Joint (geology) 3893437 215710848 2008-05-29T11:00:32Z Mikenorton 1720909 remove cleanup and unreferenced tags [[Image:Boyabat.jpg |thumb|Columnar jointed basalt in [[Turkey]]]] [[Image:Giants-causeway-in-ireland.jpg|thumb|Columnar jointing in the basalt of the [[Giant's Causeway]] in [[Ireland]]]] [[Image:Joints Caithness.JPG|thumb|Joint sets on a [[Bedding (geology)|bedding]] plane in [[flagstone]]s, [[Caithness]], [[Scotland]]]] In geology the term '''joint''' refers to a [[Fracture (geology)|fracture]] in [[Rock (geology)|rock]] where there has been no lateral movement in the plane of the fracture (up, down or sideways) of one side relative to the other. This makes it distinct from a [[geologic fault|fault]] which is defined as a fracture in rock where one side slides laterally past to the other. Joints normally have a regular spacing related to either the mechanical properties of the individual rock or the thickness of the layer involved. Joints generally occur as sets, with each set consisting of joints sub-parallel to each other. ==Formation== Joints form in solid, hard rock that is stretched such that its brittle strength is exceeded (the point at which it breaks). When this happens the rock fractures in a plane parallel to the maximum principle stress and perpendicular to the minimum principle stress (the direction in which the rock is being stretched). This leads to the development of a single sub-parallel joint set. Continued deformation may lead to development of one or more further joint sets. The presence of the first set strongly affects the stress orientation in the rock layer, causing subsequent sets to often form at a high angle to the first set. Joint sets are commonly observed to have relatively constant spacing and the spacing is roughly proportional to the thickness of the layer <ref>Ladeira,F.L. & Price,N.J. 1981. Relationship between fracture spacing and bed thickness. Journal of Structural Geology, 3, 179-183</ref>. ==Types of joint== Joints are classified by the processes responsible for their formation, if known. ===Tectonic joints=== Tectonic joints are formed during deformation episodes whenever the differential stress is high enough to induce tensile failure of the rock, irrespective of the [[Tectonics|tectonic]] regime. They will often form at the same time as [[Fault (geology)|faults]]. Measurement of tectonic joint patterns can be useful in analyzing the tectonic history of an area because of the information they give on stress orientations a the time of formation<ref>[http://www.geosc.psu.edu/~engelder/references/link029.pdf. Engelder,T. & Geiser,P. 1980. On the use of regional joint sets as trajectories of paleostress fields during development of the Appalachian Plateau, New York. Journal of Geophysical Research, 85, B11, 6319-6341.]</ref> ===Unloading joints=== Joints are most commonly formed when uplift and erosion removes the overlying rocks thereby reducing the compressive load and allowing the rock to expand laterally. Joints related to uplift and erosional unloading have orientations reflecting the principle stresses during the uplift and care needs to be taken when attempting to understand past tectonic stresses to discriminate, if possible, between tectonic and unloading joints. [[Exfoliation (geology)|Exfoliation joints]] are special cases of unloading joints formed at, and parallel to, the current land surface in rocks of high compressive strength. ===Cooling joints=== Joints can also form via cooling of hot rock masses, particularly [[lava]], forming ''cooling joints'', most commonly expressed as vertical ''columnar jointing''. The joint systems associated with cooling are typically polygonal in nature due to the cooling introducing stresses that are isotropic in the plane of the layer. ==Fractography== Joint propagation can be studied using the techniques of [[fractography]] in which characteristic marks such as hackles and plumose structures can be used to determine propagation directions and, in some cases, the principle stress orientations<ref>[http://sp.lyellcollection.org/cgi/content/abstract/92/1/175 Roberts,J.C. 1995. Fracture surface markings in Liassic limestone at Lavernock Point, South Wales. Geological Society, London, Special Publications; v. 92; p. 175-186]</ref>. ==Importance to rock strength and [[slope stability]]== Joints form one of the most important types of discontinuity within rock masses, typically having no residual strength. ==References== {{Reflist}} [[Category:Structural geology]] [[de:Kluft (Geologie)]] [[et:Lõhe (geoloogia)]] [[ja:節理]] [[nl:Diaklaas]] [[pt:Diaclase]] [[es:diaclasa]] [[fr:Diaclase]]