2. Chapter 7: Plate Tectonics
Table of Contents
Chapter 7: Plate Tectonics
Section 3: Theory of Plate Tectonics

3. Theory of Plate Tectonics
A. Plate Tectonics
In the 1960s, scientistis developed a new theory that combined continental drift and sea floor spreading.
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.

4. B. Composition of Earth’s Plates
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).

5. B. Composition of Earth’s Plates
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.

6. 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.

7. 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?

8. 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.

9. Theory of Plate Tectonics
D. Plates Moving Apart
3. That divergent boundary is called the Mid-Atlantic Ridge. It is a mid-ocean ridge.

10. 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.

11. E. Plates Moving Together
Theory of Plate Tectonics
E. Plates Moving Together
When tow plates converge, or come together, they form a convergent boundary.
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.

12. E. Plates Moving Together
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.

13. E. Plates Moving Together
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.

14. E. Plates Moving Together
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.

15. 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.

16. Theory of Plate Tectonics
F. Where Plates Collide
3. Earthquakes are common at these convergent boundaries.

17. G. Where Plates Slide Past Each Other
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.

18. G. Where Plates Slide Past Each Other
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.

19. G. Where Plates Slide Past Each Other
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.

20. H. Causes of Plate Tectonics— Convection Inside Earth
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.

21. I. Moving Mantle Material
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.

22. J. Features Caused by Plate Tectonics
Theory of Plate Tectonics
J. Features Caused by Plate Tectonics
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.

23. J. Features Caused by Plate Tectonics
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.

24. K. Normal Faults and Rift Valleys
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 fault-block mountains.

25. K. Normal Faults and Rift Valleys
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.

26. L. Mountains and Volcanoes
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.

27. L. Mountains and Volcanoes
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.

28. L. Mountains and Volcanoes
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.

29. 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.

30. 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.

31. 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.

32. N. Testing for Plate Tectonics
Theory of Plate Tectonics
N. Testing for Plate Tectonics
Only recently have scientists been able to measure exact movements of Earth’s crust.
They could study the magnetic characteristics of rocks on the seafloor.
They could study volcanoes and earthquakes.

33. N. Testing for Plate Tectonics
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.

34. N. Testing for Plate Tectonics
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.

35. 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

36. 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

37. 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

38. Section Check 3
Q 1. Answer
The answer is C. The asthenosphere is the plasticlike layer below the lithosphere.

39. 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.

40. 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.

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42. End of Chapter Summary File