1. Plate Tectonics

2. Continental drift: An idea before its time
Alfred Wegener
Proposed hypothesis in 1915
Published The Origin of
Continents and Oceans
Continental drift hypothesis
Supercontinent Pangaea began breaking apart about 200 million years ago

3. Pangaea approximately 200 million years ago

4. What would the world have been like as a supercontinent. A)
What would the world have been like as a supercontinent? A) Different climate - continental interiors hotter and drier (like Mongolia today) B) Lower species diversity due to connectivity of continents and fewer shorelines C) Differences in marine environments - less shelf area D) All of the above

5. What would the world have been like as a supercontinent. A)
What would the world have been like as a supercontinent? A) Different climate - continental interiors hotter and drier (like Mongolia today) B) Lower species diversity due to connectivity of continents and fewer shorelines C) Differences in marine environments - less shelf area D) All of the above

6. Watch Pangaea Breakup
200 Million Years Ago to present

7. Which continent moves the farthest during the breakup of Pangaea?
North America
South America
Africa
India
E) Australia

8. Which continent moves the farthest during the breakup of Pangaea?
North America
South America
Africa
India
E) Australia

9. Ocean opens between rigid plates
But oceans are parts of plates and not independent
Of continents…

10. Continental drift hypothesis
Continents "drifted" to present positions
Evidence used in support of continental drift hypothesis
Fit of continents
Fossil evidence
Rock type and mountain belts
Paleoclimatic evidence

11. The great debate Objections to drift hypothesis
Inability to provide a mechanism capable of moving continents across globe
Wegner suggested that continents broke through the ocean crust, much like ice breakers cut through ice
Geophysics suggests this was unlikely

12. Matching of mtn ranges on continents Once connected in one belt, now separated…

13. Paleoclimatic evidence for Continental Drift Dismembering of an ancient ice sheet centered over the south pole

14. Why is there no evidence for a dismembered ice sheet
at the north pole at this time also? A) Continents were not located over the north pole at this time B) An ice sheet only formed over the south pole and not the north C) Global warming from greenhouse gas emissions from countries at northern latitudes melted the northern ice sheet
D) Solar energy in the
northern hemisphere was
higher then due to sunspot
activity

15. Why is there no evidence for a dismembered ice sheet
at the north pole at this time also? A) Continents were not located over the north pole at this time B) An ice sheet only formed over the south pole and not the north C) Global warming from greenhouse gas emissions from countries at northern latitudes melted the northern ice sheet
D) Solar energy in the
northern hemisphere was
higher then due to sunspot
activity

16. The great debate Continental drift and the scientific method
Wegner’s hypothesis was correct in principle, but contained incorrect details
For any scientific viewpoint to gain wide acceptance, supporting evidence required

17. Continental drift and paleomagnetism
Renewed interest in continental drift came from rock magnetism
Magnetized minerals in rocks
Show direction to Earth’s magnetic poles
Provide a means of determining their original latitude

19. What is the orientation of the Earth’s current magnetic field in Boulder, CO?
North, inclined upward at about 45 deg
North, inclined down at
about 45 deg
C) North, inclined down at
about 90 deg
D) North, inclined upward at about 90 deg
E) None of the above

20. What is the orientation of the Earth’s current magnetic field in Boulder, CO?
North, inclined upward at about 45 deg
North, inclined down at
about 45 deg
C) North, inclined down at
about 90 deg
D) North, inclined upward at about 90 deg
E) None of the above

21. Continental drift and paleomagnetism
Polar wandering
Apparent movement of magnetic poles in volcanic rocks indicates continents move
Shows Europe was closer to equator when coal-producing swamps existed

22. Continental drift and paleomagnetism
Polar wandering
Polar wandering curves for North America and Europe have similar paths but are separated by about 24 of longitude
Differences between paths reconciled if continents next to one another

23. Apparent polar-wandering paths for Eurasia and North America

24. The scientific revolution begins
During the 1950s and 1960s technological strides permitted extensive mapping of the ocean floor
Seafloor spreading hypothesis was proposed by Harry Hess in the early 1960s

25. The scientific revolution begins
Geomagnetic reversals
Earth's magnetic field periodically reverses polarity – north magnetic pole becomes south magnetic pole, vice versa
Dates when polarity of Earth’s magnetism changed were determined from lava flows

27. Geomagnetic reversals
Geomagnetic reversals are recorded in the ocean crust
In 1963 Fred Vine and D. Matthews tied the discovery of magnetic stripes in the ocean crust near ridges to Hess’s concept of seafloor spreading

28. Paleomagnetic reversals recorded by basalt at mid-ocean ridges

29. Maps of Magnetic Stripes in Oceanic Crust

30. Why are seafloor magnetic anomalies symmetrically oriented about ridges?
Rates of plate motions on the two divergent
plates are the same for both plates
Magma is intruded into the centers of
spreading ridges and then pulled apart
in the middle
Convective cells in the mantle transfer
heat and move rock straight up beneath
ridges and then move outward
D) All of the above

31. Why are seafloor magnetic anomalies symmetrically oriented about ridges?
Rates of plate motions on the two divergent
plates are the same for both plates
Magma is intruded into the centers of
spreading ridges and then pulled apart
in the middle
Convective cells in the mantle transfer
heat and move rock straight up beneath
ridges and then move outward
D) All of the above

32. Structure of
Juan de Fuca Plate

33. Age of Oceanic Crust

34. Why is there no oceanic crust older that 180Ma?
A) Plate tectonics didn’t occur prior to this
B) Spreading ridges ran much more slowly in the past
C) Older oceanic crust has all been subducted
D) Older oceanic crust probably exists somewhere, we just
haven’t found it under the ocean yet.

35. Why is there no oceanic crust older that 180Ma?
A) Plate tectonics didn’t occur prior to this
B) Spreading ridges ran much more slowly in the past
C) Older oceanic crust has all been subducted
D) Older oceanic crust probably exists somewhere, we just
haven’t found it under the ocean yet.

36. Geomagnetic reversal
Paleomagnetism was the most convincing evidence to support concepts of continental drift and seafloor spreading

37. Plate tectonics: The new paradigm
More encompassing theory than continental drift
Mix of ideas that explained motion of Earth’s lithosphere by subduction and seafloor spreading

38. Plate tectonics: The new paradigm
Earth’s major plates
Associated with Earth's strong, rigid outer layer
Known as the lithosphere
Consists of uppermost mantle and overlying crust
Overlies a weaker region in the mantle called the asthenosphere

39. Basal tractions drive plate motions

40. Earth’s major plates Seven major lithospheric plates
Plates are in motion and change in shape and size
Largest plate is the Pacific plate
Several plates include an entire continent plus a large area of seafloor

41. Why did Pangaea break up?
A) Supercontinents act as thermal blankets for heat escaping from the mantle, which leads to extension and breakup
The Earth was hit by a giant asteroid at the time, pushing the plates apart
Stresses caused by orbital forcing
Conservative politics in Russia caused it to slide towards the right, while liberals in North America caused it to slide to the left

42. Why did Pangaea break up?
A) Supercontinents act as thermal blankets for heat escaping from the mantle, which leads to extension and breakup
The Earth was hit by a giant asteroid at the time, pushing the plates apart
Stresses caused by orbital forcing
Conservative politics in Russia caused it to slide towards the right, while liberals in North America caused it to slide to the left

43. Earth’s Tectonic Plates

44. Earth’s major plates
Plates move relative to each other at a very slow but continuous rate
Average about 5 centimeters (2 inches) per year
Cooler, denser slabs of oceanic lithosphere descend into the mantle
Motion defined by rotation around a pole

45. Plate boundaries
Interactions among individual plates occur along their boundaries
Types of plate boundaries
Divergent plate boundaries
Convergent plate boundaries
Transform fault boundaries

46. Types of Plate Margins

47. Plate boundaries
Each plate bounded by combination of all three boundary types
New plate boundaries created in response to changes in forces acting on rigid slabs

48. Divergent plate boundaries
Most are located along the crests of oceanic ridges
Oceanic ridges and seafloor spreading
seafloor is elevated forming oceanic ridges

49. Oceanic ridges and seafloor spreading
Seafloor spreading occurs along the oceanic ridge system
Spreading rates and ridge topography
Ridge systems exhibit topographic differences
Topographic differences are controlled by spreading rates (see map of age of oceanic crust for width of ridges relative to their age)

50. Which ridge spreads the fastest?
Mid Atlantic ridge
B) East Pacific Rise
C) Australia-Antarctic Ridge
D) Arctic Ridge

51. Which ridge spreads the fastest?
Mid Atlantic ridge
B) East Pacific Rise
C) Australia-Antarctic Ridge
D) Arctic Ridge

52. Divergent boundaries are located mainly along oceanic ridges

53. Divergent boundaries in Continents
Continental rifts
Splits landmasses into two or more smaller segments

54. Divergent boundaries Continental rifts
Example includes East African rifts
Produced by extensional forces acting on the lithospheric plates
Not all rift valleys develop into spreading centers
Otherwise Nevada would be an ocean!

55. The East African Rift

56. Development of Continental Rift into Ocean Basin

57. A) volcanoes B) normal faults C) calderas D) basaltic lava flows
What is the geologic evidence here (New Mexico)
for rifting?
A) volcanoes
B) normal faults
C) calderas
D) basaltic lava flows
E) all of the above

58. Convergent plate boundaries
Old portions of oceanic plates are returned to the mantle
Surface expression of descending plate is an ocean trench
Called subduction zones
Average angle at which oceanic lithosphere descends into the mantle is about 45

59. Types of convergent boundaries
All have same basic characteristics, but can have highly variable features
Types of convergent boundaries
Oceanic-continental convergence
Denser oceanic slab sinks into the asthenosphere
Bathymetry marked by trench
As plate descends, partial melting of mantle rock makes basaltic or andesitic magmas
Volcanic mountains associated with subduction of oceanic lithosphere are called continental volcanic arcs (Andes and Cascades)

60. Types of Arcs

61. Types of convergent boundaries
Oceanic-oceanic convergence
When two oceanic slabs converge, one descends beneath the other
Often forms volcanoes on the ocean floor
If the volcanoes emerge as islands, a volcanic island arc is formed (Japan, Aleutian islands, Tonga islands)

62. Fly through the Marianas Trench: Which direction is the camera flying?
A) To the West, B) To the East, C) To the North, D) To the South

63. Fly through the Marianas Trench: Which direction is the camera flying?
A) To the West, B) To the East, C) To the North, D) To the South

64. Continental Arc
Oceanic Arc
Continental
Collision

65. Types of convergent boundaries
Continental-continental convergence
Continued subduction brings continents together
Less dense, buoyant continental lithosphere does not subduct
Result is a collision between two continental blocks
Process produces mountains (Himalayas, Alps, Appalachians)

66. The collision of India and Asia produced the Himalayas

67. Transform fault boundaries
Third type of plate boundary
Plates slide past one another and no new lithosphere is created or destroyed
Transform faults
Most join two segments of a mid-ocean ridge as parts of linear breaks in the oceanic crust known as fracture zones
Accommodate simultaneous movement of offset ridges

68. Transform faults accommodate movement on offset ridge segments

69. ridge
Map view of
ridge-transform-ridge
system
transform
ridge

70. What is the sense of slip on the transform fault?
ridge
Map view of
ridge-transform-ridge
system
transform
What is the sense of slip on the
transform fault?
Left lateral strike slip
B) Right lateral strike slip
C) Thrust
D) Normal
E) Oblique
ridge

71. ? ridge Map view of ridge-transform-ridge system transform
left lateral fault?
ridge

72. ridge
Map view of
ridge-transform-ridge
system
transform
Right lateral strike slip!
ridge

73. ridge
Map view of
ridge-transform-ridge
system
transform
fracture zone
fracture zone
Right lateral strike slip!
ridge

74. Why aren’t there earthquakes on the
transform fault away from the ridge?
Volcanic rocks weld plates together
Plates are moving at same rate
Plates are hotter here
D) All of the above
ridge
transform
fracture zone
fracture zone
Right lateral strike slip!
ridge

75. Why aren’t there earthquakes on the
transform fault away from the ridge?
Volcanic rocks weld plates together
Plates are moving at same rate
Plates are hotter here
D) All of the above
ridge
transform
fracture zone
fracture zone
Right lateral strike slip!
ridge

76. Testing the plate tectonics model
Plate tectonics and earthquakes
Plate tectonics model accounts for the global distribution of earthquakes
Absence of deep-focus earthquakes along the oceanic ridge is consistent with tectonic theory
Deep-focus earthquakes associated with subduction zones
The pattern of earthquakes along a trench provides method to track plate's descent

77. Deep-focus earthquakes occur along convergent boundaries

78. Earthquakes near Japan trench

79. Locations of E-quake Hypocenters

80. Evidence from ocean drilling
Most convincing evidence confirming seafloor spreading comes from drilling directly into ocean-floor sediment
Age of deepest sediments
Thickness of ocean-floor sediments verifies seafloor spreading

81. Hot spots Caused by rising plumes of mantle material
Volcanoes form over them (Hawaiian Island chain)
Mantle plumes are long-lived structures and originate at great depth, perhaps at core-mantle boundary

82. The Hawaiian Islands form over stationary hot spot

83. What caused hot spot Hawaii’s
track to change direction
at ~45Ma?
Location of hot spot in
mantle moved to north.
Direction of plate motion
changed.
Two different hot spots
formed the bent pattern
Northern section of track
is actually an old, dead
spreading ridge.

84. What caused hot spot Hawaii’s
track to change direction
at ~45Ma?
Location of hot spot in
mantle moved to north.
Direction of plate motion
changed.
Two different hot spots
formed the bent pattern
Northern section of track
is actually an old, dead
spreading ridge.

85. Other mechanisms generate forces that contribute to plate motion
No one driving mechanism accounts for all major facets of plate tectonics
Researchers agree that convective flow in 2,900 km-thick mantle is main driving force of plate tectonics (by basal traction)
Other mechanisms generate forces that contribute to plate motion
Slab-pull
Ridge-push

86. Importance of plate tectonics
Provides a unified explanation of Earth’s major surface processes, especially oceans
Within framework of plate tectonics, we find explanations for the distribution of earthquakes, volcanoes, and mountains
Plate tectonics provides explanations for distribution/evolution of plants and animals and climate record