(58).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 solidrock — it's no wonder that rocksexperience 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 rockstogether, causing rocks to fold or fracture (break) (Figure below). Compression is the most common stress at convergent plate boundaries.

[Figure 1]

Stress caused these rocks to fracture.

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.

[Figure 2]

Shearing in rocks. The white quartz vein has been elongated by shear.

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 Figurebelow):

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.

[Figure 3]

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

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.

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