Joints

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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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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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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