Joint (geology)
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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 distinct 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.
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[edit] 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 principle stress and perpendicular to the minimum principle 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 further joint sets. The presence of the first set strongly affects the stress orientation in the rock layer, causing subsequent sets to often form at a high angle to the first set.
Joint sets are commonly observed to have relatively constant spacing and the spacing is roughly proportional to the thickness of the layer [1].
[edit] Types of joint
Joints are classified by the processes responsible for their formation, if known.
[edit] 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 of the information they give on stress orientations a the time of formation[2]
[edit] 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 principle stresses during the uplift and 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.
[edit] 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 are typically polygonal in nature due to the cooling introducing stresses that are isotropic in the plane of the layer.
[edit] 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 principle stress orientations[3].
[edit] Importance to rock strength and slope stability
Joints form one of the most important types of discontinuity within rock masses, typically having no residual strength.
[edit] References
- ^ Ladeira,F.L. & Price,N.J. 1981. Relationship between fracture spacing and bed thickness. Journal of Structural Geology, 3, 179-183
- ^ Engelder,T. & Geiser,P. 1980. On the use of regional joint sets as trajectories of paleostress fields during development of the Appalachian Plateau, New York. Journal of Geophysical Research, 85, B11, 6319-6341.
- ^ Roberts,J.C. 1995. Fracture surface markings in Liassic limestone at Lavernock Point, South Wales. Geological Society, London, Special Publications; v. 92; p. 175-186
