plate tectonics and the ocean floor
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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Plate Tectonics and
the Ocean Floor
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Continental Drift
• Theory proposed by Alfred Wegener in 1912• Continents once formed a single landmass
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Early Evidence
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Age of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
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Paleomagnetism
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Plate Tectonics – 1960’s
• Explains HOW the plates moved
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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
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Plate Movement
• “Plates” of lithosphere are moved around by the underlying hot mantle convection cells
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• Divergent
• Convergent
• Transform
Three types of plate boundary
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• Spreading ridges– As plates move apart new material is erupted to fill
the gap
Divergent Boundaries
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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
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• Black smoker at a mid-ocean ridge hydrothermal vent in the Atlantic.
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Close up at the mid-ocean ridge
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• Iceland has a divergent plate boundary running through its middle
Iceland: An example of continental rifting
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• There are three styles of convergent plate boundaries– Continent-continent collision– Continent-oceanic crust collision– Ocean-ocean collision
Convergent Boundaries
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• Forms mountains, e.g. European Alps, Himalayas
Continent-Continent Collision
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Himalayas
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• Called SUBDUCTION
Continent-Oceanic Crust Collision
TRENCH
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• Oceanic lithosphere subducts underneath the continental lithosphere
• Oceanic lithosphere heats and dehydrates as it subsides
• The melt rises forming volcanism
• E.g. The Andes
Subduction
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Trenches• Due to one plate
subducting (going below) another plate
• Earthquakes • Many volcanoes and
volcanic island arcs form here
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• 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!
Ocean-Ocean Plate Collision
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• Where plates slide past each other
Transform Boundaries
Above: View of the San Andreas transform fault
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Volcanism is mostly focused at plate margins
Pacific Ring of Fire
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Other Ocean Features
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Continental Shelf
• Part of a continent covered by water
• Gentle slope• Average depth is 60
m• Amount exposed
changes with sea level
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Continental Slope
• Steep • May be cut by
submarine canyons• Sediment piles up at
the bottom and forms continental rise
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Abyssal Plains
• Flattest area on earth• Cover ½ of deep ocean• Covered with fine
sediment
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Seamounts
• Submerged volcanic mountains
• Called oceanic island if they rise above water
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Guyot
• Seamounts that have been eroded and are now flat on top