Stress is the force applied to an object. In geology, stress is the force per unit area that is placed on a rock. There are four types of stresses that act on materials.
A deeply buried rock is pushed down by the weight of all the material above it. Since the rock is trapped in a single spot, it is as if the rock is being pushed in from all sides. This pushing causes the rock to become compressed, but it cannot deform because there is no place for it to move. This is called confining stress.
Compression is the stress that squeezes rocks together. Compression causes rocks to fold or fracture . When cars driving around a parking lot collide, compression causes the cars to crumple. Compression is the most common stress at convergent plate boundaries
Rocks that are being pulled apart are under tension (also called extension). Tension causes rocks to lengthen or break apart. Tension is the major type of stress found at divergent plate boundaries.
When forces act parallel to each other but in opposite directions, the stress is called shear. Shear stress causes two planes of material to slide past each other. This is the most common stress found at transform plate boundaries.
