1. 1 CHAPTER 2 Plate Tectonics and the Ocean Floor

2. 2 2 Density measures the mass per unit volume of a substance. Density = _Mass_ Volume Density is most commonly expressed as grams per cubic centimeter. Earth’s Interior Is Layered by Density

3. 3 3 The Study of Earthquakes Provides Evidence for Layering The behavior of seismic waves generated by earthquakes give scientists some of the best evidence about the structure of Earth. Low frequency pulses of energy generated by the forces that cause earthquakes can spread rapidly through Earth in all directions and then return to the surface.

4. 4 4 The Study of Earthquakes Provides Evidence for Layering (a) Earthquake waves passing through a homogenous planet would not be reflected or refracted (bent). The waves would follow linear paths (arrows). (b) In a planet that becomes gradually denser and more rigid with depth, the waves would bend along evenly curved paths.

5. 5 5 Earth’s Inner Structure Was Gradually Revealed The thin oceanic crust is primarily basalt, a heavy dark- colored rock. The thicker continental crust is granite, a speckled rock. The mantle is thought to consist mainly of oxygen, iron, magnesium, and silicon. The outer and inner core consist mainly of iron and nickel.

6. 6 Isostatic Adjustment

7. 7 Plate Tectonics Alfred Wegener first proposed in 1912 Called it “Continental Drift”

8. 8 Evidence for Continental Drift Wegener proposed Pangaea – one large continent existed 200 million years ago Panthalassa – one large ocean Noted puzzle-like fit of modern continents

9. 9 Evidence for Continental Drift Matching sequences of rocks and mountain chains Similar rocks on different continents

10. 10 Evidence for Continental Drift Glacial ages and other climate evidence Evidence of glaciation in now tropical regions Direction of glacial flow and rock scouring Plant and animal fossils indicate different climate than today

11. 11 Evidence for Continental Drift Distribution of organisms Same fossils found on continents that today are widely separated Modern organisms with similar ancestries

12. 12 The Fit between the Edges of Continents Suggested That They Might Have Drifted Alfred Wegener’s theory of continental drift was out of favor with the scientific community until new technology provided evidence to support his ideas. –Seismographs revealed a pattern of volcanoes and earthquakes. –Radiometric dating of rocks revealed a surprisingly young oceanic crust. –Echo sounders revealed the shape of the Mid-Atlantic Ridge

13. 13 The Fit between the Edges of Continents Suggested That They Might Have Drifted The fit of all the continents around the Atlantic at a water depth of about 137 meters (450 feet), as calculated by Sir Edward Bullard at the University of Cambridge in the early 1960s. This widely reproduced graphic was an effective stimulus to the tectonic revolution.

14. 14 Evidence for Plate Tectonics Earth’s magnetic field and paleomagnetism Earth has magnetic polarity North and South polarities Magnetic polarity recorded in igneous rocks –Magnetite in basalt

15. 15 Earth’s Magnetic Field

16. 16 Magnetic Polarity Reversals Earth’s magnetic polarity reverses periodically Recorded in ancient rocks

17. 17 Sea Floor Spreading Evidence Frederick Vine and Drummond Matthews (1963) Sea floor stripes record Earth’s magnetic polarity

18. 18 Sea Floor Spreading Harry Hess Depth recordings show sea floor features History of Ocean Basins –Sea-floor spreading –Mantle convection cells as driving mechanism

19. 19 Lithosphere A plate is the cooled surface layer of a convection current in upper mantle. Heat source

20. 20 Plate Tectonic Processes

21. 21 Sea Floor Spreading Mid-ocean ridge – spreading center Subduction zones – oceanic trench site of crust destruction

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23. 23 Age of Ocean Floor Late 1960s deep-sea drilling Radiometric dating of ocean rocks Symmetric pattern of age distribution about mid-ocean ridges Oldest ocean floor only 180 million years old

24. 24 Age of Ocean Floor

25. 25 Heat Flow Heat flow – heat from Earth’s interior released to surface Very high at mid-ocean ridges Low at subduction zones

26. 26 Earthquakes As Evidence Most large earthquakes occur at subduction zones. Earthquake activity mirrors tectonic plate boundaries.

27. 27 Distribution of Earthquakes

28. 28 Global Plate Boundaries

29. 29 Plate Tectonics Theory Lithosphere – tectonic plates that float on ductile asthenosphere Large scale geologic features occur at plate boundaries One major tectonic force –Convection One minor tectonic force –Slab pull/Slab suction

30. 30 Types of Plate Boundaries

31. 31 Examples of Plate Boundaries

32. 32 Divergent Boundary Features Plates move apart Mid-ocean ridge –Rift valley New ocean floor created Shallow focus earthquakes

33. 33 Generation of Divergent Boundary

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39. 39 Convergent Boundary Features Plates move toward each other Oceanic crust destroyed –Ocean trench –Volcanic arc Deep focus earthquakes

40. 40 Three Types of Convergent Boundaries

41. 41 Types of Convergent Boundaries Oceanic-Continental Convergence –Ocean plate is subducted –Continental arcs generated –Explosive andesitic volcanic eruptions

42. 42 Types of Convergent Boundaries Oceanic-Oceanic Convergence –Denser plate is subducted –Deep trenches generated –Volcanic island arcs generated

43. 43 Types of Convergent Boundaries Continental- Continental Convergence –No subduction –Tall mountains uplifted

44. 44 Transform Boundary Features Offsets oriented perpendicular to mid-ocean ridge –Segments of plates slide past each other Offsets permit mid-ocean ridge to move apart at different rates Shallow but strong earthquakes

45. 45 Transform Plate Boundaries Transform faults (orange) form because the axis of seafloor spreading on the surface of a sphere cannot follow a smoothly curving line. The motion of two diverging lithospheric plates (arrows) rotates about an imaginary axis extending through Earth.

46. 46 Slippage Areas (Transform Boundaries)

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48. 48 Transform Boundary Features Oceanic Transform Fault – ocean floor only Continental Transform Fault – cuts across continent All transform faults occur between mid-ocean ridge segments.

49. 49 Applications of Plate Tectonics Mantle Plumes and Hotspots –Intraplate features Volcanic islands within a plate Island chains

50. 50 Plate Movement above Mantle Plumes and Hot Spots the formation of the Hawai’ian Islands Loihi is such a newly forming island.

51. 51 Hawaiian Islands

52. 52 Global Hotspot Locations

53. 53 Plate Tectonics and Intraplate Features Seamounts –Rounded tops Tablemounts or guyots –Flattened tops Subsidence of flanks of mid-ocean ridge Wave erosion may flatten seamount

54. 54 Coral Reef Development Fringing reefs – develop along margin of landmass Barrier reefs – separated from landmass by lagoon Atolls – reefs continue to grow after volcanoes are submerged

55. 55 Coral Reef Development

56. 56 Detecting Plate Motion with Satellites

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58. 58 Future Predictions Future positions of continents and oceans –Assume same direction and rate of plate motions as now

59. 59 World Map 50 million Years in Future

60. 60 Alfred Wegener End of Chapter 2