Forces and dynamics - Physical Processes in Earth and Environmental Sciences - page 99

Nonetheless, ductile is a general, descriptive term that does
not involve a specific rheological behavior or strain mecha-
nism. It is not a synonymous term for plastic , which is a very
well-defined and particular rheological behavior. Strains pro-
duced during plastic deformations are larger in magnitude
than those produced in the elastic field and are generally
formed by dislocations of the crystalline lattices and/or dif-
fusive processes. Ductile deformations are also called ductile
flows as the material deforms or flows in a solid state (as a gla-
cier sliding downslope does, Section 6.7.5). Examples of
ductile deformation in rocks are the formation of folds and
salt diapirs. Rocks have a limited ability to change their shape
or volume, which also depends on such external parameters
as the temperature, confining pressure, and so on.
Brittle deformation happens when the internal strength
of rocks is exceeded by stresses; they bust, so internal
cohesion is lost in well-defined surfaces or fractures. Brittle
deformation can occur after the elastic limit is exceeded
not only in pure elastic bodies ( b , Fig. 3.99c) but also
when the stresses reach the plastic limit after some ductile
deformation has taken place. Such samples will be perma-
nently deformed and also fractured ( d , Fig. 3.99c).

2.0

25°C

1.5

300°C

1.0

500°C

700°C

0.5

800°C

0

0 5 10 15

Strain, e (%)

Fig. 3.101 Effect of temperature in the strain-stress diagram for
basalts under the same confining pressure (5 kbars).

plastic field can develop. In elastic-plastic materials, tem-
perature lowers the elastic limit, which is thus reached at
lower stress levels. Rocks may also behave in a viscous way
at high temperatures if the applied stresses are long lasting.
Confining pressure (lithostatic or hydrostatic pressure
acting on all sides of a rock volume) can be simulated in
laboratory experiments by introducing some fluid that
exerts a certain amount of pressure in the sample (triaxial
tests) in addition to that provided by the compressive load,
and by isolating the sample in a constraining metal jacket
to discriminate and separate the effects of the pore pres-
sure in the rock. Experiments carried out on samples of the
same lithology and at the same temperature show that
higher confining pressures increase the yield strength in a
rock, and also the plastic field, so fracturing, if it happens,
occurs after more intense straining (Fig. 3.102). This
means that rocks became more ductile at higher levels of
confining pressure.
When there is fluid trapped in the rock pores, it exerts an
additional hydrostatic pressure which has the effect of
counteracting the confining pressure by the same value of
the fluid pressure in the pores. The state of stress is lowered
and an effective stress tensor can be defined by subtracting
the values of the fluid stresses from those of the solid
normal stresses (Fig. 3.103). The Mohr circle moves
toward lower values by an amount equal to the pore pressure
( p f ) sustained by the fluid. Thus, when fluids are present in
the pores the effect is the same as lowering the confining

3.15.7 Parameters controlling rock deformation

Lithology (rock type) is a variable which may cause diverse
modes of stress-strain behavior. Different rocks or sub-
stances may need different rheological models with which
to describe their deformation. Competency is a qualitative
term used to describe rocks in terms of their inner strength
or capacity for deformation. Rocks which deform easily
and generally in a ductile way are described as incompetent ,
such as salts, shale, mudstone, or marble. Strong or compe-
tent rocks are those which are more difficult to deform,
such as quartzite, granite, quartz sandstones, or fresh
basalts. Competent rocks are stiffer and deform generally
in a brittle way. Nevertheless, competency depends not
only on lithology but also on temperature, confining pres-
sure, pore pressure, strain rate, time of application of the
stress, etc. To compare competencies of different kinds of
rocks, experiments must take place at equal temperatures
and confining pressures.
Temperature has particularly important effects in rheo-
logical behavior (Fig. 3.101). Comparing several experi-
ments on samples of the same lithology under the same
conditions of confining pressure, it is possible to compare
stress-strain relations at different temperatures. At higher
temperatures, rocks behave in a more ductile way, so com-
petence is reduced and fractures are more difficult to pro-
duce. For rocks that are elastic at low temperatures a