Microcracks in rocks: A review

Abstract In the past decade the number of studies about microcracks in rocks has rapidly increased. This review of recent work concentrates on microcracks in rock as separate entities, emphasizing microcrack morphogenesis, kinematics, dynamics, population statistics and observational techniques. Cracks are produced when the local stress exceeds the local strength. The local stress may be augmented by twin lamellae interactions, kink bands and deformation lamellae, stress concentrations at grain boundary contacts and around intracrystalline cavities. Local strength may be reduced along cleavage planes, along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids. Dislocations appear not to be a significant factor for crack nucleation below about 500°C in silicates. Spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities. The amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity. The application of hydrostatic pressure between 100 and 200 MPa effectively closes most cracks, but the closure may not be uniform if crack wall asperities exist. Hydrostatic pressure appears to stabilize cracks and make crack growth more difficult. The number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures. The application of a deviatoric stress field on the boundaries of a rock mass results, on a microscopic scale, in a very complex stress system which greatly affects nucleation and propagation paths. The relative amount of intragranular and intergranular cracking appears to depend upon mineralogy, rock type and stress state. The vast majority of stress-induced microcracks in rocks appear to be extensional. Statistically, they are predominantly oriented within 30° of the macroscopic maximum stress direction. Crack densities increase as macroscopic deviatoric stress increases above a threshold level. Crack size distributions may be either lognormal or exponential. Fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks, not the growth of a single crack. Some crack configurations are more favorable for coalescence than others. As deviatoric stress increases and rock failure is approached, the microcrack population changes spatially from random to locally intense zones of cracking. Away from the fault, the crack density dies off rapidly to the background level a few grains away. Under lesser deviatoric stresses, slow, subcritical microcrack growth can occur as a result of stress-aided corrosion at the crack tip. The rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip. Important details still remain to be worked out.