Themes of the Day:

• Plate Tectonics pre-history
• Features of the Deep Oceans
• The Seafloor Spreading Hypothesis

Plate Tectonics pre-history

• Alfred Wegener - German meteorologist (1880-1930)
• Continental Drift - Lines of Evidence
  • Fit of the Continents - e.g., Africa & South America - Pangea (Laurasia and Gondwanaland)
  • Fossil Distributions (Glossopteris, Mesosaurus, etc.)
  • Paleoclimatology (glacial deposits, desert deposits, coal swamps, etc.)
  • Distribution of Rocks and Mountain Belts
• Continental Drift - Driving Mechanism (lack thereof)
  • Wegener initially viewed continents plowing along on the surface of an Earth encircling layer of ancient oceanic crust - mountain ranges like the Andes in South America represented deformation at the leading edge of the drifting continents
  • Wegener's mechanism was physically impossible and his hypothesis was rejected and ridiculed as a result, however subsequent editions of his hypothesis incorporated more realistic mechanisms
  • Wegener froze to death in Greenland in 1930 on a meteorological expedition - still firmly believing in continental drift
  • Arthur Holmes proposes mantle convection, early 1930's (incuding a rudimentary version of seafloor spreading and subduction) as a driving force for Continental Drift - largely ignored for the next 30 years - his 1945 textbook concluded with a chapter describing continental drift by convection of the mantle, twenty years before the plate tectonics "revolution"
• Exploration of the Ocean Basins
  • World War II - development of sonar
  • Seafloor bathymetry (midocean ridges, deep sea trenches, topography & sediment thickness)

Features of the Deep Oceans

• Mid-Ocean Ridges
• Deep-sea Trenches
• Abysssal Plains
• Island Archipelagoes

The Seafloor Spreading Hypothesis

• Harry Hess (Princeton) proposes Seafloor Spreading Hypothesis - new crust produced at mid-ocean ridges (mantle upwelling) and consumed at deep-sea trenches (mantle downwelling) - based on Mantle Convection in late 1950's
• Vine & Matthews and Morley independently link seafloor magnetic stripes with Seafloor Spreading Hypothesis (1963)
  • Seafloor Magnetic Stripes (e.g., Mid-Atlantic Ridge S. of Iceland) discovered in 1950's
  • Calibration of Magnetic Polarity Reversal Time Scale
  • New oceanic crust records Earth's magnetic field when it forms
  • Spreading seafloor moves away from the ridge as newer crust is formed in a conveyor belt fashion
  • Oceanic crust acts as a "tape recorder" of Earth's magnetic polarity reversals
• Implications: oceanic crust spreads symmetrically, crust gets older as it moves away from the ridge, new crustal material is constantly being added at the ridge - rates of plate motions can be determined based on well calibrated polarity reversal time scale and mapped oceanic stripes
• Age of oceanic crust (eventually confirmed seafloor spreading hypothesis)
• Age of oceanic crust vs. depth to seafloor - cooling of crust as it ages and moves away from the ridge
• Cross section of a Mid-Ocean Ridge