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Plate Tectonics and the Ocean Floor. Continental Drift. Theory proposed by Alfred Wegener in 1912 Continents once formed a single landmass. Early Evidence. Age of Oceanic Crust. Courtesy of Paleomagnetism. Plate Tectonics – 1960’s. Explains HOW the plates moved.

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Plate Tectonics


the Ocean Floor

continental drift
Continental Drift
  • Theory proposed by Alfred Wegener in 1912
  • Continents once formed a single landmass

Age of Oceanic Crust

Courtesy of

plate tectonics 1960 s
Plate Tectonics – 1960’s
  • Explains HOW the plates moved
the crust
The Crust
  • Continental Crust
  • thick (10-70km)- buoyant (less dense than oceanic crust) - mostly old

Oceanic Crust

- thin (~7 km)- dense (sinks under continental crust)- young

plate movement
Plate Movement
  • “Plates” of lithosphere are moved around by the underlying hot mantle convection cells

Three types of plate boundary

  • Divergent
  • Convergent
  • Transform

Divergent Boundaries

  • Spreading ridges
    • As plates move apart new material is erupted to fill the gap
mid ocean ridge
Mid-Ocean Ridge
  • Underwater mountain ranges
  • Due to plates pulling away from each other
  • A Rift Valley forms in the middle where magma comes out
  • Have many fracture zones which break the ridge up

Iceland: An example of continental rifting

  • Iceland has a divergent plate boundary running through its middle

Convergent Boundaries

  • There are three styles of convergent plate boundaries
    • Continent-continent collision
    • Continent-oceanic crust collision
    • Ocean-ocean collision

Continent-Continent Collision

  • Forms mountains,e.g. European Alps, Himalayas

Continent-Oceanic Crust Collision





  • Oceanic lithosphere subducts underneath the continental lithosphere
  • Oceanic lithosphere heats and dehydrates as it subsides
  • The melt rises forming volcanism
  • E.g. The Andes
  • Due to one plate subducting (going below) another plate
  • Earthquakes
  • Many volcanoes and volcanic island arcs form here

Ocean-Ocean Plate Collision

  • When two oceanic plates collide, one runs over the other which causes it to sink into the mantle forming a subduction zone.
  • The subducting plate is bent downward to form a very deep depression in the ocean floor called a trench.
  • The worlds deepest parts of the ocean are found along trenches.
    • E.g. The Mariana Trench is 11 km deep!

Transform Boundaries

  • Where plates slide past each other

Above: View of the San Andreas transform fault


Pacific Ring of Fire

Volcanism is mostly focused at plate margins

continental shelf
Continental Shelf
  • Part of a continent covered by water
  • Gentle slope
  • Average depth is 60 m
  • Amount exposed changes with sea level
continental slope
Continental Slope
  • Steep
  • May be cut by submarine canyons
  • Sediment piles up at the bottom and forms continental rise
abyssal plains
Abyssal Plains
  • Flattest area on earth
  • Cover ½ of deep ocean
  • Covered with fine sediment
  • Submerged volcanic mountains
  • Called oceanic island if they rise above water
  • Seamounts that have been eroded and are now flat on top