In an earlier example, we described what happened to
a pencil when force was applied to both of its ends. We said that the first
sign of the force on the pencil was seen when the pencil bent slightly.
The ability of the pencil to bend shows that it has elastic
properties. This means that the pencil is allowed to be
deformed,
but returns to its original shape when the force on it is released. Like
the pencil, rocks have elastic properties. This means that when forces
are applied to rocks, such as pulling, pushing, twisting, or compression,
they change their shape. Rocks, like all other materials with elastic properties,
have an elastic
limit, a point at which any additional force will permanently
deform the object's shape. Sometimes there is plastic
deformation, which means that the shape of an object can
be changed an additional amount beyond it's elastic limit before it breaks;
other times, it breaks at its elastic limit before any plastic deformation
occurs. As we know from the pencil, when a substance with elastic properties
breaks there is some displacement
or total change in position. There is also elastic
rebound, in which the objects return to their original shape
after they have been broken apart. During an earthquake, seismic waves
are generated as a result of this type of rebound.
In the animation below, we see the fence undergo
elastic deformation until it reaches the elastic limit, and then finally
breaks during an earthquake.