A force acting upon or within a mass or rock, expressed in terms of unit weight per surface area such as tons per square inch. Stress is any force which acts to deform rocks: (1) Compression – a stress that acts to press or squeeze rocks together. (2) Tension – a stress that acts to stretch a rock, or pull a rock apart. (3) Shear – a stress which acts tangential to a plane through a body, causing two contiguous parts to slide past each other