7.1 Geological Stresses

When people have too much stress they may break. What happens if a rock gets too much stress?

With all the movement occurring on Earth's surface — slabs of crust smashing into each other, sideways movements along faults, magma rising through solid rock — it's no wonder that rocks experience stress. Rocks respond differently to different types of stress and under different conditions.

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. Four types of stresses act on materials.

• A deeply buried rock is pushed down by the weight of all the material above it. Since the rock cannot move, it cannot deform. This is called confining stress.
• Compression squeezes rocks together, causing rocks to fold or fracture (break) (Figure below). Compression is the most common stress at convergent plate boundaries.

• Rocks that are pulled apart are under tension. Rocks under tension lengthen or break apart. Tension is the major type of stress at divergent plate boundaries.
• When forces are parallel but moving in opposite directions, the stress is called shear (Figure below). Shear stress is the most common stress at transform plate boundaries.

When stress causes a material to change shape, it has undergone strain or deformation. Deformed rocks are common in geologically active areas.

A rock’s response to stress depends on the rock type, the surrounding temperature, the pressure conditions the rock is under, the length of time the rock is under stress, and the type of stress.

Responses to Stress

Rocks have three possible responses to increasing stress (illustrated in Figure below):

• elastic deformation: the rock returns to its original shape when the stress is removed.
• plastic deformation: the rock does not return to its original shape when the stress is removed.
• fracture: the rock breaks.

Under what conditions do you think a rock is more likely to fracture? Is it more likely to break deep within Earth’s crust or at the surface? What if the stress applied is sharp rather than gradual?

• At the Earth's surface, rocks usually break quite quickly, but deeper in the crust, where temperatures and pressures are higher, rocks are more likely to deform plastically.
• Sudden stress, such as a hit with a hammer, is more likely to make a rock break. Stress applied over time often leads to plastic deformation.

Summary

• Stress is the force applied to an object. Stresses can be confining, compression, tension, or shear.
• Rocks under stress may show strain or deformation. Deformation can be elastic or plastic, or the rock may fracture.
• Rocks respond to stress differently under different conditions.

Review

1. What type of stress would you find at a transform fault? At a subduction zone? What type of stress at a continental rift zone?
2. Compare and contrast fracture, plastic deformation, and elastic deformation.
3. What do you think happens with stressed rocks in an earthquake zone?

Explore More

Use this resource to answer the questions that follow.

1. What is stress defined as?
2. What are body forces and what are they proportional to?
3. What are surface forces?
4. Why does stress need to be studied in vector components?
5. In what direction do the forces go in normal stress? What about in shear stress?
6. What is strain?