Earth Structure An Introduction to Structural Geology and Tectonics

4. Deformation and Strain

A fold is a manifestation of distortion, in which originally horizontal beds are now folded. This fold, and we will see many other examples, illustrate the permanent shape changes that occur in many natural rocks. The study of these distortions, which occur in response to forces acting on bodies, is the subject of deformation. Recall, for example, the force of gravity and its many expressions. It is easy pouring syrup on pancakes because of the presence of gravity, but in the space shuttle it is quite difficult to keep the syrup in place. The fact that you are able to read this text sitting down is another convenient effect of gravity; in the space shuttle you would be floating around (probably covered by syrup). As another example, forces exerted by your body enable you to move around, but they do not provide enough acceleration to leave the Earth's surface for any length of time (because gravity pulls you back). Now let us consider a more controlled experiment to analyze the response of materials to an applied force. We can easily change the shape of a cube of clay or plasticine, by the action of, say, your hands. Where forces affect the spatial geometry of a body (syrup, you, plasticine òr rocks) we enter the realm of deformation. Simply stated: deformation of a body occurs in response to forces . From the examples given here you see that this response may have many faces. In one case the body is merely displaced or rotated (e.g., getting up from the chair and moving around the room). In other cases the body becomes distorted (e.g., plasticine cube and syrup). In this chapter we will examine such responses both qualitatively and quantitatively.