Deformation, stress and strain - Introducing Tectonics, Rock Structures and Mountain Belts - page 21

Deformation, stress and strain
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Deformation, stress and strain

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Terms and concepts
Deformation
The term 'deformation' to the struc-
tural geologist merely means a change
in shape of a rock body - contrary to
general usage, there is no implica-
tion that this change is for the worse!
The study of deformation requires an
understanding of the behaviour of
rock material under varying condi-
tions of temperature and pressure in
response to applied forces. This in
turn involves understanding the rela-
tionship between force and stress ,
and between stress and strain . Both
the latter terms are used in structural
geology, as in mechanical engineering,
in a strictly defined way ( see below) and
differently from their everyday usage,
where the meanings of stress and strain
are more or less interchangeable.

approach can be just as useful in
understanding geological structures.
It is important to realise that it is
not possible to work backwards from
the final strain to the initial unstrained
state or to the causal stress - the rela-
tionship must always be based on
a guess as to exactly how the strain
developed, and there may be an infi-
nite number of possible routes from
the initial state to the final result.

this concept is not much use to us in
understanding geological structures;
with the exception of earthquakes (to
be discussed in the following chapter)
and volcanic explosions, acceleration
is irrelevant to understanding natural
deformation in rock. For most practi-
cal purposes, the geologist will wish to
convert a force, or system of forces, into
a stress or stress field. A stress is a pair
of equal and opposite forces acting on a
unit area of a surface (Figure 4.1A) and
a stress field is the system of stresses
acting in three dimensions on a body.
The effect of a force of given mag-
nitude or strength depends on the size
of the area on which it acts. This can be

Force, stress and pressure
The term ' force ', defined strictly, means
' that which causes acceleration in a
body '; it is the product of the mass of
the body and its acceleration. However,

1kg

stress =
force/area

Stress and strain
To the structural geologist, deformation
is regarded as the result of a set of forces
that act on a body of rock and are con-
verted into a system of stresses (known
as a stress field ); it is these stresses that
are in turn responsible for changing the
shape of the rock body. This change of
shape is known as the strain . Strain is
therefore a geometrical concept and has
to be measured geometrically. To do this
accurately, of course, requires detailed
measurement and the employment
of sometimes complex mathematical
models, but often a more qualitative

1kg

A B

Figure 4.1 Force and stress. A. The stress acting on the yellow box is produced by two equal and
opposite forces, divided by the area on which the force acts (i.e. the side of the box); a smaller box,
subjected to the same forces, will be subject to a larger stress, since the force is divided by a smaller
area. B. A 1 kg weight resting on a flat slab floats, whereas the same weight resting on a smaller slab
sinks, because the force (the 1 kg weight) is spread over a smaller area.