fracturing rocks at great depth frequently becomes suppressed by pressure due to the weight of the overlying rock strata and the cementation of the formation. this suppression process is particularly significant in "tensile" (mode 1) fractures which require the walls of the fracture to move against this pressure. fracturing occurs when effective stress is overcome by the pressure of fluids within the rock. the minimum principal stress becomes tensile and exceeds the tensile strength of the material. fractures formed in this way are generally oriented in a plane perpendicular to the minimum principal stress, and for this reason, hydraulic fractures in well bores can be used to determine the orientation of stresses. in natural examples, such as dikes or vein-filled fractures, the orientations can be used to infer past states of stress.