Stress and Strain in the Life of a Rock
By Patti Hutchison
The earth's surface is constantly changing. We
don't usually see the changes because they
happen so slowly. But earth's crust is always being
pushed, pulled, folded, and broken. What causes
these changes? Stress.
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Earth's crust is made up of plates of rock. These
plates move about on the asthenosphere. This is a mostly molten layer of
the upper mantle. As they move about, the plates are pushed and pulled.
This force is called stress.
2
Stress causes the rocks of the crust to slowly change. Their volumes
change. Their shapes also change. Stress causes rocks to move up, down,
or sideways. They tilt, fold, and even break. This is called deformation.
3
There are three main types of stress. One type is called compression.
During compression rocks are pushed together. Think of how you used to
squeeze mud in your hands. As you pushed the mud together, some oozed
out the top and bottom of your hand.
4
Rocks are squeezed in the same way by stress. When this happens,
rocks are pushed higher and deeper. A rock that was once flat will bulge in
the middle as a result of compression.
5
Rocks can also be pulled apart by a type of stress called tension. Think of
having a piece of clay in your hands. As you pull both ends, the clay
stretches. It gets thinner in the middle. It stays thick on the ends. Tension
can cause rocks to stretch in this same way.
6
There is a third kind of stress called shearing. This pushes rock in
opposite, horizontal directions. The rocks bend and break. They can crack.
The crack that forms is called a fault. Rocks on one side of a fault slide past
the rocks on the other side. Earthquakes often occur along faults in the
earth's surface.
7
Sometimes stress causes the rocks to bend instead of breaking. A bend in
a rock is called a fold. An upward fold is called an anticline. A downward
fold is called a syncline. Folds in the earth's crust can be so small you can't
see them without a magnifying glass. Some are large enough to form
mountains.
8
Why do some rocks fold, and others fault? Many factors affect these
actions. One is temperature. Rocks that become very hot during
compression usually fold. Higher pressure will also likely cause rocks to
fold instead of fault.
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Some rocks are brittle. They break easily under stress. Others are ductile.
This means they bend easily. These rocks are more likely to fold. The way
the force is applied to a rock will also determine whether it will fold or fault.
If the force is applied quickly, the rock will usually fault. If it is applied
slowly, it will likely fold.
10
Stress and strain take a toll on rocks. Pressure caused by the movement
of plates changes the earth's crust. These changes usually happen over
long periods of time. Just think: some of the mountains you see today were
once flat land. The earth looks very different today than it did billions of
years ago.
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