Jointing results from relatively low levels of stress causing fractures but not displacement. The stress
comes from:
- compression, for example from the earth movements which cause folds and faults;
- decompression, for example through removal of overburden by erosion;
- contraction, for example by cooling of a lava flow.
Most exposures of any type of rock show jointing. In sedimentary rocks, joints are usually at right angles to
the bedding planes.
Strongly developed jointing is exploited by quarrying and by weathering.
Regular joint pattern
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Granite tor
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Pressure release joints
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Columnar joints
Cleavage refers to the ability of some fine-grained rocks to split into thin sheets. If cleavage is
particularly well-developed, the rock becomes slate.
This ability to split can arise from different conditions.
- True cleavage, or slaty cleavage, is developed in fine-grained rocks which are subjected to very strong
compression; good quality slate can be split into sheets of almost any thickness and is therefore suitable for
roofing and decorative stonework.
- In false cleavage, the position where the rock can be split is determined by pre-exisiting cracks
caused during folding.
- The direction of cleavage planes may change if they cross beds of slightly different grain size - refraction
of cleavage.
This potential to split is exploited by weathering, especially frost shatter.
In the crags of hilly areas, which in Britain are commonly of hard slate, cleavage planes opened up by weathering
are often confused for bedding planes. Jointing also confuses the issue.
Slaty cleavage
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False cleavage
 |
Refraction of cleavage
 |
Geopix Study Guide "Joints and Cleavage"
- gives full images of these, and other, examples to illustrate jointing and cleavage;
- includes images illustrating the effects of weathering;
- relates jointing and cleavage to the stress patterns which caused them;
- suggests how to answer exam questions on these topics.
The Structures: "Joints and Cleavage" Study Guide is part of the Geopix Structures Study Guide CD.
To order this CD go to the Geopix Online Store
The Study Guide contains the following pages:
