plate tectonics

• 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

• 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

• 1960s theory 
• molten, fresh rock pushes older rock outwards away from the ridge
• plates are moved by sea floor spreading

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

• 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

• 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

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

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