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The Shifting Crust Chapter 3

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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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continental drift
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
seafloor spreading plate tectonics
Seafloor Spreading & Plate Tectonics
  • Harry Hess - 1960s
    • new ocean basins form from volcanism
    • ocean floor forms IN BETWEEN pieces that have split
seafloor spreading plate tectonics4
Seafloor Spreading & Plate Tectonics
  • Theoretical breakthrough
    • surface of earth composed of “plates” (LITHOSPHERE) that move on a “conveyor belt” (ASTHENOSPHERE)
evidence for continental drift
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
important quote
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!
one mechanism
One Mechanism
  • seafloor-spreading
  • lithosphere
  • asthenosphere
another mechanism
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
munch and crunch
“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
sliding by
Sliding By
  • Tranforms faults
    • plates neither created nor destroyed
  • Transform faults are active
  • Fracture zones are inactive extensions of transforms
    • “fossil transforms”
young old
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?
  • “law of conservation of ocean floor”
driving forces
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
proof for seafloor spreading
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
sf spreading proof 2
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


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200 Ka


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divergent plate boundaries
Divergent Plate Boundaries
  • Divergent - seafloor spreading
    • mid-ocean ridges or rises
    • Mid-Atlantic RIDGE, East Pacific RISE
  • Convergent - subduction
    • trenches
    • Tonga Trench, Cascadia Subduction Zone
  • Translational - strike-slip faulting
    • transform faults (active) - San Andreas Fault
    • fracture zones (inactive) - Mendocino FZ
plate boundaries
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