Plate Tectonics

1. Plate Tectonics

2. I. Continental Drift A. Alfred Wegener 1. First proposed the hypothesis, (1915) 2. German Meteorologist & Geophysicist

3. 3. Published The Origin ofContinents and Oceans

4. B. Continental Drift Hypothesis • Super continent called Pangaea began breaking apart about 200 million years ago. • Continents “drifted” to present positions • Continents “broke” through oceanic crust

5. C. Evidence for Continental Drift 1. Map Fit- some of the continents seem to fit together like a jigsaw puzzle.

6. Ex: S. America & Africa

7. 2. Fossils- • Fossils of certain species of ancient organisms were found on separate continents throughout the world. Mesosaurus

8. The continents must have been as ONE in order for these species to be found on different, widespread landmasses. • Organisms were either too large or small to swim or fly to other continents without them being linked.

9. Land bridges could not have been the solution, because the remnants are nowhere to be found.

10. 3. Rock Structures • Mountain chains of comparable age that appear to be severed by the oceans (Appalachians)

11. When these landmasses are reassembled they form a continuous mountain belt. N. America & Europe S. America & Africa S. America & Antarctica

12. Ancient Climates • Glacial deposits show that most of the Southern Hemisphere was covered by ice sheets.

13. Rock deposits left behind by moving glaciers were found in all southern continents & India. • Coal fields produced by ancient tropical plant fossils indicate that the Northern Hemisphere was once a tropical environment.

15. Main objection to Wegener’s hypothesis was its inability to provide a mechanism for the movement of the continents.

16. II. Plate Tectonics A. What is the Theory? • The crust is broken into large moving sections called plates • Lithosphere: rigid layer made of the crust & upper mantle • Asthenosphere: weaker upper part of the mantle

18. Explains the formation, movements, collisions, & destruction of the Earth’s crust. • Seven major plates: Pacific (largest) N. American S. American African Eurasian Australian Antarctic

20. Plates include a continent & mostly ocean floor. • Plates are slowly moving continuously.

21. B. Plate Boundaries 1. What occurs at plate boundaries? a. Seismic activity b. Volcanism c. Mountain building

22. Rift Valley Midocean Ridge 2. Types of Plate Boundaries a. Divergent Boundaries 1. Where plates move apart (Constructive)

23. 2. Seafloor Spreading * As divergent plates spread apart creating a fissure, magma flows through the fissure (rift valley.)

24. *The lava then cools & crystallizes to form new seafloor mountains (midocean ridges.) *Younger rock is at the midocean ridges & the older rock is at the continental margins.

25. b. Convergent Boundaries 1. Where plates move together, resulting in the subduction of oceanic lithosphere into the mantle.

26. 2. Types of Convergent Boundaries a. Oceanic-Continental *Forms a subduction zone producing a deep-ocean trench.

28. * Continental volcanic arcs are mountains formed by igneous activity associated with the subduction of oceanic lithosphere beneath a continent. Ex: Andes, Cascades, Sierra Nevada

29. b.) Oceanic-Oceanic * Occurs when two oceanic plates converge, one plate subducts below another.

30. * Volcanic island arc forms as volcanoes emerge from underwater. Ex: Mariana Islands Aleutian Islands

31. c. Continental-Continental * Neither plate will subduct * Causes the continents to fold & buckle upwards forming mountains. Ex: Himalayas, Alps, Appalachians,Urals

32. c. Transform Fault Boundary 1. Where plates grind past one another without the production or destruction of lithosphere

33. 2. Most transform faults are located within the ocean basins, a few cut through continental crust San Andreas Fault

34. C. Evidence 1. Paleomagnetism *Study of ancient magnetism preserved in rocks *Shows the location of the magnetic poles at the time of the rock’s formation.

35. a. Polar Wandering * Study showing that the magnetism of older rocks point in different directions. * Showed that the magnetic poles either moved or the continents moved.

37. * The Earth’s magnetic poles should remain close to the geographic poles * If the magnetic poles don’t move, the apparent movement must have been produced by the continents moving.

38. b. Magnetic Reversals *Earth’s magnetic field reverses its polarity periodically. *These reversals have been shown in the crust of the ocean floor *Proves seafloor spreading.

39. 2. Earthquake Patterns * If earthquake epicenters are plotted on the globe, it shows the outline of the plates.

40. 3. Ocean Drilling *Drilling showed that the youngest rock is near the ridges & the oldest is further from the ridge *Proves seafloor spreading

41. 4. Hot Spots * Rising plumes of magma * As the plate moves above a hot spot, it will create volcanoes that produce underwater mountains and volcanic islands Hawaiian & Midway Islands * Used to track the direction a plate is moving.

42. Hawaiian Islands

43. D. Breakup of Pangaea 1. Fragmented landmasses collided 500 to 225 million years ago to form Pangaea 2. Pangaea began to break apart 200 million years ago during the Jurassic Period

44. 3. Break up formed two subcontinents : Laurasia & Gondwanaland 4. Those subcontinents began dividing about 165 & 100 mya

45. E. Mechanisms & Methods 1.Convection Current Hypothesis *Hotter magma rises in the mantle • *The magma cools and spreads • *The spreading magma moves the plates laterally *The cooled magma then sinks back into the mantle & reheats

47. 2. Slab-Pull & Slab-Push Hypotheses *Colder oceanic plate will sink/subduct, pulling the rest of the plate with it. *As new crust is formed at a rift valley, it pushes the plate forward.

48. 3. Hot Plumes Hypothesis * Plumes of hot magma rise through the mantle, striking and pushing the plates in different directions