Joint (geology)

Horizontal joints in the sedimentary rocks of the foreground and a more varied set of joints in the granitic rocks in the background. Image from the Kazakh Uplands in Balkhash District, Kazakhstan.
Orthogonal joint sets on a bedding plane in flagstones, Caithness, Scotland
Joints in the Almo Pluton, City of Rocks National Reserve, Idaho.
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.
Joint spacing in mechanically stronger limestone beds shows increase with bed thickness, Lilstock Bay, Somerset
Roadside weathered diorite outcrop along the Baguio-Bua-Itogon Road in the Philippines showing joints.

A joint is a break (fracture) of natural origin in a layer or body of rock that lacks visible or measurable movement parallel to the surface (plane) of the fracture ("Mode 1" Fracture). Although joints can occur singly, they most frequently appear as joint sets and systems. A joint set is a family of parallel, evenly spaced joints that can be identified through mapping and analysis of their orientations, spacing, and physical properties. A joint system consists of two or more intersecting joint sets.[1][2][3]

The distinction between joints and faults hinges on the terms visible or measurable, a difference that 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 ("Mode 2" and "Mode 3" Fractures). Thus a joint may be 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 universal geologic structures, found in almost every exposure of rock. 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 rocks, such as sandstone, limestone, quartzite, and granite. Joints may be open fractures or filled by various materials. Joints infilled by precipitated minerals are called veins and joints filled by solidified magma are called dikes.[1][2]

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