Download 3 The Changing Continents

Name
CHAPTER 10
Class
Date
Plate Tectonics
3 The Changing Continents
SECTION
KEY IDEAS
As you read this section, keep these questions in mind:
• How do the movements of tectonic plates change
Earth’s surface?
• How have the movements of tectonic plates affected
Earth’s climate and life?
• What is the supercontinent cycle?
How Do Continents Change Shape?
Tectonic plates are always moving. The movements of
the plates change Earth’s surface and affect its climate.
One of the ways plate movements affect Earth’s surface
is by changing the shapes of the continents.
The continents have not always looked the way they
look today. In the future, they will look different, too.
There are two main ways continents change: rifting and
accretion.
READING TOOLBOX
Clarify As you read this
section, underline any words
or phrases you do not
understand. Work with a
partner to figure out what
the underlined words and
phrases mean.
RIFTING
At a divergent boundary, tectonic plates move apart.
Today, most divergent boundaries are under the oceans.
However, divergent boundaries can also exist on continents. For example, there is a divergent boundary in
eastern Africa.
At the East African Rift Valley, the continent is breaking into pieces, or rifting. Rifting is the process in which
a continent breaks apart. You can also use the word
rifting to describe what happens at a mid-ocean ridge.
The East African Rift is
getting wider each year.
LOOKING CLOSER
1. Infer Draw arrows on
the picture to show which
directions the plates are
probably moving.
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The Changing Continents continued
ACCRETION
Critical Thinking
2. Infer How do geologists
likely identify a terrane?
Continents can also change shape by growing larger.
A continent can grow larger if a volcano on it erupts.
Continents can also grow by accretion. In the process
of accretion, new lithosphere is added to the edge of a
continent. The new lithosphere that is added is called a
terrane. A terrane is a piece of lithosphere that has a different geologic history from the lithosphere around it.
Scientists have studied the geologic history of the
rocks on continents. They have learned that the centers
of most continents are more than 540 million years old.
These very old, central parts of the continents are called
cratons.
The edges of the continents are made up of different
terranes. The terranes have formed over time at convergent boundaries. As one tectonic plate sinks beneath
another at a convergent boundary, terranes on the sinking
plate are scraped off. They stick to the edge of the continent on the other plate.
Terrane A
Terrane B
Terrane A
Terrane B
LOOKING CLOSER
3. Identify Label the
continent on the diagrams.
Moving plates carry terranes with them. When a plate sinks into the asthenosphere, the terranes on it are scraped off. They become part of the continent.
Many different structures can make up terranes. Some
terranes are volcanoes that have erupted on the ocean
floor. Other terranes are made up of coral or rock that
forms along beaches and on islands. Some terranes are
made up of pieces of continental crust.
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The Changing Continents continued
IDENTIFYING TERRANES
A terrane may change a lot when it is scraped off onto
a continent. Scientists can identify terranes by looking
for three features.
1. A terrane has different rocks and fossils than other
terranes or cratons.
2. Large faults, or cracks in rock, divide a terrane from
other terranes or cratons.
3. The rocks in a terrane generally have different magnetic properties than the rocks in other terranes.
Critical Thinking
4. Infer What do you think
is the reason that there are
large faults in places where
terranes meet?
How Do Continental Movements Affect
Earth?
The movements of the plates do not affect only the
shapes of the continents. As the plates move, the continents also move. Mountains form and are broken down.
Continents move closer to or farther from the equator.
These changes and movements can affect Earth’s climate.
They can also affect life on Earth.
EFFECTS ON CLIMATE
The map below shows how the water in the oceans
moves. You can see that the positions of the continents
affect how the ocean water moves.
AR C T IC OC E AN
ASIA
ASI A
NORTH
NORTH
AMERICA
AMERICA
ASIA
ASIA
EUROPE
EUROPE
Talk About It
AFRICA
AFRICA
AUSTRALIA
AUSTRALIA
P AC I FI C
OC EA N
ANTARCTICA
ANTARCTICA
SOUTH
SOUTH
AMERICA
AMERICA
ATLANTIC
OCEAN
SO U TH ERN OCEA N
INDIAN
OCEAN
ANTARCTICA
ANTARCTICA
Apply Concepts With a
partner, talk about how the
movement of ocean water
would be different if the
continents were in different
places.
The movements of ocean water affect Earth’s climate.
Cool ocean water that flows near a continent can make
the continent cooler. Warm water flowing near a continent can make the continent warmer.
The positions of the continents affect the movement of
ocean water. The movement of ocean water affects climate.
Therefore, as the continents move, climate can change.
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The Changing Continents continued
EFFECTS ON LIFE
READING CHECK
5. Explain How can plate
tectonics affect life?
Talk About It
Discuss Look up the word
cycle in a dictionary. With
a partner, discuss why the
supercontinent cycle is
considered a cycle.
When a continent breaks apart or a mountain forms,
groups of living things can be separated. For example,
one group of mice might be split into two groups. The
two groups will live in different environments. Over
time, they may evolve, or change, into new types of
living things. In this way, plate tectonics can affect life.
How Have the Continents Changed Over
Time?
Alfred Wegener thought that all the continents had
once been joined into a single supercontinent. Scientists
have found evidence that he was correct. However, scientists today think that there may have been more than one
supercontinent. They think the continents have joined
and broken apart many times over Earth’s history.
The process in which the continents come together,
form a supercontinent, and then break apart again is called
the supercontinent cycle. Right now, the continents are
broken apart. Over millions of years, the continents will
move together and form another supercontinent.
PANGAEA
The most recent supercontinent was called Pangaea.
Pangaea formed about 300 million years ago, when
the continents moved together. As the continents collided, mountains formed. The Appalachian mountains in
North America formed at this time. A large ocean called
Panthalassa surrounded Pangaea.
Siberia
Siberia
North America
Antarctica
South
America
Africa
450 million years ago
A
Europe
Europe
P A N G A
E
LOOKING CLOSER
6. Compare How were the
continents 450 million years
ago different from the
continents today?
North
America
PA N T H A LA SSA
South
America
Europe
Africa
Australia
India
IndiaAustralia
Antarctica
225 million to 200 million years ago
About 450 million years ago, the continents were separated. They
moved together and formed the supercontinent Pangaea.
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The Changing Continents continued
THE BREAKUP OF PANGAEA
About 200 million years ago, Pangaea began to break
apart. At first, a large rift split Pangaea into two pieces.
Scientists call the pieces Laurasia and Gondwanaland.
Over time, Laurasia and Gondwanaland also began to
break apart. Laurasia broke into pieces that became
North America, Europe, and Asia. Gondwanaland broke
into pieces that became South America, Africa, India,
Australia, and Antarctica.
About 50 million years ago, India collided with Asia.
The Himalaya Mountains began to form. India and Asia
are still moving together today. Thus, the Himalaya
Mountains are still growing.
North
America
7. Identify Which continents
did Laurasia form?
Asia
A S
I A
R
U
L A
READING CHECK
Europe
GO
Africa
ND
South
America
W
India
A
N
A
LA
Australia
N
Antarctica
D
160 million to 140 million years ago
North
America
Europe
LOOKING CLOSER
Asia
South
America
Africa
India
Australia
Antarctica
8. Illustrate Look closely
at the map showing the
continents 160 million to
140 million years ago. Draw
lines to show where the
continents broke apart to
form the Atlantic Ocean.
70 million to 50 million years ago
By about 160 million years ago, Pangaea had broken into two large pieces. Over
time, those pieces broke apart to form the modern continents. The dark lines in the
second map show where the continents are today.
SUPERCONTINENT OF THE FUTURE
Scientists can use information about how the plates
are moving to predict what Earth will look like in the
future. They think that in about 250 million years, the
continents will form a new supercontinent. Africa will
collide with Europe and Asia. North America and South
America will collide with Africa. Australia and Antarctica
will come together.
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Section 3 Review
SECTION VOCABULARY
Pangaea the supercontinent that formed
300 million years ago and that began to break
up 200 million years ago
Panthalassa the single, large ocean that
covered Earth’s surface during the time the
supercontinent Pangaea existed
rifting the process by which Earth’s crust breaks
apart; can occur within continental crust or
oceanic crust
supercontinent cycle the process by which
supercontinents form and break apart over
millions of years
terrane a piece of lithosphere that has a unique
geologic history and that may be part of a
larger piece of lithosphere, such as a continent
1. Compare Describe the difference between Pangaea and Panthalassa.
2. Describe The continents that existed before Pangaea formed were much smaller
than the continents today. Describe how the continents have grown larger.
3. Explain How can the movements of the continents affect Earth’s climate?
4. Infer The oldest rocks on Earth are found on the continents. Are these rocks
most likely found in the centers of continents or at their edges? Explain your
answer.
5. Describe Relationships Describe how convergent and divergent boundaries are
related to the supercontinent cycle.
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