geological stresses

responses to stress

Rocks have three possible responses to increasing stress (illustrated in Figure 1.3): 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 Earths crust or at the surface? What if the stress applied is sharp rather than gradual? At the Earths 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. Click image to the left or use the URL below. URL:

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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 1.1). Compression is the most common stress at convergent plate boundaries. 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 1.2). Shear stress is the most common stress at transform plate boundaries. 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 rocks 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.

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instructional diagrams

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questions

force per unit area that is placed on an object.

a) strain

b) pressure

-->  c) stress

d) deformation

a rocks response to stress depends on

a) the type of rock and the type of stress

b) the surrounding temperature

c) the amount of time the rock is under stress

-->  d) all of the above

rocks that are pulled apart are under

-->  a) tension

b) shearing

c) compression

d) confining

parallel forces moving in opposite directions are

a) tension

-->  b) shear

c) compression

d) confining stress

a sudden stress is more likely to make a rock fracture than a show increasing stress.

-->  a) true

b) false

stress that causes a material to change its shape is called

a) elasticity

b) plasticity

c) fracture

-->  d) deformation

rock that returns to its original shape when the stress is removed.

a) fracture

b) plastic deformation

-->  c) elastic deformation

d) shearing

rocks that break under a lot of stress.

-->  a) fracture

b) plastic deformation

c) elastic deformation

d) shearing

tension occurs at divergent boundaries.

-->  a) true

b) false

this type of stress occurs at convergent boundaries.

a) shearing

-->  b) compression

c) tension

d) no stress

diagram questions

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