Environmental Engineering Reference
In-Depth Information
Figure 1
An outline of the geological resources of the British Isles.
ROCK DEFORMATION: FOLDING AND FAULTING
Tectonic activity sets up huge stresses in rocks which result in
strain
or deformation. This
is quite different from denudation and the associated processes of rock disintegration,
which are the subject of later chapters. Deformation occurs along planar structures (folds,
faults, etc.) and the vast bulk of intervening rock mass may remain intact, even if
relocated
en masse
. If we focus on any small cube of rock within the crust we can
measure the force applied by the surrounding rock to each of its six faces. A three-
dimensional
orthogonal
or right-angled pattern of forces emerges, with a pair of opposing
forces in each
principal stress
direction. Assuming that internal constituents are packed
as tightly as possible, the cube will retain its shape provided that the confining forces (σ
2
,
σ
3
) are equal and resist the compressive force (σ
1
), creating an
isotropic
force field
(Figure 12.20).
Deformation occurs when the forces are
anisotropic
(unequal) and the rock is able to
deform. This depends on its
rheologic properties
or ability to flow and occurs, as we
have seen, in crustal convection as slow granular creep. This is strongly influenced by
temperature and fluid content, with high pore fluid pressures improving the plastic
response. Rock may respond to progressive increases in stress by
elastic
and
plastic
deformation and
brittle failure
(Figure 12.21a). Elastic strain is recoverable once the
stress is removed, whereas plastic strain is permanent, as seen after stretching an 'elastic'
