rock goes back to its original shape, like a squeezed rubber ball. If more stress is applied
to the rock, it will eventually bend plastically. In this instance, the rock bends, but does
not return to its original shape when the stress is removed. If the stress continues, the
rock willfracture; that is, it breaks. When a material changes shape, it has undergone
deformation.Deformed rocks are common in geologically active areas (Figure7.3).
Figure 7.3: When stress is applied to a material, it initially deforms elastically. With more
stress, the material deforms plastically and when the material’s strength is exceeded, it
fractures. The amount of stress that can be applied before the material transitions to the
next type of deformation depends on the material and the conditions where it is located.
( 30 )
Whatarockdoesinresponsetostressdependsonmanyfactors: therocktype; theconditions
the rock is under, primarily the surrounding temperature and pressure; the length of time
the rock is under stress; and the type of stress. It seems difficult to imagine that rocks
would not just simply break when exposed to stress. At the Earth’s surface, rocks usually
break quite quickly once stress is applied. But deeper in the crust, where temperatures and
pressures are higher, rocks are more likely to deform plastically. Sudden stress, like a hit
with a hammer, is more likely to make a rock break. Stress applied over time, often leads
to plastic deformation.
Geologic Structures
Sedimentary rocks are often found in layers. This is most magnificently displayed at the
Grand Canyon, where the rock layers are exposed like a layer cake (Figure7.4). Each layer
is made of sediments that were deposited (laid down) in a particular environment, perhaps a
lake bed, shallow offshore region, or a sand dune. Sediments are deposited horizontally. the
lowest layers are the oldest and the highest layers are the youngest. Some volcanic rocks,