joint (geology)

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1. A discrete brittle fracture in a rock along which there has been little or no movement parallel to the plane of fracture, but slight movement normal to it. Fracture may be caused by shrinkage, due to cooling or desiccation, or to the unloading of superincumbent rocks by erosion or tectonism. A group of joints of common origin constitutes a ‘joint set’ and the joints are usually planar and parallel or sub-parallel in orientation. ‘Joint systems’ comprise two or more joint sets, which are usually arranged systematically with respect to the principal stress axes of regional deformation. Cooling joints (shrinkage joints), such as those which split a rock into long prisms or columns to form ‘columnar joints’, most commonly found in lavas, are due to differential volume changes in cooling and contracting magmas. Unloading joints result from erosional unloading of the crust and form flat-lying, sheet-like joint sets, e.g. those found in granitic rocks.