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Theory of plate tectonics Continental drift hypothesis Before the theory of plate tectonics in the 1960’s, there was Alfred Wegener’s hypothesis of continental drift. He thought all the continents were once connected to make a single supercontinent called Pangea. Pangea 300 million years ago Evidence that supported Pangea Why do the continents move? The theory of plate tectonics states that the Earth’s lithosphere is broken up into plates. These plates continually move and change shape and size. When plates move, they cause earthquakes and volcanoes along the plate boundaries. Plates move slow-on average 5 cm a year! Plate motion is caused by convective currents in the asthenosphere Types of plate boundaries Divergent- plates pull apart Real life example: atlantic ocean and east african rift valley Causes: earthquakes, sea floor spreading, rift valleys, and mid ocean ridges divergent Mid ocean ridge East African rift valley Convergent continental continental 2 continents collide-push together Example: how Appalachian mountains formed and the Himalayan mountains Causes earthquakes and mountains Appalachian mountains form Himalayan mountains Convergent oceanic oceanic 2 ocean plates collide-push together Example: islands of japan, indonesia Causes: earthquakes, subduction zones, tsunamis, volcanic islands Convergent oceanic continental An ocean plate and continental plate collide Examples: Cascade volcanic mountains in northwestern US (Mt st Helens) Causes: earthquakes, volcanic eruptions, tsunamis Transform fault 2 plates slide past each other Example: San andreas fault in California Causes only earthquakes What evidence supports the theory of plate tectonics? Paleomagnetism- alternating layers of rock at the bottom of ocean floors show normal and reverse polarity Earthquake patterns Hot spots