plate tectonics

• Oceanic crust


  • More dense
  • 30-70km thick
  • Less than 200 mil old
  • Mainly basalt
  • Silicon aluminium and oxygen components
• Continental crust


  • 6-10km thick
  • Less dense
  • Over 1.5 million years old
  • Mainly granite
  • Silicon, magnesium and oxygen
• Theory of plate tectonics

  • Jigsaw fit of the coastline
  • Mountain ranges that connects
  • Found newer rick close to the plate boundary
  • Glacial evidence seen in countries that has no ice before
  • Seen rocks from sea floor that are emerging - sea floor spreading
  • Fossil evidence
  • Found the same species across continents across the ocean
• Palaeomagnetism


  • Formation of rocks in relation to the magnetic field
  • Iron in these rocks will align themselves in the direction of the earth magnetic pole
  • When the magnetic field switches the rocks will align the other way to face the magnetic field
  • Creates a banding pattern seen in mid-atlantic ridge
• Convection currents


  • Thought that convection currents in the mantle moves the plates
  • Convection currents is when rising heat moves throughout the mantle and cause the lithospheric plates to move with them
  • The mantle is not liquid so it was partly discounted
• Ridge pull/ gravitational sliding

  1. Constructive plate boundaries upwelling hot magma generates a bouyancy effect causing mid-ocean ridges to form

  2. New oceanic crust cools and thickens with age and gets pushed downhill as new rock emerges from the active zone of divergence behind it

  3. Gravity acts don on the slope of this ridge meaning the plates experiences a force that acts away from the plate boundary
• Slab pull


  When the older and denser oceanic plate is subducting under the mantle the force that is pulling the plate is known as slab pull
• Plate boundaries

  • Convergent (Oceanic-continental, Oceanic-oceanic, Continental-continental)
  • Divergent (Oceanic-oceanic, Continental-continental)
  • Transformational (Oceanic-oceanic, Oceanic-continental)
• Mid-ocean ridges

  • Plates moving apart in oceans
  • Space will fill with basaltic lava upwelling to form a ridge
  • Transform faults cut along the ridge
  • Occurs at right angles causing frictional stress building up using earthquakes
  • Can be up to 4000m tall
  • Can have hydrothermal activities
• Mid atlantic ridge


  • Between the pacific and nazca plate
  • Volanoes and shallow earthquakes occurs here
• Rift valleys

  • Plates move away forming a rift valley on land
  • Lithosphere stretches so it causes the lithosphere to fracture and create fault lines
  • The blocks the falls known as horst
  • Walls of the valley is between 500-800m
• East african rift valley

  • Volcanoes and new plates can form here
• Fold mountains

  • Continental plates move inwards
  • No volcanoes
  • No subduction
  • continental crust has a lower density than underlying layers
• Oceananic trench

  • Oceanic meets oceanic plates
  • Subduction occurs
  • Leads to formation of deep ocean trenches
  • Forms island arc
• Island arcs


  • During subduction descending plates encounters hotter surroundings
  • Heart generated due to friction
  • Begins to melt the plates
  • Magma rises towards the surface
  • Reaches the surface and forms complex composite nad explosive volcanoes
• Magma plumes/hotspots

  • Plumes of magma rises though the earths mantle
  • Mantle plumes are long lives areas of heart flow in the mantle
  • Alternative theory: Lithosphere is thinning so the it experiences local extensions
  • Pulled hotspots forwards due to the oceanic pate subducting
• Hawaiian islands


  • With pacific plate