Chapter Menu

1
Chapter Menu
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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2
Chapter Introduction

• What is the scientific theory of plate tectonics?

3
Chapter Introduction

• 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.
4
Chapter Introduction
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.

5
Chapter Introduction
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.

6
Lesson 1 Reading Guide - KC
The Continental Drift Hypothesis

• What evidence supports continental drift?
• Why did scientists question the continental drift
  hypothesis?

7
Lesson 1 Reading Guide - Vocab
The Continental Drift Hypothesis

• Pangaea
• continental drift

8
Lesson 1-1
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.

9
Lesson 1-1
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.

10
Lesson 1-2

• The eastern coast of South America mirrors the
  shape of the west coast of Africa.

11
Lesson 1-2
Evidence That Continents Move

• Evidence to support Wegeners 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.

12
Lesson 1-2
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.

13
Lesson 1-2

• The presence of an ice sheet covering Pangea
  could explain glacial features found on some
  continents today.

14
Lesson 1-2
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.

15
Lesson 1-2

• The orange area shows where fossils of
  Glossopteris have been found.
• Fossils provide evidence for continental drift.

16
Lesson 1-2
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.

17
Lesson 1-2

• The Caledonia mountain range in northern Europe
  and the Appalachian Mountains in eastern North
  America are similar in age, structure, and rock
  type.

18
Lesson 1-2
Evidence That Continents Move (cont.)
How were similar rock types used to support the
continental drift hypothesis?
19
Lesson 1-3
What was missing?

• Wegeners 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.

20
Lesson 1-3
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
21
Lesson 1-3
What was missing? (cont.)
Why did scientists argue against Wegeners
continental drift hypothesis?
22
Lesson 1 - VS

• All continents were once part of a supercontinent
  called Pangaea.

23
Lesson 1 - VS

• Alfred Wegener proposed that continents move
  around on Earths surface.

24
Lesson 1 LR1
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
25
Lesson 1 LR2
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
26
Lesson 1 LR3
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
27
Lesson 1 - Now
Do you agree or disagree?

• 1. India has always been north of the equator.
• 2. All the continents once formed one
  supercontinent.

28
Lesson 2 Reading Guide - KC
Development of a Theory

• What is seafloor spreading?
• What evidence is used to support seafloor
  spreading?

29
Lesson 2 Reading Guide - Vocab
Development of a Theory

• mid-ocean ridge
• seafloor spreading
• normal polarity
• magnetic reversal
• reversed polarity

30
Lesson 2-1
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 oceans surface.

31
Lesson 2-1
Seafloor Topography
32
Lesson 2-2
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.

33
Lesson 2-2
Seafloor Spreading (cont.)

• Magma erupts on Earths 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.

34
Lesson 2-2
Seafloor Spreading (cont.)
35
Lesson 2-2
Seafloor Spreading (cont.)

• Scientists argued that if the seafloor spreads,
  the continents must also be moving.

What is seafloor spreading?
36
Lesson 2-2
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.

37
Lesson 2-2
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.

38
Lesson 2-2

• Continents move as the seafloor spreads along a
  mid-ocean ridge.

39
Lesson 2-3
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.
• Earths magnetic field today is described as
  having normal polaritya state in which
  magnetized objects, such as compass needles, will
  orient themselves to point north.

40
Lesson 2-3
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.

41
Lesson 2-3
42
Lesson 2-3
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 Earths
  magnetic field.
• Scientists have discovered parallel patterns in
  the magnetic signature of rocks on either side of
  a mid-ocean ridge.

43
Lesson 2-3

• Minerals in fresh lava record Earths magnetic
  signature.

44
Lesson 2-3
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.

45
Lesson 2-3
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.

46
Lesson 2-3
Seafloor Spreading Theory
47
Lesson 2-3
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.

48
Lesson 2-3
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.

49
Lesson 2 - VS

• New ocean crust forms along mid-ocean ridges.
• Mid-ocean ridges are large mountain ranges that
  extend throughout Earths oceans.

50
Lesson 2 - VS

• A magnetic reversal occurs when Earths magnetic
  field changes direction.

51
Lesson 2 LR1
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
52
Lesson 2 LR2
Todays magnetic field is described as having
which of these?
A. reversed polarity B. normal polarity C. magneti
c reversal D. no polarity
53
Lesson 2 LR3
In which state do magnetized objects reverse
themselves to point south?
A. reversed polarity B. normal polarity C. seafloo
r spreading D. magnetic polarity
54
Lesson 2 - Now
Do you agree or disagree?
3. The seafloor is flat. 4. Volcanic activity
occurs only on the seafloor.
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?

56
Lesson 3 Reading Guide - Vocab
The Theory of Plate Tectonics

• plate tectonics
• lithosphere
• divergent plate boundary
• transform plate boundary
• convergent plate boundary

57
Lesson 3 Reading Guide - Vocab
The Theory of Plate Tectonics

• subduction
• convection
• ridge push
• slab pull

58
Lesson 3-1
The Plate Tectonics Theory

• The theory of plate tectonics, proposed in the
  late 1960s, states that Earths surface is made
  of rigid slabs of rock, or plates, that move with
  respect to each other.
• Plate tectonics suggests that Earths surface is
  divided into large plates of rigid rock and each
  plate moves over Earths hot and semiplastic
  mantle.

59
Lesson 3-1
The Plate Tectonics Theory (cont.)
What is plate tectonics?
60
Lesson 3-1
The Plate Tectonics Theory (cont.)

• Geologists use the word tectonic to describe the
  forces that shape Earths 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.

61
Lesson 3-1
Earths Tectonic Plates
62
Lesson 3-1
The Plate Tectonics Theory (cont.)

• The lithosphere is thin below mid-ocean ridges
  and thick below continents.
• Earths 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 Earths plates to
  move.

63
Lesson 3-2
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
64
Lesson 3-2
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.

65
Lesson 3-3
66
Lesson 3-2
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.

67
Lesson 3-3
68
Lesson 3-2
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.

69
Lesson 3-2
Plate Boundaries (cont.)
subduction from Latin subductus, means to lead
under, removal
70
Lesson 3-2
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.

71
Lesson 3-3
72
Lesson 3-3
73
Lesson 3-2
Plate Boundaries (cont.)
What are the three types of plate boundaries?
74
Lesson 3-3
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.

75
Lesson 3-3
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.

76
Lesson 3-3

• Notice the relationship between earthquake
  epicenters, volcanoes, and plate boundaries.

77
Lesson 3-3
Evidence for Plate Tectonics (cont.)
How are earthquakes and volcanoes related to the
theory of plate tectonics?
78
Lesson 3-4
Plate Motion

• Earths 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 Earths crust.

79
Lesson 3-4
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.

80
Lesson 3-4
Plate Motion (cont.)
What causes convection?
81
Lesson 3-4
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.

82
Lesson 3-4

• As a slab, or dense plate, sinks, it pulls on the
  rest of the plate with a force called slab pull.

83
Lesson 3-5
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?

84
Lesson 3-5

• 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.

85
Lesson 3 - VS

• Tectonic plates are made of cold and rigid
  slabs of rock.
• Mantle convectionthe circulation of mantle
  material due to density differencesdrives plate
  motion.

86
Lesson 3 - VS

• The three types of plate boundaries are
  divergent, convergent, and transform boundaries.

87
Lesson 3 LR1
What word do geologists use to describe the
forces that shape Earths surface?
A. semiplastic B. lithosphere C. tectonic D. mantl
e
88
Lesson 3 LR2
Which of these is made up of the crust and the
solid, uppermost mantle?
A. seafloor B. lithosphere C. biosphere D. hydrosp
here
89
Lesson 3 LR3
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
90
Lesson 3 - Now
Do you agree or disagree?
5. Continents drift across a molten
mantle. 6. Mountain ranges can form when
continents collide.
91
Chapter Review Menu
Key Concept Summary Interactive Concept
Map Chapter Review Standardized Test Practice
92
The BIG Idea

• The scientific theory of plate tectonics states
  that Earths lithosphere is broken up into rigid
  plates that move over Earths surface.

93
Key Concepts 1
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.

94
Key Concepts 2
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.

95
Key Concepts 3
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.

96
Chapter Review MC1
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
97
Chapter Review MC2
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. seafloo
r spreading D. magnetic reversal
98
Chapter Review MC3
What is the rigid outermost layer of Earth called?
A. mantle B. lithosphere C. plate D. transform
plate boundary
99
Chapter Review MC4
What forms where two plates slide past each other?
A. transform plate boundary B. divergent plate
boundary C. convergent plate boundary D. semiplast
ic mantle
100
Chapter Review MC5
What is the process in which a denser plate sinks
below a more buoyant plate?
A. seafloor spreading B. convergence C. convection
D. subduction
101
Chapter Review STP1
What is the name of the supercontinent that all
continents were once part of?
A. Wegener B. Pangaea C. mantle D. seafloor
102
Chapter Review STP2
Where did scientists find glacial features that
are usually associated with cooler climates?
A. Antarctica B. plate boundaries C. mountain
ranges D. Africa
103
Chapter Review STP3
Which term refers to vast mountain ranges on the
seafloor?
A. volcanoes B. oceanic crust C. pillow
lava D. mid-ocean ridges
104
Chapter Review STP4
Which of these states that Earths 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
105
Chapter Review STP5
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