Chapter 9outline

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Chapter 9outline

  1. 1. Chapter 9 Plate tectonics
  2. 2. An Idea Before Its Time <ul><li>An Idea Before Its Time </li></ul><ul><li>1915 – _____________ proposed a radical idea he called __________________ . </li></ul><ul><li>Wegener’s continental drift hypothesis stated that the continents had once been joined to form a single supercontinent . He called the supercontinent _______________ (meaning all land ). He hypothesized that about 200 million years ago, Pangea began breaking into smaller continents which then drifted to their present positions. </li></ul><ul><li>His hypothesis was not officially recognized until the 1960s. Why? </li></ul>
  3. 3. Evidence: The Continental Puzzle <ul><li>1. ______________________________ . </li></ul><ul><li>Example: South America and Africa </li></ul><ul><li>Critics of this idea have rightly stated the shorelines are under constant change by erosion processes. </li></ul>
  4. 5. <ul><li>2. ________________ – This evidence includes several fossils found on different landmasses. Wegener concluded that these organisms could not have crossed the oceans that are separating the continents. </li></ul><ul><li>Example: Mesosaurus – whose fossil remains, are found on the eastern half of South America and southern Africa. This reptile could not have swum the Atlantic Ocean to reach these locations. </li></ul>
  5. 7. Evidence for Continental Drift
  6. 8. <ul><li>Extra credit – There is a frog that lives on the Seychelles Islands of the eastern coast of Africa. Are these frogs evidence of continental drift? Is so, how? </li></ul><ul><li>Send your answer by email or place your name on paper with your answer </li></ul>
  7. 9. <ul><li>3. _____________________ – Some rock types and mountain belts match up. </li></ul><ul><li>Does this mean that they were once connected on Pangea? </li></ul><ul><li>Example: The _____________ Mountains along with the British Isles and Caledonian Mountains in Scandinavia form a single mountain chain when the landmasses are placed, as they would have been in Pangea. </li></ul><ul><li>Example: Rocks in South America and Africa are of the same type, composition, and age. </li></ul>
  8. 11. <ul><li>4. Ancient Climates (________________) – Example: Wegener found evidence of ice ages 220 – 300 million years ago which covered large sections of the Southern Hemisphere. Glacial (till) deposits are found on South America, Africa, India, and Australia. </li></ul><ul><li>The scratches (striations) and grooves appear to have moved from what is now the sea onto land. Much of the land that appears to have been glaciated near the equator. </li></ul><ul><li>Example: During this same period, large tropical swamps existed in the Northern Hemisphere that resulted in coal formation later on. Pennsylvania was once covered in these swamps. </li></ul>
  9. 13. <ul><li>Fossil __________ </li></ul><ul><li>_________________ are well preserved in the sediment layers in the bottom of a pond, lake or ocean, an analysis of the pollen grains in each layer tell us what kinds of plants were growing at the time the sediment was deposited. </li></ul>
  10. 15. <ul><li>Wegener’s hypothesis met stiff resistance because he could not explain how continents could move. </li></ul><ul><li>A New Theory Emerges </li></ul><ul><li>As technology advanced new evidence like mapping the ocean floor led to a new theory called ____________________ . </li></ul>
  11. 16. <ul><li>Earth Major Plates – According to the plate tectonic theory, the uppermost mantle, along with the overlying crust, behaves as a strong, rigid layer. This is known as the _______________ . </li></ul><ul><li>The lithosphere lies over a weaker portion of the mantle known as the ______________ . </li></ul><ul><li>The lithosphere is divided in segments called _______________ , which move and continually change shape. </li></ul><ul><li>The lithospheric plates move relative to each other at a very slow but continuous rate that averages 5 centimeters per year. </li></ul>
  12. 17. <ul><li>But how do the plates move? </li></ul><ul><li>There is an _________________________ within the Earth. </li></ul><ul><li>Hot mantle material becomes ____________ and begins to rise. </li></ul><ul><li>As this material approaches the lithosphere it begins to cool and slowly sinks deeper into the mantle where it begins to heat up again. </li></ul><ul><li>Doesn’t this sound like a ______________ ? </li></ul><ul><li>As the mantle material moves along the bottom of the lithosphere, _____________ is created provides the force which moves the plates. </li></ul><ul><li>As the plates move, they grind together creating volcanoes and earthquakes. </li></ul>
  13. 19. Types of Plate Boundaries <ul><li>There are three main types of boundaries </li></ul><ul><li>______________________ (spreading centers) – these occur where plates are moving apart. This process creates upwelling and forms new seafloor. </li></ul><ul><li>Examples: the mid-Atlantic ridge and the east coast of Africa. </li></ul>
  14. 21. <ul><li>________________________ – form where two plates move together. </li></ul><ul><li>This process has two scenarios </li></ul><ul><li>1. Ocean crust colliding with continental crust </li></ul><ul><li>Example: west coast of South America, Japan, Aleutian Islands, Philippines </li></ul><ul><li>This generally produces a ______________ resulting in an oceanic trench. </li></ul><ul><li>2. Continent to continent collision </li></ul><ul><li>Example: _____________ and Asia collided to create the Himalayas. </li></ul>
  15. 23. <ul><li>__________________________ – these occur where two plates are grinding past each other without the production or destruction of the lithosphere. </li></ul><ul><li>Example: The ______________ fault in California and sections of the mid-Atlantic ridge move independently of each other along transform boundaries. </li></ul>
  16. 25. <ul><li>What will the future look like? </li></ul><ul><li>Will the continents ever form a large supercontinent again? </li></ul><ul><li>Evidence suggests that if movement continues as it is today, a new supercontinent will from some where in the Pacific Ocean in about 300 million years. </li></ul>
  17. 26. Actions at Plate Boundaries <ul><li>_______________________ – most are located along the crests of oceanic ridges. These boundaries are ___________________ plate margins because new oceanic lithosphere is generated here. </li></ul>
  18. 27. <ul><li>Oceanic Ridges and Seafloor Spreading </li></ul><ul><li>Along well-developed divergent plate boundaries, the seafloor is elevated, forming the oceanic ridge. </li></ul><ul><li>These ridges are the longest physical feature on Earth. </li></ul><ul><li>______________ are found on most ridge segments. Spreading centers like these can be found on land. </li></ul><ul><li>Can you name a place on land where this is happening? _____________ </li></ul>
  19. 29. <ul><li>Seafloor spreading is the process by which plate tectonics produces new oceanic lithosphere. </li></ul><ul><li>Typically, spreading averages around 5 centimeters per year. This speed is fast geologically in that there is no ocean floor older than 180 million years. </li></ul>
  20. 31. <ul><li>Continental Rifts </li></ul><ul><li>When spreading centers develop within a continent, the land mass may split into two or more smaller segments. </li></ul><ul><li>Examples: _____________ rift valley, ________ Valley in Northwest Europe. </li></ul>
  21. 33. <ul><li>Most accepted model of continental break up – ___________ , where plumes of magma pushes into the crust, uplifting and stretching the crust. </li></ul><ul><li>A rift valley forms. If the stretching continues, the rift valley will lengthen and deepen, until the continent splits. At this point the rift becomes a sea. </li></ul><ul><li>Example: ____________ formed when the Arabian Peninsula rifted from Africa about 20 million years ago. </li></ul>
  22. 35. Evidence of hot spot rifting <ul><li>Hot spot rifts usually occur with three arms. </li></ul><ul><li>Arms that link together are ________ arms….. </li></ul><ul><li>Arms that do not link are inactive arms… </li></ul><ul><li>What happens to the inactive arms? </li></ul><ul><li>They tend to form ______________ </li></ul>
  23. 36. How are plates broken ????? <ul><li>Hotspots - 3 armed centers of volcanism. They can fracture the crust </li></ul>
  24. 37. If Hot Spots Connect….. <ul><li>Active arms connect </li></ul>Failed arms do not connect
  25. 38. Failed Arms Become ……………. <ul><li>River valleys </li></ul>
  26. 41. <ul><li>Convergent Boundaries </li></ul><ul><li>These boundaries return older portions of the lithosphere back to the mantle. </li></ul><ul><li>Because lithosphere is “destroyed” at convergent boundaries, they are called _______________plate margins . </li></ul><ul><li>Destructive plate margins where oceanic crust is being pushed into the mantle are _____________________ . </li></ul><ul><li>The surface feature created is a ______ . </li></ul>
  27. 42. <ul><li>There are three types of convergent boundaries: </li></ul><ul><li>1. Oceanic – Continental - When an oceanic plate subducts beneath continental lithosphere, a _________________ develops along the margin of the continent. </li></ul><ul><li>Oceanic crust subducts because it is denser than continental crust. Oceanic crust sinks into the asthenosphere and melts. </li></ul><ul><li>The melted magma begins to rise (being less dense that the surrounding rock) toward the surface and may be part of a volcanic eruption someday. </li></ul>
  28. 43. <ul><li>Example: west coast of ______________ where the Nazca plate descends beneath South America producing magma that helped make the Andes Mountains. </li></ul><ul><li>The ___________ are an example of a continental volcanic arc . </li></ul>
  29. 45. <ul><li>2. Oceanic-Oceanic – When two oceanic slabs converge, one descends beneath the other. </li></ul><ul><li>Volcanoes build on the ocean floor. If activity continues, it will eventually build a chain of volcanic structures that become islands. </li></ul><ul><li>This newly formed land consisting of an arc shaped chain of small volcanic islands is called a ___________________ . </li></ul><ul><li>The Aleutian Islands is an example of this activity. </li></ul>
  30. 47. <ul><li>3. Continent – Continent – When two continents collide the lithosphere is buoyant, which prevents it from being subducted. </li></ul><ul><li>The result is the formation of ___________________ such as the Himalayas. </li></ul><ul><li>In this case, oceanic crust was deformed and metamorphosed sedimentary rocks. </li></ul><ul><li>At the present time, India is colliding with Eurasia. </li></ul><ul><li>The Himalayas are presently still on the rise. </li></ul><ul><li>Other examples of continent-to-continent collision include the Alps, Appalachians, and Urals. </li></ul>
  31. 49. <ul><li>Transform Fault Boundaries </li></ul><ul><li>Here, plates grind past each other without destroying the lithosphere. </li></ul><ul><li>Most transform faults join two segments of a mid-ocean ridge. The seafloor produced at one ridge axis moves in the opposite direction as seafloor is produced at an opposing ridge segment. </li></ul><ul><li>Another example of a transform fault boundary is the _______________ Fault. </li></ul><ul><li>Here the Pacific plate is moving to the northwest at about the same rate, as the North American plate is moving west. </li></ul><ul><li>If this movement continues, all the land west of the fault will break and become an island and could eventually reach Alaska. </li></ul>
  32. 51. Testing Plate Tectonics <ul><li>Evidence of Plate Tectonics   </li></ul><ul><li>1. __________________ – The magnetic poles align closely, but not exactly with the geographic poles. In may ways the Earth’s magnetic field is much like a bar magnet. </li></ul><ul><li>Certain rocks contain iron-rich minerals, such as magnetite. </li></ul><ul><li>When heated above a certain temperature, these magnetic minerals loose their magnetism. </li></ul>
  33. 52. <ul><li>When the iron-rich mineral grains cool down they become magnetized in the direction parallel to the existing magnetic field. </li></ul><ul><li>Once the minerals solidify, the magnetism they possess stays frozen in this position. </li></ul><ul><li>If the rock is moved or if the magnetic pole changes position, the magnetism retains its original alignment. </li></ul><ul><li>Rocks formed millions of years ago show the location of the magnetic poles at the time of their formation. </li></ul>
  34. 55. <ul><li>Geophysics learned that Earth’s magnetic field periodically reverses polarity . A rock forming at this time will be magnetized with the polarity opposite that of the rocks formed today. </li></ul><ul><li>Rocks formed today have normal polarity . </li></ul><ul><li>Rocks that show opposite magnetism are said to have reverse polarity . </li></ul><ul><li>The discovery of strips of alternating polarity, which lie as mirror images across the ocean ridges, is among the strongest evidence of seafloor spreading. </li></ul>
  35. 57. <ul><li>2. _________________________ </li></ul><ul><li>Scientists found a close link between deep-focus earthquakes and ocean trenches. </li></ul><ul><li>Also, the absence of deep-focus earthquakes along the oceanic ridge system was shown to be consistent with the new theory. </li></ul>
  36. 61. <ul><li>3. Ocean Drilling </li></ul><ul><li>Glomar Challenger drilled hundreds of holes into the ocean floor removing core samples. </li></ul><ul><li>When the oldest sediment from each drill site was plotted against distance from the ridge crest, it was revealed that the age of the sediment _______________with increasing distance from the ridge . </li></ul><ul><li>This data confirmed the seafloor-spreading hypothesis. </li></ul><ul><li>The youngest oceanic crust is at the ridge crest and the oldest oceanic crust is at the continental margins. </li></ul>
  37. 62. <ul><li>4. Hot Spots </li></ul><ul><li>Mapping the Pacific seafloor volcanoes revealed a chain of volcanic structures extending from the Hawaiian Islands to Midway Islands and then north to the Aleutian trench. </li></ul><ul><li>Volcanoes in this chain, age with increasing distance from Hawaii. </li></ul><ul><li>The Suiko Seamount (Aleutian trench) is 65 million years old. </li></ul><ul><li>Midway Island is 27 million years old. </li></ul><ul><li>The island of Hawaii is less than a million years old. </li></ul>
  38. 63. <ul><li>A rising plume of mantle material is located below Hawaii. </li></ul><ul><li>Melting of this hot rock as it nears the surface creates a volcanic area, or hot spot . </li></ul><ul><li>As the Pacific plate moves over the hot spot, successive volcanic mountains have been created. </li></ul><ul><li>Kauai is the oldest of the large islands and its volcanoes are extinct. </li></ul><ul><li>Hawaii has two active volcanoes Mauna Loa and Kilauea. </li></ul>
  39. 67. Mechanics of Plate Motion <ul><li>Scientists generally agree that convection occurring in the mantle is the basic driving force for plate movement.   </li></ul><ul><li>The motion of matter resulting from convection is called ________________ . </li></ul>
  40. 68. <ul><li>Slab-Pull and Ridge-Push </li></ul><ul><li>1. _____________ – Occurs when old ocean crust, which is relatively cool and dense, sinks into the asthenosphere and “pulls” the trailing lithosphere along. This is thought to be the primary downward arm of convection flow in the mantle . </li></ul><ul><li>2. ____________ – results from the elevated position of the oceanic ridge system. Ridge-push causes oceanic lithosphere to slide down the sides of the oceanic ridge. </li></ul>
  41. 69. <ul><li>Mantle Convection </li></ul><ul><li>The __________________________ within the Earth causes the thermal convection on the mantle that ultimately drives plate motion. </li></ul>
  42. 72. <ul><li>Earth's Formation: </li></ul><ul><li>1 - When the earth originated it contained no continents, and consisted of only a few kinds of igneous rock.   </li></ul><ul><li>2. A second significant idea derived from plate tectonic theory is that as the continents have grown through time they have alternately come together to form super continents . </li></ul><ul><li>Last super continent was named – ___________ , 300 million years ago. </li></ul><ul><li>Prior to Pangea another super continent called _________ existed about 600 million years ago. </li></ul>
  43. 73. Beginning 4 billion years ago with small volcanic chains (arcs), created by convection cells and subduction zones that evolved into proto-continents through many individual subduction zones    
  44. 74. When the proto-continents collided micro-continents were created that evolved into the major continents today (such as North and South America, Siberia, Australia).

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