Stress and strain are related but distinct concepts: stress is the force applied to a rock, while strain is the resulting deformation of that rock.
Understanding Stress
Stress, in the context of earth science, refers to the force acting upon a rock body per unit area. It's a measure of the intensity of the forces that cause deformation. Stress can be caused by various factors, including:
- Tectonic forces: Forces related to the movement of Earth's plates.
- Gravity: The weight of overlying rocks.
- Fluid pressure: Pressure exerted by fluids within the rock.
There are three main types of stress:
- Tensional stress: Pulls the rock apart. (Extension)
- Compressional stress: Squeezes the rock. (Shortening)
- Shear stress: Causes the rock to slide past one another. (Translation)
Stress is what causes rocks to deform.
Understanding Strain
Strain is the deformation of a rock body caused by stress. It is the change in size or shape of a rock as a result of applied stress. Strain is a response to stress. Different types of strain include:
- Elastic Strain: Temporary and reversible deformation. The rock returns to its original shape when the stress is removed.
- Plastic (Ductile) Strain: Permanent deformation. The rock does not return to its original shape after the stress is removed. This type of deformation usually occurs at high temperatures and pressures deep within the Earth.
- Brittle Strain: Occurs when the rock fractures or breaks. This type of deformation usually occurs at low temperatures and pressures near the Earth's surface.
Strain is often expressed as a ratio of the change in length to the original length (for linear strain) or as an angle of distortion (for shear strain).
Stress vs. Strain: A Table for Comparison
| Feature | Stress | Strain |
|---|---|---|
| Definition | Force per unit area acting on a rock. | Deformation of a rock. |
| Nature | Cause | Effect |
| Types | Tensional, Compressional, Shear. | Elastic, Ductile (Plastic), Brittle. |
| Measurement | Pascals (Pa) or pounds per square inch (psi). | Dimensionless (ratio) or angle. |
| Analogy | Pushing on a door | The door bending or breaking |
Analogy
Think of squeezing a rubber ball. The squeezing force you apply is the stress. The squishing of the ball is the strain.
Conclusion
In summary, stress is the force applied to a rock, while strain is the resulting deformation. Stress causes strain, and the type of strain depends on the type and magnitude of the stress, as well as the properties of the rock itself.
