# Stress and Strain

Hello this is doctor Keith Miller, and I will be discussing the formation
and classification of folds. But first, why do rocks sometimes fault or fracture in response to deforming
forces and another cases form folds? To understand how rocks deform, we need to understand two terms — stress and strain. Stress is
simply an applied force. Stress is described by the
magnitude of the force, that is, how strong is the
force — by pressing it a little or pressing it alot. And the direction, that is, is the force compressional, is it
tensional, or is it shearing. Strain on the other hand is how the material responds, or changes
its shape in response to, an applied stress. There
are three types of strain — elastic, plastic, and brittle. In elastic strain, the material would
original shape when that force is removed. So, for example, this deformation is reversible. An example of such an elastic material is a rubber band. When I apply a stress the rubber band changes shape, but when I remove that stress it returns back to its original shape. When I apply the stress, it changes shape. Remove the stress and it returns back to its original shape. In brittle deformation, when a stress is applied, the material will
break or fracture. Although initially elastic, this piece of wood will fracture if I rapidly apply a strong force. Elastic materials — including rocks — will change their behavior and become
brittle under the right conditions, And yes, rocks do behave elastically. When the force being applied increases
an elastic material will eventually reach its elastic limit. Energy will
continue to be stored in elastic deformation until its elastic limit is reached. At that point will behave
like a brittle material and break or fracture. This can be illustrated again by our rubber band. I again apply a stress to the rubber band, and it will change its shape. Remove the stress — it turns back to its original shape again.
However, if I keep applying a stress — keep applying a stress — eventually I’ll reach a point at which it reaches its elastic limit. Ow!!! And when the rubber band breaks it releases all of that stored energy.
This is what happens in an earthquake Rocks begin deforming elastically and when they reach their elastic limit
they fracture and the result is a fault. Lastly, in plastic deformation the material change in shape when a
force is applied but that chang of shape as permanent. Examples of plastic materials are silly putty, or this steel rod. So I apply a force and they change
their shape. But when I stop applying the force these
materials remain deformed. I’ll do the same with silly putty. Apply a force — it changes shape. I stop the force and the deformation is
permanent. When rocks deform plastically they create folds. Rocks are more likely
to behave plastically when they are hot. As a
result, folds typically occur in rocks deeply buried below the
surface where temperature are high.

## 2 comments on “Stress and Strain”

1. Udita Rawat Post author

mr miller could u please make a video on clouds, earthquake, plate tectnoics theory?…..