A to Z of North Wales Geology
J is for..............JOINTING

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.

Sheet Jointing in Granodiorite

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.
Joint sets in Granodiorite

©Jonathan Wilkins, 08.01