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The Mechanisms of Continental Drift: Seafloor Spreading and Plate Tectonics

This chapter explores Alfred Wegener's theory of continental drift and the concept of Pangea, which posits that a large supercontinent once existed before breaking apart. It discusses the evidence supporting this theory, including fossil distribution and glacial deposits, while noting Wegener's inability to provide a mechanism for his ideas. The development of plate tectonics and seafloor spreading in the 1960s, notably through Harry Hess's research, provided the necessary framework. The chapter also touches on concepts like subduction zones, transform faults, and the dynamic nature of Earth’s lithosphere and asthenosphere.

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The Mechanisms of Continental Drift: Seafloor Spreading and Plate Tectonics

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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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