The document discusses the history of theories of plate tectonics. It describes how early theories viewed the Earth's crust as rigid and unmoving, but accumulating evidence from seafloor mapping, core sampling, and studies across scientific disciplines demonstrated that the crust is made up of mobile tectonic plates that move and interact along boundaries. The modern theory of plate tectonics explains continental drift, mountain building, volcanism and earthquakes based on the dynamics of divergent, convergent, and transform plate boundaries.

1. Plate Tectonics
Chapter 9

2. Rigid Earth Theory

3. Rigid Earth Theory
• It was once believed that Earth’s crust was
hard and brittle and could not bend

4. Rigid Earth Theory
• It was once believed that Earth’s crust was
hard and brittle and could not bend

5. Rigid Earth Theory
• It was once believed that Earth’s crust was
hard and brittle and could not bend
• Plasticity

6. Rigid Earth Theory
• It was once believed that Earth’s crust was
hard and brittle and could not bend
• Plasticity
– We now know that Earth’s crust can bend (like
a tough plastic) before breaking

7. Isostacy
• “The maintenance of hydrostatic
equilibrium in the crust”
– hydrostatics—branch of physics related to the
pressure and equilibrium of liquids (hydro)
• statics—bodies not active; at rest; in equilibrium;
as opposed to dynamics

8. Isostacy

9. Isostacy
• Addition or removal of crustal material causes a sinking
or rebounding of crust

10. Isostacy
• Addition or removal of crustal material causes a sinking
or rebounding of crust
– Add or remove continental mass and the crust will sink
or rise to accommodate the added/removed weight

11. Isostacy
• Addition or removal of crustal material causes a sinking
or rebounding of crust
– Add or remove continental mass and the crust will sink
or rise to accommodate the added/removed weight
• a glacier growing or remelting, crust eroding off the
surface, sediment deposits, water bodies on land,
esp. those created by dams

12. Alfred Wegener and
His Continental Drift Theory

13. Alfred Wegener and
His Continental Drift Theory
• German meteorologist, 1920s
“The present continents were
originally connected as one
enormous landmass that has
broken up and drifted apart over
the last few 100 million years.
The drifting continues….”

14. Alfred Wegener and
His Continental Drift Theory
• German meteorologist, 1920s
“The present continents were
originally connected as one
enormous landmass that has
broken up and drifted apart over
the last few 100 million years.
The drifting continues….”
• Pangaea (Gk. “whole land”)

15. Alfred Wegener and
His Continental Drift Theory
• German meteorologist, 1920s
“The present continents were
originally connected as one
enormous landmass that has
broken up and drifted apart over
the last few 100 million years.
The drifting continues….”
• Pangaea (Gk. “whole land”)

16. Wegener’s Lines of Evidence

17. Wegener’s Lines of Evidence
• Similar geology (rocks and rock structures)…

18. Wegener’s Lines of Evidence
• Similar geology (rocks and rock structures)…
• …petrology (rock chemistry),

19. Wegener’s Lines of Evidence
• Similar geology (rocks and rock structures)…
• …petrology (rock chemistry),
• …paleontology (fossilized plants and animals),

20. Wegener’s Lines of Evidence
• Similar geology (rocks and rock structures)…
• …petrology (rock chemistry),
• …paleontology (fossilized plants and animals),
• …matching glacial features (U-shaped valleys,
glacial deposits, etc.) on continents separated by
oceans

21. Wegener’s Lines of Evidence
• Similar geology (rocks and rock structures)…
• …petrology (rock chemistry),
• …paleontology (fossilized plants and animals),
• …matching glacial features (U-shaped valleys,
glacial deposits, etc.) on continents separated by
oceans
• …continent shapes that seem to fit together,

22. Wegener’s Lines of Evidence
• Similar geology (rocks and rock structures)…
• …petrology (rock chemistry),
• …paleontology (fossilized plants and animals),
• …matching glacial features (U-shaped valleys,
glacial deposits, etc.) on continents separated by
oceans
• …continent shapes that seem to fit together,
• …patterns in the locations of volcanoes

23. Wegener’s Lines of Evidence
• Similar geology (rocks and rock structures)…
• …petrology (rock chemistry),
• …paleontology (fossilized plants and animals),
• …matching glacial features (U-shaped valleys,
glacial deposits, etc.) on continents separated by
oceans
• …continent shapes that seem to fit together,
• …patterns in the locations of volcanoes

24. Wegener’s Lines of Evidence
• Similar geology (rocks and rock structures)…
• …petrology (rock chemistry),
• …paleontology (fossilized plants and animals),
• …matching glacial features (U-shaped valleys,
glacial deposits, etc.) on continents separated by
oceans
• …continent shapes that seem to fit together,
• …patterns in the locations of volcanoes
Ex.: S. America/Africa, Madagascar/India, Australia/Antarctica

26. …but no one bought it.

27. …but no one bought it.
The
crust is
too
rigid!

28. …but no one bought it.
The So why don’t we
crust is see the crust
too ripping apart
rigid! right now?

29. …but no one bought it.
The So why don’t we
crust is see the crust
too ripping apart
rigid! right now?
What do you
mean, “The
continents are
floating???”

30. …but no one bought it.
The So why don’t we
crust is see the crust
too ripping apart
rigid! right now?
And hey, what’s the
What do you power source driving
mean, “The these movements of all
continents are the land masses,
floating???” anyway???

31. …but no one bought it.
The So why don’t we
crust is see the crust
too ripping apart
rigid! right now?
And hey, what’s the
What do you power source driving
mean, “The these movements of all
continents are the land masses,
floating???” anyway???
What a
knucklehead.

32. Then along came Oceanographer
Harry Hess in the 1960s…

33. Then along came Oceanographer
Harry Hess in the 1960s…

36. The evidence continued to mount…

37. The evidence continued to mount…
• Military seafloor mapping: Seafloor geology—
structure, chemistry, and age

38. The evidence continued to mount…
• Military seafloor mapping: Seafloor geology—
structure, chemistry, and age
– Oceanic crust: only 100 m.y.o

39. The evidence continued to mount…
• Military seafloor mapping: Seafloor geology—
structure, chemistry, and age
– Oceanic crust: only 100 m.y.o
– Continental crust: 4.1 b.y.o.

40. The evidence continued to mount…
• Military seafloor mapping: Seafloor geology—
structure, chemistry, and age
– Oceanic crust: only 100 m.y.o
– Continental crust: 4.1 b.y.o.
• Core sampling

41. The evidence continued to mount…
• Military seafloor mapping: Seafloor geology—
structure, chemistry, and age
– Oceanic crust: only 100 m.y.o
– Continental crust: 4.1 b.y.o.
• Core sampling
• Seafloor sediment

42. The evidence continued to mount…
• Military seafloor mapping: Seafloor geology—
structure, chemistry, and age
– Oceanic crust: only 100 m.y.o
– Continental crust: 4.1 b.y.o.
• Core sampling
• Seafloor sediment
• Rigid Earth folks retired—paradigm shift to plasticity

43. The evidence continued to mount…
• Military seafloor mapping: Seafloor geology—
structure, chemistry, and age
– Oceanic crust: only 100 m.y.o
– Continental crust: 4.1 b.y.o.
• Core sampling
• Seafloor sediment
• Rigid Earth folks retired—paradigm shift to plasticity
• Convection currents as mechanism/power source

44. The evidence continued to mount…
• Military seafloor mapping: Seafloor geology—
structure, chemistry, and age
– Oceanic crust: only 100 m.y.o
– Continental crust: 4.1 b.y.o.
• Core sampling
• Seafloor sediment
• Rigid Earth folks retired—paradigm shift to plasticity
• Convection currents as mechanism/power source
• Geologists, geophysicists, seismologists,
oceanographers, physicists, and paleontologists all
agree the theory fits the evidence gathered within
their respective fields

45. The Theory of Plate Tectonics
Tectonic (crustal) plates
• Pulling apart (spreading/diverging)
• Slamming together and sinking
(subducting/converging)
• Sliding laterally (sideways)

46. Divergent Plate Boundaries
• Spreading centers
– Crust pulling apart, magma rising to the surface

47. Convergent Plate Boundaries

48. Convergent Plate Boundaries
• Subduction zones

49. Convergent Plate Boundaries
• Subduction zones
– Crust being forced together

50. Convergent Plate Boundaries
• Subduction zones
– Crust being forced together
– Lightest material rises (mountain-building) while
the heaviest stuff sinks (pushed back into the
mantle)

51. Convergent Plate Boundaries
• Subduction zones
– Crust being forced together
– Lightest material rises (mountain-building) while
the heaviest stuff sinks (pushed back into the
mantle)
– Remelting (mostly from friction) creates volcanoes

52. Convergent Plate Boundaries
• Subduction zones
– Crust being forced together
– Lightest material rises (mountain-building) while
the heaviest stuff sinks (pushed back into the
mantle)
– Remelting (mostly from friction) creates volcanoes
– Intense, deep-focus earthquakes

54. Three Types of Subduction Zones

55. Three Types of Subduction Zones
1. Continental crust meets oceanic crust

56. Three Types of Subduction Zones
1. Continental crust meets oceanic crust
– Oceanic crust sinks

57. Three Types of Subduction Zones
1. Continental crust meets oceanic crust
– Oceanic crust sinks
– Big trench offshore

58. Three Types of Subduction Zones
1. Continental crust meets oceanic crust
– Oceanic crust sinks
– Big trench offshore
– Volcanoes on the continental margin

59. Three Types of Subduction Zones
1. Continental crust meets oceanic crust
– Oceanic crust sinks
– Big trench offshore
– Volcanoes on the continental margin
– Big earthquakes (potential for tsunamis)

61. Continental-Oceanic Subduction

62. Three Types of Subduction Zones

63. Three Types of Subduction Zones
2. Oceanic crust meets oceanic crust

64. Three Types of Subduction Zones
2. Oceanic crust meets oceanic crust
– The older and colder crust will probably sink

65. Three Types of Subduction Zones
2. Oceanic crust meets oceanic crust
– The older and colder crust will probably sink
– Big earthquakes and volcanic islands (called
“island arcs”)

66. Three Types of Subduction Zones
2. Oceanic crust meets oceanic crust
– The older and colder crust will probably sink
– Big earthquakes and volcanic islands (called
“island arcs”)
– Deep ocean trench

67. Three Types of Subduction Zones
2. Oceanic crust meets oceanic crust
– The older and colder crust will probably sink
– Big earthquakes and volcanic islands (called
“island arcs”)
– Deep ocean trench
– Potential for tsunamis

68. Oceanic-Oceanic Subduction

69. Three Types of Subduction Zones

70. Three Types of Subduction Zones
3. Continental crust meets continental crust

71. Three Types of Subduction Zones
3. Continental crust meets continental crust
– Too light to subduct

72. Three Types of Subduction Zones
3. Continental crust meets continental crust
– Too light to subduct
– Mountain-building

73. Three Types of Subduction Zones
3. Continental crust meets continental crust
– Too light to subduct
– Mountain-building
– Big earthquakes

74. Three Types of Subduction Zones
3. Continental crust meets continental crust
– Too light to subduct
– Mountain-building
– Big earthquakes
– Little if any volcanism (mostly intrusive)

75. Continental-Continental Subduction

76. Transform Fault Boundaries

77. Transform Fault Boundaries
• Tectonic plates slide past one another

78. Transform Fault Boundaries
• Tectonic plates slide past one another
– Earthquakes are less intense than subduction

79. Transform Fault Boundaries
• Tectonic plates slide past one another
– Earthquakes are less intense than subduction
– No volcanoes

80. Transform Fault Boundaries
• Tectonic plates slide past one another
– Earthquakes are less intense than subduction
– No volcanoes
– Little or no mountain-building

81. “Hot spots”

82. “Hot spots”
• Also called magma plumes

83. “Hot spots”
• Also called magma plumes
• Generally occur some distance from any
other type of plate boundary

84. “Hot spots”
• Also called magma plumes
• Generally occur some distance from any
other type of plate boundary
• Unrelated to convergent, divergent, or
transform boundaries

85. “Hot spots”
• Also called magma plumes
• Generally occur some distance from any
other type of plate boundary
• Unrelated to convergent, divergent, or
transform boundaries
• Anomalous (odd) “balloons” of rising magma

86. “Hot spots”
• Also called magma plumes
• Generally occur some distance from any
other type of plate boundary
• Unrelated to convergent, divergent, or
transform boundaries
• Anomalous (odd) “balloons” of rising magma
– Hot spot stays in one position as the moving,
island-covered crustal plate rides away from it

89. Accreted Terranes
• A moving continent may
pick up new land material
as lighter (felsic) material
scrapes off of a
subducting plate

90. Accreted Terranes
• A moving continent may
pick up new land material
as lighter (felsic) material
scrapes off of a
subducting plate

91. Craton
• These terranes were added to the original
material first formed from magma that rose
out of Earth’s earliest crust
– Craton--the name given to these ancient proto-
continents
cratons
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92. Continental Shields
• More magma material was added to the
cratons, forming continents.
– Continental shields: Where the earliest
continental material still exists intact and is
exposed at the surface.
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93. 31

94. Topography
• Right from the very beginning, the crust
was affected by stresses and strains that
caused crustal deformations
• Over time, the crust has continued to be
folded, faulted, broken, eroded and further
built upon, creating the topography, the
ups and downs of land relief, that we see
today
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