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Plates of the Lithosphere Wegener’s Hypothesis • Pangaea – Giant supercontinent of the Paleozoic and Mesozoic eras • Continental Drift – The breakup of Pangaea where plates moved away from each other [Insert Fig. 30.1 - Pangaea] Evidence of Continental Drift • Fossils – Animal and plant fossils appeared to have evolved in the same geographic region but now scattered on separate continents Certain fossils appear in continuous bands across continents that are now separated across oceans. © Earth Observatory NASA Dismissal of Continental Drift • Wegener could not produce a plausible mechanism as to how the continents drifted South America and Africa continents separated by the Atlantic Ocean appear to fit together like pieces of a puzzle. © Earth Observatory NASA Emergence Plate Tectonics Theory • Seafloor Spreading – New basaltic rock forms along midocean ridges and old seafloor is destroyed at deep ocean trenches Evidence of Seafloor Spreading • Mid-ocean ridge – Linear mountains present in all ocean basics • Age of ocean floor rocks – Young rocks near ridges and become progressively older with increasing distance from the ridges Continental Drift Re-examined • As a rift develops in a continental landmass, an ocean begins to open up • As new seafloor is created along mid-ocean ridges, ocean floor diverges moving older seafloor away Plate Tectonics Theory • Lithospheric Plates – Large segments of lithosphere • Plate Tectonics – The formation, movement and destruction of lithospheric plates Location of Plate Boundaries • Earthquakes, volcanoes and mountain ridges mark the location of plate boundaries Movement of Plates Cross section illustration of the three main types of plate boundaries: Divergent, convergent and transform plate boundaries © USGS Divergent Plate Boundaries • Tensional forces result in plates moving apart Convergent Plate Boundaries • Compressional forces result in plates colliding with each other • Depending on the types of crust that are colliding, convergent boundaries produce different land features Convergent Plate Boundaries Convergent Plate Boundaries Transform Boundaries • Shear forces result in two plates sliding past each other