within Earth. The rocks of the crust are cool
and rigid. As the lithosphere begins to stretch, the crust breaks into
large blocks. As stretching continues, the blocks of rock move along
the faults that separate them. The illustrations on page 83 show how
this process forms fault-block mountains.
Fault-Block Mountains
How do fault-block mountains form?
Fault-block mountains form along normal faults as blocks of continental
crust are pulled apart. In this activity, you will use wooden blocks to demonstrate
the processes that form fault-block mountains.
PROCEDURE
1
Use the triangular blocks to demonstrate how
movements along normal faults form two
mountains separated by a valley. Start
with the blocks arranged as shown.
Move the outer blocks apart to form
two mountains separated by a valley. Draw a diagram of your results.
2 Use the rectangular blocks to demonstrate how a row of tilted
fault-block mountains forms along normal faults. (Hint: You can
tilt the blocks as they move.) Draw a diagram of your results.
WHAT DO YOU THINK?
• How do your diagrams show that fault-block mountains form as
the crust is being stretched?
• Along which type of plate boundary would fault-block mountains
be most likely to form—divergent, convergent,
or transform? Explain.
CHALLENGE Why do fault-block mountains not form at strike-slip faults?
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82 Unit: The Changing Earth
SKILL FOCUS
Modeling
MATERIALS
• 3 triangular
blocks
• 3 rectangular
blocks
TIME
15 minutes
Page 7 of 8
1
An area of the lithosphere can arch upward when, for example, it
is heated by material rising in the mantle beneath it. As the crust
stretches, it breaks into many blocks separated by faults.
2
As the lithosphere is pulled apart, some blocks tilt. The edges of
the blocks that tilt upward form mountains, and the edges that tilt
downward form valleys. Other blocks drop down between faults,
forming valleys. The edges of the blocks next to blocks that drop
down are left standing high above the valleys as mountains.
Fault-block mountains form as stress repeatedly builds up in the
crust and then is released during earthquakes. Even the most powerful
earthquakes can move blocks of rock only a few meters up or down at
one time. Fault-block mountains can be kilometers high. Millions of
years and countless earthquakes are needed for them to form.
check your reading
Describe two ways that blocks of rock can move along faults and
form mountains.
Fault-Block Mountains
Fault-block mountains form as the crust stretches and breaks into
blocks that move along faults.
1
Stretching Begins
The crust breaks into blocks as it is stretched.
2
Blocks Tilt or Drop Down
As the crust is stretched more, the blocks
move along the normal faults between them.
This block has
dropped down.
This block
has tilted.
Chapter 3: Mountains and Volcanoes 83
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Page 8 of 8
The Sierra Nevada
moved up along one
side of the fault.
Approximate location of fault
The land on the
other side of the
fault dropped down.
The Sierra Nevada in California is a fault-block mountain range.
The range moved up along a normal fault along its eastern edge. The
block on the other side of the fault dropped down. This combination
of upward and downward movement formed the steep eastern side of
the Sierra Nevada. The western side of the range tilts down gently
toward California’s Central Valley.
In summary, both folded mountains and fault-block mountains
form over millions of years. Folded mountains are pushed up by slow,
continual stress that causes rock to gradually bend. Fault-block mountains form, earthquake by earthquake, as stress built up in the crust is
released by the movement of rock. Folded mountains form where
continental crust is being compressed, and fault-block mountains
form where it is being stretched.
CRITICAL THINKING
1. How is the formation of
mountain belts related to
tectonic plate boundaries?
4. Analyze The Ural Mountain
belt is no longer along the edge
of a tectonic plate. Would you
expect the Urals to be tall and
steep or low and rounded?
Why?
2. How do folded mountains
form?
3. How do fault-block mountains
form?
5. Synthesize How could it be
possible for a mountain range
to be continually pushed up
but not get any higher?
CHALLENGE
6. Analyze This graph shows
how the heights of two mountains changed as they formed.
Which line shows the formation of a folded mountain? a
fault-block mountain? Explain.
Height
KEY CONCEPTS
Time
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84 Unit: The Changing Earth