Joints are fractures along which no significant displacement has occurred. They are found in most of the consolidated rocks of igneous, sedimentary and metamorphic origin.
The tectonic mode of jointing is mostly due to large-scale twisting or torsion of the earth’s crust involving both compressional and tensional forces. Joints divide the rocks into parts or blocks, without any movement of the blocks past each other.
A rock may be traversed by a number of joints, but some of them may appear well developed and continuous for considerable length than the others. Such conspicuous joints are called Master Joints or Major Joints.
Ascries of parallel joints is called a Two or more joint sets intersecting each other produce a Joint-system. Two sets of joints, nearly at right angles to one another, are said to form a conjugate-joint system.
ADVERTISEMENTS:
Types of Joints
Genetically the following types of joints have been recognised in various types of rocks
(a) Tensional joints
These are also known as shrinkage-joints. The polygonal cracks found on the dried-out mud flats due to contraction are the examples of Tensional Joints in sedimentary rocks.
ADVERTISEMENTS:
In igneous rocks, such joints are developed due to cooling and contraction of the magma -mass i.e. the originally hot rock. The joints may be of radiating, irregular, hexagonal or polygonal type.
Fine grained uniform rocks like basalt sometime show a remarkable kind of Columnar-jointing. The most perfect forms are composed of hexagonal columns which extend inwards from a cooling surface of the molten mass.
In granites and granodiorites several sets of joints may be observed, but commonly three set are prominent-one horizontal and two vertical at right angles to each other and to the horizontal set.
When these sets are more or less equally spaced, the fracture planes give rise to cubical or rectangular blocks. Such a jointing is called mural jointing.
ADVERTISEMENTS:
Tensional joints may also be due to deformation. Such joints are commonly developed along the axis of the anticlines, where the stretching effect is more pronounced.
Sometimes joints, more or less parallel to the surface of the ground, are developed in plutonic igneous rocks like granites as they are exposed at the surface.
They originate mainly due to unloading effect, when the cover is removed through the processes of erosion. These joints are called Sheet joints or exfoliation joints.
(b) Shear joints
ADVERTISEMENTS:
These are also known as tectonic joints and are formed in a rock under compression. They form as a direct result of folding or thrusting in rocks.
Two sets of shear fractures generally develop under compression and it has been established, through a number of studies that there exists much relation between the orientation of the joint patterns and the orientation of the stress system which operates in mountain building or continental uplift or local twists etc.