1. Understanding Earth PLATE TECTONICS: The Unifying Theory
Grotzinger • Jordan
Understanding Earth
Seventh Edition
Chapter 2:
PLATE TECTONICS:
The Unifying Theory
© 2014 by W. H. Freeman and Company

2. Chapter 2
Plate Tectonics:
The Unifying Theory

3. About Plate Tectonics
It is the movement of plates and the forces acting on them.
It explains the distribution of volcanoes, earthquakes, mountain chains, rock assemblages, and seafloor structures.
The forces that drive plate motions arise from the mantle convection system.

4. Lecture Outline The discovery of plate tectonics
2. Plates and their boundaries
3. Rates and history of plate motion
4. The grand reconstruction
5. Mantle convection: the engine of plate tectonics

5. Lecture Outline
6. Theory of plate tectonics and the scientific method

6. 1. Discovery of Plate Tectonics
Continental
drift:
“jig-saw puzzle” fit of continents

7. 1. Discovery of Plate Tectonics
Continental
drift:
similarity of rock assemblages and ages across oceans

8. 1. Discovery of Plate Tectonics
Continental drift:
distribution of certain fossils

9. 1. Discovery of Plate Tectonics
Seafloor spreading:
geological activity in mid-ocean ridges

10. 1. Discovery of Plate Tectonics
Seafloor spreading:
new crust formed there

11. Thought questions for this chapter
What mistakes did Wegener make in formulating his theory of continental drift? Do you think the geologists of his era were justified in rejecting his theory?

12. 2. The Plates and Their Boundaries
● mosaic of rigid plates

13. 2. The Plates and Their Boundaries
● three types of boundaries

14. 2. The Plates and Their Boundaries
● divergent, convergent, transform

15. 2. The Plates and Their Boundaries
● next: a detailed look at the above

16. rifting, volcanoes, and earthquakes
1. Divergent Boundaries
(a) Oceanic plate separation
rifting, volcanoes, and earthquakes
Mid-
Atlantic
Ridge
North American
Plate
Eurasian
Plate

17. rift valleys, volcanoes, and earthquakes
1. Divergent Boundaries
(b) Continental plate separation
rift valleys, volcanoes, and earthquakes
East African
Rift Valley
Somali Subplate
African Plate

18. oceanic trench, volcanic island arc, and deep earthquakes
2. Convergent Boundaries
(a) Ocean-ocean convergence
oceanic trench, volcanic island arc, and deep earthquakes
Mariana Islands
Marianas Trench
Philippine
Plate
Pacific Plate

19. volcanic mountain chain, folded mountains, and deep earthquakes
2. Convergent Boundaries
(b) Ocean-continent convergence
volcanic mountain chain, folded mountains, and deep earthquakes
Andes
Mountains
Peru-Chile Trench
South
American
Plate
Nazca Plate

20. crustal thickening, folded mountains, and earthquakes
2. Convergent Boundaries
(c) Continent-continent convergence
crustal thickening, folded mountains, and earthquakes
Himalaya
Mountains
Tibetan
Plateau
subduction
Indian-Australian Plate
Eurasian
Plate

21. lateral (transform) fault and earthquakes
3. Transform-Fault Boundaries
(a) Continental transform fault
lateral (transform) fault and earthquakes
Pacific Plate
North American Plate

22. lateral (transform) faults and earthquakes
3. Transform-Fault Boundaries
(b) Mid-ocean ridge transform fault
lateral (transform) faults and earthquakes
Eurasian Plate
North American Plate

23. Thought questions for this chapter
Why are there active volcanoes along the Pacific coast in Washington and Oregon but not along the east coast of the United States?
How do the differences between continental and oceanic crust affect the way lithospheric plates interact?

24. 3. Rates and History of Plate Motion
Ship towing a
sensitive magnetometer
Magnetic anomalies:
seafloor areas of high and low magnetic values
Mid-Atlantic Ridge
high intensity
low intensity

25. 3. Rates and History of Plate Motion
Iceland
Mid-Atlantic
Ridge
Mid-Atlantic Ridge
high intensity
low intensity
● seafloor as a magnetic tape recorder

26. 3. Rates and History of Plate Motion
● magnetic time scale developed

27. 3. Rates and History of Plate Motion
● velocity of seafloor spreading = d / t

28. 3. Rates and History of Plate Motion
● example area: mid-ocean ridge, south of Iceland

29. 3. Rates and History of Plate Motion
● Velocity = 60 km / 3.3 mil. yr. = km / mil. yr. (or 18 mm / yr)

30. 3. Rates and History of Plate Motion
Example relative plate velocities:
East Pacific Rise (Pacific and Nazca plates) – 138 to 150 mm/yr
South Atlantic (Mid-Atlantic Ridge) – to 35 mm/yr
Southern Ocean, south of Australia – to 75 mm/yr
Southern Ocean, south of Africa – mm/yr

31. Thought questions for this chapter
In Figure 2.15, the isochrons are symmetrically distributed in the Atlantic Ocean, but not in the Pacific Ocean. For example, seafloor as much as 180 million years old (in darkest blue) is found in the western Pacific, but not in the eastern Pacific. Why?

32. 4. The Grand Reconstruction
Reconstructing the history of plate motions:
Assembly and breakup of the supercontinent Rodinia
Assembly and breakup of the supercontinent Pangaea

33. 4. The Grand Reconstruction
Earth’s geography 1 billion years ago. Let’s see continental motion!

34. ASSEMBLY OF RODINIA
Late Proterozoic (750 Ma)
Formed about 1.1 billion years ago; began to break up about 750 million years ago

35. ASSEMBLY OF PANGAEA
Late Proterozoic (650 Ma)
The distribution of continents and oceans between Rodinia and the assembly of Pangaea

36. The distribution of continents and oceans about 458 million years ago
ASSEMBLY OF PANGAEA
Middle Ordovician (458 Ma)
The distribution of continents and oceans about 458 million years ago

37. The distribution of continents and oceans about 390 million years ago
ASSEMBLY OF PANGAEA
Early Devonian (390 Ma)
The distribution of continents and oceans about 390 million years ago

38. ASSEMBLY OF PANGAEA
Early Triassic (237 Ma)
The distribution of continents and oceans about 237 million years ago; Pangaea is formed

39. BREAKUP OF PANGAEA
Early Jurassic (195 Ma)
The breakup of the super-continent about 195 million years ago; Pangaea is being rifted

40. The distribution of continents and oceans about 152 million years ago
BREAKUP OF PANGAEA
Late Jurassic (152 Ma)
The distribution of continents and oceans about 152 million years ago

41. BREAKUP OF PANGAEA
Late Cretaceous-Early Tertiary
(66 Ma)
The distribution of continents and oceans about 66 million years ago; much like today in some ways

42. The distribution of continents and oceans as we know them today
PRESENT DAY
The distribution of continents and oceans as we know them today

43. 5. Mantle Convection: The Engine of Plate Tectonics
Upper
mantle
Theory 1: whole mantle convection
700 km
Lower
mantle
Plate recycling extends to the core-mantle boundary.
2900 km
Outer core

44. 5. Mantle Convection: The Engine of Plate Tectonics
Theory 2: stratified convection
Boundary near
700 km separates
the two different convection
systems.
The lower mantle convects more sluggishly than the upper mantle.

45. 5. Mantle Convection: The Engine of Plate Tectonics
spreading centers and hot spots

46. 6. Theory of Plate Tectonics and the Scientific Method
Plate tectonics is not a dogma, but a confirmed theory whose strength lies in its simplicity, its generality, and its consistency with many types of observations.
This theory has survived so many attempts to prove it wrong and has been so important in explaining and predicting so many phenomena that geologists treat the theory as fact.
Reasons why proof and acceptance took so long: very cautious approach of many scientists studying this issue; global scale of the problem; and specialized technology required to gain data took time to develop.

47. Thought questions for this chapter
Would you characterize plate tectonics as a hypothesis, a theory, or a fact? Why?
The theory of plate tectonics was not widely accepted until the banded patterns of magnetization on the ocean floor were discovered. In light of earlier observations – the jigsaw-puzzle fit of the continents, the occurrence of fossils of the same life-forms on both sides of the Atlantic, and the reconstruction of ancient climate conditions – why are these banded patterns of magnetism such key pieces of evidence?

48. Key terms and concepts Convergent boundary Divergent boundary Geodesy
Continental drift
Convergent boundary
Divergent boundary
Geodesy
Island arc
Isochron
Magnetic anomaly
Magnetic time scale
Mantle plume
Mid-ocean ridge
Pangaea
Plate tectonics
Relative plate velocity
Rodinia

49. Key terms and concepts Spreading center Subduction Transform fault
Seafloor spreading
Spreading center
Subduction
Transform fault