Plate Tectonics and Continental Drift: Earth's Changing Landforms

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2. Some questions we will answer in this chapter: • How is the earth always changing? • What forces inside the earth create and change landforms on the surface? • What is the theory of plate tectonics and how does it work? • What two theories help make up the theory of plate tectonics? • What is continental drift and sea floor spreading? • What happens when the plates crash together, pull apart, and slide against each other? SNK Geography 2012-13

3. Different theories about phenomena on earth’s surface • Most of them contradict each other • Not reliable • Eg. • Roman Church • Copernicus • Aristotle • Galileo SNK Geography 2012-13

4. Bases of studies of every discipline SNK Geography 2012-13

5. The Plate Tectonic Theory • One single theory • explains almost all phenomena prevalent in the lithosphere • volcanism, • mid-oceanic ridges, • deep sea trenches, • earthquakes, • mineralization, • mountain-building and many more. • “Paradigm Shift” Yet tip of the iceberg SNK Geography 2012-13

6. To really understand how the earth became to look as it does today, and the theory of plate tectonics, you also need to become familiar with two other ideas: Continental Drift and Seafloor Spreading. SNK Geography 2012-13

7. Continental Drift Theory • In the early 1900s a German explorer and scientist proposed the continental drift theory. He proposed that there was once a single “supercontinent” called Pangaea. SNK Geography 2012-13

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9. The idea of ‘moving’ continents SNK Geography 2012-13

10. Alfred Wegener Proposed hypothesis in 1915 Published The Origin of Continents and Oceans Continental drift hypothesis Supercontinent Pangaea began breaking apart about 200 million years ago Continental drift: An idea before its time SNK Geography 2012-13

11. Evidence used in support of continental drift hypothesis Fit of the continents Fossil evidence Matching rock type and mountain belts Paleoclimatic evidence SNK Geography 2012-13

12. Evidence for Continental Drift • Jigsaw Puzzle fit of continents Alfred Wegener during Greenland expedition SNK Geography 2012-13

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14. More evidence • Matching geologic structures including: • Mountain chains • Ore deposits • Rocks of same age SNK Geography 2012-13

15. The Evidence for Continental Drift- Fossils SNK Geography 2012-13

16. Wegener was not believed!!! Why?? • What could possibly force the continents to move across the ocean floor in this way. They would be crushed. • He was a meteorologist, not a geologist SNK Geography 2012-13

17. Mapping the Mid-Ocean Ridge SNK Geography 2012-13

18. What device is used to map the ocean floor? • The sonar is used to map the ocean floor • Sonar bounces sound waves off underwater objects and then records the echoes of these sound waves • The time it takes for the echo to arrive indicates the distance to the object SNK Geography 2012-13

19. Some thought-provoking observations SNK Geography 2012-13

20. Trend of the Mid-Atlantic Ridge • Similar to the coastlines of adjacent continents SNK Geography 2012-13

21. Oceanic Crust is thin • Both sides of the ridge, oceanic crust is hardly 4 to 5 km thick • Rest all oceans –between 10 to 20 km SNK Geography 2012-13

22. Volcanoes • Continuous chain of volcanoes all along the mid-Atlantic ridge • What would be the effect of this at the ridge? SNK Geography 2012-13

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26. Age of rocks • Youngest near the ridge • As we move away from the ridge, rocks are older in age • No rock in the Atlantic ocean is more than 200 million years old ( The crust is not more than 200 million years old) SNK Geography 2012-13

27. Developments 50s and 60s • World war 2 submarines found mountains under the oceans – the mid-ocean ridges • Theory of seafloor spreading suggested by Princeton professor Dr. Harry Hess SNK Geography 2012-13

28. Sea-Floor Spreading • Sea-floor spreading: The process by which molten material adds new oceanic crust to the ocean floor SNK Geography 2012-13

29. Seafloor spreading First look at the earth’s layers as shown here. SNK Geography 2012-13

30. Pangaea revisited • By piecing together this information, we can see how the continents have moved over the past 200 million years, due to seafloor spreading SNK Geography 2012-13

31. Some questions!! SNK Geography 2012-13

32. If new crust develops from mid-oceanic ridges………. • Where does the old crust go? • Why the amount of lava coming out and forming crust does not reduce? SNK Geography 2012-13

33. Deep sea coast • The sea near the east coast of China was very deep. • Yellow Sea • South China Sea • East China Sea SNK Geography 2012-13

34. Ring of Fire • Chain of volcanic islands running parallel to the east coast of Eurasia • Aleutian • Japan • Phillipines SNK Geography 2012-13

35. Volcanic activity • Intense • Explosive. • Lava different than that of the Mid-Atlantic ridge SNK Geography 2012-13

36. Frequent earthquakes • Japan gets at least 2 earthquakes every day SNK Geography 2012-13

37. Conclusion • As Eurasia is pushed from Mid-Atlantic Ridge, • It pressed against and subsided under the crust which make up the floor of the Pacific Ocean • Constant rubbing of both the plates melts some rocks, magma forms and erupts out forming the islands. Process known as?????????????????????????? SNK Geography 2012-13

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39. Deep-Ocean Trenches • Deep-Ocean Trenches: A deep valley along the ocean floor through which oceanic crust slowly sinks towards the mantle SNK Geography 2012-13

40. What happens to the ocean floor at deep ocean trenches? • At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle, over tens of millions of years SNK Geography 2012-13

41. Plate Tectonics SNK Geography 2012-13

42. According to the theory of plate tectonics, the earth’s outer shell is not one solid piece of rock. Instead the earth’s crust is broken into a number of moving plates. The plates vary in size and thickness. SNK Geography 2012-13

43. 2 Types of Plates • Ocean plates - plates below the oceans • Continental plates - plates below the continents SNK Geography 2012-13

44. What is the Asthenoshere? • The plastic layer below the lithosphere = asthenosphere • The plates of the lithosphere float on the asthenosphere SNK Geography 2012-13

45. Sea-Floor Spreading • Mid oceanic ridges • Magma comes out • Forms new crust • Pushes existing crust on both sides • Drifting continents SNK Geography 2012-13

46. Plate tectonics • Sea floor spreading provides the driving mechanism for movement • However, it is not the continents that are moving, but the “plates” of lithosphere “floating” in effect on the asthenosphere • The lithosphere is made up of about 20 plates which move relative to each other in several ways • Let’s look at a generalized sketch SNK Geography 2012-13

47. The Plates SNK Geography 2012-13

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49. Types of Plate Margins SNK Geography 2012-13

50. DIVERGENT PLATE BOUNDARY • At a divergent plate boundary lithospheric plates move away from each other. • The mid-Atlantic Ridge, a topographically high area near the middle of the Atlantic Ocean, is an example of a divergent plate boundary. • New crustal material SNK Geography 2012-13