Jointing of Rocks

typical shapes of the chunks, blocks, and fragments into which rocks break when split naturally and artificially. The shape of the jointing is determined by the orientation and density of the cracks that bound it; the dimensions vary (from centimeters to meters in cross section).

In sedimentary rocks rectangular, cubical, parallelepipedal, slab, prismatic, spheroidal, and scaly jointings are common. The formation of jointings in sedimentary rocks is primarily a result of the cracks that form in the process of lithogenesis and during the deformation and weathering of the rocks. Igneous rocks have prismatic (usually hexagonal) or columnar, spheroidal, rectangular, parallelepipedal, slab, and pillow jointings that develop during the cooling and compression of lavas and intrusions. In metamorphic rocks the most common jointings are slab, platy, and ribbed jointings that develop during rock deformation. The distribution of joints throughout the rock mass determines whether chunks of consistent size and shape can be obtained; this is important in the quarrying of block rubble. The jointing of rocks is taken into account in planning blasting operations during the working of mineral deposits and the cutting of mine shafts by machine with a minimum expenditure of power.