Download 9.2 Plate Tectonics

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Section 9.2
9.2 Plate Tectonics
1 FOCUS
Section Objectives
9.4
9.5
9.6
Explain the theory of plate
tectonics.
Describe lithospheric plates.
Identify the three types of
plate boundaries.
Key Concepts
◆
What are lithospheric
plates?
◆
What are the three types
of plate boundaries?
Reading Focus
Build Vocabulary
◆
Comparing and Contrasting Copy the
table. After you read, compare the three types
of plate boundaries by completing the table.
Boundary Type
Relative Plate Motion
convergent
a.
?
divergent
b.
?
transform fault
c.
?
According to the plate tectonics theory, the uppermost mantle,
along with the overlying crust, behaves as a strong, rigid layer. This
layer is known as the lithosphere. The outer shell lies over a weaker
region in the mantle known as the asthenosphere. The lithosphere is
divided into segments called plates, which move and continually
change shape and size. Figure 8 on pages 256-257 shows the seven
major plates. The largest is the Pacific plate, covering most of the
Pacific Ocean. Notice that several of the large plates include an entire
continent plus a large area of the seafloor. This is a major departure
from Wegener’s continental drift hypothesis, which proposed that the
continents moved through the ocean floor, not with it. Note also that
none of the plates is defined entirely by the margins of a continent.
The lithospheric plates move relative to each other at a very slow but
continuous rate that averages about 5 centimeters per year—about as
fast as your fingernails grow. This movement is driven by the unequal
distribution of heat within Earth. Hot material found deep in the mantle
moves slowly upward as part of Earth’s internal convection system. At the
same time, cooler, denser slabs of oceanic lithosphere descend into the
mantle, setting Earth’s rigid outer shell into motion. The grinding movements of Earth’s lithospheric plates generate earthquakes, create
volcanoes, and deform large masses of rock into mountains.
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2 INSTRUCT
Earth’s Major Plates
L2
Using Analogies
Crack the shell of a
hard-boiled egg. Ask
students if the egg
reminds them of anything. The egg can
be seen as a tiny model of Earth. The
thin eggshell is analogous to Earth’s
crust, divided into plates. Within the
shell is the firm mantle. Have students
move the pieces of shell around. They
should notice how the shell buckles in
some places and exposes “mantle”
in other places. This movement is
analogous to the movement of Earth’s
crust. However, Earth’s movement
results in the formation of mountains,
earthquakes, and new ocean floor.
Kinesthetic, Visual
254 Chapter 9
◆
Reading Strategy
Earth’s Major Plates
a. plates move together
b. plates move apart
c. plates slide past each other
Build Science Skills
◆
plate tectonics
plate
divergent boundary
convergent
boundary
transform fault
boundary
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Concept Map Have students make
a concept map using the term plate
tectonics as the starting point. All the
vocabulary terms in this section should
be used.
Reading Strategy
Vocabulary
What is the theory of plate
tectonics?
What is plate tectonics?
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Types of Plate Boundaries
Figure 7 Three Types of Plate
Boundaries
All major interactions among individual plates occur along their
boundaries.
The three main types of boundaries are convergent,
divergent, and transform fault boundaries.
A
Build Reading Literacy
Convergent boundaries Convergent boundaries form where
Transform fault boundaries Transform fault boundaries are
margins where two plates grind past each other without the production
or destruction of lithosphere, as shown in Figure 7C. The San Andreas
Fault zone in California is an example of a transform fault boundary.
Each plate contains a combination of these three types of boundaries. Although the total surface area of Earth does not change, plates
may shrink or grow in area. This shrinking or growing depends on the
locations of convergent and divergent boundaries. The Antarctic plate
is growing larger. The Philippine plate is descending into the mantle
along its margins and is becoming smaller. New plate boundaries can
be created because of changes in the forces acting on these rigid slabs.
Visualize Have students read the
section on types of plate boundaries and
then try to form a mental picture of each
type of boundary. Ask: In which type of
boundary do the plates move without
changing the lithosphere? (transform
fault boundary)
Visual, Verbal
Divergent boundary
B
two plates move together. This process results in oceanic lithosphere
plunging beneath an overriding plate, and descending into the mantle,
as shown in Figure 7B. At other locations, plates carrying continental
crust are presently moving toward each other. Eventually, these continents may collide and merge. Thus, the boundary that once separated
two plates disappears as the plates become one.
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Refer to p. 502D in Chapter 18, which
provides the guidelines for this
visualizing strategy.
Divergent boundaries Divergent boundaries (also called
spreading centers) occur when two plates move apart. This process
results in upwelling of material from the mantle to create new seafloor,
as shown in Figure 7A. A relatively new divergent boundary is located
in Africa, in a region known as the East African Rift valley.
Types of Plate
Boundaries
3 ASSESS
Convergent boundary
Evaluate
Understanding
C
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To assess students’ knowledge of section
content, have them draw diagrams of
the three types of plate boundaries.
Each diagram should have a caption
describing the movement of the plates.
Transform fault boundary
L1
Reteach
Have students use any materials they
wish, such as blocks of wood, to
illustrate the plate movements in the
three types of boundaries.
Section 9.2 Assessment
Reviewing Concepts
1.
2.
3.
Define the term lithospheric plate.
List the three types of plate boundaries.
What theory proposes that Earth’s outer
shell consist of a number of rigid slabs?
In Wegener’s hypothesis, the continents
moved through the ocean floor. The
ocean floor did not move and was not
part of the block of continental crust.
The boundaries of the continents were
defined by either the shorelines or
continental shelves. In plate tectonics,
the plates are divided by boundaries
along which different types of motion
and deformation occur. The ocean floors
are part of the plates and move along
with the continents.
5. Drawing Conclusions What is the major
difference in the role of the ocean floor
between the continental drift hypothesis and
the theory of plate tectonics?
Critical Thinking
4. Comparing and Contrasting Compare
the plate motions in the three types of
boundaries.
Plate Boundaries Use what you have
learned about plate tectonics to compare
Wegener’s continental drift hypothesis to
the theory of plate tectonics.
Plate Tectonics
Section 9.2
Assessment
1. a section of the crust and upper mantle
(the lithosphere) that moves as a unit
2. convergent, divergent, and transform fault
boundaries
3. the theory of plate tectonics
4. In convergent boundaries, the two plates
move together. In divergent boundaries, the
two plates move apart. In transform fault
boundaries, the two plates grind past each
other.
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5. In the continental drift hypothesis, the
continents plowed through the ocean floors;
in the plate tectonics theory, the ocean floors
are an integral part of the lithospheric plates
and move with the continents.
Answer to . . .
A theory that states that
Earth’s rigid outer shell is
broken into plates made up of the crust
and upper mantle, also known as the
lithosphere. A plate moves as a unit
with respect to the surrounding plates.
Plate Tectonics 255
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Section 9.2 (continued)
Earth’s Tectonic Plates
North American
plate
Answer
Locate South American plate. Divergent:
Caribbean plate, Mid-Atlantic Ridge;
Southeast Indian Ridge, Antarctic plate.
Convergent: Nazca plate, South American
plate; Australian-Indian plate, Eurasian
plate. Transform fault: Antarctic plate,
Pacific plate; Caribbean plate, North
American plate
Use Visuals
Urals
Aleutian Arc
Eurasian plate
Baikal Rift
Japan Arc
Himalayas
Arabian
plate
L1
M
id
African plate
st
we
uth
So
Pacific
plate
ian Ridge
Figure 8 Have students examine the
figure. They may need help from an
atlas or a globe to locate the features
in the questions. Ask: How do you think
the Andes Mountains were formed?
(The Nazca plate collided with the South
American plate.) How do you think the
Red Sea was formed? (The African plate
and the Arabian plate moved apart,
forming a rift that became the Red Sea.)
Which plate is the largest? Where is
it located? (The Pacific plate; it is mostly
within the Pacific Ocean.)
Visual
d
-In
East
African
Rift
Philippine
plate
India
Em
per
or-H
awa
iian
Cha
in
Mariana Arc
In
an
di
g
Rid
Australian-Indian plate
e
So
uth
e
as
t I
nd
ian
Ridge
Tonga
Arc
Kermadec
Arc
Alpine Fault
Antarctic plate
Figure 8
Location None of the
A Convergent Model
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Purpose Students will observe what
happens when two plates collide in a
model of a convergent boundary.
Materials two slabs of modeling clay,
wax paper
Procedure Place the slabs of clay on
the wax paper on a table so they will
slide easily. Push the two slabs of clay
together to model a collision of two
plates.
Expected Outcome The clay slabs will
buckle up to create folds and breaks that
resemble mountains.
Kinesthetic, Visual
plates are defined entirely
by the margins of a
continent. Over a dozen
smaller plates have been
identified but are not
shown.
Locate Find a major plate
that includes an entire
continent plus a large area
of seafloor. Name two
other examples of a
divergent boundary, a
convergent boundary, and
a transform fault boundary.
Seven Major Plates
Intermediate Plates
North American
Eurasian
Caribbean
South American
Australian-Indian
Nazca
Cocos
Pacific
Antarctic
Philippine
Scotia
African
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Customize for English Language Learners
Students who are learning disabled will benefit
by having globes in the classroom. You might
want to mark the major plates on a globe so
256 Chapter 9
students can better visualize how Earth’s
lithosphere is divided into tectonic plates.
Arabian
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L2
Students may think that the movement
of lithospheric plates causes Earth’s
surface area to become either larger or
smaller (depending on which way the
plates move.) Explain that individual
plates can become larger or smaller, but
Earth’s total surface area cannot change.
Give students a basketball or soccer ball
and ask them to imagine what will
happen to the interior of the ball if the
surface area changed. (If surface area
decreased, the interior would become
compressed and internal pressure would
increase. If surface area increased, either
the interior would expand or the lithosphere would separate from the mantle.)
Visual, Logical
North American
plate
Iceland
Eurasian plate
Canadian Shield
c
Ro
ky
ou
M
Juan de Fuca
plate
ins
nta
Basin
and
Range
lac
pa
Ap
San Andreas
Fault
nM
hia
Alps
ts.
Caribbean
plate
African plate
dMi
Cocos
plate
Pacific plate
tic
R
idge
South American
An
plate
de
s
ountains
Nazca
plate
lan
Antilles
Arc
M
East Pacific Rise
Galapagos
Ridge
At
Use Community
Resources
L2
Invite a geologist or physicist in your
community to talk to the class about
global positioning systems (GPS), how
they work, and what they are used for.
Encourage students to think about and
ask questions about how GPS can be
used to measure the movements of
landmasses.
Verbal, Interpersonal
Chile Ridge
Scotia plate
Antarctic plate
Plate Tectonics
257
Facts and Figures
The continents are still moving, and eventually
they will probably collide to form a single
landmass again. Earth scientists predict that
the continents will probably merge again
somewhere in the Pacific Ocean. When will
this happen? Research suggests that, based on
the current rate of plate movements, a single
landmass is formed about once every 500
million years. Since it has been about 200
million years since Pangaea broke up, the
next supercontinent may form in a few
hundred million years.
Plate Tectonics 257