| Joint (geology) |
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in Ireland ]] In geology the term joint refers to a Fracture in rock where there has been no lateral movement (up, down or sideways) of one side relative to the other. This makes it distinct from a Fault which is defined as a fracture in rock where one side slides laterally past to the other. Joints form in solid, hard rock that is stretched past its Elastic Modulus (the point at which it breaks). When this happens the rock fractures in a plane perpendicular to the extensional stress (the direction in which the rock is being stretched) and parallel with compressive stress (the direction in which the rock is being compressed). The direction of compressive stress is most commonly vertical if due only to the weight of overlying rock and atmosphere. Joints are most commonly formed when erosion removes the overlying rocks thereby reducing the compressive load and allowing the rock to expand laterally. Joints can also form via cooling of hot rock masses, particularly Lava , forming ''cooling joints'', most commonly expressed as vertical ''columnar jointing''. Measurement of joint patterns can be useful in analyzing the tectonic history of an area because of the relationship between the direction of stretch and compression. Joint propagation directions can be analyzed by characterizing ''plumose structures'' on the faces of joints, which manifest as fan-shaped irregularities which originate at the point of rupture and propagate toward the site of minimum compressive stress. Joints can be classified by their orientation and by the process of their formation. |
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