Continental Drift

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The Shifting Crust Chapter 3
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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
• SEAFLOOR SPREADING

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Seafloor Spreading Plate Tectonics

• Theoretical breakthrough
• PLATE TECTONICS
• 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 1960s
• 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
• worlds most explosive volcanos are formed over
  subduction zones
• Oregons 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?
• SUBDUCTION
• law of conservation of ocean floor

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Driving Forces

• Slab pull versus ridge push

• Slab Pull - gravity pulls cooled, dense plates
  back down into mantle at subduction zone

• 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
• Earths magnetic field a mystery
• currents in liquid core one hypothesis

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SF Spreading Proof - 2

• Earths 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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Grocery bar code
Last reversal 200 Ka
Calibrated by age- dating of rocks
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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

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

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Translational

• 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

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