1. Plate Tectonics: The Unifying Theory
Peter W. Sloss, NOAA-NESDIS-NGDC

2. Plate Tectonics Fundamental concept of geoscience
Integrates from many branches
First suggested based on geology and paleontology
Fully embraced after evidence from geophysics

3. What tectonics theory explains
• Distribution of earthquakes and volcanoes
• Relationship of age and height of mountain belts
• Age distribution of oceanic crust
• Magnetic information in rocks

4. What is Plate Tectonics Theory?
Earth’s upper mantle (lithosphere) is broken into rigid plates which move with respect to each other
Plates rest on and move in the asthenosphere
There are 3 kinds of plate margins that are marked by earthquakes and volcanoes
Divergent
Convergent
Transform
The movement is driven by uneven distribution of heat within Earth and the mechanism that drives plate movement is not well known

5. Mosaic of Earth’s Plates
Peter W. Sloss, NOAA-NESDIS-NGDC

6. Theory evolved from 2 concepts:
Continental Drift - by Alfred Wegener
Fit of continents - Pangaea
Ancient climatic evidence - glacial deposits
Fossil evidence - Glossopteris & Mesosaurus
Matched mineral zones & mountain chains
(Problem - no acceptable method of motion)

7. “Fit” of the Continents

8. Anomalous Distribution of Fossils (Mesosaurus)

9. AND PALEOMAGNETIC EVIDENCE
Polar Wandering - either
the North Pole or the continents had moved
both Europe and North America had apparently moved as a single continent for several hundred million years

10. Apparent polar wandering curves

11. Seafloor Spreading “Geopoetry” of Harry Hess & Robert Dietz
New seafloor forms by upwelling at the center of MOR and moves laterally
Older crust is destroyed in the subduction zones at the trenches
Seafloor is younger than 200 MY
Solved Continental Drift problem

12. Modern Proof of Seafloor Spreading
Deep Sea Drilling Project - sampled sea floor sediment & crust
Age & thickness increased with distance from MOR
Ages were symmetrical about MOR
Magnetic Anomalies - found weak & strong signal
Alternating normal & reversed polarization
Stripes || to & symmetrical about MOR

14. Magnetic Anomalies

15. Ocean crust records magnetic reversals

16. Age of Seafloor Crust
Fig
R. Dietmar Muller, 1997

17. More Proofs
Hot Spots - deep, long-lived, stationary mantle magma sources
Expressed at the surface by:
linear chain of volcanoes
aged with distance from hot spot
Over 100 identified
Used as fixed points against which plate motion is measured

19. Hawaiian-Emperor chain

20. Long-lived global hot spots

21. PUTTING IT ALL TOGETHER

22. I. Plates Group of rocks all moving in the same direction
Can have both oceanic and continental crust or just one kind
Plate interior are relatively quiet
Most activity is located at plate margins

23. II. Types of plate boundaries
• divergent: mid-ocean ridges
• convergent: collision zones volcanic arcs
• strike-slip: San Andreas Fault Alpine Fault, N.Z.

24. A. Divergent plate boundaries
Usually begin within continents as hot spot burns thru crust and eventually grows to become ocean basin
Two kinds of divergent margins
Mid Ocean Ridges (MOR)
Continental Rift Valleys

25. Features of Mid Ocean Ridges
• Central rift valley (width is inversely proportional to the rate of spreading)
• Region of high heat flow
Shallow-focus earthquakes
• Almost exclusively basalt volcanism

26. Earthquakes Associated with Divergent Margins

27. Rifting and Seafloor Spreading Along the Mid-Atlantic Ridge
Peter W. Sloss, NOAA-NESDIS-NGDC

28. Rifting and Seafloor Spreading

29. Features of Continental Rifts
• East Africa, Rio Grande rift
• Beginning of ocean formation (may not get that far)
• Rifting begins at a triple junction (two spreading centers get together to form ocean basin, one left behind).
• Rock types: basalt and sandstone

30. Hot Spot induced rifting
Hot spot burns thru crust -> 3 branched rift
Divergence begins
2 branches are active
1 branch is not active
Linear sea develops

32. Inception of Rifting Along the East African Rift System
Peter W. Sloss, NOAA-NESDIS-NGDC

33. Inception of Rifting Within a Continent

34. Gulf of ‘Aqaba Gulf of Suez
Nile Delta
Gulf of
‘Aqaba
Gulf of
Suez
Red Sea
Earth Satellite Corp.

35. The Gulf of California Formed by Rifting of Baja California from Mainland Mexico
Worldsat International/Photo Researchers

36. B. Convergent boundaries
New crust created at MOR—old crust destroyed (recycled) at subduction zones
2 kinds: subduction & collision
Relative important densities:
continental crust ≈ 2.8 g/cm3
oceanic crust ≈ 3.2 g/cm3
asthenosphere ≈ 3.3 g/cm3

37. Convergent boundaries
Three types:
ocean–ocean Philippines
ocean–continent Andes
continent–continent Himalaya

38. Ocean–Ocean Island arcs: Chain of volcanic islands
• Highly seismic tectonic belt of
shallow to deep earthquakes
• High heat flow arc of active andesitic volcanoes
• Bordered by a submarine trench

39. Ocean–Ocean Subduction Zone

40. Ocean–Continent Continental arcs:
• Magmatic belt of active volcanoes (andesite to rhyolite)
• Often accompanied by compression of upper crust which builds mountains
Bordered by a submarine trench

41. Ocean-Continent Subduction Zone

42. Continent–Continent
Continent–continent boundaries, convergence is accommodated by
• Folding (shortening and thickening)
• Strike-slip faulting
• Underthrusting (intracontinental subduction)

43. Continent–Continent Convergent Boundary

44. Continent-Continent Collision

45. Himalayas and Tibetan Plateau
• Product of the collision between India and Asia.
• Collision began about 45 M yr. ago, continues today.
• Before collision, southern Asia looked something like the Andes do today.

46. Stages in the collision of India with Asia

47. C. Transform Boundary Offsetting Spreading Centers

48. III. Rates of plate motion
Mostly obtained from magnetic
anomalies on seafloor
Slow spreading: 3 cm/year
Fast spreading: 10 cm/year
Very fast spreading: 17cm/year

49. Relative Velocity and Direction of Plate Movement
Data from C. Demets, R.G> Gordon, D.F. Argus, and S. Sten, Model Nuvel-1, 1990

50. IV. Rock assemblages and plate tectonics
Each plate tectonic environment produces a distinctive group of rocks.
By studying the rock record of an area, we can understand the tectonic history of the region.

51. Layer of ophiolite suite.

52. Precambrian Ophiolite Suite
Pillow basalt
M. St. Onge/Geological Survey of Canada

53. V. Exotic or Microplate Terranes
Large blocks that contrast sharply with surrounding area
Wrong faulting, folding, fossils, rock types, metamorphism, magnatism
Thought to be fragments of continents, seafloor, seamounts, island arcs that rafted in & docked in new place

54. Approaching Arc or Microcontinent

55. Collision

56. Accreted Microplate Terrane

57. Microplate terranes Added to Western North America Over the Past 200 Million Years
After Hutchinson,

58. VI. Driving mechanism of plate tectonics
Thought to be convection of the mantle.
Friction at base of the lithosphere transfers energy from the asthenosphere to the lithosphere.
Convection may have overturned asthenosphere 4–6 times.

62. Other factors
• Trench pull
Plate sliding
• Ridge push

63. Three possible driving factors

64. Three possible mechanisms for the movement of lithosphere over the asthenosphere
Fig

65. VII. Tectonic reconstructions
A variety of evidence traces the motion of continents over time:
Paleomagnetism
Deformational structures
Environments of deposition
Fossils
Distribution of volcanoes

66. Assembly of Pangaea
I.W.D. Dalziel, 1995

67. Breakup of Pangaea
200 million years ago
After Dietz & Holden, 1970

68. Breakup of Pangaea
140 million years ago
After Dietz & Holden, 1970

69. Breakup of Pangaea
65 million years ago
After Dietz & Holden, 1970

70. Breakup of Pangaea
Today
After Dietz & Holden, 1970

71. Examining Deep-sea Drill Cores
Texas A&M University

73. Questions about plate tectonics
• What do we really know about convection cells in the mantle?
• Why are some continents completely surrounded by spreading centers?
• Why are tectonics in continental crust and oceanic crust so different?

74. Cross Section of Western Canada

75. Fig a

76. Formation of Magnetic Anomalies
Fig

77. Himalayas and Tibetan Plateau
Models
• Underthrusting
• Distributed shortening
•Strike-slip faulting

78. Wilson cycle Plate tectonics repeats itself: rifting, sea-
floor spreading, subduction, collision,
rifting, …
Plate tectonics (or something like it)
seems to have been active since the
beginning of Earth’s history.

79. After Hutchinson,
Fig

80. Examples of Plate Boundaries
O-C
convergent
O-O
divergent
C-C
divergent
O-O
divergent
O-O
convergent
O-O
divergent
O-C
convergent
Fig. 20.8a,b

81. Volcanic and Nonmarine sediments are deposited in rift valleys
Fig a

82. Cooling and subsidence of rifted margin allows sediments to be deposited
Fig b

83. Carbonate platform develops
Fig c

84. Continental margin continues to grow supplied from erosion of the continent
Fig d

85. Ocean–Continent Convergent Boundaries

86. Opening of the Atlantic by Plate Motion
After Phillips & Forsyth, 1972
Fig

87. Idealized Ophiolite Suite - Oc. Crust
Deep-sea sediments
Pillow basalt
Gabbro
Peridotite

88. Model for Forming Oceanic Crust at Mid-ocean Ridges

89. The growth of oceanic basin 1

90. The growth of oceanic basin 2

91. The growth of oceanic basin 3

92. Age of the Ocean Basins
After map by Sclater & Meinke

93. Parts of an Ocean–Ocean Convergent Plate Boundary
Fig

94. Parts of an Ocean–Continent Convergent Plate Boundary
Fig

95. Continued Subduction
Fig a

96. Continent– Continent Collision
Fig b