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a combination of fractures in rocks, along which there has been little or no movement.
Joints may be open, closed, or blind. The blocks and pieces of rock encompassed by joints are called joint blocks. Joints may be vertical, inclined, or horizontal, depending on their spatial orientation. In laminar rock strata, joints may be transverse, diagonal, or parallel relative to the strata.
Jointing may develop during the original formation of the rock (original jointing), or it may result from later exogenic or endogenic processes. Original joints form in sedimentary rock during diagenesis, accompanied by compaction and dehydration of the sediment. In magmatic rock, original contraction joints occur to compensate for the reduction in volume of cooling magmatic bodies. Exogenic processes lead to the formation of weathering joints, joints associated with expansion of the rock upon removal of loads (on the slopes and floors of river valleys and ravines), and joints accompanying the formation of landslides, cave-ins, and collapses. Shear and tension joints form during endogenic processes. Tension joints develop in the direction of maximum normal tension stresses, perpendicular to the rock tension or in the direction of its compression; they are short, with an uneven, coarse surface, and are common along the hinges of folds on the vaults of domes and limbs of fractures. Shear joints occur in the direction of maximum tangential stress at an angle of about 45° to the axis of compression or tension; they are even and straight, often have traces of grinding of rock, and extend for dozens and even hundreds of meters along and beneath the surface. Cleavage is a special form of shear joint.
REFERENCESBelousov, V. V. Strukturnaia geologiia, 2nd ed. Moscow, 1971.
Mikhailov, A. E. Strukturnaia geologiia i geologicheskoe kartir-ovanie, 3rd ed. Moscow, 1973.
A. E. MIKHAILOV