Joints

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Joints are produced by tensional forces.

These forces may be the result of folding, cooling or unloading of rocks. They give rocks a 'blocky' appearance when they outcrop at the Earth's surface.

Joints will only form in competent rocks, which are brittle and will break under tension. Competent rocks include sandstone, limestone and granite. Joints will not occur in incompetent rocks such as mudstone or clay. Joints in sedimentary rocks are normally found perpendicular to the beds.

Joints produced by folding

Tectonic joints are 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 aroud a fold to be under tension. The rocks fracture because the outer surface of the bed is stretched more than the inner surface. The ock is competent so it does not change in thickness as it is bent, so brittle fractures form.

Two types of tectonic joints are common:

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 fracture plane. Where there are a number of joints with a similar orientation, they are described as a joint set.

Joints produced by cooling

Cooling joints are 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 vertical. The igneous rock is insulated and then cooled with evenly spaced cooling centres.

As the rock cooled and crystallised it shrank slightly and this resuoltde in a series of column-like structures, which are polygonal in plan.

Joints produced by unloading of rocks

Unloading joints 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 rocks - this is called load pressure. When the granite is exposed at the surface, as a result of uplift and erosion, the lack of load pressure from the overlying rocks allows them to expand. Joints form roughly parallel to the Earth's surface. Most of these joints are approximately horizontal but they can also be vertical.

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