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Plate Tectonics

Plate Tectonics. 9 .1 Continental Drift. Evidence for Continental Drift Pangaea Alfred Wegener – proposed continental drift as a theory Continental Drift – continents have moved slowly to their current locations Pangaea – one large landmass that broke apart 200 million years ago.

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Plate Tectonics

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  1. Plate Tectonics 9.1 Continental Drift

  2. Evidence for Continental Drift • Pangaea • Alfred Wegener – proposed continental drift as a theory • Continental Drift – continents have moved slowly to their current locations • Pangaea – one large landmass that broke apart 200 million years ago. • Pangaea means “all land” • Continental drift was accepted long after Wegener’s death in 1930. • Fossil Clues • Mesosaurus found on both Africa and South America. • Glossopteris found on Africa, Australia, India, South America, and Antarctica.

  3. Climate Clues • Warm weather plants were found on Spitzbergen in the Artic Ocean. • Wegner hypothesized that Spitzbegen drifted from the tropic regions. • Glacial clues • South America, Africa, India, and Australia were onece covered in glaciers • Rock Clues • Rock make up of the continents should be the same. • Similar rock sturctures are found on different continents. • Appalachian Moutnains in US are similar to Greenland and western Europe

  4. How could continents drift? • Although there was evidence, Wegener couldn’t explain how or why continental drift happened. • Continental drift was rejected. • After Wegner’s death, more clues were found. • Seafloor spreading helped explain how continental drift could occur. Pg 251 1-3

  5. Plate Tectonics 9.2 Seafloor Spreading

  6. Clues on the Ocean Floor • Little was known before 1950s • Has mountains and valleys • mid-ocean ridges form an underwater mountain range that stretches along the center

  7. The Seafloor Moves • Seafloor spreading • Hot les-dense material below Earth’s curst is forced upward toward the surface • Turns and flows sideways carrying the seafloor away • As seafloor spreads apart, magma moves upward and flows from the cracks • Becomes solid as it cools and forms a new seafloor • The seafloor cools, contracts, and becomes more dense. • Colder seafloor sinks down • Age Evidence • Seafloor rocks are no older than 180 million years • Youngest rocks are at the mid-ocean ridges

  8. Magnetic Clues • Earth’s magnetic field has a north and a south pole • Magnetic lines leave Earth near the north pole and enter near the south pole • Magnetic reversal – magnetic forces run the opposite way • Earth’s magnetic field has reversed many times • Iron-bearing minerals show the reversal of magnetic field • Magnetic reversals form parallel strips to the mid-ocean ridges

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