(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...