With regard to the manner of their production, joints may be classified into two series: (1) those which are due to tension, the rock usually parting in planes normal to the directions of tension; (2) those which are due to compression, the cracks forming in the shearing-planes.
In igneous rocks joints are caused by the cooling and consequent contraction of the highly heated mass. This shrinkage sets up tensile stresses in the mass to which the rock yields by cracking and parting, the shape of the blocks being largely controlled by the coarseness or fineness of the mass. Igneous rocks are subject to all the vicissitudes which affect other kinds of rocks; they are faulted, compressed, exposed to tension, etc. Hence, systems of joints may occur in them, which were formed subsequently to the shrinkage-joints due to the contraction of cooling. In some cases the jointing of sedimentary rocks may perhaps be caused by a shrinkage of the mass on drying, but this cannot be an important method of producing systems of joints.
The convex sides of anticlinal and synclinal folds are stretched, and (provided they are not too deeply buried) the stretching may result in a system of cracks radial to the curves which follow the strike of the beds. Folds are not horizontal, but pitch in the direction of their axes. This complex folding may produce two sets of tensile stresses perpendicular to each other, and thus cause two series of joints, one following the strike and the other the dip of the beds. Complex folding must produce a twisting and warping of the strata, and it has been experimentally shown that a brittle substance, when twisted, cracks in two sets of fractures which intersect nearly at right angles. How slight is the twisting and warping needful to produce joints is shown by the fact that strata which are perfectly horizontal, so far as can be detected, are jointed. The modern limestones which are formed in coral-reefs are jointed, even in cases where the movements resulting in fracture must have been minimal.
Tension joints produce either rough, or smooth and sharply cut surfaces, which is determined by the character of the rock. In sandstones which are weakly cemented the cracks pass between the grains, while in hard and firm rocks the fractures are clean.
Fig. 197. - Jointing in limestone, Black Hills, South Dakota. (U. S. G. S).
Compression Joints are caused when the rocks yield along the shearing-planes. In simply folded strata are produced two sets of strike joints which are inclined toward each other, but whether dip joints will be made by complex folding is not certain. In some conglomerates the joint planes pass through the hard quartz pebbles and leave a smooth, even, shining face. Tension would pull such a pebble out of its socket and only by shearing could it be cleanly cut.
The whole subject of joints in sedimentary rocks is a difficult one and the explanations given of them are not altogether satisfactory, for several other agencies may be involved in their production. It is, however, highly probable that the master joints which roughly follow the strike and dip of the strata, have been caused by the forces which produce folding.
Fig. 198. - Joints dying away downward, shown by pinching out of white calcite veins. (Photograph by van Ingen).
Joints cannot occur in the shell of flowage, and are best developed in the shell of fracture, being of less importance in the transition belt between the two.