Deformation: Stress and Strain

Tectonic activity (i.e., the movement of the Earth’s tectonic plates) has an important effect – it applies stress on the surrounding rock of the Earth’s crust.

Rocks change as they undergo stress. Stress is a force applied to a given area. Since stress is a function of area, changing the area to which stress is applied will change the resulting stress. For example, imagine the stress created at the tip of the heel of a high-heeled shoe and compare it to the bottom of an athletic shoe. In the high-heeled shoe heel, the area is very small, so much stress is concentrated at that point. The stress is more spread out in an athletic shoe. If stress is not concentrated at one point in a rock, the rock is less likely to change (break or bend).

There are three main types of stress: compression, tension, and shear. When compressional forces are at work, rocks are pushed together. Tensional forces operate when rocks pull away from each other. Simple shear force is created when rocks move horizontally past each other in opposite directions. Rocks can withstand much more compressional stress than tensional stress.

Applying stress creates a deformation in the rock, known as strain. Initially, as rocks are subjected to increased stress which begins the process of strain, they behave elastically, meaning they return to their original shape after deformation ceases. This elastic behavior continues until the rocks reach their elastic limit, which will begin to deform plastically.

Plastic deformation may lead to the rocks bending into folds, or if too much strain accumulates, the rocks may behave in a brittle manner and fracture. An example of brittle behavior is a hammer hitting glass, which shatters the glass. With plastic deformation, the rocks do not return to their original shape when the stress is removed. The deformation that results from applied stress depends on many factors, including the type of stress, the type of rock, pressure and temperature conditions (e.g., rocks deeper in the crust will be subject to higher pressures and temperatures), and the length of time the rock is subjected to the stress. Rocks behave very differently at depth than at the surface. Rocks tend to deform in a more plastic manner at depth and in a more brittle manner near Earth’s surface. The faster the event occurs (like an earthquake), the more likely rocks will experience brittle deformation.

Figure Above: A stress and strain diagram. As stress and strain increase, rocks first experience elastic deformation and will return to their original shape if the stress is released. When the elastic limit is reached (point X), if stress accumulates as strain, the rocks will deform plastically and will not return to their original shape if the stress is released. If rocks are subjected to stress greater than they can accommodate with strain, they will fracture (brittle deformation, yellow star)