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The Shifting Crust Chapter 3

The Shifting Crust Chapter 3. Continental Drift. Alfred Wegener -1912 large “supercontinent” (Pangea) existed and then split into pieces fossil & glacial deposit evidence Wegener not able to provide MECHANISM for his theory Major mechanism later found in the OCEANS.

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The Shifting Crust Chapter 3

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  1. The Shifting Crust Chapter 3

  2. Continental Drift • Alfred Wegener -1912 • large “supercontinent” (Pangea) existed and then split into pieces • fossil & glacial deposit evidence • Wegener not able to provide MECHANISM for his theory • Major mechanism later found in the OCEANS

  3. Seafloor Spreading & Plate Tectonics • Harry Hess - 1960s • new ocean basins form from volcanism • ocean floor forms IN BETWEEN pieces that have split • SEAFLOOR SPREADING

  4. Seafloor Spreading & Plate Tectonics • Theoretical breakthrough • PLATE TECTONICS • surface of earth composed of “plates” (LITHOSPHERE) that move on a “conveyor belt” (ASTHENOSPHERE)

  5. Evidence for Continental Drift • Fit of the continents • Correlation of mountain belts • Diversity of species • correlation of dinosaur species (Pangea) • isolation of mammal species (after breakup) • Glaciers

  6. Important Quote • “Relationships known since early in the century yet the validity of continental drift was not generally recognized until the late 1960’s” • It took ocean floor data to solidify ideas and convince scientific community! • The oceans rule!

  7. One Mechanism • seafloor-spreading • lithosphere • asthenosphere

  8. Another Mechanism • subduction • world’s most explosive volcanos are formed over subduction zones • Oregon’s Cascades Mtns. over Cascadia Subduction Zone • Juan de Fuca plate under N. American plate

  9. “Munch and Crunch” • “MUNCH” - subduction of oceanic plate under continent or ocean • oceanic crust is thin and dense • dives • “CRUNCH” - collision of continental plates • India into Tibet and China • continental crust is thick and light

  10. Sliding By • Tranforms faults • plates neither created nor destroyed • Transform faults are active • Fracture zones are inactive extensions of transforms • “fossil transforms”

  11. Young & Old • ocean floors - 200 million years • land - billions of years • With seafloor spreading is the earth expanding? • Why is seafloor so young relative to continents? • SUBDUCTION • “law of conservation of ocean floor”

  12. Driving Forces • Slab Pull - gravity pulls cooled, dense plates back down into mantle at subduction zone • “Slab pull” versus “ridge push” • Ridge Push - rising, hot rock pushes plates apart at spreading center

  13. Proof for Seafloor Spreading • What made people believe in seafloor spreading? (poor Wegener!) • changes in inclinations of magnetic field • Earth’s magnetic field a mystery • currents in liquid core one hypothesis

  14. SF Spreading Proof - 2 • Earth’s magnetic field flips back & forth • magma freezes magnetic minerals • minerals lines themselves up w/ prevailing field of earth • anomaly in field is the key - normal (positive) or reversed (negative) • Vine and Matthews noted this in the ‘60s and flagged this as PROOF for seafloor spreading

  15. Grocery bar code Last reversal 200 Ka Calibrated by age- dating of rocks

  16. Divergent Plate Boundaries • Divergent - seafloor spreading • mid-ocean ridges or rises • Mid-Atlantic RIDGE, East Pacific RISE

  17. Convergent • Convergent - subduction • trenches • Tonga Trench, Cascadia Subduction Zone

  18. Translational • Translational - strike-slip faulting • transform faults (active) - San Andreas Fault • fracture zones (inactive) - Mendocino FZ

  19. Plate Boundaries • How do we know where these boundaries are? • bathymetry • earthquakes • deepest earthquakes at CONVERGENT boundaries (subduction zones/trenches) • slab breakage causes earthquake • like potato chip breaking off in bean dip

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