1. PLATE TECTONICS

3. THEORY OF PLATE TECTONICS 1968 Continental Drift –Hypothesis first proposed by Alfred Wegener in 1915 Sea Floor Spreading –Hypothesis first proposed by Harry Hess in 1962 Plate Tectonics 1968 – combination of these two hypotheses

4. BASIC PROPOSITIONS Earth’s crust is divided into rigid plates that float buoyantly on the denser plastic mantle Elevation of the crust depends on density (rock type and temperature) & thickness Plates move laterally over surface generating unstable earthquake belts where they interact

5. Isostacy Recall that the crust (density 2.7-3.0 g/cm 3 ) floats on the upper mantle (density 3.3 g/cm 3 ) with elevation dependent on: Density (Rock type & Temperature) Thickness

6. Earthquake Foci

7. STABLE & UNSTABLE CRUST Stable Plates Granitic Crust – Continental Craton Basaltic Crust – Abyssal Plains Unstable Plate Boundaries Divergent – Rift Zones (Midocean Ridges) Convergent – Subduction Zones Shear Zones – Transform Faults

8. Eight Major Plates North America South America AfricaEurasiaAntarctica Consist of both granitic & basaltic crust Australia – India Two granitic continents plus basaltic crust Pacific- all basaltic crust Nazca – all basaltic crust

10. A World Cruise

12. A Note Lithosphere Rigid outer layer of Earth down to depth of about 60 mls. Includes crust and uppermost mantle. Forms plates, generates earthquakes due to faulting Asthenosphere Hot plastic zone in upper mantle (60-400 mls) which forms base of lithospheric plates No earthquakes generated

14. PLATE TECTONICS & THE EVOLUTION OF OCEANS & CONTINENTS

15. Evolution of Ocean Basins according to Dr. Chowns Baby Ocean

16. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea

17. The Red Sea Rift

18. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea Adolescent

19. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic

20. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic Mature

21. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic MaturePacific

23. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic MaturePacific Senile

25. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic MaturePacific SenileMediterranean

26. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic MaturePacific SenileMediterranean Dead

28. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic MaturePacific SenileMediterranean DeadHimalayas

29. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic MaturePacific SenileMediterranean DeadHimalayas Dead, rotted and dismembered (Yuck!)

31. Evolution of Ocean Basins according to Dr. Chowns Baby OceanRed Sea AdolescentAtlantic MaturePacific SenileMediterranean DeadHimalayas Dead, rotted and dismemberedUrals, Appalachians (Yuck!)

32. Age, Temperature & Depth of Oceans Hot rock is less dense than cold rock Density affects ocean depth Age and depth increase away from Ridges Young, hot basalt forms high Mid-Ocean Ridges Old, cold basaltic crust is subducted in trenches

33. Heat Flow from Pacific Sea-floor

34. Three types of convergent boundary Basalt – Basalt (Ocean-Ocean) Island Arc-Trench (Japan, Marianas) Basalt-Granite (Ocean-Continent) Mountain Chain-Trench (Andes) Granite-Granite (Continent-Continent) Mountain Chain (Himalayas)

35. Convergent Boundaries

36. Continental Collision

37. OPHIOLITES Usually oceanic crust is subducted but- Sometimes a slab of upper mantle and ocean crust gets caught up in continental collisions (obducted) and preserved in the mountains Examples Cyprus, Alps, Appalachians

38. DRIVING MECHANISM?

39. CONVECTION CURRENTS IN MANTLE Driven by internal heat resulting from radioactivity Analogy of convection in a pot of soup Estimates of temperature made from analysis of Earthquake waves Exact details unknown because it is difficult to collect data from the mantle

40. Simple Convection Currents In Mantle

41. More Complex Models

42. PLATE TECTONICS THE SUPPORTING EVIDENCE Wegener’s original evidence for Continental Drift 1913 Evidence compiled since 1950, especially paleomagnetism

44. WEGENER’S EVIDENCE 1. Distribution of Mountains & Earthquakes Circumpacific Ring of Fire Alpine-Himalayan

45. 2. Remarkable fit between continental margins (especially across Atlantic)

46. 3. Continuation of old mountains from one continent to another (eg. Appalachians)

47. 4. Similarities in rock layers and history between continents that were formerly contiguous (eg. North America & Europe, Africa & South America)

48. 5. Similarity in fossils between continents ( eg. Mesosaurus)

49. 6. Peculiarities in distribution of modern organisms

51. Distribution of Marsupials 200 Ma

52. 7. Paleoclimatic Evidence (eg. Ancient Glacial Deposits)

53. EVIDENCE SINCE 1950 1. Direct measurements of plate movement by laser, satellite and GPS 2. Tracking of hot spots like Hawaii

54. The Hawaiian Hot Spot

55. Hot Spots and Island Chains Movement of plates over hot spots in mantle produces chains of volcanoes. As old volcanoes move away from hot spots they cool and subside to form atolls or guyots Eg. Hawaiian chain & Emperor seamounts

56. 3.Tracking of continents by position relative to the magnetic poles

57. PALEOMAGNETISM

58. Modern Magnetic Pole Two modern continents with magnetic rocks aligned to magnetic poles

59. Modern Magnetic Pole Pole positions inferred from ancient magnetic rocks

60. Ancient Magnetic Pole Continents repositioned so that ancient pole matches modern pole position

61. Apparent Polar Wandering

62. 4. Proof of spreading rates from record of magnetic reversals in seafloor basalts

63. Magnetic stripes produced by sea floor spreading

64. Age of the Ocean Floor

65. THE BIG PICTURE

67. Evolution of the Oceans 200 Ma

68. Breakup of Pangaea 200-100 Ma

69. Modern oceans 50-0 Ma