Geoscience Reference
In-Depth Information
Plate 12.11
Emperor Trajan's Market (Forum Traiani), dedicated in January
AD
113, reveals some of the massive use and
understanding of Earth materials in Rome. Structural walls of travertine blocks (left foreground), with holes for iron tie braces
providing earthquake-proofing, and terracotta bricks in the market hemicircle, contrast with the decorative use of a variety of more
expensive marbles, lighter travertine slabs and granite for pillars and flooring.
Photo: Ken Addison
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 (
2
,
bending a soft metal rod. Rock may behave elastically at
very low pressures and temperatures and plastically at
higher pressures and temperatures (i.e. at greater depth),
especially if the strain rate or deformation speed is slow,
typically changing length at 1 per cent per 10
4
years!
Brittle failure, or fracture, occurs if the strain rate exceeds
the plastic deformation rate. Earthquakes occur when this
happens abruptly.
The principal forms of plastic deformation are
foliations
(including cleavage) and
folds
, which produce
banded and wave-like structures respectively. Foliation
tends to occur at small to intermediate scales, seen
typically in microscope to hand specimens. Folds occur
at all scales, with wavelengths up to 10
1-2
km, and in a
variety of wave forms (
Plate 12.12
an
d
Figure 12.22b,
c
).
Material between each foliation or fold plane is stretched
or compressed and particles are realigned parallel to the
deformation. This creates strength anisotropy, which is
3
)
are equal and resist the compressive force (
1
), creating
an
isotropic
force field (
Figure 12.21
).
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 12.22a
).
Elastic strain is recover-
able once the stress is removed, whereas plastic strain is
permanent, as seen after stretching an 'elastic' band and
