A joint is defined as a fracture in a rock between the sides of which there is no observable relative movement. They are present is most consolidated rocks of igneous, metamorphic and sedimentary origin. Joints may form as a result of either diastrophism or contraction. Description: (i) A series of parallel joints is called a ‘joint set’. (ii) Two or more joint sets intersecting each other produce a ‘joint system’. (iii) Two sets of joints nearly at right angles to one another, produced by the same stress system, is known as conjugate system.
iv) A persistent joint or set which may be horizontal or vertical is called ‘master joint’. Classification: 1. According to the mode of origin, three types of joints have been recognised, as follows: (a) Tensional joints: These are also known as ‘shrinkage joints’. In igneous rocks, they are produced as a consequence of contraction due to cooling. ‘Columnar Structure’ which characterises many basic extrusive and intrusive, consists of long hexagonal blocks closely packed together.
In granites and granodiorites several sets of joints may be observed, but commonly three sets are prominent-one horizontal and two vertical at right angles to each other and to the Horizontal set. If these sets are more or less equally spaced, the fracture planes give rise to cubical blocks; the jointing is then termed ‘Mural jointing’. Joints formed in little deformed sedimentary rocks are due to tension caused by compaction and shrinkage as sediments are considerable into sedimentary rocks. Tensional joints may also be due to deformation. b) Sheet joints: These joints develop in sets and are more or less parallel to the surface of the ground, especially in plutonic igneous intrusions such as granite. They may originate due to unloading of the rock mass when the cover is removed through the processes of erosion. (c) Tectonic joints: These are also known as shear joints. They are formed in a rock under compression. They originate as a direct result of folding or thrusting in rocks. Generally they are of three types: (i) Strike set:
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Longitudinal joints parallel to the fold axis. (ii) Dip-set: Also known as cross-joints, perpendicular to the longitudinal joints. (iii) Diagonal set: Which is a conjugate set of oblique joints, -which lie at rather less than 45° to the direction of tectonic-axis. 2. According to the geometric classification of joints, there are three important varieties, like strike-joints, dip joints and diagonal joints, which are totally with respect to the regional strike and dip of the country rocks. Joints may be open or closed.
The closed joints are also known as latent, blind or incipient joints. They may become open as a result of weathering, which is commonly found in jointed lime- Stones. Recognition of joints in the field and their effects on out crops: Joints are generally recognised in the field as ‘faults without displacement’. Their dimension varies within wide-limits. Sometimes they are very short in their extension, but in certain cases they are found to extend for miles together. Joints commonly control the drainage pattern of an area.
They also determine the shape of coastlines, because they provide a passage, whereby water may penetrate deeply into the rockmass, thus allowing weathering to take place. Jointed rocks are pervious to fluids and hence may act as aquifers or reservoir rocks for oil or natural gas. The presence or absence of joints in a region matters much to quarrymen and miners because it determines the ease with which query and mining can be accomplished. Sometimes joints act as avenues for molten rock materials to come above the surface. It also determines the localisation of some mineral deposits.
This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2012) Geologic provinces of the world (USGS) Shield Platform Orogen Basin Large igneous province Extended crust Oceanic crust: 0–20 Ma 20–65 Ma >65 Ma Igneous rock (derived from the Latin word ignis ...