Joint (geology)

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Joint sets on a bedding plane in flagstones, Caithness, Scotland
A rock in Abisko fractured along existing joints possibly by mechanical frost weathering
Columnar jointed basalt in Turkey
Columnar jointing in basalt, Marte Vallis, Mars
Recent tectonic joint intersects older exfoliation joints in granite gneiss, Lizard Rock, Parra Wirra, South Australia.

A joint is a break (fracture) of natural origin in the continuity of either a layer or body of rock that lacks any visible or measurable movement parallel to the surface (plane) of the fracture. Although they can occur singly, they most frequently occur as joint sets and systems. A joint set is family of parallel, evenly spaced joints that can be identified through mapping and analysis of the orientations, spacing, and physical properties. A joint system consists of two or more interlocking joint sets. The distinction between joints and faults hinges on the terms visible or measurable which depends on the scale of observation. Faults differ from joints in that they exhibit visible or measurable lateral movement between the opposite surfaces of the fracture. As a result, a joint may have been created by either strict movement of a rock layer or body perpendicular to the fracture or by varying degrees of lateral displacement parallel to the surface (plane) of the fracture that remains “invisible” at the scale of observation.[1][2][3]

Joints are among the most abundant of geologic structures as they are found in most every exposure of rock. Although they are among the most ubiquitous of geological features, joints are, at the same time, the most confusing of geological features. They vary greatly in appearance, dimensions, and arrangement, and occur in quite different tectonic environments. Often, the specific origin of the stresses that created certain joints and associated joint sets can be quite ambiguous, unclear, and sometimes controversial. The most prominent joints occur in the most well-consolidated, lithified, and highly competent (stiff) rocks, such as sandstone, limestone, quartzite, and granite. Joints may be open fractures or filled by various materials. Joints, which are infilled by precipitated minerals are called veins and joints filled by solidified magma are called dikes (dykes).[1][2]

Formation[edit]

Joints result from brittle failure of a rock body or layer as the result of tensional stresses. These tensional stresses either were induced or imposed from outside, e.g. by the stretching of layers, or the rise of pore fluid pressure by external compression or fluid injection, or the result of internal stresses induced by the shrinkage caused by the cooling or desiccation of a rock body or layer whose outside boundaries remained fixed.[1][2]

When tensional stresses stretch a body or layer of rock such that its brittle strength is exceeded, 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.[1][2]

Types of joints[edit]

Joints are classified by the processes responsible for their formation, or their geometry.

Types with respect to formation[edit]

Tectonic joints[edit]

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.[4]

Unloading joints (Release joints)[edit]

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[edit]

Exfoliation joints may be a special case of unloading joints formed at, and parallel to, the current land surface in rocks of high compressive strength, although there is as yet no general agreement on a general theory of how they form.

Cooling joints[edit]

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

Types with respect to attitude and geometry[edit]

Joints can be classified into three groups depending on their geometrical relationship with the country rock:

  • Strike joints – Joints which run parallel to the direction of strike of country rocks are called "strike joints"
  • Dip joints – Joints which run parallel to the direction of dip of country rocks are called "dip joints"
  • Oblique joints – Joints which run oblique to the dip and strike directions of the country rocks are called "oblique joints".

Fractography[edit]

Plumose structure on a fracture surface in sandstone

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.[5]

Importance to soil and rock mass strength[edit]

In geotechnical engineering a joint forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of soil and rock masses in, for example, tunnel, foundation, or slope construction.

Importance in the production of geofluids[edit]

It is long been recognized that joints (fractures) play a major role in the subsurface fluid flow of water in aquifers and petroleum in oil fields. Major industry research projects have been dedicated during the last decades to the study of faulted and fractured reservoirs.

See also[edit]

References[edit]

  1. ^ a b c d Mandl, G. (2005) Rock Joints: The Mechanical Genesis. Springer-Verlag, Heidelberg, Germany. 221 pp. ISBN 978-3-540-24553-7
  2. ^ a b c d Davis, G.H., S.J. Reynolds, and C. Kluth (2012) Structural Geology of Rocks and Regions (3rd Edition): John Wiley and Sons, Inc., New york, New York. 864 pp. ISBN 978-0471152316
  3. ^ Goudie, A.S. (2004) Encyclopedia of Geomorphology volume 2 J–Z. Routledge New York, New York. 578 pp. ISBN 9780415327381
  4. ^ 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.]
  5. ^ 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]