ABSTRACT
The first important concepts to understand are stress and strain. Although most people use these words interchangeably, they have a very specific meaning in geology. A stress is a force applied per unit area. This is familiar with units such as pounds per square inch in the English units, or the kilobar (kbar) and Pascal (Pa) in the metric system. These stresses (Figure 4.1) can be compressional (forces coming together and squeezing the material between them), tensional (forces pulling something apart), or shear (forces sliding past one another). Strain is the deformation resulting from stress. Thus, stress is the force and strain is the response. Even though people use both words interchangeably, they are not synonyms in geology. An analogy might be
to say that when a person is under a lot of stress, that person shows the strain by loss of sleep, erratic behavior, or nervousness.
