# 1: Rheology of Rocks

• 1.0: Introduction to Rheology
An understanding of rheology is crucial for understanding important concepts in geophysics, such as what happens during an earthquake, how mountains and faults are formed, and how rocks behave in the mantle. Rheology is an important concept in structural geology and can help geophysicist to make observations about what forces lead to the creation of a geologic structure, and how those forces may act over a regional scale.
• 1.1: Stress and Strain (1-D)
When a force is applied to a rock, this produces stress. Stress is a force per unit area applied on a rock. There are several types of stress, all of which indicate differing directions the force is applied. Normal stress acts at right angles to surfaces, and can either push a rock together (compression) or pull it apart (tension).
• 1.2: 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.
• 1.3: 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.
• 1.4: 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.
• 1.5: 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).