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Plate Tectonics Living with a Dynamic Planet Alfred Wegener: a man before his time • Meteorologist • Research on arctic climates • • • • – Greenland Evidence for past climates preserved in rocks A super-continent: Pangea Pangea broke apart 200 million years ago Continents drifted to their present locations Wegener’s Continental Drift Evidence for Pangea: glaciation Evidence for Pangea: paleontology (fossils) How Science Works: Continental Drift Disputed • Lack of Mechanism – what makes • • • • continents move? Alternate explanations possible Anti-German sentiments in the U.S. What are the credentials of Wegener? Continental Drift largely dismissed until 1960’s Oceanography: a new science and new evidence • Following WW II – a convergence of • • • • questions, technology, manpower and money Defense needs – mapping the seafloor Anti-submarine technology GI Bill – a wave of scientists and engineers Booming postwar economy and support for science A new view of the Seafloor • Completely surprising! • Mountains on the sea floor • Trenches – near some continental edges, near islands More puzzles: Magnetic stripes near the Mid-Ocean Ridges A new hypothesis: Sea-Floor Spreading – Making New Oceanic Lithosphere Continental Drift Revised: the New Global Tectonics • Earth’s lithosphere is made of crust and • • • • uppermost mantle Lithosphere is broken into plates Plates float on moving asthenosphere (upper mantle) Plate motion driven by heat in mantle Plates interact – especially at edges Lithosphere and Asthenosphere Plates • Made of Continental and Oceanic Crust – attached to uppermost part of mantle • Plates move • Plate interactions produce specific features (e.g., mountains, etc.) and processes (e.g., earthquakes) • Motion is also related to heat dissipation and density differentiation Balancing the System • Sea Floor (oceanic lithosphere) is created at MOR (divergence) • Either Earth is expanding (no evidence!) or • Sea Floor must be destroyed somewhere – where plates converge Subduction – destroying sea floor The sinking of cold, dense, oceanic lithosphere into the asthenoshere, beneath an over-riding plate. • Associated: Trenches, Volcanoes, Mountains, Earthquakes Convergence Brings Continents into Collision – Forming Mountains • Subduction of oceanic lithosphere brings landmasses together • Collision of continents (or continent/island arc) • Due to low densities, neither can subduct • Crash!! TRANSFORM BOUNDARIES – Two Plates Passing • Transform boundaries link other types of plate boundaries: • Transform boundaries allow a single plate edge to have multiple types of interactions w/other plates • Transforms are major Faults – Earthquakes! The Big Picture of Plate Interactions Plate Tectonics Explains: • • • • Formation of Oceans Formation of Mid-ocean Ridges Formation of Trenches Formation of Mountains • Formation of Metamorphic Rocks • Formation of Sedimentary Rocks as mountains erode • Earthquakes: cause and distribution • Volcanos: cause, types and distribution • Formation of Igneous Rocks