Stress in the Earth’s Crust - When you think about enormous plates of lithosphere traveling around on the planet’s surface, you can probably imagine that the process is not smooth. Most geological activity takes place where two plates meet, at plate boundaries. In the Earthquakes chapter, you will learn that nearly all earthquakes, volcanic eruptions, and mountain building occur at plate boundaries.
In this chapter, you will learn more about the geological activity that occurs because of plate tectonics, specifically mountain building and earthquakes. When plates are pushed or pulled, the rock is subjected to stress. Stress can cause a rock to change shape or to break. When a rock bends without breaking, it folds. When the rock breaks, it fractures. Mountain building and earthquakes are some of the responses rocks have to stress.
Causes and Types of Stress
Stress is the force applied to an object. In geology, stress is the force per unit area that is placed on a rock. There are four types of stresses that act on materials.
- A deeply buried rock is pushed down by the weight of all the material above it. Since the rock is trapped in a single spot, it is as if the rock is being pushed in from all sides. This pushing causes the rock to become compressed, but it cannot deform because there is no place for it to move. This is called confining stress.
- Compression is the stress that squeezes rocks together. Compression causes rocks to fold or fracture (break). When cars driving around a parking lot collide, compression causes the cars to crumple. Compression is the most common stress at convergent plate boundaries.
- Rocks that are being pulled apart are under tension (also called extension). Tension causes rocks to lengthen or break apart. Tension is the major type of stress found at divergent plate boundaries.
- When forces act parallel to each other but in opposite directions, the stress is called shear. Shear stress causes two planes of material to slide past each other.
This is the most common stress found at transform plate boundaries. If the amount of stress on a rock is greater than the rock’s internal strength, the rock bends elastically. This type of change is called elastic because when the stress is eliminated the rock goes back to its original shape, like a squeezed rubber ball. If more stress is applied to the rock, it will eventually bend plastically.
In this instance, the rock bends, but does not return to its original shape when the stress is removed. If the stress continues, the rock will fracture; that is, it breaks. When a material changes shape, it has undergone deformation. Deformed rocks are common in geologically active areas.
What a rock does in response to stress depends on many factors:
- the rock type;
- the conditions
- the rock is under,
- primarily the surrounding temperature and pressure;
- the length of time the rock is under stress;
- and the type of stress.
It seems difficult to imagine that rocks would not just simply break when exposed to stress. At the Earth’s surface, rocks usually break quite quickly once stress is applied. But deeper in the crust, where temperatures and pressures are higher, rocks are more likely to deform plastically. Sudden stress, like a hit with a hammer, is more likely to make a rock break. Stress applied over time, often leads to plastic deformation.
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