1. Understanding Earth Sixth Edition Chapter 2: PLATE TECTONICS: The Unifying Theory © 2011 by W. H. Freeman and Company Grotzinger Jordan

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

4. Lecture Outline 1.The discovery of plate tectonics 2. Plates and their boundaries 3. Rates and history of plate movements 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. Evolution of the Theory Continentaldrift: “jig-saw puzzle” fit of continents

7. 1. Evolution of the Theory Continental drift: similarity of rock assemblages and ages across oceans

8. 1. Evolution of the Theory Continental drift: distribution of certain fossils

9. 1. Evolution of the Theory Seafloor spreading: geological activity in mid- ocean ridges

10. 1. Evolution of the Theory 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 Mosaic of Earth’s Crustal Plates ● mosaic of rigid plates

13. 2. The Mosaic of Earth’s Crustal Plates ● three types of boundaries

14. 2. The Mosaic of Earth’s Crustal Plates ● divergent, convergent, transform

15. 2. The Mosaic of Earth’s Crustal Plates ● next: a detailed look at the above

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

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

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

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

20. 2. Convergent Boundaries (c) Continent-continent convergence TibetanPlateau Indian-Australian Plate EurasianPlateEurasianPlate crustal thickening, folded mountains, and earthquakes HimalayaMountains subduction

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

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

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 Magnetic anomalies: seafloor areas of high and low magnetic values low intensity high intensity Mid-Atlantic Ridge Ship towing a sensitive magnetometer

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

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

27. ● magnetic isochrons on the seafloor Figure 2.15

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

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

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

31. 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) – 34 to 35 mm/yr Southern Ocean, south of Australia – 70 to 75 mm/yr Southern Ocean, south of Africa – 14 mm/yr

32. 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?

33. 4. The Grand Reconstruction Reconstructing the history of plate motions: 1.Assembly and breakup of the supercontinent Rodinia 2.Assembly and breakup of the supercontinent Pangaea

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

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

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

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

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

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

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

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

42. 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

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

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

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

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

47. 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.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.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.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.

48. 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?

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

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