EarthStructure An Introduction to Structural Geology and Tectonics

5. Rheology

Strain is the shape change that a body undergoes in the presence of a stress field. But what do we really know about the corresponding stress? And is stress independent of strain? In this final chapter of the block on Fundamentals, we turn to the final and perhaps most challenging aspect: the relationship between stress and strain. While it is evident that there is no strain without stress, their actual relationship is not easy to define on a physical basis. In other words, stating that stress and strain in rocks are related is quite a different matter from physically determining to what extent and their actual relationship(s). In materials science and geology we use the term rheology to describe the ability of materials under stress to deform or to flow, which involves fundamental parameters as strain rate (strain per time) and viscosity. These and several other concepts will be discussed in this chapter, and especially we will look at their significance for understanding of rock deformation.

Rheology is the study of flow of matter. Flow is an everyday phenomenon; in the previous chapter we used syrup on pancakes and human motion as actualistic examples of deformation in the presence of stresses. Rocks don't seem to do much by comparison, but remember that geologic processes take place over hundreds of thousands to millions of years. For example, considerable deformation has occurred in rocks along the San Andreas Fault during the last 700,000 years, and lateral displacements on the order of tens of kilometers may have occurred in the Paleozoic Appalachian fold-and-thrusts belt over a time period of about 1 million years (1 m.y.). Geologically speaking, time is available in large supply and given sufficient amounts of it, rocks are able to flow somewhat like syrup. One everyday example of flow in what we consider solid material can be found in an unexpected place. When you look carefully at the windows in an old house you may find that the glass distorts your view. The reason is that, with time, the glass has sagged under its own weight (driven by gravity), giving rise to a wavy image. If the window glass is very old, you actually find that the top part of the glass is thinner than the bottom part. Eventually, the glass should leave the window frame and form a glass ornament on the window sill. There is no immediate reason for concern, however, because this process will take many thousands of years.