Themes of the Day:

• Continental Drift to Seafloor Spreading: The Plate Tectonics Revolution
• Plate Tectonics Paradigm: Driving Force, Plate Boundaries

Continental Drift to Seafloor Spreading: The Plate Tectonics Revolution

• 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 - Development of a Hypothesis
  • 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)
• The Seafloor Spreading Hypothesis
  • In the late 1950's 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
  • 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
    • 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)

Plate Tectonics: Driving Forces, Plate Boundaries

• Heat loss from the interior of the Earth drives convection of the mantle, which in turn drives Plate Tectonics A B
• Basic Principles of Plate Tectonics
  • Lithosphere forms a number of rigid plates that undergo little internal deformation
  • Lithospheric plates move relative to each other atop the ductile asthenosphere in response to convection in the Earth's mantle
  • Plate motion rates are on the order of centimeters per year (cm/yr) - about as fast as fingernails grow
  • The vast majority of geologic activity (earthquakes, volcanoes, etc.) occurs at plate boundaries
• Earth's Plates - in Google Earth
  • Plate Tectonics is a unifying paradigm that explains many topographic/bathymetric features of Earth's surface - Figure 1.13
• Types of Plate Boundaries
  • Divergent - Mid-ocean spreading ridges - new crust created
  • Convergent - Subduction zones or Continent-continent collisions - deep-sea trenches, volcanic arcs, and major earthquakes at subduction zones - Subduction zone examples: Andes Mts on W edge of South America, Japan, Aleutian Islands - Continent-Continent collision examples: Himalayas, Alps
  • Transform - plates slide past each other - Strike-slip faults - San Andreas Fault
• Hot Spots - independant of Plate Boundaries, useful for measuring plate motions (direction, speed)
• For more on Plate Tectonics, see This Dynamic Earth: the Story of Plate Tectonics, from the U.S. Geological Survey.
• Note: Surficial geologic processes are the result of a combination of Plate Tectonics - driven by heat loss from the interior of the Earth - and atmospheric phenomena (weather) - driven by heat input from the Sun.