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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

Plate Tectonics

and

the Ocean Floor


Continental drift

Continental Drift

  • Theory proposed by Alfred Wegener in 1912

  • Continents once formed a single landmass


Plate tectonics and the ocean floor

Early Evidence


Plate tectonics and the ocean floor

Age of Oceanic Crust

Courtesy of www.ngdc.noaa.gov


Paleomagnetism

Paleomagnetism


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


Plate tectonics and the ocean floor

Three types of plate boundary

  • Divergent

  • Convergent

  • Transform


Plate tectonics and the ocean floor

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


Plate tectonics and the ocean floor

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


Close up at the mid ocean ridge

Close up at the mid-ocean ridge


Plate tectonics and the ocean floor

Iceland: An example of continental rifting

  • Iceland has a divergent plate boundary running through its middle


Plate tectonics and the ocean floor

Convergent Boundaries

  • There are three styles of convergent plate boundaries

    • Continent-continent collision

    • Continent-oceanic crust collision

    • Ocean-ocean collision


Plate tectonics and the ocean floor

Continent-Continent Collision

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


Plate tectonics and the ocean floor

Himalayas


Plate tectonics and the ocean floor

Continent-Oceanic Crust Collision

  • Called SUBDUCTION

TRENCH


Plate tectonics and the ocean floor

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

Trenches

  • Due to one plate subducting (going below) another plate

  • Earthquakes

  • Many volcanoes and volcanic island arcs form here


Plate tectonics and the ocean floor

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!


Plate tectonics and the ocean floor

Transform Boundaries

  • Where plates slide past each other

Above: View of the San Andreas transform fault


Plate tectonics and the ocean floor

Pacific Ring of Fire

Volcanism is mostly focused at plate margins


Other ocean features

Other Ocean Features


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


Seamounts

Seamounts

  • Submerged volcanic mountains

  • Called oceanic island if they rise above water


Guyot

Guyot

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


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