Plate Tectonics

1. Plate Tectonics

2. Topics of Discussion Layers of Earth Continental Drift Seafloor Spreading Plate Tectonics Mountains

3. Layers of Earth

4. Clues to Earth�s Interior Although the best way to find out what�s inside Earth might be to dig a tunnel to its center, that isn�t possible. Geologists must use indirect observations to gather clues about what Earth�s interior is made of and how it is structured. This indirect evidence includes information learned by studying earthquakes and rocks that are exposed at Earth�s surface helped to locate the layers of Earth.

6. Rock Clues Certain rocks are found in different places on Earth�s surface. The rocks formed far below the surface are made of material similar to what is thought to exist deep inside Earth. Forces inside Earth pushed them closer to the surface, there they eventually were exposed by erosion.

7. Earth�s Layers Based on evidence from earthquake waves and exposed rocks, scientists have produced a model of Earth�s interior. The model shows that Earth�s interior has at least four distinct layers�the inner core, the outer core, the mantle, and the crust.

8. Crust Earth�s outermost layer is the crust. The crust is 5 to 100 km thick, and is the thinnest layer of the Earth. It is thinnest under the oceans and thickest through the continents.

9. Crust There are two types of crust�continental and oceanic. Oceanic crust is thinner and denser than continental crust.

10. Mantle The layer in Earth�s interior above the outer core is the mantle. The mantle is the largest layer of Earth�s interior.

11. Outer Core The outer core lies above the inner core and is thought to be composed mostly of molten metal. The outer core stops one type of seismic wave and slows down another. Because of this, scientists have concluded that the outer core is a liquid.

12. Inner Core The innermost layer of Earth�s interior is the solid inner core. This part of the core is dense and composed mostly of solid iron. At about 5,000�C, the inner core is the hottest part of Earth.

13. Continental Drift

14. Evidence for Continental Drift

15. Pangaea In 1912, German scientist Alfred Wegener (VEG nur) proposed the hypothesis of continental drift. According to the hypothesis of continental drift, continents have moved slowly to their current locations. Wegener suggested that all continents once were connected as one landmass that broke apart about 200 million years ago.

16. Pangaea He called this large landmass Pangaea, which means �all land.�

17. A Controversial Idea Wegener�s ideas about continental drift were controversial. It wasn�t until long after Wegener�s death in 1930 that his basic hypothesis was accepted.

18. A Controversial Idea He was unable to explain exactly how the continents drifted apart. He proposed that the continents plowed through the ocean floor, driven by the spin of Earth. Physicists and geologists of the time pointed out that continental drift would not be necessary to explain many of Wegener�s observations.

19. Fossil Clues Fossils provided support for continental drift. Fossils of the reptile Mesosaurus have been found in South America and Africa.

20. Fossil Clues This swimming reptile lived in freshwater and on land. How could fossils of Mesosaurus be found on land areas separated by a large ocean of salt water? Wegener hypothesized that this reptile lived on both continents when they were joined.

21. A Widespread Plant This fossil plant Glossopteris (glahs AHP tur us) has been found in Africa, Australia, India, South America, and Antarctica. The presence of Glossopteris in so many area also supported Wegener�s idea that all of these regions once were connected and had similar climates.

22. Climate Clues Wegener hypothesized that Spitsbergen, an island in the Artic Ocean, drifted from tropical regions to the arctic since fossils of warm-weather plants were found there. Glacial deposits and rock surfaces created by glaciers are found in South America, Africa, India, and Australia. This shows that parts of these continents were covered with glaciers in the past.

23. Rock Clues Similar rock structures are found on different continents. Parts of the Appalachian Mountains of the eastern United States are similar to those found in Greenland and western Europe. Rock clues like these support the idea that the continents were connected in the past.

24. How could continents drift? Although Wegener provided the fossil, rock, and climate evidence to support his hypothesis of continental drift, he couldn�t explain how, when, or why these changes took place.

25. How could continents drift? Wegener�s idea of continental drift was initially rejected Rock, fossil, and climate clues were NOT enough to �Prove� continents drifted. After Wegener�s death, more clues were found, largely because of advances in technology, and new ideas that related to continental drift were developed. The idea that helped his theory the most was seafloor spreading (next section).

26. Seafloor Spreading

27. Mapping the Ocean Floor If you were to lower a rope from a boat until it reached the seafloor, you could record the depth of the ocean at that particular point. This is exactly how it was done until World War I, when the use of sound waves was introduced by German scientists to detect submarines.

29. The Seafloor Moves In the early 1960s, Princeton University scientist Harry Hess suggested an explanation- the ocean floor moves. His now-famous theory is known as seafloor spreading. Hess proposed that hot, less dense material below Earth�s crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions.

30. The Seafloor Moves As the seafloor spreads apart at a mid-ocean ridge, new seafloor is created. The older seafloor moves away from the ridge in opposite directions.

31. Evidence for Spreading In 1968, scientists aboard the research ship Glomar Challenger began gathering information about the rocks on the seafloor. Scientists found that the youngest rocks are located near the mid-ocean ridges. The ages of rocks become increasingly older in samples obtained farther from the ridges, adding to the evidence for seafloor spreading.

32. Evidence for Spreading As molten material is forced upward along the ridges, it brings heat and chemicals that support exotic life-forms in deep, ocean water.

33. Magnetic Clues During a magnetic reversal, the lines of magnetic force run the opposite way. Scientists have determined that Earth�s magnetic field has reversed itself many times in the past. The reversals are recorded in rocks forming along mid-ocean ridges.

34. Magnetic Time Scale

35. Magnetic Time Scale This discovery provided strong support that seafloor spreading was indeed occurring. This helped explain how the crust could move�something that the continental drift hypothesis could not do.

36. Plate Tectonics

37. Plate Tectonics The idea of seafloor spreading showed that more than just continents were moving, as Wegener had thought. It was now clear to scientists that sections of the seafloor and continents move in relation to one another.

38. Plate Tectonics In the 1960s, scientist developed a new theory called plate tectonics that combined the ideas of continental drift and seafloor spreading. The basis being that Earth�s crust and upper mantle are divided into 30 large sections called plates. According to the new theory, Earth�s crust and part of the upper mantle are broken into sections called plates that move on a plastic-like layer of the mantle.

39. Composition of Earth�s Plates These two parts combined are the lithosphere The plastic-like layer below the lithosphere is called the asthenosphere.

40. Earth�s Plates Earth�s plates fit together like the pieces of a jigsaw puzzle. The plates have not always been their current size and shape, and continents have moved great distances.

42. Causes of Plate Tectonics� Convection Inside Earth The cycle of heating, rising, cooling, and sinking is called a convection current (think of cooking soup in a pan). A version of this same process, occurring in the mantle, is thought to be the force behind plate tectonics. Scientists suggest that differences in density cause hot, plastic-like rock to be forced upward toward the surface.

43. Moving Mantle Material

44. Plate Boundaries The places where the edges of different plates meet are called plate boundaries. When plates move, they interact in several ways. They can collide, pull apart, or slide past each other. When the plates interact, the result of their movement is seen at the plate boundaries. Movement along any plate boundary means that changes must happen at other boundaries.

45. Plate�s Boundaries The constant movement of plates creates forces that affect Earth�s surface at the boundaries of the plates. At some boundaries, these forces are large enough to cause mountains to form. Other boundaries form huge rift valleys with active volcanoes.

46. Plate�s Boundaries At a third type of boundary, huge faults form. Faults are large fractures in rocks along which movement occurs. The movement can cause earthquakes.

47. Divergent Boundary The boundary between two plates that are moving apart is called a divergent boundary. In the Atlantic Ocean, the North American Plate is moving away from the Eurasian and the African Plates this divergent boundary is called the Mid-Atlantic Ridge. The pulling force is called tension. The most common result of plates separating is the formation of new crust when magma pushes up between the plates.

48. Convergent Boundary The boundary between two plates that are moving together is called a convergent boundary. The force of pushing to plates together is called compression. As new crust is added in one place, it disappears below the surface at another. The disappearance of crust can occur when seafloor cools, becomes denser, and sinks.

49. When plates move toward each other, they collide, causing several different things to occur. The outcome depends on the density of the two plates involved. When an oceanic plate converges with a less dense continental plate, the denser oceanic plate sinks under the continental plate. The area where an oceanic plate subducts, or goes down, into the mantle is called a subduction zone. This type of boundary can create a deep-sea trench Some volcanoes form above subduction zones High temperatures cause rock to melt around the subducting slab as it goes under the other plate. Convergent Boundary

50. Convergent Boundary A subduction zone also can form where two oceanic plates converge. In this case, the colder, older, denser oceanic plate bends and sinks down into the mantle.

51. Convergent Boundary

52. Transform Boundary The boundary between two plates that are sliding past each other is called a transform boundary. They move in opposite directions or in the same direction at different rates. The force of plates sliding is called shearing. Faults and earthquakes form between the plates.

53. Plate Tectonic Features The interaction of plates produces forces that change Earth�s surface Folding Faulting Build mountains Rift Valleys Ocean Features Mountains and Volcanoes Earthquakes

54. Folding The bending of rock layers because of stress in the Earth�s crust is called folding. Depending on how rock layers deform, different types of folds are made. The major types of folds are: Anticlines are upward-arching folds. Synclines are downward, trough like folds. Monoclines are horizontal folds

55. Features of Plate Tectonics Divergent Boundaries cause Normal faults Rift Valleys Mid-Ocean Ridges Convergent Boundaries cause Reverse Faults Mountains (covered in next section) Volcanoes (covered in following unit) Transform Boundaries cause Strike Slip Faults

56. Faulting The surface along which rocks break and slide past each other is called a fault. The blocks of crust on each side of the fault are called fault blocks.

57. Testing for Plate Tectonics Until recently, the only tests scientists could use to check for plate movement were indirect. They could study the magnetic characteristics of rocks on the seafloor, volcanoes, earthquakes. This supported the theory but did not prove that the plates were moving.

58. Testing for Plate Tectonics One new method uses lasers and satellites. Using such methods, scientists have observed that the plates move at rates ranging from about 1 cm to 12 cm per year. Satellite Laser Ranging System data show that Hawaii is moving toward Japan at a rate of about 8.3 cm per year.

59. Mountains

60. Building Mountains There are four main types of mountains� fault-block, folded, upwarped, and volcanic. Each type forms in a different way and can produce mountains that vary greatly in size. The ruggedness of a mountain chain depends largely on whether or not it is still forming. Mountains like the Himalaya are currently forming at a rate of several centimeters per year, while much older mountains like the Ouachita Mountains in Arkansas stopped forming millions of years ago and are now being eroded by geological processes.

61. Fault-Block Mountains Fault-block mountains are made of huge, tilted blocks of rock that are separated from surrounding rock by faults. Example: Teton Range in Wyoming When rock layers are pulled by opposite directions (tension), large blocks slide downward, creating peaks and valleys.

62. Folded Mountains This causes the rock layers to buckle and fold, forming folded mountains.

63. Upwarped Mountains Sedimentary rock layers on top erode exposing the igneous or metamorphic rocks underneath. The rocks can erode further to form sharp peaks and ridges.

64. Mountains and Volcanoes When two oceanic plates converge, the denser plate is forced beneath the other plate. Curved chains of volcanic islands called island arcs form above the sinking plate. If an oceanic plate converges with a continental plate, the denser oceanic plate slides under the continental plate. Folding and faulting at the continental plate margin can thicken the continental crust to produce mountain ranges.

65. Volcanic Mountains Occasionally, magma from inside Earth reaches the surface. When this happens, the magma is called lava. When hot, molten lava flows onto Earth�s surface, volcanic mountains can form. Layer upon layer of lava piles up until a cone-shaped feature called a volcanic mountain forms.

66. Underwater Volcanic Mountains Underwater eruptions can produce mountains beneath the sea. Eventually, if enough lava is erupted, these mountains grow above sea level. The Hawaiian Islands are a series of volcanic mountains that have been built upward from the seafloor.