Plate Tectonics & The Ocean Floor

1. Plate Tectonics & The Ocean Floor

2. Oceanic Crust Composed of basalt Crust beneath oceans Younger crust More dense ~ 3 g/cm3 Continental Crust Composed of granite Crust beneath continents Older crust Less dense ~ 2.7 g/cm3 Recall: Structure of the Earth Crust: Thinnest & outermost layer of the Earth

3. Lithospheric Plates * Lithosphere: made up of the crust and the upper mantle

5. Continental Drift Theory • First proposed by Alfred Wegener in 1915 • Proposed that a supercontinent, Pangaea, once existed and has since broken apart into the continents we know today • Panthalassa “all seas” was the single ocean. • Tethys Sea was a small inland sea that eventually divided Pangaea into two land masses. Laurasia in the north and Gonwanaland in the south.

6. 200 million years ago

7. What Evidence supports Continental Drift? • Jigsaw puzzle-like continental coastlines • Fossils on opposite coastlines were similar in type and age.

8. What Evidence supports Continental Drift? • Similar rock types andstructures of matching mountainranges

9. What Evidence supports Continental Drift? • Evidence of glaciation in tropical areas in low • latitudes. Today 200 mya

10. Objections to Continental Drift • One of the biggest objection was his explanation of “how” the continents could have drifted apart. • He suggested that gravity and tidal forces allowed for continental crust to plow through oceanic crust of the ocean basins.

11. Plate Tectonics • Movement of lithospheric plates due to convection currents and mantle plumes • Sinking of dense, cool magma due to gravity is replaced with less dense, warmer magma • Movement of these huge plates creates mountains, volcanoes, oceanic ridges, and trenches

12. Evidence for Plate Tectonics • Paleomagnetism • Sea Floor Spreading • Ocean drilling • Earthquake Patterns • Hot Spots

13. Paleomagnetism • Earth’s magnetic field has reversed many times since Earth’s formation • Iron is magnetic. • As magma cools to form igneous rocks, the atoms of iron align them selves with the magneticfield of the earth.

14. Paleomagnetism • Using the position of the iron atoms, scientist can determine the location of the magnetic pole has reversed in the past. • Using radiometric dating, they have determined that the Earth’s magnetic field has reversed about 1000 times over the past 76 million years.

15. Magnetic Pole Reversal Are we currently experiencing a polar reversal? In the news….

16. Sea Floor Spreading • Sea floor features were discovered from depth recordings of Capt. Harry Hess. • His soundings (depth recorder) showed • Extensive mountain ranges near the center of ocean basins. • Deep trenches near the edge of ocean basins where crustal plates are subducted and destroyed.

17. Sea Floor Spreading • From the soundings, Hess suggested that new crust forms at the mid-ocean ridges, while older crust is found at the trenches, where the process of subduction occurs.

18. Ocean Drilling • Showed that the age and thickness of sediment increased with increasing distance from the mid-ocean ridge

19. Earthquake Patterns • Earthquakes occur in areas of plate movement, mainly along plate boundaries.

21. Hot Spots • Shows movement of volcanic activity in middle of oceanic plates

22. Acceptance of a Theory • Finally in the 1960’s the continental drift theory and sea floor spreading were united as the Plate Tectonics Theory. • The theory explains that tectonicplates that make up the lithosphere “float” on the somewhat “fluid” asthenosphere. Click Me

23. Types of Plate Boundaries • Divergent Plate Boundaries • Convergent Plate Boundaries • Transform Faults

24. Divergent Plate Boundaries  • Mid-ocean ridges form along divergent boundaries. • Magma pushes continental crust up causing volcanic activity. • Plates begin to move apart in opposite directions causing crustal material to slump downward, creating a rift valley. See animation.. Click here!!

25. Divergent Plate Boundaries  • A linear sea is formed as water fills in the valley. • Through continued divergence, an oceanbasin forms, creating an ocean. See animation.. Click here!!

27. Divergent Plate Boundaries  • Mid-ocean ridges have oceanicrises and oceanic ridges. • Oceanic rises  gentlysloping due to a fast rate of spreading. Ex: East Pacific Rise

28. East Pacific Rise

29. Divergent Plate Boundaries  • Oceanic ridges  have steeperslopes due to a slower rate of spreading. Ex: Mid-Atlantic Ridge

30. Iceland and Mid-ocean Ridge

31. Convergent Plate Boundaries  • Plates are pushed together • Oceanic crust is subducted and absorbed into the mantle creating a subduction zone • Producing a deep-ocean trench and volcanic arc.

32. Three types of Convergence • Oceanic/ Continental (Fig 1) • Creates a subduction zone • Subduction creates a deeptrench • On the continental plate, a continentalarc forms parallel to the trench. • Ex: Cascade Mountains

33. Cascade Mountains

34. Three types of convergence • Oceanic/ Oceanic (Fig 2) • Subduction creates a deep trench. • As one oceanic plate subducts, a chain of volcanic mountains form, producing an island arc. Ex: Aleutian Islands & Mariana Trench

35. Aleutian Island Arc

36. Marianas Trench

37. Three types of convergence • Continental/ Continental (Fig 3) • As two continental plates collide, they buckle, fold and push upward to produce a mountain range. Ex: Himalayas

38. Transform Faults/Boundaries • Plates slide past one another along faults in the lithospheric plate • Crustal material is neither created nor destroyed Ex: San Andreas Fault

39. San Andreas Fault

40. Hot Spots & Mantle Plumes • Hotspots are areas of continued volcanic activity NOT associated with plate boundaries. Ex: Hawaiian Islands & YellowstoneNationalPark • Hot spots are fueled by regions of rising molten rock called mantleplumes. (pg 62)

41. Convection Cells and Magma Plumes

42. Seamounts & Tablemounts • Underwater volcanoes are called seamounts. • As seamounts break the surface of the ocean, they may become islands. • Over time, the seamount’s peak erodes away as the seamount subsides, eventually becoming a flat-top tablemount or Guyot.

43. Hawaiian-Emperor Seamount

44. Coral Reef Formation • Fringing Reef: grow close to land, associated with active volcanic islands. Ex: Kurumba Island in the Maldives

45. Coral Reef Formation • Barrier Reef: Linear or circular reefs separated from land by a lagoon of water. Ex: Great Barrier Reef

46. Coral Reef Formation • Atoll: By this stage, the volcano is completely submerged. All that remains is the reef and a lagoon. Ex: Atafuin S. Pacific Click Me

48. The Big Picture

49. 1-Asthenosphere 2-Lithosphere 3-Hot spot 4-Oceanic crust 5-Subducting plate 6-Continental crust 7-Continental rift zone (young plate boundary) 8-Convergent boundary plate 9-Divergent boundary plate 10-Transform plate boundary 11-Shield volcano 12-Oceanic spreading ridge 13-Convergent plate boundary 14-Strato volcano 15-Island arc 16-Plate 17-Asthenosphere 18-Trench

50. And finally . . .