# 1: Rheology of Rocks

• 1.0: Introduction to Rheology
The movement of tectonic plates is often thought of on a very large scale, but what actually happen to rocks at plate boundaries? In simple terms, rheology tells us how a rock will behave when a force is applied to it. Forces on rocks can come from a variety of places, such as the weight of overlying rock or the pulling apart of crust at divergent boundaries. An understanding of rheology is crucial for understanding important concepts in geophysics.
• 1.1: Stress
In introductory physics courses we learn about how objects move when forces act on them. However, we usually consider that the object itself is perfectly rigid and we are not concerned with the internal deformation of the object. However, when we want to consider how an object deforms internally when a force is applied we have to consider the rheology of the material, the orientation of the applied forces and where these forces are applied.
• 1.2: Strain and Strain Rate
In introductory physics we learn to relate the forces applied to an object to the resulting displacement or velocity of the object, but we do not consider how an object deforms internally in response to the applied forces. To understand how rocks deform through the slow movement of tectonic plates, or rapid movement occurring during an earthquake, we need to consider the internal deformation of the rocks.
• 1.3: Elastic Deformation
In geology, we measure strain without knowing about the applied stress or type of deformation (elastic, viscous) the rock experienced. So how do we get from the observed strain to the magnitude of stress causing the deformation? We can first investigate the type of deformation, which is either elastic or viscous. Elastic deformation is shallow and has a low magnitude of strain.
• 1.4: Failure of Rocks
Let's relate the previously discussed stresses to fracture and faulting. Failure can occur by frictional sliding or fracture. Byerlee's law defines the shear stress needed to cause sliding, essentially it defines the point of failure given an applied normal stress. The law tells us that shear stress increases about linearly with the normal stress.
• 1.5: Viscous Deformation
Rocks deform by different mechanism depending on the physical conditions. As pressure and temperature increase with depth through the crust and into the the lithosphere the dominant mechanism of deformation changes from elastic and brittle rheology to viscous and plastic rheology. This means that the deformation of rocks at these conditions is best described as solid-state viscous flow with failure through plasticity, which limits the strength of the rock.
• 1.6 Inclined Plane Jupyter Notebook
An interactive example of how to calculate the velocity and strain rate for flow down an inclined plane. See how the angle and viscosity change the velocity and strain rates within the flow.
• 1.7: Summary

Thumbnail: A vertical viewpoint of a rock outcrop that has undergone ductile deformation to create a series of asymmetric folds. (CC BY 2.0; Mike Beauregard via Wikipedia).