Joint (geology)
[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.
They vary greatly in appearance, dimensions, and arrangement, and occur in quite different tectonic environments.
The most prominent joints occur in the most well-consolidated, lithified, and highly competent rocks, such as sandstone, limestone, quartzite, and granite.
[1][2] When tensional stresses stretch a body or layer of rock such that its tensile strength is exceeded, it breaks.
[2][4] Within regions that have experienced tectonic deformation, systematic joints are typically associated with either layered or bedded strata that have been folded into anticlines and synclines.
They are often oriented perpendicular to either the upper surface and base of lava flows and the contact of the tabular igneous bodies with the surrounding rock.
This type of jointing is typical of thick lava flows and shallow dikes and sills.
[7] Joints can be classified according to their origin, under the labels of tectonics, hydraulics, exfoliation, unloading (release), and cooling.
Such joints form when directed tectonic stress causes the tensile strength of bedrock to be exceeded as the result of the stretching of rock layers under conditions of elevated pore fluid pressure and directed tectonic stress.
[2][4][8] Hydraulic joints are formed when pore fluid pressure becomes elevated as a result of vertical gravitational loading.
A stress builds up which eventually exceeds the tensile strength of the bedrock and results in jointing.
Because of the absence of diagnostic ornamentation or the lack of any discernible movement or offset, they can be indistinguishable from joints.
Such fractures occur in planar parallel sets at an angle of 60 degrees and can be of the same size and scale as joints.
[2] Joints are important not only in understanding the local and regional geology and geomorphology but also in developing natural resources, in the safe design of structures, and in environmental protection.
Regional and local joint systems exert a strong control on how ore-forming hydrothermal fluids (consisting largely of H2O, CO2, and NaCl — which formed most of Earth's ore deposits) circulated within its crust.
As a result, understanding their genesis, structure, chronology, and distribution is an important part of finding and profitably developing ore deposits.
Finally, joints often form discontinuities that may have a large influence on the mechanical behavior (strength, deformation, etc.)
