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  1. 1. Continents Change Positions Over Time
  2. 2. Continental Drift <ul><li>Theory developed in 1912 by German scientist Alfred Wegener. </li></ul><ul><li>States that Earth’s continents were once joined in a single landmass and gradually moved, or drifted apart . </li></ul>
  3. 3. Evidence for Continental Drift- Fossils <ul><li>Fossils of an ancient reptile were discovered in eastern South America and western Africa. </li></ul><ul><ul><li>These fossils were not found anywhere else in the world. </li></ul></ul>
  4. 4. Evidence for Continental Drift- Climate <ul><li>Fossils of tropical plants can be found on Greenland’s shores. </li></ul><ul><li>South Africa’s rocks were deeply scratched by ice sheets that once covered the area. </li></ul>
  5. 5. Evidence for Continental Drift- Geology <ul><li>Types of rock found in Brazil matched rock found in western Africa. </li></ul><ul><li>Limestone rocks in the Appalachians were exactly like those in Scotland’s Highlands. </li></ul>
  6. 6. Pangaea <ul><li>from the Greek word meaning “all lands”; Wegener’s name for the joined continents (supercontinent) </li></ul>
  7. 7. Mid-Ocean Ridge <ul><li>Huge underwater mountain range. </li></ul>
  8. 8. Sea-Floor Spreading <ul><li>Ridges form cracks along the crust. </li></ul><ul><li>Melted rock rises through these cracks, cools and forms new oceanic crust. </li></ul><ul><li>These areas are called spreading centers . </li></ul>
  9. 9. Age of the Sea Floor <ul><li>Evidence that the sea floor is spreading apart came from the age of rocks in the crust. </li></ul><ul><li>The youngest rock is closest to the ridge, while the oldest rock is farthest away. </li></ul><ul><li>Continental crust (4 billion years old) is much older than oceanic crust (160-180 million </li></ul>
  10. 10. Ocean Trenches <ul><li>In these deep trenches, dense oceanic crust is sinking into the asthenosphere. </li></ul><ul><li>Old crust is being destroyed at the same rate that new crust is being formed. </li></ul>
  11. 11. Causes of Plate Movement <ul><li>Tectonic plates rest on the asthenosphere, a layer of soft, hot rock. </li></ul><ul><li>Rock in this layer moves by convection . </li></ul>
  12. 12. Causes of Plate Movement <ul><li>Rocks in the asthenosphere act in a similar way. </li></ul><ul><ul><li>Hot, soft rock rises, cools and sinks, only to be heated and rise again. </li></ul></ul><ul><ul><li>This sinking and rising motion, if it continues, is called a convection current . This current moves rocks in the mantle only a few centimeters a year. Over millions of years, this moves plates thousands of kilometers. </li></ul></ul>
  13. 13. Causes of Plate Movement <ul><li>Slab pull and ridge push also move these huge plates. </li></ul><ul><ul><li>slab pull - gravity pulls the edge of a cool, dense plate into the asthenosphere and the entire plate is dragged along. </li></ul></ul><ul><ul><li>Ridge push - material from a mid-ocean ridge slides downhill from the ridge, pushing the rest of the plate. </li></ul></ul>
  14. 14. Theory of plate tectonics <ul><li>States the Earth’s lithosphere is made up of huge plates that move over the surface of the Earth. </li></ul><ul><ul><li>Plates can move apart, push together, or scrape past each other. </li></ul></ul><ul><ul><li>Most major earthquakes, volcanoes, and mountain ranges appear where tectonic plates meet. </li></ul></ul>
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