II. Plate Tectonics

A. Earth Structure

  1. lithosphere - hard, rigid, rocky outer layer (about 100km)
  2. asthenosphere - hot, soft, mushy layer underneath

B. Plate Tectonics

  1. Earth's surface is split into plates 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. huge mountains form when two continents collide
  5. continental rocks are neither created nor destroyed, therefore may be very old

C. Plate Boundaries

  1. divergent  - plates move away from each other (usually 2-15 cm/year)
    1. new ocean lithosphere created at mid-ocean ridges
    2. relatively gentle, lava flow type of volcanoes
    3. moderate magnitude, shallow depth (<40 km )earthquakes
    4. mid-ocean ridges begin as continental rifts
  2. convergent - plates move toward each other
    1. subduction zone
      • ocean crust destroyed at depth
      • forms volcanic arc or volcanic island arc of explosive, dangerous volcanos
      • huge, deep earthquakes (up to 700km and mag 9)
    2. continental collision
      • folding and faulting as huge mountains form
      • continents sutured together
  3. Transform boundaries - lateral motion of plates
    1. lithosphere neither created nor destroyed
    2. earthquakes of variable size and depth, but less than subduction zone
    3. seldom forms volcanoes

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