plate tectonics

  • Created by: beaw18
  • Created on: 11-06-19 16:22

structure of the earth

  • core: centre of the earth, made of iron and nickle giving the earth it's magnetic field. inner-core is solid, outer-core is liquid 
  • mantle: 80% of the volume of the earth. semi-solid rock. 2 layers - asthenosphere (plastic properties which allow it to flow under pressure) and lithosphere (above and more rigid)
  • crust: outer shell consisting of oceanic and continental crust. continental crust is less dense than oceanic
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continental drift and plate tectonics

  • 1912: Alfred Wegner came up with the theory of continental drift
  • theorised that the continents had once all been attached as a large continent called 'pangaea'
  • geological evidence
    • South America and Africa fit together like a jigsaw
    • similar patterns of glaciation from Africa, Australia, India, South America suggest these were once located closely together
    • mountain chains and some rock sequences on either sides of oceans the same
  • biological evidence
    • similar marine fossils found in Australian and Indian limestone
    • similar reptile fossils found in South America and South Africa
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sea-floor spreading

  • 1960s theory 
  • molten, fresh rock pushes older rock outwards away from the ridge
  • plates are moved by sea floor spreading
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palaeomagnetism and the age of sea-floor rocks

palaeomagnetism

  • as lava erupts it cools and the magnetic orientation of iron particles within the lava is 'locked' into the rock
  • direction of the earth's magnetic field changes every 400,000 to 500,000 years

sea-floor rocks 

  • 1960s ocean drilling programme showed that the thickest and oldest and thickest sediments were around the continents and the newest further out in oceans
  • further supports sea floor spreading
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global patterns of plates and plate boundaries

  • most quakes are concerntrated in narrow areas between plate margins
  • 10 tectonic plates
  • destrucive plate margins move towards eachother
  • construcive plate margins move away from eachother
  • conservative plate margins slide alongside eachother
  • most plate movement is slow and continuous but sudden movements produce earthquakes
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divergent (or constructive) plate boundaries

  • plates move away from eachother
  • rising plumes of magma between gaps of plates results in:
    • intense volcanic activity
    • mid-ocean ridges - when plates move apart in oceanic areas, space between filled with lava which turns to rock
    • rift valleys - when plates move apart in on land
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convergent (destructive) plate boundaries

when the plates move towards eachother

  • oceanic-continental margins:
    • denser oceanic plate subducts under the lighter oceanic plate
    • forms deep ocean trenches
    • continental crust buckles and forms mountain chains
    • if the angle of subduction on the ocean plate is between 30 and 70 degrees, faulting occurs in the Benioff zone, releasing energy to trigger an earthquake
  • oceanic-oceanic plate margin
    • the denser plate will subduct and create a trench
    • descending plate melts, magma rises and chains of volcanoes or island arcs form
  • continental-continental margins
    • little if any subduction because of similar densities
    • impact and pressure tends to form fold mountains
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conservation plate boundaries

  • two plates slide against eachother
  • movement can be violent and an additional build up of pressure which leads to powerful earthquakes
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