Plate tectonics II

1. Plate tectonics II The oceans are where most of the action takes place

2. Theory of Plate Tectonics • Surface of the Earth is covered by a series of plates • Creation of seafloor occurs at mid-ocean ridges, where volcanism creates the seafloor. • Consumption of the seafloor occurs at convergent margins, where plates are subducted back into the mantle. • Continents never subduct; they are passengers riding on the plates

3. Symmetric stripes of “normal” and “reverse” magnetism in ocean crust on either side of the ridge

6. How ocean basins formed

7. Sediment thickness increases away from ridge crest

8. Confirmation of seafloor spreading: Age of oceanic crust

10. Seafloor is created at ridges; it is destroyed at subduction zones

11. Plate Tectonics

12. Plates of the Earth’s crust

13. Plate Structure: The crust is not the plate! The plate is the “lithosphere” Fig. 1.11

14. Plate Tectonics • Lithosphere: the outer rigid shell of the earth (~ 100 km). The plates are composed of this material • Asthenosphere: part of mantle beneath lithosphere • The lithosphere rides on top of the asthenosphere

15. Plates • Group of rocks all moving in the same direction • Can have both oceanic and continental crust or just one kind.

16. Three Types of Plate Boundaries Divergent Convergent Transform Fig. 2.5

17. Plate boundaries • Divergent • Convergent • Strike-Slip

18. Divergent Plate Boundary Usually start within continents— grows to become ocean basin Fig. 2.6

19. Divergent Plate Boundaries Fig. 2.8

20. Plate boundaries • Divergent • Convergent • Strike-Slip

22. Ocean-Continent Convergence Ocean-Ocean Convergence Andes Unimak Island, AK

23. Convergent Boundaries Three types: ocean–ocean Japan ocean–continent Andes continent–continent Himalaya

24. Ocean–Ocean Island arcs: • Tectonic belts of high seismicity • High heat flow arc of active volcanoes • Bordered by a submarine trench

25. Convergent plate boundary Fig. 2.9

26. Ocean–Continent Continental arcs: • Active volcanoes • Often accompanied by compression of upper crust

27. Convergent Plate Boundary Fig. 2.9

28. Continent–Continent • In ocean–continent boundaries, collision convergence is taken up by subduction • In continent–continent boundaries, convergence is accommodated by deformation of the crust without subduction (both plates are too buoyant to be subducted)

30. Transform Faults

31. Transform Boundaries The San Andreas Fault

32. Transform Plate Boundary Fig. 2.10

33. Measured Plate Velocities correspond well with velocities inferred from magnetic stripes. Plate Movement is an observed fact.