JOINTS are of enormous importance in predicting the behaviour
of rock masses when they are excavated or quarried, since they are weaknesses which may be
exploited or cause difficulties in construction. For this reason, geological surveys of
outcrops and boreholes will pay great attention to the jointing which is present.
JOINTS are also found in igneous rocks which would otherwise be almost featureless. Jointing
in igneous rocks, such as granite may be caused by tectonic forces after the magma has
cooled, or as a result of tension due to shrinkage after complete crystallisation. Joints
due to shrinkage may be parallel to the roof or walls of an intrusion, or result in striking features such as the
columnar jointing which is famously displayed in the Giant's Causeway of Antrim, or less
famously in the sills of Snowdonia.
Jointing parallel to the wall of an intrusion is shown in the photo on the left - with
construction lines added for clarity.
A major joint set in the same outcrop is illustrated in the right-hand photo. It has a
number of regularly-spaced members (pointing to 2 o'clock), and is intersected by another
set (at about 11 o'clock) which is much less frequent.
©Jonathan Wilkins, 08.01