Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 144

sediment layer thickness, and C is fractional grain concentration.
Readers may be familiar with industrial and technological uses
of fluidization, notably the passage of hot gases through pow-
dered coal beds to optimize burn efficiency. In this case gas
fluidization is often accompanied by bubble formation, which
is uncommon in liquid fluidization. Gas-induced fluidization
is thought by some to have an important role in the mainte-
nance of certain volcanic eruptive flows, notably pyroclastic
density currents (Section 5.1).
2 It may result from the temporary collapse of grain-to-
grain contacts in loosely packed sand. The grains shaken
apart are momentarily suspended in their own porewater.
The energy for granular disaggregation is provided by the
high accelerations experienced during cyclic shock caused

by earthquake ground motions (Section 4.17). Pressure
changes due to the passage of large storm waves over a
sandy or muddy (including “fluid mud”) bottom may also
cause liquefaction, as can the sudden arrival of a turbidity
current or repeated impact of feet on saturated sands on
the beach or river bed. Should the liquefied sediment be
resting on a slope, however slight, then downslope flow
will inevitably result, the flow transforming as it does so
into a debris or turbidity flow (Section 4.12). Post-
liquefaction resettlement causes a net upward displacement
of pore fluid of volume proportional to the difference in
pre- and post-liquefaction porosity. The spatial funneling of
this flow is the cause of the violent fluidization witnessed
after earthquakes (Fig. 4.69).

4.14 Fractures

In Section 3.15 the concept of brittle versus ductile behavior
was explained. Brittle behavior involves the loss of cohe-
sion in rocks when deviatoric stresses higher than the rock
resistance are applied. The planes where rocks have lost
their cohesion are called fractures . In other words the
rocks break under certain conditions and fractures are the
places where the rocks break. Nevertheless, the definition
of fractures is a little more complicated than that. Pure
brittle fractures are well-defined surfaces where the cohe-
sion has been lost, without any distortion; if we could glue
together the pieces of the broken rock, its shape would be
the same as before (Fig. 4.70a). Brittle fractures are char-
acteristic of rocks exhibiting elastic behavior and form in
the upper part of the crust (about 10 km), although sud-
den stresses can cause brittle failure in solid viscous mate-
rials (Section 3.15). There is a broad and very intriguing
transition between the brittle and ductile fields, where the
definition of fractures is not so obvious; for example the
material in Fig. 4.70b shows some ductile deformation
accompanying the fracture since some bending occurred
before the fracture was produced. In this case, joining
together the fragments of the original piece of rock will
not give the rock's original shape. Fractures in the brittle-
ductile transition ( semi-brittle behavior) often show intense
deformation where some cohesive loss is achieved in lim-
ited, non-continuous surfaces, instead of developing a well-
defined unique fracture surface. Semi-brittle fractures do
not experience a total loss of coherence on well-defined
surfaces. Further increase in ductility, but still in the brittle-
ductile transition, produces shear bands where a cohort
of oblique, lenticular-shaped, discrete fractures is formed
(tension gashes, Fig. 4.70c). Ductile shear zones are charac-
terized by intense deformation, without apparent loss of

cohesion at macroscopic scale (visual or outcrop scales),
concentrated in a fairly narrow band (2D) surrounded by
distinctively less deformed rocks (Fig. 4.70d). Such shear
zones form at deep levels in the crust.

4.14.1 Types of fractures

The first important analysis of fractures involves how the
rock bodies on either side of the fracture move. According
to this criterion they can be classified broadly into two

(a) (b)

(c) (d)

Fig. 4.70 Different kinds of shear features: (a) Pure brittle shear
fracture, (b) in the brittle-ductile transition, fractures occur after
some previous material distortion, (c) gash fractures in the
brittle-ductile transition, and (d) ductile shear zone.