6.1: Geological Stresses

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

Lithospheric plates do a lot of moving on Earth's surface. Slabs of lithosphere smash into each other. They move sideways past each other along faults. Where conditions are right, magma rises through solid rock. It's no wonder that rocks experience stress! Rocks respond differently to different types of stress and under different conditions.

Types of Stress

Stress is the force applied to a rock. Plates experience stress when they collide, move apart, or slide past each other. Plates moving on a rounded surface experience stress. Stress happens to rocks on a smaller scale, too. Local movements can cause stress in rocks.

There are four types of stresses that affect rocks:

Confining stress comes from the weight of all the overlying rock. This weight pushes down on a deeply buried rock. The rock is being pushed in from all sides, which compresses it. The rock will not deform because there is no place for it to move.
Compression stress squeezes rocks together. Compression causes rocks to fold or fracture (Figure below). When two cars collide, compression causes them to crumple. Compression is the most common stress at convergent plate boundaries.

Stress caused these rocks to fold

Stress caused these rocks to fold.

Tension stress pulls rocks apart. Tension causes rocks to lengthen or break apart. Tension is the major type of stress found at divergent plate boundaries.
Shear stress happens when forces slide past each other in opposite directions (Figure below). This is the most common stress found at transform plate boundaries.

Rock that has undergone shearing

This rock has undergone shearing. The pencil is pointing to a line. Stresses forced rock on either side of that line to go in opposite directions.

Responses to Stress

The amount of stress on a rock may be greater than the rock's strength. In that case, the rock will undergo strain or deformation (Figure below). Deep within the Earth, the pressure is very great. A rock behaves like a stretched rubber band. When the stress stops, the rock goes back to its original shape. If more stress is applied to the rock, it bends and flows. It does not return to its original shape. Near the surface, if the stress continues, the rock will fracture (rupture) and break.

$Graph of stress, going from elastic deformation to plastic deformation to fracture$

With increasing stress, the rock undergoes: (1) elastic deformation, (2) plastic deformation, and (3) fracture.

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

What type of stress would you find at a transform fault?
What type of stress would you find at a subduction zone?
Under what conditions will a rock fracture?