Joint (geology)

Joints arise from brittle fracture of a rock or layer due to tensile stress. This stress may be imposed from outside; for example, by the stretching of layers, the rise of pore fluid pressure, or shrinkage caused by the cooling or desiccation of a rock body or layer whose outside boundaries remained fixed. When tensional stresses stretch a body or layer of rock such that its tensile strength is exceeded, it breaks. When this happens the rock fractures in a plane parallel to the maximum principal stress and perpendicular to the minimum principal 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 additional joint sets. The presence of the first set strongly affects the stress orientation in the rock layer, often causing subsequent sets to form at a high angle, often 90°, to the first set. == Types ==

Joints are classified by their geometry or by the processes that formed them. By geometry The geometry of joints refers to the orientation of joints as either plotted on stereonets and rose-diagrams or observed in rock exposures. In terms of geometry, three major types of joints are recognized: columnar jointing, systematic joints, and nonsystematic joints. but rare cases of columnar jointing have also been reported in sedimentary strata. The width of these prismatic columns ranges from a few centimeters to several metres, and they are often oriented perpendicular to surfaces of contact between the igneous rock and its cooler surroundings. They can thus usually be seen at the top and base surfaces of lava flows, and the contacts of tabular igneous intrusions with the surrounding rock. Systematic Systematic joints are planar, parallel, joints that can be traced for some distance, and occur at regularly, evenly spaced distances on the order of centimeters, meters, tens of meters, or even hundreds of meters. As a result, they occur as families of joints that form recognizable joint sets. Typically, exposures or outcrops within a given area or region of study contains two or more sets of systematic joints, each with its own distinctive properties such as orientation and spacing, that intersect to form well-defined joint systems. Hydraulic Hydraulic joints are formed when pore fluid pressure becomes elevated as a result of vertical gravitational loading. In simple terms, the accumulation of either sediments, volcanic, or other material causes an increase in the pore pressure of groundwater and other fluids in the underlying rock when they cannot move either laterally or vertically in response to this pressure. This also causes an increase in pore pressure in preexisting cracks that increases the tensile stress on them perpendicular to the minimum principal stress (the direction in which the rock is being stretched). If the tensile stress exceeds the magnitude of the least principal compressive stress the rock will fail in a brittle manner and these cracks propagate in a process called hydraulic fracturing. Hydraulic joints occur as both nonsystematic and systematic joints, including orthogonal and conjugate joint sets. In some cases, joint sets can be a tectonic - hydraulic hybrid. Unloading Unloading joints or release joints arise near the surface when bedded sedimentary rocks are brought closer to the surface during uplift and erosion; when they cool, they contract and become relaxed elastically. A stress builds up which eventually exceeds the tensile strength of the bedrock and results in jointing. In the case of unloading joints, compressive stress is released either along preexisting structural elements (such as cleavage) or perpendicular to the former direction of tectonic compression. == Fractography ==

Joint propagation can be studied through the techniques of fractography in which characteristic marks such as hackles and plumose structures are used to determine propagation directions and, in some cases, the principal stress orientations. Plumose fracture.jpg|Plumose structure on a fracture surface in sandstone, Arizona. Plumose Fracture 1.JPG|Detail of plumose fracture. Plumose_Fracture_2.JPG|Detail of plumose fracture. == Shear fractures ==

Some fractures that look like joints are actually shear fractures, which in effect are microfaults. They do not form as the result of the perpendicular opening of a fracture due to tensile stress, but through the shearing of fractures that causes lateral movement of the faces. Shear fractures can be confused with joints because the lateral offset of the fracture faces is not visible in the outcrop or in a specimen. Because of the absence of diagnostic ornamentation or the lack of any discernible movement or offset, they can be indistinguishable from joints. Such fractures occur in planar parallel sets at an angle of 60 degrees and can be of the same size and scale as joints. As a result, some "conjugate joint sets" might actually be shear fractures. Shear fractures are distinguished from joints by the presence of slickensides, the products of shearing movement parallel to the fracture surface. The slickensides are fine-scale, delicate ridge-in-groove lineations on the surface of fracture surfaces. == Importance ==

Joints are important not only in understanding the local and regional geology and geomorphology but also in developing natural resources, in the safe design of structures, and in environmental protection. Joints have a profound control on weathering and erosion of bedrock. As a result, they exert a strong control on how topography and morphology of landscapes develop. Understanding the local and regional distribution, physical character, and origin of joints is a significant part of understanding the geology and geomorphology of an area. Joints often impart a well-develop fracture-induced permeability to bedrock. As a result, joints strongly influence, even control, the natural circulation (hydrogeology) of fluids, e.g. groundwater and pollutants within aquifers, petroleum in reservoirs, and hydrothermal circulation at depth, within bedrock. Thus, joints are important to the economic and safe development of petroleum, hydrothermal, and groundwater resources and the subject of intensive research relative to these resources. Regional and local joint systems exert a strong control on how ore-forming hydrothermal fluids (consisting largely of , , and NaCl — which formed most of Earth's ore deposits) circulated within its crust. As a result, understanding their genesis, structure, chronology, and distribution is an important part of finding and profitably developing ore deposits. Finally, joints often form discontinuities that may have a large influence on the mechanical behavior (strength, deformation, etc.) of soil and rock masses in, for example, tunnel, foundation, or slope construction. As a result, joints are an important part of geotechnical engineering in practice and research. ==Image gallery==

PICT1709.JPG|Horizontal joints in the sedimentary rocks of the foreground and a more varied set of joints in the granitic rocks in the background. Image from the Kazakh Uplands in Balkhash District, Kazakhstan. Joints City of Rocks NR.jpg|Joints in the Almo Pluton, City of Rocks National Reserve, Idaho. Recent joint intersection.JPG|Recent tectonic joint intersects older exfoliation joints in granite gneiss, Lizard Rock, Parra Wirra, South Australia. Joint spacing varying with bed thickness.jpg|Joint spacing in mechanically stronger limestone beds shows increase with bed thickness, Lilstock Bay, Somerset. Jointed diorite outcrop, Dalupirip, Itogon, Benguet 01.jpg|Roadside weathered diorite outcrop showing joints. Baguio-Bua-Itogon Road, Philippines ==See also==