1. Theory of Plate Tectonics The Modern Synthesis

3. The modern synthesis:

4. The Earth’s surface is broken into about a dozen major plates

6. Plates “float” or “slide” on a highly viscous layer -- the asthenosphere

7. Plates are made up of both continental and oceanic crust,

8. Main action in plate tectonics occurs at the edges of the plates

9. Over the course of geologic time these boundaries are the site of the splitting, shifting and crumpling of the continents

11. In a human time scale these boundaries are marked by the presence of volcanoes and the occurrence of earthquakes

12. Volcanoes of the World (Smithsonian Institution Global Volcanism Program)

13. Types of Plate Motion

15. Divergent Plates

16. Divergent Plate Characteristics Spreading boundary Rift Valleys volcanoes, earthquakes

17. Divergent Plates examples: Rift Valley of eastern Africa Mid-Atlantic ridge

20. Heezen and Tharp

21. Convergent Plates

22. At convergent plate boundaries the response to plate collision will vary with the type of crust involved There are three possible combinations:

23. Oceanic crust converging with oceanic crust:

24. characteristics subduction of one plate formation of an island arc Volcanoes, earthquakes

25. Oceanic crust converging with oceanic crust: example  Japanese Islands

27. Oceanic crust converging with continental crust:

28. Characteristics Formation of mountain ranges Volcanoes, earthquakes Trenches

29. Oceanic crust converging with continental crust: example  Andes Mountains

31. Continental crust converging with continental crust:

32. Characteristics Overriding of one plate formation of mountain ranges earthquakes

33. Continental crust converging with continental crust: example  Himalayas

35. Transform Boundaries

36. Characteristics “side-slip boundary” create fault zones earthquakes

37. Transform Boundaries example  San Andreas

39. Recap Picture of the Three Types of Tectonic Plate Boundaries

40. Cross section by José F. Vigil from This Dynamic Planet

41. 250 million years

42. 150 million years

43. 50 million years

44. Causes of Plate Motions

45. The Driving Force of Plate Movements is Mantle Convection

47. The rising part of a convection current causes up and out forces driving plates away from one another by a process called RIDGE PUSH

49. The downward part of a convection current causes downward forces pulling plates down at convergent boundaries.

50. The weight of the subducting plate helps pull the remaining lithosphere down. This process is called SLAB PULL

52. Most scientists agree that convection currents in the mantle cause plate movement but it is debated; what causes these convection currents and whether or not these currents change positions.