Geo 105, Jan. 13 to 15. Continental Drift to... ...plate tectonics.

Succession of ideas:

• Continental Drift: key person is Alfred Wegener
• Evidence:
  • Geography: curious matching of continental outlines across Atlantic Ocean
    ...Click for one of the first attempts at closing the Atlantic Ocean
    ...Click for Wegener's reconstruction
  • Paleontology: distribution of animals & plants from 200 million years ago.
    ... Click for "landbridges".
  • Geology: distribution of ancient glaciation
    ... Click for "reconstructed" southern Continents.
  • Geophysics: apparent polar wander paths (though this work is from 1950's)
    ... Click for direction of magnetic field across the globe.
    ... Click for Basic notions of Paleomagnetism.
    ... Click for Polar Wander Paths.
• Sea-floor spreading: key person is A. Holmes
• Evidence:
  • oceans & continents are quite different
  • compared to continents:
  • ¥ oceanic crust is thinner ( 6 vs. 30-35 km)
  • ¥ rock type is "basalt"
  • ¥ oceanic crust is younger
  • in addition:
  • "active" mountains are at the edges of continents
  • ocean has ridges in middle, trenches at edges
  • earthquakes are at (some) continent-ocean boundaries
    ... Click for Sea-Floor Spreading a la Holmes.
• Plate Tectonics: many people, perhaps Vine & Matthews and J. Tuzo Wilson are ones to remember
• Evidence:
  • relationship between fracture zones & ridge segments
  • the magnetic lineations on sea-floor
    ...reversals of the Earth's Magnetic field
    ... Click for Magnetic Stripes southwest of Iceland.
    ...how stripes are made (each panel jumps in time to show spreading & new oceanic crust formation)
    ...how stripes are made (fancy plot with paleomagnetic time scale)
    
  • where earthquakes occur, where they do not.
    ...where earthquakes occur along fracture zones
    ...Wilson's view of how transform faults work
    
  • the fault and slip orientation of earthquakes
  • where mid-ocean volcanos occur
  • Then Isacks, Oliver, & Sykes showed how subduction zones work:
    ...Distribution of deep ocean trenches
    ...detail of bathymetry, volcanos, & earthquakes at Tonga subduction zone (north of New Zealand)
    ..."Cross-section" of an idealized subduction zone (but quite similar to the Tonga subduction zone)
    ...A famous cartoon that shows rigid plates and the three types of plate boundaries
    
  • *** success of the geometric theory of plate tectonics in explaining new observations.
    

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