Theories and plate movement

• There are two types of crusts

  • thin oceanic, underlines ocean basins, mostly made of basalt
  • thicker continental, underlines continents, mostly made of granite
• The lower density of continental crusts allows it to ’float’ higher on the much higher density mantle underneath it
• The temperature of the mantle increases with depth so rocks in the upper mantle are cool and brittle but rocks in the lower mantle are hot and plastic
• The Benioff Zone

  • an area of seismicity corresponding with the slab being thrust downwards in a subduction zone
  • the different speeds and movements of rock at this point produce numerous earthquakes, that are usually intermediate/ deep focused
  • this theory is an important factor in determining earthquake magnitude as it determines the position and depth of the hypocentre
• Paleomagnetism

  • refers to magma 'locking in' the Earth's magnetic polarity before it cools
  • scientists use this to determine historic periods of large-scale tectonic activity through the reconstruction of relative plate motions and creating a geo timeline
• A thrust/ mega thrust fault

  • when plates are in contact with each other and can cause frictional stress and when that exceeds a threshold, a sudden failure occurs along the fault that can cause a 'mega thrust' earthquake
• Locked fault

  • a fault that isn't slipping because of frictional resistance on the fault being greater than the stress on the fault
  • these faults can store strain for long periods of time that is eventually released in a large magnitude earthquake when the frictional resistance is overcome
  • eg the 2004 Indian Ocean tsunami was caused by a mega thrust locked fault