Table of Contents
Chapter 7: Plate Tectonics
Section 3: Theory of Plate Tectonics
Theory of Plate Tectonics
A. Plate Tectonics
1. In the 1960s, scientistis developed a new
theory that combined continental drift and
sea floor spreading.
2. According to the theory of plate tectonics,
Earth’s crust and part of the upper mantle are
broken into plates, or sections, that move
around on a plasticlike layer of the mantle.
Theory of Plate Tectonics
B. Composition of Earth’s Plates
1. Plates are made of the crust and a part of the
upper mantle.
2. These two
parts, the crust
and the rigid
upper mantle
form the
lithosphere
(LIH thuh
sfihr).
Theory of Plate Tectonics
B. Composition of Earth’s Plates
3. The plasticlike layer below the lithosphere
is called the asthenosphere (as THE nuh
sfihr).
4. The rigid
plates of the
lithosphere
float and move
around on the
asthenosphere.
Theory of Plate Tectonics
C. Plate Boundaries
1. When plates move, they can interact in
several ways. They can move toward
each other and converge, or collide.
2. They also can pull apart or slide
alongside one another. When the plates
interact, the result of their movement is
seen at the plate boundaries.
Theory of Plate Tectonics
C. Plate Boundaries
3. Movement along any plate boundary means
that changes must happen at other boundaries.
4. What is
happening to
the Atlantic
Ocean floor
between the
North
American and
African Plates?
Theory of Plate Tectonics
D. Plates Moving Apart
1. The boundary between two plates that are
moving apart is called a divergent boundary.
2. In the Atlantic
Ocean, the
North American
Plate is moving
away from the
Eurasian and
the African
Plates.
Theory of Plate Tectonics
D. Plates Moving Apart
3. That divergent boundary is called the MidAtlantic Ridge. It is a mid-ocean ridge.
Theory of Plate Tectonics
D. Plates Moving Apart
4. As the plates pull apart, magma pushes up
and becomes new seafloor. At some divergent
plate boundaries, rift valleys form as the
plates pull apart and crust sinks.
Theory of Plate Tectonics
E. Plates Moving Together
1. When tow plates converge, or come
together, they form a convergent boundary.
2. What happens to the plates when they
come together? For example, oceanic
plates are denser than continental plates.
When an oceanic plate converges with a
less dense continental plate, the denser
oceanic plate sinks under the continental
plate.
Theory of Plate Tectonics
E. Plates Moving Together
3.The area where an
oceanic plate
subducts, or goes
down, into the
mantle is called a
subduction zone.
4. Some volcanoes
form above
subduction zones.
Theory of Plate Tectonics
E. Plates Moving Together
5. This type of convergent boundary creates a
deep-sea trench where one plate bends and
sinks beneath the other.
6. High temperatures cause rock to melt
around the subducting slab as it goes under
the other plate.
7. The newly formed magma is forced
upward along these plate boundaries,
forming volcanoes.
Theory of Plate Tectonics
E. Plates Moving Together
8. When two oceanic plates converge, the colder,
older, denser plate bens and sinks down into
the mantle. A subduction zone also can form
where two oceanic plates collide.
9. Volcanoes can form and, over time, some
volcanoes form islands. The Mariana Island in
the western Pacific Ocean are a chain of
volcanic islands that formed where two
oceanic plates collide.
Theory of Plate Tectonics
F. Where Plates Collide
1. When two continental plates collide or
converge, neither of he plates sinks under the
other. Subduction usually does not occur.
2. The continental plates are less dense than the
asthenoshphere below them. As a result,
when these two plates collide, they fold and
crumple to form mountain range.
Theory of Plate Tectonics
F. Where Plates Collide
3. Earthquakes are common at these convergent
boundaries.
Theory of Plate Tectonics
G. Where Plates Slide Past Each Other
1. The third type of plate boundary is called
a transform boundary.
2. Transform boundaries occur where two
plates slide past one another.
3. In one type of transform boundary, two
plates slide past each other in opposite
directions.
Theory of Plate Tectonics
G. Where Plates Slide Past Each Other
4. In another type,
two plates are
moving in the
same direction,
but at different
rates. When one
plate slips past
another suddenly,
earthquakes
occur.
Theory of Plate Tectonics
G. Where Plates Slide Past Each Other
5.The San Andreas Fault is part of a transform
plate boundary. It has been the site of many
earthquakes.
Theory of Plate Tectonics
H. Causes of Plate Tectonics—
Convection Inside Earth
1. The cycle of heating, rising, cooling, and
sinking is called a convection current.
2. A version of this same process, occurring
in the mantle, is thought to be the force
behind plate tectonics.
3. Scientists suggest that differences in density
cause hot, plasticlike rock to be forced
upward toward the surface.
Theory of Plate Tectonics
I. Moving Mantle Material
1. In one
hypothesis,
convection
currents
occur
throughout
the mantle.
2. Such convection currents (see arrows) are the
driving force of plate tectonics.
Theory of Plate Tectonics
J. Features Caused by
Plate Tectonics
1. Earth is an active planet with a hot interior.
The heat inside Earth causes convention that
powers the movement of Earth’s plates.
2. The interaction of plates produces forces
that build mountains, create ocean basins,
and cause volcanoes.
Theory of Plate Tectonics
J. Features Caused by
Plate Tectonics
3. When rocks in Earth’s crust break and
move, energy is released in the form of
seismic waves.
4. Humans feel this release as earthquakes.
Theory of Plate Tectonics
K. Normal Faults and Rift Valleys
1. When rocks break and move along surfaces,
a fault forms.
2. Faults interrupt rock layers by moving
them out of place.
3. Entire mountain
ranges can form
in the process,
called faultblock mountains.
Theory of Plate Tectonics
K. Normal Faults and Rift Valleys
4. Rift valleys and mid-ocean ridges can form
where Earth’s crust separates.
5. Examples of rift
valleys are the
Great Rift Valley
in Africa, and the
valleys that occur
in the middle of
mid-ocean ridges.
Click image to view movie.
Theory of Plate Tectonics
L. Mountains and Volcanoes
1. As continental
plates collide, the
forces that are
generated cause
massive folding
and faulting of
rock layers into
mountain ranges
such as the
Himalaya.
Theory of Plate Tectonics
L. Mountains and Volcanoes
2. Usually compression forces cause a reverse
fault.
3. In a reverse fault, rock layers above the
fault surface move up when compared with
the rock layers below the fault. This is the
opposite of a normal fault.
Theory of Plate Tectonics
L. Mountains and Volcanoes
4. If an oceanic plate converges with a
continental plate, the denser oceanic
plate slides under the continental plate.
5. Mountains and volcanoes can form as a
result of the folding and faulting that
occurs at the plate boundaries.
Theory of Plate Tectonics
M. Strike-Slip Faults
1. Strike-slip faults occur where two plates stick,
or strike, and then slip by one another. It
occurs at transform boundaries.
2. A transform
boundary is
where two
plates slide past
one another.
Theory of Plate Tectonics
M. Strike-Slip Faults
3. The plates can slide by in opposite directions
or they may slide by in same direction, but
different rates.
4. When plates
move suddenly,
vibrations are
generated
inside Earth
that are felt as
an earthquake.
Theory of Plate Tectonics
M. Strike-Slip Faults
5. Earthquakes, volcanoes,
and mountain ranges are
evidence of plate motion.
6. Plate tectonics
explains how
activity inside
Earth can affect
Earth’s crust
differently in
different
locations.
Theory of Plate Tectonics
N. Testing for Plate Tectonics
1. Only recently have scientists been able
to measure exact movements of Earth’s
crust.
2. They could study the magnetic
characteristics of rocks on the seafloor.
3. They could study volcanoes and
earthquakes.
Theory of Plate Tectonics
N. Testing for Plate Tectonics
4. One new method uses lasers and a satellite.
Now, scientists can measure exact movements
of Earth’s
plates of as
little as 1 cm
per year.
Theory of Plate Tectonics
N. Testing for Plate Tectonics
5. Satellite Laser Ranging System data show
that Hawaii is moving toward Japan at a
rate of about 8.3 cm per year.
6. Using such methods, scientists have
observed that the plates move at rates
ranging from about 1 cm to 12 cm per year.
Section Check
3
Question 1
Which of the following is made up of Earth’s
crust and part of the upper mantle?
A. asthenosphere
B. continental crust
C. lithosphere
D. plastisphere
Section Check
3
Question 2
________ is the theory that Earth’s crust and
part of the upper mantle are broken into
sections.
A. Continental drift
B. Pangaea effect
C. Plate tectonics
D. Seafloor spreading
Section Check
3
Question 3
The boundary between two plates that are
moving apart is a __________ boundary.
A. convergent
B. creeping
C. divergent
D. tectonic
Section Check
3
Q 1. Answer
The answer is C. The asthenosphere is the
plasticlike layer below the lithosphere.
Section Check
3
Q2. Answer
The answer is C. The sections are called plates
and are thought to move on a plasticlike layer
of Earth’s mantle.
Section Check
3
Q3. Answer
The answer is C. An example of a divergent
boundary is the Mid-Atlantic Ridge, where the
seafloor is spreading.
Help
To advance to the next item or next page click on any
of the following keys: mouse, space bar, enter, down or
forward arrow.
Click on this icon to return to the table of contents
Click on this icon to return to the previous slide
Click on this icon to move to the next slide
Click on this icon to open the resources file.
Click on this icon to go to the end of the presentation.
End of Chapter Summary File