Plate Tectonics
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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 www.ngdc.noaa.gov. Paleomagnetism. Plate Tectonics – 1960’s. Explains HOW the plates moved.

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Plate Tectonics and the Ocean Floor

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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 www.ngdc.noaa.gov


Paleomagnetism


Plate Tectonics – 1960’s

  • Explains HOW the plates moved


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

  • “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

  • 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


  • Black smoker at a mid-ocean ridge hydrothermal vent in the Atlantic.


Close up at the mid-ocean ridge


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


Himalayas


Continent-Oceanic Crust Collision

  • Called SUBDUCTION

TRENCH


Subduction

  • Oceanic lithosphere subducts underneath the continental lithosphere

  • Oceanic lithosphere heats and dehydrates as it subsides

  • The melt rises forming volcanism

  • E.g. The Andes


Trenches

  • 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


Other Ocean Features


Continental Shelf

  • Part of a continent covered by water

  • Gentle slope

  • Average depth is 60 m

  • Amount exposed changes with sea level


Continental Slope

  • Steep

  • May be cut by submarine canyons

  • Sediment piles up at the bottom and forms continental rise


Abyssal Plains

  • Flattest area on earth

  • Cover ½ of deep ocean

  • Covered with fine sediment


Seamounts

  • Submerged volcanic mountains

  • Called oceanic island if they rise above water


Guyot

  • Seamounts that have been eroded and are now flat on top


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