Faults and Joints

Fractures are planar discontinuities, i.e. interruption of the rock physical continuity, due to stresses. The geological fractures occur at every scale, so that any large volume of rock has some or many. These discontinuities are related to sudden elastic relaxation of elastic energy stored in the rock. The geological fractures have their economic importance. The loss of continuity in intact rocks provides the necessary permeability for migration and accumulation of fluids such as groundwater and petrol. Fractured reservoirs and aquifers are typically anisotropic since their transmissivity is controlled by the conductive properties of fractures, which the local stress field partially controls. Geological fractures may be partially or wholly healed by introduction of secondary minerals, often giving rise to ore deposits, or by recrystallization of the original minerals. Planar discontinuities along which rocks lose cohesion during their brittle behaviour are called: joints if there is no component of displacement parallel to the plane (there may be some very small orthogonal parting; joints are extension fractures). faults if rocks on both sides of the plane have moved relative to each other, parallel to the plane (faults are shear fractures). veins if the fractures are filled with secondary crystallisation. Joints and faults divide the rocks in blocks whose size and shape must be taken into consideration for engineering, quarrying, mining and geomorphology.