1.0: Introduction to Rheology
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The movement of tectonic plates is often thought of on a very large scale, but what actually happens to rocks at the plate boundaries as the plates slowly slide past each other? How a rock responds when a force is applied is determined by both the physical properties of the rock and the conditions at which the rock is deforming (e.g., temperature and pressure). The physical properties, such as elasticity and fracture strength also depend on the rock type. Rheology describes how rock will deform when subjected to a force and takes into account both the physical properties of the rock and the conditions at which it is deforming. Examples of a rheology include elastic (for materials that deform like a spring) and viscous (for materials that deform like a fluid). Forces on rocks are the results of the weight of overlying rock (also known as the lithostatic stress), tectonic forces originating from convection in the Earth's mantle, which moves the tectonic plates, and gravitational forces related to density differences within the crust and lithosphere.
The term rheology was coined by Eugene Bingham in 1929, inspired by the Greek phrase, panta rhei, meaning everything flows. In some disciplines it is used only to describe the flowing behavior of material. In the geosciences it is used more generally to describe all types of deformation experienced by earth materials.
An understanding of rheology is crucial for understanding concepts in geology, geophysics and planetary science as well as petrology, such as what happens during an earthquake, how mountains and faults are formed, how rocks deform in the mantle, the origin of tectonic plates, how magma ascends through the crust and how lava flows.
In physics, we learn to consider how forces are applied to particles or blocks and we are interested in the resulting displacement or velocity of the particle or block. In geophysics we turn our attention to how a block (or a rock) deforms internally when forces are applied at its boundaries. In this context, in the place of force we consider the stress, which is defined as the force applied over an area. Likewise, in the place of displacement we consider strain, which is defined as the change in shape or size of a rock relative to the original shape or size. Finally, in the place of velocity, we consider the strain-rate, which is defined as the change in strain with respect to time.
By the end of this chapter, you should be comfortable with the various ways in which we describe the stresses that act on a rock, some ways a rock can respond to those stresses, and how these stresses are related to geologic structures such as faults or shear zones. First, we will examine stress and strain in a one dimensional (1-D: along a line) context. Second, we will look at various types of rheology (elastic, viscous, and plastic), the timescales over which deformation occurs and the effect this has on some geologic structures. Finally, we will cover how rocks fail (brittle or plastic failure).