Physics for Geologists, Second edition

96 Stress and strain
In geology, viscosities will be very large, probably of the order of 10 x
10'' Pa s, but even these could lead to catastrophic rates of sliding of perhaps
10m a year on quite small slopes. When Smoluchowski (1909) addressed
the problems of sliding he wrote:
Suppose a layer of plastic material, say pitch, interposed between the
block and the underlying bed; or suppose the bed to be composed of
such material: then the law of viscous liquid friction will come into play,
instead of the friction of solids; therefore any force, however small, will
succeed in moving the block. Its velocity may be small if the plasticity is
small, but in geology we have plenty of time; there is no hurry.
Bending and folding
There are several ways of bending layered material, and it is one of the
retarding influences on geology that many seem to think of bending only in
terms of lateral compression (Figure 9.5). Carey (1954) in this regard is still
worth reading. (Since we are concerned with geology, we shall talk about
folding or deformation rather than bending.)
Folding is certainly possible by crumpling a thin sheet under lateral com-
pression, but it arises more commonly from mechanical instability in the
outer parts of the Earth, and in layered sequences under conditions where
gravity plays a part. In an unstable layered system (such as one in which
the upper layers are more dense than the lower), deformation will result in
folding of the type shown in the upper part of Figure 9.5 - and such folding
may be induced in stable layers above and below the unstable layer.
Folding requires compensation in space, either by sliding on surfaces, or
flowing of the less viscous material, or both. Bending a beam puts the con-
cave side in compression and the convex side in extension. Under some
circumstances this can cause fracture, but in some cases natural rocks can-
not sustain such stresses over long periods of time, and behave more like
liquids.
Copyright 2002 by Richard E. Chapman
Figure 9.5 Folding can be by bending or buckling.