1. Lesson 1 The Continental Drift Hypothesis
Chapter Introduction
Lesson 1 The Continental Drift Hypothesis
Lesson 2 Development of a Theory
Lesson 3 The Theory of Plate Tectonics
Chapter Wrap-Up
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Chapter Menu

2. What is the theory of plate tectonics?
Chapter Introduction

3. What do you think?
Before you begin, decide if you agree or disagree with each of these statements. As you view this presentation, see if you change your mind about any of the statements.
Chapter Introduction

4. Do you agree or disagree?
1. India has always been north of the equator.
2. All the continents once formed one supercontinent.
3. The seafloor is flat.
Chapter Introduction

5. Do you agree or disagree?
4. Volcanic activity occurs only on the seafloor.
5. Continents drift across a molten mantle.
6. Mountain ranges can form when continents collide.
Chapter Introduction

6. Lesson 1 Reading Guide - KC
The Continental Drift Hypothesis
What evidence supports continental drift?
Why did scientists question the continental drift hypothesis?
Lesson 1 Reading Guide - KC

7. Lesson 1 Reading Guide - Vocab
The Continental Drift Hypothesis
Pangaea
continental drift
Lesson 1 Reading Guide - Vocab

8. Pangaea
Nearly 100 years ago, Alfred Wegener proposed that all the continents were once part of a supercontinent called Pangaea.
Over time, Pangaea began breaking apart and the continents slowly moved to their present position.
Lesson 1-1

9. Pangaea (cont.)
Wegener proposed the hypothesis of continental drift, which suggested that continents are in constant motion on the surface of Earth.
Wegener observed the similarities of continental coastlines now separated by oceans and how they could fit together like pieces of a puzzle.
Lesson 1-1

10. The eastern coast of South America mirrors the shape of the west coast of Africa.
Lesson 1-2

11. Evidence That Continents Move
Evidence to support Wegener’s hypothesis is found in
climate clues;
fossil clues;
rock clues.
When Wegener pieced Pangaea together, he proposed that the continents were located closer to the South Pole 250 million years ago.
Lesson 1-2

12. Evidence That Continents Move (cont.)
Wegener suggested that a large sheet of ice covered the continents.
Wegener studied the sediments left behind and the glacial grooves that formed when the ice sheets melted and Pangaea spread apart.
This provided climate evidence for continental drift.
Lesson 1-2

13. The presence of an ice sheet covering Pangea could explain glacial features found on some continents today.
Lesson 1-2

14. Evidence That Continents Move (cont.)
Animals and plants that live on separate continents can be unique to that continent alone.
Fossils of similar organisms have been found on several continents separated by oceans.
Fossils of a plant called Glossopteris have been found on continents that are now separated by oceans.
Lesson 1-2

15. The orange area shows where fossils of Glossopteris have been found.
Fossils provide evidence for continental drift.
Lesson 1-2

16. Evidence That Continents Move (cont.)
Wegener observed that mountain ranges and rock formations on different continents had common origins, providing rock evidence for continental drift.
Volcanic rock that is identical in chemistry and age has been found on both the western coast of Africa and the eastern coast of South America.
Lesson 1-2

17. The Caledonia mountain range in northern Europe and the Appalachian Mountains in eastern North America are similar in age, structure, and rock type.
Lesson 1-2

18. Evidence That Continents Move (cont.)
How were similar rock types used to support the continental drift hypothesis?
Lesson 1-2

19. What was missing?
Wegener’s ideas were not widely accepted until nearly four decades later.
Scientists questioned continental drift because it was a slow process and Wegener could not measure how fast continents moved or how they moved.
Scientists could not understand how continents could push their way through the solid rock of the mantle and the seafloor.
Lesson 1-3

20. What was missing? (cont.)
mantle
Science Use the middle layer of Earth, situated between the crust above and the core below
Common Use a loose, sleeveless garment worn over other clothes
Lesson 1-3

21. What was missing? (cont.)
Why did scientists argue against Wegener’s continental drift hypothesis?
Lesson 1-3

22. All continents were once part of a supercontinent called Pangaea.
Lesson 1 - VS

23. Alfred Wegener proposed that continents move around on Earth’s surface.
Lesson 1 - VS

24. C. continental movement D. continental drift
What term did Wegener use to describe the constant motion of continents on the surface of Earth?
A. Pangaea
B. continental drag
C. continental movement
D. continental drift
Lesson 1 – LR1

25. D. continental coastlines
In which of these did Wegener observe similarities that suggested continents might fit together like the pieces of a puzzle?
A. fossils
B. ice sheets
C. plates
D. continental coastlines
Lesson 1 – LR2

26. Wegener proposed that 250 million years ago, South America, Africa, India, and Australia were located closer to what?
A. equator
B. South Pole
C. North Pole
D. Pacific ocean
Lesson 1 – LR3

27. 1. India has always been north of the equator.
Do you agree or disagree?
1. India has always been north of the equator.
2. All the continents once formed one supercontinent.
Lesson 1 - Now

28. Lesson 2 Reading Guide - KC
Development of a Theory
What is seafloor spreading?
What evidence is used to support seafloor spreading?
Lesson 2 Reading Guide - KC

29. Lesson 2 Reading Guide - Vocab
Development of a Theory
mid-ocean ridge
seafloor spreading
normal polarity
magnetic reversal
reversed polarity
Lesson 2 Reading Guide - Vocab

30. Mapping the Ocean Floor
During the late 1940s scientists were able to determine the depth of the ocean using a device called an echo sounder.
Once ocean depths were determined, scientists used these data to create a topographic map of the sea floor that revealed vast mountain ranges, called mid-ocean ridges, that stretch for many miles deep below the ocean’s surface.
Lesson 2-1

31. Seafloor Topography
Lesson 2-1

32. Seafloor Spreading
By the 1960s, scientists discovered the process of seafloor spreading.
Seafloor spreading is the process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge.
When the seafloor spreads, the mantle below melts and forms magma.
Lesson 2-2

33. Seafloor Spreading (cont.)
Magma erupts on Earth’s surface as lava, which cools and crystallizes on the seafloor, forming rock.
Because the lava erupts into water, it cools rapidly and forms rounded structures called pillow lavas.
As the seafloor continues to spread apart, the older oceanic crust moves away from the mid-ocean ridge.
Lesson 2-2

34. Seafloor Spreading (cont.)
Lesson 2-2

35. Seafloor Spreading (cont.)
Scientists argued that if the seafloor spreads, the continents must also be moving.
What is seafloor spreading?
Lesson 2-2

36. Seafloor Spreading (cont.)
The rugged mountains that make up the mid-ocean ridge system can form in two different ways.
Large amounts of lava can erupt from the center of the ridge, cool, and build up around the ridge.
Lesson 2-2

37. Seafloor Spreading (cont.)
Or, as the lava cools and forms new crust, it cracks and the rocks move up or down along these cracks in the seafloor, forming jagged mountain ranges.
The abyssal plain, the smooth part of the seafloor, is made when the layer of sediment that accumulates far from the mid-ocean ridge becomes thick enough.
Lesson 2-2

38. Continents move as the seafloor spreads along a mid-ocean ridge.
Lesson 2-2

39. Development of a Theory
The first evidence used to support seafloor spreading was discovered in rocks on the seafloor.
Scientists studied the magnetic signature of minerals in these rocks.
Earth’s magnetic field today is described as having normal polarity—a state in which magnetized objects, such as compass needles, will orient themselves to point north.
Lesson 2-3

40. Development of a Theory (cont.)
Sometimes a magnetic reversal occurs and the magnetic field reverses direction.
The opposite of normal polarity is reversed polarity: a state in which magnetized objects reverse direction and orient themselves to point south.
Lesson 2-3

41. Lesson 2-3

42. Development of a Theory (cont.)
Volcanic rock on the seafloor contains iron-rich minerals that are magnetic.
Magnetic minerals in cooling lava from the mid-ocean ridge record the direction of Earth’s magnetic field.
Scientists have discovered parallel patterns in the magnetic signature of rocks on either side of a mid-ocean ridge.
Lesson 2-3

43. Minerals in fresh lava record Earth’s magnetic signature.
Lesson 2-3

44. Development of a Theory (cont.)
Scientists studied magnetic minerals in rocks from the seafloor using a magnetometer to measure and record the magnetic signature.
They discovered parallel magnetic stripes on either side of the mid-ocean ridge.
Lesson 2-3

45. Development of a Theory (cont.)
Each pair of stripes has a similar composition, age, and magnetic character.
The pairs of magnetic stripes confirm that the ocean crust formed at mid-ocean ridges is carried away from the center of the ridges in opposite directions.
Lesson 2-3

46. Seafloor Spreading Theory
Lesson 2-3

47. Development of a Theory (cont.)
Other measurements made on the seafloor confirm seafloor spreading.
Measuring the amount of thermal energy leaving the Earth shows that more thermal energy leaves Earth near mid-ocean ridges than is released from beneath the abyssal plains.
Lesson 2-3

48. Development of a Theory (cont.)
Sediment collected from the seafloor can be dated to show that the sediment closest to the mid-ocean ridge is younger than the sediment farther away from the ridge.
Lesson 2-3

49. New ocean crust forms along mid-ocean ridges.
Mid-ocean ridges are large mountain ranges that extend throughout Earth’s oceans.
Lesson 2 - VS

50. A magnetic reversal occurs when Earth’s magnetic field changes direction.
Lesson 2 - VS

51. As the seafloor continues to spread apart, the older oceanic crust moves in which direction with respect to the mid-ocean ridge?
A. toward it
B. above it
C. away from it
D. under it
Lesson 2 – LR1

52. Today’s magnetic field is described as having which of these?
A. reversed polarity
B. normal polarity
C. magnetic reversal
D. no polarity
Lesson 2 – LR2

53. In which state do magnetized objects reverse themselves to point south?
A. reversed polarity
B. normal polarity
C. seafloor spreading
D. magnetic polarity
Lesson 2 – LR3

54. 4. Volcanic activity occurs only on the seafloor.
Do you agree or disagree?
3. The seafloor is flat.
4. Volcanic activity occurs only on the seafloor.
Lesson 2 - Now

55. Lesson 3 Reading Guide - KC
The Theory of Plate Tectonics
What is the theory of plate tectonics?
What are the three types of plate boundaries?
Why do tectonic plates move?
Lesson 3 Reading Guide - KC

56. Lesson 3 Reading Guide - Vocab
The Theory of Plate Tectonics
plate tectonics
lithosphere
divergent plate boundary
transform plate boundary
convergent plate boundary
Lesson 3 Reading Guide - Vocab

57. Lesson 3 Reading Guide - Vocab
The Theory of Plate Tectonics
subduction
convection
ridge push
slab pull
Lesson 3 Reading Guide - Vocab

58. The Plate Tectonics Theory
The theory of plate tectonics, proposed in the late 1960s, states that Earth’s surface is made of rigid slabs of rock, or plates, that move with respect to each other.
Plate tectonics suggests that Earth’s surface is divided into large plates of rigid rock and each plate moves over Earth’s hot and semiplastic mantle.
Lesson 3-1

59. The Plate Tectonics Theory (cont.)
What is plate tectonics?
Lesson 3-1

60. The Plate Tectonics Theory (cont.)
Geologists use the word tectonic to describe the forces that shape Earth’s surface and the rock structures that form as a result.
The cold and rigid outermost rock layer of the Earth is called the lithosphere.
The lithosphere is made up of the crust and the solid, uppermost mantle.
Lesson 3-1

61. Earth’s Tectonic Plates
Lesson 3-1

62. The Plate Tectonics Theory (cont.)
The lithosphere is thin below mid-ocean ridges and thick below continents.
Earth’s tectonic plates are large pieces of the lithosphere that fit together like the pieces of a giant jigsaw puzzle.
The layer of Earth below the lithosphere, called the asthenosphere, is so hot that it behaves like a plastic material and enables Earth’s plates to move.
Lesson 3-1

63. The Plate Tectonics Theory (cont.)
plastic
Science Use capable of being molded or changing shape without breaking
Common Use any of numerous organic, synthetic, or processed materials made into objects
Lesson 3-2

64. Plate Boundaries
A divergent plate boundary forms where two plates separate.
When the seafloor spreads at a mid-ocean ridge, lava erupts, cools, and forms new oceanic crust.
Divergent plate boundaries can also exist in the middle of a continent, where they pull continents apart and form rift valleys.
Lesson 3-2

65. Lesson 3-3

66. Plate Boundaries (cont.)
A transform plate boundary forms where two plates slide past each other.
As they move past one another, the plates can get stuck and stop moving.
Stress builds up where the plates are stuck until they eventually break and suddenly move apart, resulting in a rapid release of energy as earthquakes.
Lesson 3-2

67. Lesson 3-3

68. Plate Boundaries (cont.)
Convergent plate boundaries form where two plates collide.
The denser plate sinks below the more buoyant plate in a process called subduction.
The area where a denser plate descends into Earth along a convergent plate boundary is called a subduction zone.
Lesson 3-2

69. Plate Boundaries (cont.)
subduction
from Latin subductus, means “to lead under, removal”
Lesson 3-2

70. Plate Boundaries (cont.)
When an oceanic plate and a continental plate collide, the denser oceanic plate subducts under the edge of the continent, creating a deep ocean trench and a line of volcanoes above the subducting plate on the edge of the continent.
When two continental plates collide, neither plate is subducted, and mountains form from uplifted rock.
Lesson 3-2

71. Lesson 3-3

72. Lesson 3-3

73. Plate Boundaries (cont.)
What are the three types of plate boundaries?
Lesson 3-2

74. Evidence for Plate Tectonics
Scientists can measure how fast continents move using a network of satellites called the Global Positioning System.
Because plates are rigid, tectonic activity occurs where plates meet.
Lesson 3-3

75. Evidence for Plate Tectonics (cont.)
Volcanoes form where plates separate along a mid-ocean ridge or continental rift or collide along a subduction zone.
Mountains can form where two continents collide.
Lesson 3-3

76. Notice the relationship between earthquake epicenters, volcanoes, and plate boundaries.
Lesson 3-3

77. Evidence for Plate Tectonics (cont.)
How are earthquakes and volcanoes related to the theory of plate tectonics?
Lesson 3-3

78. Plate Motion
Earth’s plates move because the asthenosphere moves underneath the lithosphere.
Convection is the circulation of material caused by differences in density.
Hot mantle material rises upward and comes in contact with Earth’s crust.
Lesson 3-4

79. Plate Motion (cont.)
As the mantle cools, it becomes denser and then sinks, forming a convection current.
Convection currents in the asthenosphere act like a conveyor belt moving the lithosphere above it.
There are three forces that interact to cause plate motion: basal drag, ridge push, and slab pull.
Lesson 3-4

80. Plate Motion (cont.)
What causes convection?
Lesson 3-4

81. Plate Motion (cont.)
Basal drag refers to how convection currents in the asthenosphere circulate and drag the lithosphere like a conveyor belt.
Rising mantle material at mid-ocean ridges creates the potential for plates to move away from the ridge with a force called ridge push.
Lesson 3-4

82. As a slab, or dense plate, sinks, it pulls on the rest of the plate with a force called slab pull.
Lesson 3-4

83. A Theory in Progress
Plate tectonics has become the unifying theory of geology, but several unanswered questions remain.
Why is Earth the only planet in the solar system that has plate tectonic activity?
Why do some earthquakes and volcanoes occur far away from plate boundaries?
Lesson 3-5

84. What forces dominate plate motion?
What will scientists study next? This is a 3-D image of seismic wave velocities from a new technique called anisotropy.
Lesson 3-5

85. Tectonic plates are made of cold and rigid slabs of rock.
Mantle convection— the circulation of mantle material due to density differences—drives plate motion.
Lesson 3 - VS

86. The three types of plate boundaries are divergent, convergent, and transform boundaries.
Lesson 3 - VS

87. What word do geologists use to describe the forces that shape Earth’s surface?
A. semiplastic
B. lithosphere
C. tectonic
D. mantle
Lesson 3 – LR1

88. Which of these is made up of the crust and the solid, uppermost mantle?
A. seafloor
B. lithosphere
C. biosphere
D. hydrosphere
Lesson 3 – LR2

89. What type of boundary forms where two plates separate?
A. convergent plate boundary
B. transform plate boundary
C. new plate boundary
D. divergent plate boundary
Lesson 3 – LR3

90. 5. Continents drift across a molten mantle.
Do you agree or disagree?
5. Continents drift across a molten mantle.
6. Mountain ranges can form when continents collide.
Lesson 3 - Now

91. Interactive Concept Map Chapter Review Standardized Test Practice
Key Concept Summary
Interactive Concept Map
Chapter Review
Standardized Test Practice
Chapter Review Menu

92. The theory of plate tectonics states that Earth’s lithosphere is broken up into rigid plates that move over Earth’s surface.
The BIG Idea

93. Lesson 1: The Continental Drift Hypothesis
The puzzle piece fit of continents, fossil evidence, climate, rocks, and mountain ranges supports the hypothesis of continental drift.
Scientists were skeptical of continental drift because Wegener could not explain the mechanism for movement.
Key Concepts 1

94. Lesson 2: Development of a Theory
Seafloor spreading provides a mechanism for continental drift.
Seafloor spreading occurs at mid-ocean ridges.
Evidence of magnetic reversal in rock, thermal energy trends, and the discovery of seafloor spreading all contributed to the development of the theory of plate tectonics.
Key Concepts 2

95. Lesson 3: The Theory of Plate Tectonics
Types of plate boundaries, the location of earthquakes, volcanoes, and mountain ranges, and satellite measurement of plate motion support the theory of plate tectonics.
Mantle convection, ridge push, and slab pull are the forces that cause plate motion. Radioactivity in the mantle and thermal energy from the core produce the energy for convection.
Key Concepts 3

96. Which of these have been found on several continents separated by oceans, suggesting that they were once geographically closer together?
A. animals
B. ice sheets
C. plant fossils
D. plate boundaries
Chapter Review – MC1

97. Which of these refers to the process by which new oceanic crust forms along a mid-ocean ridge as older oceanic crust moves away from the ridge?
A. continental drift
B. plate tectonics
C. seafloor spreading
D. magnetic reversal
Chapter Review – MC2

98. What is the rigid outermost layer of Earth called?
A. mantle
B. lithosphere
C. plate
D. transform plate boundary
Chapter Review – MC3

99. What forms where two plates slide past each other?
A. transform plate boundary
B. divergent plate boundary
C. convergent plate boundary
D. semiplastic mantle
Chapter Review – MC4

100. What is the process in which a denser plate sinks below a more buoyant plate?
A. seafloor spreading
B. convergence
C. convection
D. subduction
Chapter Review – MC5

101. What is the name of the supercontinent that all continents were once part of?
A. Wegener
B. Pangaea
C. mantle
D. seafloor
Chapter Review – STP1

102. Where did scientists find glacial features that are usually associated with cooler climates?
A. Antarctica
B. plate boundaries
C. mountain ranges
D. Africa
Chapter Review – STP2

103. Which term refers to vast mountain ranges on the seafloor?
A. volcanoes
B. oceanic crust
C. pillow lava
D. mid-ocean ridges
Chapter Review – STP3

104. A. the theory of plate tectonics B. the theory of mid-ocean ridges
Which of these states that Earth’s surface is made of rigid slabs of rock that move with respect to one another?
A. the theory of plate tectonics
B. the theory of mid-ocean ridges
C. the theory of divergent plate boundaries
D. the theory of transform plate boundaries
Chapter Review – STP4

105. What is the name for the force that pulls on the rest of a plate as part of a dense plate sinks?
A. ridge push
B. basal drag
C. slab pull
D. plate tectonics
Chapter Review – STP5