Process of breaking up rocks under pressure by introducing water or other fluids; usually done to increase permeability in oil, gas, and geothermal reservoirs. The pressure opens joints, cracks, and bedding planes which can be kept open by the introduction of sand, glass beads, or aluminium balls. Hydrofracturing may occur naturally as a result of internal hydraulic overpressures, e.g. in the formation of porphyry deposits, and is often used to increase recovery from oil and gas wells. See geothermal field; porphyry copper.