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Joint (geology)
A joint is a break (fracture) of natural origin in the continuity of either a layer or body of rock that lacks any visible or measurable movement parallel to the surface (plane) of the fracture. Although they can occur singly, they most frequently occur as joint sets and systems. A joint set is family of parallel, evenly spaced joints that can be identified through mapping and analysis of the orientations, spacing, and physical properties. A joint system consists of two or more interlocking joint sets. The distinction between joints and faults hinges on the terms ''visible'' or ''measurable'' which depends on the scale of observation. Faults differ from joints in that they exhibit visible or measurable lateral movement between the opposite surfaces of the fracture. As a result, a joint may have been created by either strict movement of a rock layer or body perpendicular to the fracture or by varying degrees of lateral displacement parallel to the surface (plane) of the fracture that remains “invisible” at the scale of observation.〔Mandl, G. (2005) ''Rock Joints: The Mechanical Genesis.'' Springer-Verlag, Heidelberg, Germany. 221 pp. ISBN 978-3-540-24553-7〕〔Davis, G.H., S.J. Reynolds, and C. Kluth (2012) ''Structural Geology of Rocks and Regions (3rd ed.)'': John Wiley and Sons, Inc., New york, New York. 864 pp. ISBN 978-0471152316〕〔Goudie, A.S. (2004) ''Encyclopedia of Geomorphology volume 2 J–Z.'' Routledge New York, New York. 578 pp. ISBN 9780415327381〕
Joints are among the most universal geologic structures as they are found in most every exposure of rock. They vary greatly in appearance, dimensions, and arrangement, and occur in quite different tectonic environments. Often, the specific origin of the stresses that created certain joints and associated joint sets can be quite ambiguous, unclear, and sometimes controversial. The most prominent joints occur in the most well-consolidated, lithified, and highly competent rocks, such as sandstone, limestone, quartzite, and granite. Joints may be open fractures or filled by various materials. Joints, which are infilled by precipitated minerals are called veins and joints filled by solidified magma are called dikes.〔〔
== Formation ==
Joints result from brittle fracture of a rock body or layer as the result of tensile stresses. These tensile stresses either were induced or imposed from outside, e.g. by the stretching of layers; the rise of pore fluid pressure as the result of either external compression or fluid injection; or the result of internal stresses induced by the 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.〔〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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