Unit03a_GEOS218_S25 PDF: Plate Tectonics and Geological Disasters | Course Notes

Welcome to Geos 218 Geological Disasters & Society Today: Unit 3a Plate Tectonics Overview History of the Theory Evidence 1 Main Concepts in Plate Tectonics: The layered Earth geochemical layers (crust/mantle/core) vs...

Welcome to Geos 218 Geological Disasters & Society Today: Unit 3a Plate Tectonics Overview History of the Theory Evidence 1 Main Concepts in Plate Tectonics: The layered Earth geochemical layers (crust/mantle/core) vs physical layers (rigid solid/ mushy solid/ stiff solid/ liquid/ solid) 2 1  Main Concepts in Plate Tectonics: Zooming in 3 Main Concepts in Plate Tectonics: Rigid lithospheric plates float on the mushy (solid but soft) asthenosphere Ocean crust Cont. lithosphere Crust Lithosphere asthenosphere 4 2  Strength Layering: Plate Tectonics nLithosphere (rigid!) n Crust + Uppermost ocean CRUST continent Mantle ß à n Brittle – will break l it h phere n ~100 km thick osp lithos h ere uppermost Lithospheric plates mantle (rigid/brittle) overlie (float on) à ß asthenosphere (plastic) asthenosphere (weak/plastic) n Asthenosphere is solid (mushy, not liquid!) n Plastic – will flow,move, n Asthenosphere is 100-300 km thick change like Silly Putty n Asthenosphere is within mantle 5 Main Concepts in Plate Tectonics: Evidence for plate tectonics Tectonic Plates Lithosphere/asthenosphere Sea Floor Spreading Uniformitarianism 6 3  Before Plate Tectonics 1620: Francis Bacon noted parallelism of Atlantic coastlines of Africa and South America Late 1800s: Eduard Suess suggests ancient supercontinent Gondwanaland (South America, Africa, Antarctica, Australia, India and New Zealand) 1915: Alfred Wegener’s book supports theory of continental drift – all the continents had once been supercontinent Pangaea, and had since drifted apart Theory of continental drift was rejected because mechanism for movement of continents could not, at the time, be visualized 7 Continental Drift Evidence Supporting Continental Drift Early proponent: 1. Shape/fit of coastlines. Alfred Wegener: 2. Identical rocks, same type and age, are German Meteorologist found on both sides of the Atlantic. The Origin of Continents and Oceans, 1912 3. Mountain ranges with same rock types, structures, and ages are now on opposite sides of the Atlantic. 4. Ancient fossils of the same species of extinct plants and animals are found in rocks of the same age but on continents that are now widely separated. 5. Grooves and rock deposits left by ancient glaciers are found today on different continents very close to the equator. 6. Coral reefs and coal-forming swamps are found in warm environments, but ancient coal seams and coral reefs are found in locations where it is much too cold today. HOWEVER – the theory could not explain: HOW DO CONTINENTS MOVE? 8 4  Continental Drift was REJECTED (until ~1950) Mechanism: how could continents move? Continents can’t plow through hard rock of ocean basins! If they did we should see evidence – scars along the ocean floor Although, geology of ocean basins poorly known – no maps of ocean floor! 9 How Continental Drift Became Plate Tectonics Improved science methods, tech and discoveries Technology: oceanography from WWII technologies (sonar) radioactive isotopic dating of rocks seismometer network **paleomagnetism Key discoveries: seafloor topography seafloor ages magnetic reversals distribution of earthquakes, volcanoes 10 5  Evidence of Plate Tectonics (maps) The Fit of the Continents Outlines of continents at the 1,800 m water depth line match up very well – 1,800 m water depth line marks boundary between lower-density continental rocks above and higher-density oceanic rocks below – Continental masses cover 40% of Earth’s surface, ocean basins cover other 60% 11 Evidence of Plate Tectonics (maps) Oceanic mountain ranges and deep trenches Ocean bottom is mostly about 3.7 km deep, with two areas of exception: Continuous mountain ranges extend more than 65,000 km along ocean floors Volcanic mountains that form at spreading centers, where plates pull apart and magma rises to fill gaps Narrow trenches extend to depths of more than 10 km Tops of subducting plates turn downward to enter mantle 12 6  Evidence of Plate Tectonics (maps) Systematic increases in seafloor depth Ocean floor depths increase systematically with seafloor age, moving away from mid-ocean ridges As oceanic crust gets older, it cools and becomes denser, therefore sinking a little lower into mantle Weight of sediments on plate also cause it to sink a little into mantle Figure 2.24 Warm, buoyant (low density) Old, cold less buoyant Thin sediment (higher density) Thick sediment 13 Evidence of Plate Tectonics (ages) Ages from ocean basins Oldest rocks on ocean floor are only ~200 my old (