Joint and jointing

Fractures in rock are classified according to the type of relative motion that has occurred across the fracture. Extensional fractures, also known as joints, are characterized by movement perpendicular to the fracture. The masses of rock separated by a joint moved away from each other, even if imperceptibly, when the joint was formed. Joints stand in contrast to faults, which are shear fractures across which the opposite sides slide past (rather than away from) each other. Rocks can undergo more than one episode of deformation during their existence, so it is possible for a fracture to begin as a joint and evolve into a fault as the stresses acting on the joint change through geologic time . The precise definition of a joint, however, is not universal and some geologists classify fractures as joints if there seems to have been only a small, but measurable, amount of shearing.

Joints formed in coal are known as cleats, and joints (or faults) filled with mineral deposits are known as veins. Volcanic rocks such as basalt contract as they cool, forming networks of columnar joints that divide the rock into regular polygonal columns such as those seen in Devil's Tower and Devil's Postpile National Monuments, USA.

Joints that are exposed at Earth's surface are often enlarged by chemical dissolution and weathering , particularly in soluble rocks such as limestone and, to a lesser degree, rocks such as calcite-cemented sandstones. Therefore, the width of joints in outcrops does not necessarily reflect the movement that created the joint.

Groups of joints sharing a similar three-dimensional geometry are known as joint sets. Like all fractures in rock, joints are irregular or wavy features rather than perfect planes. Therefore, joints in a set will have slightly different geometries and the separation of joints into sets can pose a difficult task for geologists. The combination of two or more sets of joints in a rock mass is a joint system.

Joints can also be described as being systematic or non-systematic. Systematic joints are those that are nearly planar (although never perfectly so) and occur in sets with regular spacing and orientation. Non-systematic joints are those with irregular or seemingly random geometry, spacing, and orientation. Although the terms are similar, systematic joints do not necessarily belong to joint systems and joint systems are not necessarily composed of systematic joints. It is possible, for example, for there to be a joint system composed of two sets of non-systematic joints.

The origin of joints and faults is studied by making detailed maps of rock fracture systems in the field and then applying mathematical techniques developed in the discipline of fracture mechanics. In fracture mechanics terminology, joints are known as Mode I fractures. Faults are Mode II or III fractures, depending on the direction of movement parallel to the fault surface. Studies have shown that rocks, like all other materials, contain innumerable microscopic flaws. When a rock is subjected to stress, either within Earth's crust or a laboratory-testing device, the fractures that most efficiently dissipate the stress grow in length and combine at the expense of other, less efficiently oriented, fractures. Fractures also perturb the distribution and intensity of stress in the adjacent rock, and the shape of a growing fracture can therefore be strongly influenced by the growth of its neighbors. Evidence for this phenomenon can be found in the field, where neighboring joints curve and then abruptly terminate against each other to form complicated, but understandable, patterns.

Because they are discontinuities in otherwise solid rock masses, joints can influence fluid flow through rocks and rock mass stability. Jointed rock has much higher porosity and permeability than intact rock of the same type, and can more easily transmit petroleum , groundwater , or ore-bearing geothermal fluids. Joints also form mechanical discontinuities that decrease the strength of rock, which is particularly important when rock is excavated during construction projects. Rockslides and rock falls also occur as a result of extensive joint sets or systems. Therefore, the identification and analysis of discontinuities such as joints, faults, and bedding planes is a critically important part of many applied geologic studies.

See also Faults and fractures; Field methods in geology