Continental Drift and the Theory of Plate Tectonics

1. CONTINENTAL DRIFT and the Theory of PLATE TECTONICS notes #14

2. Contracting Earth Theory - early 20th century • The Earth started out as a molten blob and gradually cooled. • As it cooled, heavier metals (Fe) sank down and formed the core, while lighter metals (Al) stayed up in the crust. • Pressure produced by contraction caused some parts of the crust to buckle upwards, forming mountains. • Other parts buckled downwards, creating ocean basins. • Picture in your mind a grape turning into a raisin as it dries out.

3. Continentaldrift • In 1912, Alfred Wegener proposed the idea that prior to 200 mya, the continents were all joined together in a single Supercontinent called PANGAEA. • He thought they had moved apart over the last 200 million years by sliding across the ocean floor.

6. Evidence for Continental Drift 1. The Shapes Match • The continents look as if they were pieces of a giant jigsaw puzzle. • Because they were one continent that broke up. • Don’t match exactly due to erosion

7. 2. The Plants and Animals Match - Wegener noted that plant and animal fossils of late Paleozoic age found on several different continents were quite similar. Lived on Pangaea.

8. 3. The Rocks Match • Broad belts of rocks in North America and Europe are the same type. These regions match when the edges of the continents are joined. • Mountains formed from collision when Pangaea formed.

9. 4. The Ice Matches • Wegener was aware that an ice sheet covered parts of South America, Africa, India, and Australia about 300 million years ago. Glacial evidence seems random. The direction of the “striations” line up when Pangaea reconstructed

10. His theory was not accepted at the time because he could not explain how the continents moved.

11. New Evidence in the 1950’s • Leads to the proposal and scientific acceptance of the Theory of Plate Tectonics 1. The Earth’s crust is not one solid piece but is rather composed of a number of individual crustal/lithospheric/tectonic plates. Main Points of the Theory

12. Main Points of the Theory 2. The plates “float” on the partially molten and denser asthenosphere 3. They move over the mantle at an average rate of 2-5 cm/year (this is the like the growth rate of your fingernails). 4. The movement is caused by convection currents in the mantle.

15. The earth’s crust floats on the denser mantle beneath it – pg. 10 ESRT 2.7-3.0 g/cm3 3.4-5.6 g/cm3 Glenco McGraw-Hill

16. Plates and Plate Boundaries • The crustal (lithospheric) plates typically contain oceanic and continental crust. • As the plates move, they can separate, collide, or slide past one another. • This results in three kinds of plate boundaries animations • Divergent -apart • Convergent-together • Transform-slide side by side • Look at page 5 in ESRT

18. Where did this Theory come from? • In the 1950’s the global mid-ocean ridge system was discovered. • It is an immense submarine mountain chain that zigzags between the continents around the globe. • It is more than 50,000 kilometers (km) long and up to 800 km across.

19. Mid-ocean ridges circle the globe like the seams on a baseball

20. Divergent Boundaries on land Plates move away from each other, crust cracks. Gap is replaced by lava (volcanoes or fissures) -cools and creates new basaltic crust. 2. The widening region is called a rift zone or rift valley.

21. This process caused the breaking up of Pangaea. • Short lived on land because valley becomes ocean floor. Ex. The Great Rift Valley in Africa

24. Since basalt is denser and thinner than the continents, it gets covered with sea water. • The ocean widens along a mid ocean ridge, called “sea-floor spreading”, and becomes large enough for a ridge/rift system.

25. link

27. Mid-Atlantic Ridge-longest mountain chain in the world

28. “New” pillow basalt from the Mid-Atlantic Ridge

29. Convergent Boundaries • When two plates come together they collide • The edge of each plate may be continental or oceanic crust so three types of convergent boundaries exist. • Oceanic Plate - Oceanic Plate • Oceanic plate - Continental Plate • Continental Plate - Continental plate

30. A. Oceanic - Oceanic Convergence (common) • When two ocean plates converge, one is a little denser and subducts, is taken down, beneath the other. • This creates deep trenches and basaltic volcanic island arcs due to the partial melting of the subducting plate. • Japan, the Philippines, and the Aleutian Islands

32. The “Ring of Fire” is a string of volcanic arcs & islands caused by subduction zones that surround most of the Pacific Plate/Ocean

33. These ocean trenches are the deepest parts the world’s ocean • The deepest is “Challenger Deep” in the Mariana’s Trench at 11,035 meters

34. B. Oceanic - Continental Convergence link • Oceanic crust is thinner and denser than continental, so it “subducts” underneath the continental crust. • This creates shallow trenches that fill in with sedimentary “clastic wedge”. • A volcanic arc is formed on the continent.

36. C. Continental-Continental (pretty rare) • When continental plates meet, neither one is subducted because both plates are thick with low density. • Instead, one may slide under the other and the crust buckles upward or sideways and forms tall mountain ranges. • (animation)

37. The Himalayan Mountains (Everest) formed when the Indian Plate collided with the Eurasian Plate starting 10 mya

38. Mt. Everest is the world’s highest point at 29,028 ft.

40. Transform Boundaries • Two plates slide past one another • Most are found on the ocean floor as “fracture zones” that help offset the movement of seafloor spreading along divergent boundaries.

41. A few occur on land • The San Andreas Fault in CA is about 1,300 km long. • The Pacific Plate has been grinding past the North American Plate at the rate of 5 cm/yr, • resulting in often severe earthquake activity

43. Cause of TectonicsConvection Currents • Heated material from within the earth rises since it is less dense • As this heated material reaches the surface, it spreads out, carrying the crustal plates with it. • It then cools, increases density and sinks back into the mantle

44. Mechanisms of the movement • Ridge-push causes oceanic lithosphere to slide down the sides of the raised oceanic ridge under the pull of gravity - It may contribute to plate motion • Slab-pull is when cool, dense oceanic crust sinks into the mantle and “pulls” the trailing lithosphere along.

45. The Future • What is the predicted future position of the continents? • No one knows for sure but using current plate motions, scientists can predict what they thing the earth will look like.

46. Supporting Evidence for the Theory of Plate Tectonics

47. 1. Earthquakes and volcano patterns

49. Mid-Ocean Ridges/Transform • Earthquakes and volcanoes occur at ridges when rock breaks while being uplifted and split. • Earthquakes occur at transform boundaries when rocks grind against opposite plate.

50. Subduction Zones • Earthquakes occur in the oceanic plate as it subducts, bends, and breaks • This results in shallow and deep focus earthquakes • 670 km is maximum depth due to softening as temperatures rise.