Lecture 3 Plate Tectonics: The Unifying Theory

1. Lecture 3 Plate Tectonics: The Unifying Theory

2. Take-Away Points • How we know plate tectonics happens • Most earthquakes and volcanoes occur along plate boundaries • New ocean crust is created along mid-ocean ridges • Old ocean crust is recycled at subduction zones • Subduction zones are where mountain-building (orogeny) occurs

3. About Plate Tectonics • It is the movement of plates and the forces acting on them. • It explains the distribution of volcanoes, earthquakes, mountain chains, rock assemblages, and seafloor structures. • The forces that drive plate motions arise from the mantle convection system.

4. Discovery of Plate Tectonics Continental drift: “jig-saw puzzle” fit of continents

5. Discovery of Plate Tectonics Continental drift: similarity of rock assemblages and ages across oceans

6. Discovery of Plate Tectonics Continental drift: distribution of certain fossils

7. Continental Drift • Continental Drift theory was proposed by Alfred Wegener in 1912. He used continental fit, distribution of fossils, ancient climates, and the wandering of Earth's polar regions to support his Theory. He stated the present continents were part of a super continent – Pangaea.

8. Pangaea

9. Discovery of Plate Tectonics Seafloor spreading: geological activity in mid-ocean ridges

10. Discovery of Plate Tectonics Seafloor spreading: new crust formed there

11. Sea-Floor Spreading • Harry Hess proposed that new ocean floor is formed at the rift of mid-ocean ridges. The ocean floor, and the rock beneath it, are produced by magma that rises from deeper levels. Hess suggested that the ocean floor moved laterally away from the ridge and plunged into an oceanic trench along the continental margin.

12. Sea-Floor Spreading Subduction zone = destruction of lithosphere Mid oceanic ridge = construction of lithosphere

13. The Plates and Their Boundaries ● mosaic of rigid plates

14. Types of Plates • Ocean plates - plates below the oceans • Continental plates - plates below the continents

15. What are tectonic plates made of? • Plates are made of rigid lithosphere. The lithosphere is made up of the crust and the upper part of the mantle.

16. The Plates and Their Boundaries ● three types of boundaries

17. The Plates and Their Boundaries ● divergent, convergent, transform

18. The Plates and Their Boundaries ● next: a detailed look at the above

19. Three Basic Types of Plate Boundaries Using hands to show relative motion Divergent Transform Convergent USGS Graphics

20. Three Basic Types of Plate Boundaries Divergent Transform Convergent USGS Graphics

21. Divergent Boundaries (a) Oceanic plate separation rifting, volcanoes, and earthquakes Mid- Atlantic Ridge North American Plate Eurasian Plate

22. Divergent Boundaries (b) Continental plate separation rift valleys, volcanoes, and earthquakes East African Rift Valley Somali Subplate African Plate

23. Divergent Boundaries • Boundary between two plates that are moving apart or rifting   • RIFTING causes SEAFLOOR SPREADING

24. Features of Divergent Boundaries • Mid-ocean ridges • rift valleys • fissure volcanoes

26. Iceland: An example of continental rifting • Iceland has a divergent plate boundary running through its middle

28. Convergent Boundaries • Oceanic-Continental Convergence • Continent-Continent Convergence • Oceanic-Oceanic Convergence

29. Convergent Boundaries (a) Ocean-ocean convergence oceanic trench, volcanic island arc, and deep earthquakes Marianas Trench Mariana Islands Philippine Plate Pacific Plate

30. Convergent Boundaries (b) Ocean-continent convergence volcanic mountain chain, folded mountains, and deep earthquakes Andes Mountains Peru-Chile Trench South American Plate Nazca Plate

31. Convergent Boundaries (c) Continent-continent convergence crustal thickening, folded mountains, and earthquakes Himalaya Mountains Tibetan Plateau subduction Indian-Australian Plate Eurasian Plate

32. Convergent Boundaries • Boundaries between two plates that are colliding   • There are 3 types…

33. Type 1 • Ocean plate colliding with a less dense continental plate • Subduction Zone: where the less dense plate slides under the more dense plate • VOLCANOES occur at subduction zones

34. Subduction • As new ocean floor is created by sea-floor spreading at the mid-oceanic ridges it is consumed at the subduction zones where the lithosphere sinks under the asthenosphere.

36. Andes Mountains, South America

37. Type 2 • Ocean plate colliding with another ocean plate • The less dense plate slides under the more dense plate creating a subduction zone called a TRENCH

39. Type 3 • A continental plate colliding with another continental plate • Have Collision Zones: • a place where folded and thrust faulted mountains form.

42. Himalaya

43. Transform-Fault Boundaries (a) Continental transform fault lateral (transform) fault and earthquakes Pacific Plate North American Plate

44. Transform-Fault Boundaries (b) Mid-ocean ridge transform fault lateral (transform) faults and earthquakes Eurasian Plate North American Plate

45. Transform Fault Boundaries • Boundary between two plates that are sliding past each other • EARTHQUAKES along faults

47. San Andreas Fault, CA