JOINTS, in geology, are the dividing planes by which nearly all rocks are traversed more or less completely and which extend in various directions and at various inclinations, generally at a high angle, to the horizontal. Unlike faults, joints are fissures where little or no movement between the blocks has taken place, though the presence of slickensides—striae on the smooth f aces— show that in some cases there has been some rubbing movement. Jointing cannot extend to a very great depth: for towards the base of the "zone of fracture," estimated by Van Hise at about 12 km. in rigid rocks, joints and faults cannot form since the rocks tend to flow in relief of strain.
In unweathered rocks, joints, though present, are inconspicuous, but on weathering become marked, especially in a soluble rock such as limestone. Sedimentary rocks usually show two sets of joints perpendicular to the bedding and to one another, one set in the direction of dip, the other of strike. The more prominent set, if well developed and running for long distances, forms a set of master joints. Individual fissures cannot be traced very far, since joints run into one another, though their approximate general parallelism is evident. The distance between joints varies from an inch or so to several yards : in alternating strata the degree of jointing may vary from bed to bed. In igneous rocks jointing is generally quite irregular, but in granite two vertical sets at right angles to one another, with another set of cross joints approxi mately horizontal, are frequently found. These cross joints are of a different origin : they are the effect of weathering since they con form to the surface of the ground and die out with depth. In trusive sills and dikes in many places show a columnar structure perpendicular to the cooling surfaces, where jointing has been so symmetrical that the sheets weather out into polygonal prismatic columns: in the well known Giant's Causeway the columns are hexagonal implying three equally developed sets of joints.
Near the surface joints are more or less open, especially in lime stones. Solution by percolating water controlled by joints has led to the formation of large caves and underground rivers several miles in length. Highly jointed rocks are objectionable as a source of potable water; since such water is likely to be contami nated by surface impurities. Quarrying operations are much helped by the presence of a well developed joint system combined with the bedding planes of weakness. Large monoliths are obtain able from granite where the vertical jointing, widely spaced, is not interrupted by cross jointing.