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is a catastrophic event that most certainly modifies the surface
of Earth. What the principle of uniformity does state is that the
physical and chemical laws we observe today operated exactly
the same way in the past. The rates of operation may or may not
have been the same in the past, but the events you see occur-
ring today are the same events that occurred in the past. Given
enough time, you can explain the formation of the structures of
Earth’s surface with known events and concepts.
The principle of uniformity has been used by geologists
since the time of Hutton. The concept of how the constant
changes occur has evolved with the development of plate tec-
tonics, but the basic frame of reference is the same. You will see
how the principle of uniformity is applied by first considering
what can happen to rocks and rock layers that are deeply buried.

19.2 DIASTROPHISM
All the possible movements of Earth’s plates, including drift
toward or away from other plates, and any process that deforms
or changes Earth’s surface are included in the term diastrophism.
Diastrophism is the process of deformation that changes Earth’s
surface. It produces many of the basic structures you see on the
surface, such as plateaus, mountains, and folds in the crust. The
movement of magma is called vulcanism or volcanism. Diastro-
phism, volcanism, and earthquakes are closely related, and their
occurrence can usually be explained by events involving plate
tectonics (chapter 18). The results of diastrophism are discussed
next in the section on stress and strain, which is followed by
a discussion of earthquakes, volcanoes, and mountain chains.
Again, remember that diastrophism, volcanism, earthquakes,
and the movement of Earth’s plates are very closely related.
FIGURE 19.1 An aerial view from the south of the eruption of
All are involved with the shapes, arrangements, and interrela-
Mount St. Helens volcano on May 18, 1980.
tionships of different parts of Earth’s crust and the forces that
change it. We will begin with a discussion of some of these forces
before discussing what the forces can do.

STRESS AND STRAIN
Any solid material responds to a force in a way that depends
on the extent of coverage (force per unit area, or pressure), the
nature of the material, and other variables such as temperature.
Consider, for example, what happens if you place the point of a
ballpoint pen on the side of an aluminum pop (soda) can and
apply an increasing pressure. With increasing pressure, you can
observe at least four different and separate responses:
1. At first, the metal successfully resists a slight pressure and

nothing happens.
2. At a somewhat greater pressure, you will be able to deform

or bend the metal into a concave surface. The metal will
return to its original shape, however, when the pressure

is removed. This is called an elastic deformation since the
FIGURE 19.2 Would you believe that this rock island has metal was able to spring back into its original shape.
“always” existed where it is? Would you believe it was formed by
3. At a still greater pressure, the metal is deformed to a
a sudden, single event? What evidence would it take to convince
you that the rock island formed ever so slowly, starting as a part of concave surface, but this time the metal does not return to

southern California and moving very slowly, at a rate of centimeters its original shape. This means the elastic limit of the metal
per year, to its present location near the coast of Alaska? has been exceeded, and it has now undergone a plastic
19-3 CHAPTER 19 Building Earth’s Surface 479