15.2: What Is Stress and Strain?
What Is Stress and Strain?
Rocks change as they undergo stress, which is just a force applied to a given area. Since stress is a function of area, changing the area to which stress is applied makes a difference. For example, imagine a single steel nail and a bed made entirely of those same nails. If someone were to lay down on a single nail, the stress becomes concentrated at the point, and they would surely hurt themselves. However, lay down on the bed of nails, and the stress becomes distributed and spread out producing a much safer outcome. In geology, there are three main types of stress (Figure 15.2 and Table 15.1):
- Compressional forces Links to an external site. operate when rocks are pushed together. Typically, rocks can withstand compressional stress better than tensional stress.
- Tensional (extensional) forces Links to an external site. operate when rocks pull away from each other.
- Shear forces Links to an external site. are created when rocks move horizontally past each other in opposite directions.
Figure 15.2: Top, a block of rock. Top middle, the block of rock has undergone extensional (tensional) stress and has elongated and thinned. Bottom middle, the original block has undergone compressional stress and has shortened and thickened. Bottom, the original block has undergone shear stresses which resulted in each side of the block moving in opposite directions. (CC-BY 4.0; Emily Haddad, own work)
In response to these stresses, rocks may undergo three different types of strain:
- Elastic strain Links to an external site. is reversible. Rock that has undergone only elastic strain will go back to its original shape if the stress is released.
- Ductile strain (or plastic deformation) Links to an external site. is irreversible. A rock that has undergone ductile strain will remain deformed even if the stress stops. Rocks that undergo ductile deformation typically result in folding Links to an external site..
- Fracture (or brittle deformation) Links to an external site. is also irreversible. A rock that has fractured or ruptured has abruptly broken into distinct pieces. If the pieces are offset, shifted in opposite directions from each other, the fracture is referred to as a fault Links to an external site..
| Type of Stress | Crustal Response | Geologic Structures | Associated Plate Boundary |
| Compression | Shortening & thickening |
Reverse faults, thrust faults Folding |
Convergent |
| Tension (extension) | Elongation (stretching) & thinning | Normal faults, detachment faults | Divergent |
| Shear | Tearing | Strike-slip faults | Transform |
The resulting deformation depends on many factors, including the type of stress, the type of rock, the depth of the rock (pressure and temperature), and the length of time the rock endures the stress. Rocks behave very differently at depth than at the surface. Generally, rocks at depth deform in a more plastic manner (folds), and in a more brittle manner near the Earth’s surface (faults).