EarthStructure An Introduction to Structural Geology and Tectonics

3. Force and Stress

We are all too familiar with stress -- it is a term that we use on many occasions. Stress from yet another homework assignment, a test, or maybe an argument with a roommate or spouse. We are also aware that we are subject to the forces of nature. The force of gravity tries to keep us on the Earth's surface and the force of impact destroys our car. Just like us, rocks experience the pull of gravity throughout their history, beginning with deposition followed by deformation and erosion. Forces arising from plate interactions result in a broad range of geological structures that include fabrics and textures on the microscale, faults and folds on the mesoscale, and mountain ranges and oceans on the macroscale. In geology, the terms force and stress have very specific meaning. In this chapter we will discuss these concepts, followed by a more detailed look at the components of stress that ultimately are responsible for the formation of geological structures. To understand the processes associated with the structural geology of rocks and regions, we must be acquainted with some fundamental principles of mechanics. Mechanics is concerned with the action of forces on bodies and their effect; you can say that it is the science of motion. Classical or Newtonian mechanics describes the action of forces on rigid bodies. The equations of Newtonian mechanics explain everything from the entertaining interaction between colliding balls at a game of pool to the galactic dance of the planets in our solar system. However, in rocks we often deal with processes that result both in movement and distortions, i.e., displacements between as well as within bodies. The previously mentioned encounter between a wall and a car comes to mind as an unfortunate example of displacement and distortion. The theory associated with this type of material behavior is the topic of continuum mechanics. In continuum mechanics a materials is treated as a continuous medium, i.e., there are no discontinuities that affect its behavior. Immediately this may seem inappropriate for rocks, because we know that they consists of many grains whose boundaries are material discontinuities. At scale of these discontinuities (the grain scale) one might argue that continuum mechanics does not be apply. Yet, on the scale of the rock body we can consider the system statistically homogeneous, and the predictions from continuum mechanics theory give us adequate first-order descriptions of the changes in rocks. The primary advantage of a continuum mechanics approach is that it allows for the mathematical description of deformation in relatively simple terms. However, if the behavior of rocks is dominated by small-scale discontinuities, such as is the case in fracture mechanics (Chapter 6), continuum mechanics theory no longer applies. For now, however, we will consider it an adequate approach.

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