CHAPTER 2 Plate Tectonics and the Ocean Floor

1. CHAPTER 2Plate Tectonics and the Ocean Floor Fig. 2-32

2. Plate tectonics • Thin, rigid blocks move horizontally • Interactions of plates build major features of Earth’s crust • Mountain ranges, trenches, volcanoes, etc.

3. Theory of Plate Tectonics explains: • Global distribution of • Volcanoes • Earthquakes • Faults • Mountain belts • Features of seafloor • Evolution of continents and oceans http://www.geo.lsa.umich.edu/~crlb/COURSES/270/plate_tectonics.jpeg

4. Continental drift • Alfred Wegener proposed continental drift hypothesis in 1912 stating: • One large continent – Pangaea • Surrounded by single large ocean • Panthalassa • About 200 million years ago • As you can imagine, was NOT widely accepted at first http://pubs.usgs.gov/publications/graphics/wegener.gif

5. Evidence for continental drift • Puzzle-like fit of continents • Sir Edward Bullard fit continents at 2000m water depth (1965) • Used edge of continental shelves • Better fit than Wegener’s using coastlines Fig. 2.3

6. Evidence for continental drift • Wegener matched sequences of rocks and mountain chains • Similar age, rock types, structures http://www.lee.edu/~cguldenzopf/Images/Historical/Historicalmw32.jpg http://geology.csupomona.edu/drjessey/class/Gsc101

8. Evidence for continental drift • Wegener noted glacial ages and other climate evidence • Ancient glaciation in modern tropical regions • If there is evidence of ancient glacial activity, that area had to be closer to poles at some point • Direction of glacial flow

9. Evidence for continental drift • Distribution of organisms • Same land plants and animals distributed in different continents (e.g., South America and Africa) Similar fossils on separate southern continents

10. Similar fossils on separate southern continents http://pubs.usgs.gov/publications/graphics

11. Objections to Wegener’s continental drift hypothesis: • Continents cannot “plow” through ocean crust • Celestial mechanism –gravitational forces associated with tides too small • Proposed a celestial mechanism for moving plates – incorrrect , lost favor in 1930’s • More data was needed, collected using new technologies • sonar for mapping ocean bottom • measurements of Earth’s magnetism

12. Evidence for plate tectonics • Echo-sounding • Showed mountain ranges in middle of oceans, deep trenches near continents • Radiometric dating of bottom cores • Ocean is 4 billion years old but ocean rocks are less than 180 million years old – why? http://oceanexplorer.noaa.gov/history/quotes/tech/media

13. http://www.fau.edu/divdept/physics/jordanrg/LLS

15. Evidence for plate tectonics • Earth’s magnetic field • Paleomagnetism – study of earth’s ancient magnetism • N or S magnetic alignment of magnetite particles when rock hardens • Magnetic inclination (magnetic dip) • Latitude • If rocks move on plates, the latitude of origin is reflected in the rocks magnetic dip

16. Apparent polar wandering • When rocks from different continents were aligned as Pangea, polar wandering curves matched, showing a single stationary pole, but moving plates Fig. 2.8

17. Magnetic polarity reversals • Reversals aged by radiometric dating • About 1000 reversals over last 76 my (once per 200K years) Fig. 2.9

18. Sea floor spreading • Harry Hess(1962)

19. Fig. 2.11 More recent

20. Seafloor Spreading and Magnetization

21. Magnetic anomalies Fig. 2.12

22. Evidence to support sea floor spreading • Heat flowis highest at crest of mid-ocean ridge • Most large earthquakes occur along plate margins

23. Global distribution of earthquakes Fig. 2.13

24. Plate Boundary Features

25. Plate tectonics theory • Lithospheric plates “float” on the asthenosphere • Large scale geologic features occur at plate boundaries • Two major tectonic forces • Slab pull • Generated by subducting plate that pulls rest behind it • Slab suction • Subducting plate drags across mantle and sucks it down toward subduction zone

26. Types of plate boundaries Divergent Convergent Transform Fig. 2.14

27. Fig. 2.15 Divergent boundary features • Plates move apart • Mid-ocean ridge • Rift valley • New ocean floor created • Shallow earthquakes • As opposed to large, deep earthquakes found at convergent boundaries

28. Fig. 2.17 Divergent boundary features

29. Divergent BoundariesTypes of spreading centers • Oceanic rise • Fast-spreading • Gentle slopes • Oceanic ridge • Slow-spreading • Steep slopes • Ultra-slow • Deep rift valley • Widely scattered volcanoes For example, not all of the mid-Atlantic ridge moves at same rate, some areas are “rises” same are “ridges”

30. Convergent boundary features • Plates move toward each other • Oceanic crust destroyed • Ocean trench • Volcanic arc • Deep earthquakes • Types: • Oceanic-continental • Continental-continental

31. Motion at Plate Boundaries

32. Fig. 2.20

33. Types of convergent boundaries • Oceanic-continental convergence • Ocean plate subducted under continental • Continental arc • Find volcanoes on continent • Oceanic trench • Deep earthquakes

34. Fig. 2.21a,b

35. Tectonic Settings and Volcanic Activity

36. Types of convergent boundaries • Continental-continental convergence • Uplifted mountain ranges • Deep earthquakes

37. 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

38. Types of transform faults • Oceanic—wholly in ocean floor • Continental—extends from mid-ocean ridge across continent • San Andreas Fault Fig. 2-23

39. Motion at Plate Boundaries

40. Applications of plate tectonics model to intraplate features • Mantle plumes and hotspots • Volcanic islands within a plate • Island chains • Systematic variation of age • Record ancient plate motions As plate moves over hot spot, new islands are formed, notice how the island of Hawaii is newer than Kauai

41. Fig. 2.24

42. Tectonic Settings and Volcanic Activity

43. Applications of plate tectonics model to intraplate features • Seamounts and tablemounts • Seamounts – conical on top • Tablemounts – flat on top • Wave erosion may flatten seamount

44. Applications of plate tectonics model to intraplate features • Coral reefsassociated with subsiding seafloor • Fringing • Barrier • Atoll

45. Movement from mid-ocean ridge, mountain submersed Fig. 2.27

46. Seamounts/Tablemounts and Coral Reef Stages

47. Measuring plate motion by satellites Fig. 2.30

48. Paleoceanography • Reconstructing paleogeography • Continental accretion • Continental material added to edges of continents through plate motion • Bits and pieces of volcanoes, islands, etc added to landmasses • Continental separation or rifting • Continents move apart

49. Paleo- reconstructions Fig. 2.31

50. Global Geography Through Geologic Time