The Shifting Crust
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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

  • 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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Seafloor Spreading & Plate Tectonics

  • Harry Hess - 1960s

    • new ocean basins form from volcanism

    • ocean floor forms IN BETWEEN pieces that have split


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Seafloor Spreading & Plate Tectonics

  • Theoretical breakthrough


    • surface of earth composed of “plates” (LITHOSPHERE) that move on a “conveyor belt” (ASTHENOSPHERE)

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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

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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!

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One Mechanism

  • seafloor-spreading

  • lithosphere

  • asthenosphere

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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

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“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

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Sliding By

  • Tranforms faults

    • plates neither created nor destroyed

  • Transform faults are active

  • Fracture zones are inactive extensions of transforms

    • “fossil transforms”

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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”

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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

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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

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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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bar code



200 Ka


by age-

dating of


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Divergent Plate Boundaries

  • Divergent - seafloor spreading

    • mid-ocean ridges or rises

    • Mid-Atlantic RIDGE, East Pacific RISE

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  • Convergent - subduction

    • trenches

    • Tonga Trench, Cascadia Subduction Zone

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  • Translational - strike-slip faulting

    • transform faults (active) - San Andreas Fault

    • fracture zones (inactive) - Mendocino FZ

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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