Joint (geology)

In the study of structural geology, a joint is a natural fracture within a rock body or layer. The defining characteristic of a joint is that it lacks visible or measurable movement parallel to the fracture surface. This specific type of breakage is known as a Mode 1 fracture. It is important to distinguish joints from faults. In a fault, the opposite sides of the fracture exhibit visible or measurable lateral movement. This difference often depends on the scale of observation. A joint might actually involve tiny amounts of displacement, but if that movement is invisible at the scale being studied, it is classified as a joint.

Joints are among the most universal geologic structures. They appear in almost every rock exposure. These fractures vary greatly in their dimensions, appearance, and arrangement. They can occur in many different tectonic environments. Joints are most prominent in highly competent, well-consolidated, and lithified rocks. Examples of such rocks include granite, limestone, sandstone, and quartzite. Some joints remain as open fractures. Others are filled with different materials. If a joint is infilled by precipitated minerals, it is called a vein. If it is filled by solidified magma, it is called a dike.

Most joints form through brittle fracture caused by tensile stress. Tensile stress occurs when a rock is stretched until it exceeds its tensile strength. This stretching can be caused by several different factors. External forces might stretch the layers of rock. An increase in pore fluid pressure can also cause stretching. Additionally, a rock body might shrink due to cooling or desiccation while its outer boundaries remain fixed. When the rock breaks under this tension, the fracture plane forms parallel to the maximum principal stress. It also forms perpendicular to the minimum principal stress, which is the direction of the stretching. This process typically creates a single sub-parallel joint set. As deformation continues, additional sets may develop. The first set of joints often influences the orientation of later sets. This often causes new sets to form at a high angle, frequently 90 degrees, to the first set.

Geologists classify joints by their geometry or by the processes that created them. Geometrically, joints are categorized as columnar, systematic, or nonsystematic. Systematic joints are planar and parallel. They can be traced over long distances and occur at regular, even intervals. These intervals might be measured in centimeters, meters, or even hundreds of meters. When these parallel joints form recognizable families, they are called joint sets. A joint system consists of two or more intersecting joint sets. Geologists measure the angles where these sets meet, calling them dihedral angles. If the dihedral angles are nearly 90 degrees, the sets are called orthogonal. If the angles are between 30 and 60 degrees, they are called conjugate sets. Nonsystematic joints are the opposite; they are irregular in form, orientation, and spacing. They cannot be easily grouped into distinctive sets.

Columnar jointing is a unique geometric type of joint. This process splits a rock body into long, prismatic columns. These columns are characterized by triple joint junctions. At these junctions, the joint planes typically meet at angles of about 120 degrees. While the columns are usually hexagonal in section, they can also have 3, 4, 5, or 7 sides. This type of jointing is common in thick lava flows and shallow dikes or sills. It is also seen in some sedimentary strata. The width of these columns can range from just a few centimeters to several meters. They often form perpendicular to the contact surfaces between igneous rock and its cooler surroundings. This means they are frequently visible at the top and base of lava flows.

Joints also form through specific physical processes like hydraulic fracturing and exfoliation. Hydraulic joints occur when pore fluid pressure becomes elevated due to vertical gravitational loading. This happens when the accumulation of sediments or volcanic material increases the pressure of groundwater. If the pressure becomes high enough, it increases tensile stress on existing cracks. When this stress exceeds the least principal compressive stress, the rock fails and cracks propagate. Exfoliation joints are large, curved, and flat-lying fractures. They are typically found in massively exposed rock faces in deeply eroded landscapes. These fan-shaped fractures can be tens of meters in size. They are driven by the vertical gravitational load of massive mountains and the release of pressure as erosion removes overlying rock. This process is also known as unloading or release.

Understanding these fractures allows scientists to reconstruct the history of the Earth's crust. Tectonic joints, for example, reflect the local stresses associated with folding and faulting. In regions with tectonic deformation, systematic joints often relate to folded strata. These can be classified by their orientation to fold axes. Longitudinal joints run roughly parallel to the axes, while cross-joints are perpendicular. Diagonal joints trend obliquely, and strike joints run parallel to the strike of the axial plane. By studying the specific types and angles of joints, geologists can determine the direction and magnitude of the forces that shaped the landscape.