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.
2. See VOIDS, TYPES OF.
joint (in geology)
joint, in geology, fracture in rocks along which no appreciable movement has occurred (see fault). Nearly vertical, or sheet, joints that result from shrinkage during cooling are commonly found in igneous rocks. Similar joints occur in thick beds of sandstone and gneiss, with the sheets resembling the structure of a sliced onion. The prismatic joints of the Palisades of New Jersey and Devil's Tower, Wyoming, are examples of joints caused by contraction during the cooling of fine-grained igneous rock masses. Deep-seated igneous rocks often have joints approximately parallel to the surface, suggesting that they formed by expansion of the rock mass as overlying rocks were eroded away. Some joints in sedimentary rocks may have formed as the result of contraction during compaction and drying of the sediment. In some cases, jointing of the rock may result from the action of the same forces that cause folds and faults. In relatively undisturbed sedimentary rocks, such joints are often in two vertical sets perpendicular to one another. Commonly, streams develop along zones of weakness caused by joints in rocks, and thus the regional pattern of joint orientation often exerts a strong control on the development of drainage patterns.