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, or have its shape changed, but returns to its original shape when the force on it is released. Like the pencil, or a rubber band, 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, if the substance is brittle, it breaks at its elastic limit before any plastic deformation occurs. As we know from the pencil and rubber band, 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.
Last modified on 8/13/98 by Maggi Glasscoe (firstname.lastname@example.org)