3.1 Elementary Plate Tectonics

• Lithosphere


  A rigid layer made up of the uppermost part of the mantle and the crust.
• Crust


  Earth's outermost layer.
• Mantle


  The layer of hot, solid material between Earth's crust and core. (82% of Earth Volume)
• Continental Crust


  35-70km thick on average.

  Very old, mainly over 1500mn yrs.

  Light in colour with density of 2.6

  Silica, Aluminium, Oxygen

  Numerous types of rocks (Granitic rocks most common)
• Oceanic Crust


  6-10km thick on average

  Young, mainly under 200mn yrs.

  Dark in colour with density of 3.0.

  Silica, iron and magnesium.

  Mainly balsatic
• Plate Boundaries


  Six major plates and numerous minor ones.
• Convection Current Theory


  States that huge convection currents occur on the Earth's interior. Magma rises through the core to the surface and then spreads out at mid-ocean ridges.

  
  

  Movement is caused by radioactive decay in the core.
• Dragging Theory


  States that plates are dragged or subducted by their oldest edges which become cold and heavy.

  
  

  Complete cooling takes over 1mn yrs.
• Hotspots


  Plumes of lava that rise vertically through the mantle.

  Usually found near plate boundaries but not always the case (Hawaii).

  
  

  Flow of hotter magma can lead to movement.
• Sea-floor spreading


  Dietz and Hess (1960s) - continents move due to a growth of crust beneath them.

  
  

  Vine and Matthews discovered that anomalies were due to magnetic grains in the lava lining up with Earth's magnetic field. (Paleomagnetism)
• Slow-spreading Ridges


  Slow-spreading ridges (Mid-Atlantic) have a pronounced rift down the centre.

  
  

  They are fed by small and discontinuous magma chambers.
• Fast-spreading ridges


  These ridges (East Pacific Rise) lack a central rift and have a smooth topography.
• Mountain Building

  Linear mountain chains are associated with convergent plate boundaries where an ocean plate meets a continental plate.

  
  

  E.g. The Himalayas.
• Ocean Ridges


  Longest uplifted features on earth surface - length exceeds 60,000km and are 1000-4000km wide.

  
  

  E.g. Mid-Atlantic Ridge
• Ocean Trenches

  Subduction zones form when oceanic lithospheric plate collides with another plate. Subduction continues as perpetuated by the weight and coolness of the subducted plate.

  
  

  Trenches are the point subduction is occurring.

  
  

  They can be 11,000 meters deep (Challenger Deep) and can be quite long.
• Island Arcs


  Island arcs are a series of volcanic islands, formed in an arc-shape, as in the Caribbean when oceanic lithosphere is subducted beneath oceanic lithosphere.
• Plates are large pieces of the lithosphere that move relative to one another at plate boundaries.
• The asthenosphere is the lower part of the lithosphere that can flow slowly over time.
• The lithosphere is the rigid outer layer of Earth, consisting of the crust and uppermost mantle.
• Fold Mountains:

  • They occur with the compression squeezing layers of rock.
  • Upfolds (anticlines) form the ridges.
  • Downfolds (synclines) form valleys.
• Ocean Trenches:

  • Material can also be forced town.
  • Material may be scraped onto the accretionary prism.
  • The lowest point in the meeting of the oceanic and continental plate is the trench.
• Oceanic - Oceanic Boundaries:

  • Processes are all similar to Continental - Oceanic, but there is no fold mountain formation.
  • Volcanic Island Arcs and Trenches are most common features.
  • Mariana Islands and Mariana Trench are example of this kind of boundary.
• Divergent Plate Boundaries:

  • Plates move away.
  • East African Rift Valley
  • Somalian Plate moving away from Nubian Plate.
  • Transform faults at right angles to rift.
  • Mid-Atlantic ridge causes volcanic action to build up ridges along the fault. Eurasian / African moving away from Pacific