Joint

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Joint [geological]: defined by the Oxford English Dictionary (1989) as a crack or fissure intersecting a mass of rock; usually occurring in sets of parallel planes, dividing the mass into more or less regular blocks, with first usage attributed to Holland (1601). Price (1966) defines joints as cracks and fractures in rock along which there has been extremely little or no movement, with some authors describing shear joints having a shear displacement (e.g., Hancock, 1985). Pollard and Segall (1987) show that joints have two parallel surfaces that meet at the fracture front, that these surfaces are approximately planar, and that the relative displacement of originally adjacent points across the fracture is small compared to fracture length. Pollard and Aydin (1988) suggest that joints are generally associated with the opening mode, whereas faults are associated with the shearing modes, and propose that the word joint be restricted to those fractures with field evidence for dominantly opening displacements. We suggest that fractures with dominantly opening displacements but that have mineral fills should be called veins. Pollard and Aydin (1988) criticise definitions of joints as having no discernible displacement, because they would not exist without some displacement. They are particularly critical of the concept of shear joints, which they regard as small faults. Pollard and Aydin (1988) define, describe and illustrate various joint geometries (e.g., cross, diagonal, dip, ladder, normal, polygonal and strike joints), mechanisms (e.g., contraction, cooling, shrinkage and unloading joints) and surface patterns (e.g., hackle and rib marks and plumose structures).(From Peacock et al., 2016, Glossary of fault and other fracture networks, J. Structural, Geology, 92, 12-29).