Environmental Engineering Reference
In-Depth Information
Figure 3.2.
Some features commonly seen in dyke of intrusive rock.
(c) The intrusive rock has developed a “planar” foliation parallel to its contacts and,
within this, a lineation, or linear arrangement of mineral grains, parallel to the direc-
tion of flow during intrusion;
(d) Joints in the intrusive body occur in at least 3 sets, as shown on Figure 3.2. Set (i)
joints are parallel to the contacts (and to the foliation). Set (ii) joints are normal to the
lineation i.e. to the direction of magma flow and also to the contacts. Set (iii) joints are
normal to the contacts and parallel to the lineation direction. The joints in all sets
commonly show extension characteristics (i.e. rough or plumose surfaces) and
are either slightly open or infilled with secondary minerals including calcite and zeo-
lite minerals. It can be inferred that shrinkage was an important factor in their for-
mation, although extension during viscous flow seems a likely initiating factor for
Set (ii).
It is sometimes found that both the host and intrusive rock are sheared or crushed,
along and near the contact zone. In some cases this appears to be as a result of viscous
drag, but more generally tectonically induced movements after solidification appear to be
the likely cause.
In some cases, open joints in the body and contact zones may render the intrusive mass
highly permeable. If continuous, such permeable masses represent potential leakage zones
beneath dams or from storages.
