Joints

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  • Created by: naomi
  • Created on: 16-05-13 10:14

joints produced by folding - Tectonic joints

  • produced by the tension created in rocks when they are folded by Earth movements
  • compressive forces fold the rocks but cause some of the rocks around a fold to be under tension
  • the rocks fracture becase the outer surface of the bed i stretched more than the inner surface
  • the rock is competent so it does not change in thickness as it is bent, so brittle fractures form
  • two types of tectonic joints are commin: 
  • tension joints, parallel to the axial plane trace of the fold
  • cross joints, at an angle to the axial plane trace of the fold
  • tension joints and cross joints show no displacement across the frature plane
  • where there are a number of joints with a similar orientation, they are described as a joint set
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joints produced by cooling - cooling joints

  • caused by tensional forces set up in an igneous rock as it cools steadily and contracts
  • the joints will be perpendicular to the cooling surfaces of thick basalt lava flows, dykes and sills, so will often be verticle
  • the igneous rock is insulated and then cooled with evenly spaced cooling centres
  • as the rock cooled and crystallised it shrank slightly and this resulted in a series of colum-like structure, which are polygonal in plan 
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joints produced by unloading of rocks - unloading

  • often form in granite, an igneous rock which cooled deep below the surface where the pressure is high
  • the weight of the overlying rocks 'compressed' the rock - this is called the load pressure
  • when the granite is exposed at the surface, as a result of uplift and erosion, the lack of load pressure from overlying rocks allows them to expand
  • joints form roughly parall to the Earth's surface
  • most of these joints are approximately horizontal, but they can also be verticle 
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