Geology Reference
In-Depth Information
Figure 3.38
Collapsed
columnar joints.
This has apparently
happened after the
pattern of columns
had been de
ned in
the cooling lava
sheet but whilst the
lava was still
plastic. Am
Buachaille (the
Herdsman), Isle of
Staffa, Scotland.
major role in allowing the failure to develop, had no preferred orien-
tation, other than it was at right angles to the planar shistocity
(
Figure 3.40).
These are secondary joints, probably formed during
a relatively late stage of the regional tectonics responsible for the
schistosity.
Pollard & Aydin (1988) dismiss the concept of shear joints as
'
sheer
nonsense
but this seems to be a bit tongue-in-cheek (the paradox being
that once shear takes place a joint becomes a fault by de
'
nition).
Fractures certainly do develop in shear directions as they do in triaxial
testing (
Chapter 5).
In the example shown in
Figure 3.41,
some sec-
tions of the shear joints show no visible displacement but over other
lengths of the same discontinuity, there is displacement. The argument
about shear joints is largely academic in that whereas the joints pro-
pagate in the shearing direction predicted from Mohr
s circle repre-
sentation, in detail, the joint is probably made up of coalesced sections,
which are strictly tensile, originating from minor
'
flaws in the rock
(Kulander & Dean, 1995). Engelder (1999) extends the discussion to
hybrid joints. The appearance of shear joints on a stereonet, before and
after tilting, is shown in
Figure 3.42.
As discussed above, many joints follow some systematic geometrical
pattern relating to the principal stress directions and magnitudes at the
time of their formation. In some
field exposures, however, the fracture
network can be very complex and dif
cult to unravel, especially when
a rock mass has been through several structural events, with each event
