Optional – Overview of Folds, Faults, and Unconformities
Optional – Overview of Folds, Faults, and Unconformities
Note: this section is OPTIONAL readings that provides further information on structural geology. If you continue with Earth Science, you will need to know this material; however, it is not required for the completion of EESC 111 Lab 8, nor is it tested on the Lab 8 Pre-Lab Reading Quiz.
Stress and Strain
Rocks change as they experience stress, defined as a force applied to a given area. Because stress is a function of area, changing the area over which a force is applied will change the resulting stress. For example, imagine the stress that is created at the tip of the heel of a high heeled shoe and compare it to the bottom of an athletic shoe. In the high heeled shoe heel, the area is very small, so much stress is concentrated at that point. The stress is more spread out in an athletic shoe. If stress is not concentrated at one point in a rock, the rock is less likely to break or bend because of that stress.
There are three main types of stress: compression, tension, and shear. When compressional forces are at work, rocks are pushed together. Tensional forces operate when rocks pull away from each other. Simple shear force is created when rocks move horizontally past each other in opposite directions. Rocks can withstand much more compressional stress than tensional stress, as is apparent in some aspects of classical architecture (Figure 7.14).
Applying stress to a rock can create deformation in that rock, known as strain. Initially, as rocks are subjected to increased stress, they behave in an elastic manner, meaning that once the stress is removed, they will return to their original shape (the first part of the curve in Figure 7.15). Deformation is elastic until the rocks reach their elastic limit (point X on Figure 7.15), at which point the rock will begin to deform plastically. Plastic deformation means that the deformation does not go away when stress is removed. It may lead to the rocks bending into folds, or if too much strain accumulates, the rocks may fracture. Deformation that does not involve a rock breaking is called ductile deformation. Deformation that results in breaking is called brittle deformation.
The deformation that results from applied stress depends on many factors, including the type of stress, the type of rock, pressure and temperature conditions, and how rapidly the stress is applied. Tension is more likely to cause brittle deformation than compression. Rocks at higher pressures and temperatures deeper within the crust are more likely to undergo ductile deformation. Sudden and rapid application of stress is more likely to produce brittle deformation.
Folds: Geologic Structures Formed by Ductile Deformation
Folds are geologic structures created by ductile (plastic) deformation of Earth’s crust. To demonstrate how folds are generated, take a piece of paper and hold it up with a hand on each end. Apply compressional forces by push the ends towards each other. You hav