Joint (geology)

Columnar jointed basalt in Turkey

Columnar jointing in basalt, Marte Vallis, Mars

Columnar jointing in the basalt of the Giant's Causeway in Ireland

Columnar jointing in the andesite of the Tōjinbō in Japan

Joint sets on a bedding plane in flagstones, Caithness, Scotland

In geology the term joint refers to a fracture in rock where there has been no lateral movement in the plane of the fracture (up, down or sideways) of one side relative to the other. This makes it different from a fault which is defined as a fracture in rock where one side slides laterally past to the other. Joints normally have a regular spacing related to either the mechanical properties of the individual rock or the thickness of the layer involved. Joints generally occur as sets, with each set consisting of joints sub-parallel to each other.

Formation

Joints form in solid, hard rock that is stretched such that its brittle strength is exceeded (the point at which it breaks). When this happens the rock fractures in a plane parallel to the maximum principal stress and perpendicular to the minimum principal stress (the direction in which the rock is being stretched). This leads to the development of a single sub-parallel joint set. Continued deformation may lead to development of one or more additional joint sets. The presence of the first set strongly affects the stress orientation in the rock layer, often causing subsequent sets to form at a high angle to the first set.

Joint sets are commonly observed to have relatively constant spacing, which is roughly proportional to the thickness of the layer .

Types of joints

Joints are classified by the processes responsible for their formation, if known.

Tectonic joints

Tectonic joints are formed during deformation episodes whenever the differential stress is high enough to induce tensile failure of the rock, irrespective of the tectonic regime. They will often form at the same time as faults. Measurement of tectonic joint patterns can be useful in analyzing the tectonic history of an area because they give information on stress orientations at the time of formation

Unloading joints

Joints are most commonly formed when uplift and erosion removes the overlying rocks thereby reducing the compressive load and allowing the rock to expand laterally. Joints related to uplift and erosional unloading have orientations reflecting the principal stresses during the uplift. Care needs to be taken when attempting to understand past tectonic stresses to discriminate, if possible, between tectonic and unloading joints.

Exfoliation joints are special cases of unloading joints formed at, and parallel to, the current land surface in rocks of high compressive strength.

Cooling joints

Joints can also form via cooling of hot rock masses, particularly lava, forming cooling joints, most commonly expressed as vertical columnar jointing. The joint systems associated with cooling typically are polygonal because the cooling introducing stresses that are isotropic in the plane of the layer.

Fractography

Joint propagation can be studied using the techniques of fractography in which characteristic marks such as hackles and plumose structures can be used to determine propagation directions and, in some cases, the principal stress orientations.

Importance to rock strength and slope stability

Joints form one of the most important types of discontinuity within rock masses, typically having no tensile strength.

See also

• Tessellated pavement