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Chapter 2: Plate Tectonics

Chapter 2: Plate Tectonics. Tectonic Plates of Earth. Pangaea. Pangaea: Pan = all Gaea = Earth Panthalassa Thalassa = sea AKA Tethys Sea. Continental Drift. Progressive breakup of Pangaea into modern continents Similar to sea ice. Wegner’s Evidence.

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Chapter 2: Plate Tectonics

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  1. Chapter 2: Plate Tectonics

  2. Tectonic Plates of Earth

  3. Pangaea • Pangaea: • Pan = all • Gaea = Earth • Panthalassa • Thalassa = sea • AKA Tethys Sea

  4. Continental Drift • Progressive breakup of Pangaea into modern continents • Similar to sea ice

  5. Wegner’s Evidence • Present-day shorelines fit like puzzle pieces • Better fit from continental shelves

  6. Fossil Evidence • Identical fossils present in S. Amer. & Africa • e.g. Mesosaurus • e.g. Glossopteris • e.g. Marsupials

  7. Geologic Evidence • Identical rocks on different continents • e.g. 2.2Ga igneous rocks in Brazil & Africa • Similar Mountain Ranges • e.g. Appalachian Mts ~ Caledonian Mts

  8. Identical package of rocks & fossils found in S.Amer., Africa, Australia & Antarctica Mesozoic Supergroup

  9. Paleoclimatic Evidence • Paleo = ancient • Climate = weather conditions • Glacially transported sediments • Glacial striations

  10. Rejection of Continental Drift Hypothesis • No evidence of continents “breaking through” oceanic crust • Tidal forces necessary would halt Earth’s rotation • Danish scientists found no astronomical evidence of drift from 1927-1948

  11. Earth’s Magnetic Field • Similar to bar magnet • Magnetic materials align themselves to magnetic field

  12. Magnetic orientation has 2 dimensions North-South Dip angle (Inclination) Curie point (T) Fossil Magnetism / Paleomagnetism N-S Orientation & Dip

  13. Magnetization = degrees from N pole Magnetization + Latitude = 90° e.g. Lavas from Puerto Rico show 75 ° from N pole & Puerto Rico = 15 ° from equator Magnetic Inclination

  14. Apparent Polar Wander

  15. Seafloor Spreading

  16. Paleomagnetism

  17. Progression of Seafloor Spreading

  18. Plate Boundaries

  19. Plate Boundaries • Corresponds to Earthquakes & Volcanoes

  20. Plate Boundaries • Three Types • Divergent • AKA Rift • Convergent • AKA Subduction • Transform • AKA Strike-slip

  21. Divergent Boundaries • AKA Spreading Centers • AKA Rifts • Largest mountain chains • Plates move apart due to eruption of lava • New lava = new oceanic crust • Oldest oceanic crust 180Ma

  22. Pillow Basalts • Form when lava extruded under water • Immediately outer layer freezes • New material pushes through like toothpaste

  23. Rifts also can form in continental settings Linear depressions Lakes, valleys, etc. Asthenosphere thins due to tension e.g. East Africa Rift Zone, Mt. Kilimanjaro Continental Rifting

  24. Continental Rifting (con’t) • If tension continues, eventually continental rift develops into oceanic spreading • e.g. Red Sea, Sea of Cortez

  25. Convergent Boundaries • Old oceanic crust dense & heavy • Heavy vs. light => subduction • AKA destructive margins • Large earthquake & explosive volcanoes • Melting triggered at ~100km depth

  26. Oceanic-Oceanic Convergence e.g. Virgin Islands, Japan, Philippines

  27. Oceanic-Continental e.g. Andes, MesoAmerica, Italy

  28. Continental-Continental e.g. India

  29. Hot Spots • Caused by mantle plumes • Plumes do not move, plates do • Bend at 40Ma • Major change in plate motion

  30. Relative Plate Motions • Relative to Hot Spots & other plates • Measure motions with Paleo-magnetism

  31. Forces Driving Plate Motions • Convection of Mantle • Upwelling Mantle • Ridge-push • Slab Suction • AKA Slab-pull

  32. Layer-Cake Model • Two zones of convection, above & below ~660km • Explains why mid-ocean ridge basalt different than hot spot basalt

  33. Whole Mantle Convection • Cold oceanic crust descend to bottom of mantle, “stirring” it • Hot plumes rise from core-mantle boundary • Bring “primitive” mantle to surface • Not popular b/c complete mixing in 100s Ma

  34. Deep Layer Model • Heat from Earth’s interior causes two layers to shrink & swell • Similar to lava lamp • Small amt of material rises to surface to create hot spots • Little seismic evidence to support this model

  35. Importance of Plate Tectonics • First theory to provide comprehensive view & explain: • Earth’s major surficial processes • Geologic distribution of earthquakes, volcanoes & mountain ranges • Distribution of mineral resources & ancient organisms

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