VI. Plate Tectonics

A. Continental Drift and Pangaea

1915 - Alfred Wegener

  1. evidence
    1. fit of continental shorelines (fig. 9.3)
    2. continuity of mountain ranges (fig. 9.5)
    3. continuity of glacial moraines and striations(fig. 9.6)
    4. similar rock types (fig. 9.4)
    5. similar fossils (fig. 9.7)
  2. problems
    1. HOW? no mechanism for plate movement
    2. prejudice

B. Sea Floor spreading  - modification of continental drift

  1. evidence
    1. mid-ocean ridges
    2. volcanic iron-rich lavas (magnetic)
    3. creating new oceanic rocks
  2. problems
    1. is Earth getting bigger?
    2. are ocean rocks destroyed?

C. Plate Tectonics - modification of seafloor spreading

  1. Earth's surface is split into plates (around 125km thick) of rigid lithosphere "floating" on hot, mushy asthenosphere
  2. plates move apart at mid-ocean ridges where new oceanic rocks are created by volcanism
  3. oceanic rocks are destroyed at subduction zones
  4. continental rocks are neither created nor destroyed, therefore may be very old

D. Plate Boundaries

  1. divergent - plates move away from each other (fig. 9.18)
    1. new ocean lithosphere created at mid-ocean ridges
    2. magnetic anomalies form as a result of magnetic reversals (fig. 9.11, 9.12, 9.14, 9.15) can use magnetic anomalies to determine past plate motion
    3. mid-ocean ridges begin as continental rifts (fig. 9.18)
  2. convergent - plates move toward each other
    1. subduction zone (fig. 9.20 & 9.21)
    2. continental collision (fig. 9.22)
  3. Transform boundaries - lateral motion of plates (fig. 9.25) lithosphere neither created nor destroyed

E. Hot Spots (fig. 9.27 )

  1. stationary magma plume creates chain of volcanoes, with youngest on top of the plume
  2. used to determine the direction and rate of plate motion

F. Driving Mechanisms

  1. convection (fig. 9.28)
    1. asthenosphere
    2. entire mantle
  2. slab-pull (fig. 9.29)
  3. ridge-push

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