Joints Types & Definitions

Joints refer to fractures or cracks in rocks where there has been no significant displacement or movement along the fracture surfaces. These are important because they influence the permeability, strength, and weathering of rocks.

Joints are caused by various stresses acting on the rock, such as tectonic forces, cooling, and erosion. They can be important in many geological processes, such as fluid flow, weathering, and slope stability.

Joint characteristics

• Orientation: This refers to the direction and dip of the joint surface, typically described using strike and dip angles.
• Spacing: This refers to the distance between individual joints in a set.
• Aperture: This refers to the width of the joint opening.
• Filling: This refers to any material that may be present within the joint, such as minerals, clay, or gouge.
• Surface roughness: This refers to the texture of the joint surface, which can be smooth, rough, or undulating.

Types of Joints 

Systematic Joints

A set of parallel or near-parallel joints that are regularly spaced. These are often related to tectonic stresses within the Earth's crust. They can be found in patterns, either as single sets or in multiple sets that intersect at various angles.

• Orthogonal Joints: Joints that form at right angles to each other, creating a grid-like pattern.
• Conjugate Joints: Pairs of joint sets that intersect at acute angles, often related to stress fields.

Non-Systematic Joints

Joints that do not form a consistent pattern or orientation. They are less predictable and often occur due to local stress conditions or rock inhomogeneities. They are irregularly spaced and oriented, often appearing randomly distributed.

Extension Joints

These joints form when rocks are subjected to tensile stress, pulling the rock apart, which leads to the opening of cracks. They are perpendicular to the direction of the least principal stress. They are common in areas of crustal extension.

Shear Joints

Formed under shear stress where one part of the rock slides past another, creating fractures that are oriented at an angle to the shear stress direction. They often occur in conjugate sets, with angles of approximately 30 degrees or 60 degrees from the direction of maximum shear stress.

Release Joints (Unloading Joints)

Develop as overlying rock or ice is removed, reducing the vertical pressure and allowing the rock to expand upward and fracture. They are typically sub-horizontal and can be seen in areas where there has been significant erosion or deglaciation.

Columnar Joints

Form in cooling lava or other igneous intrusions. The contraction during cooling creates fractures that form a polygonal pattern when viewed from above. They often result in the formation of near-vertical columns, as seen in columnar basalt formations.

Tension Joints

Similar to extension joints, these form under tension but are often associated with the bending or flexure of rock layers. They are generally perpendicular to the bedding in sedimentary rocks where the layers are bent into folds.

Conjugate Joints

Sets of joints that form at an angle to each other due to the same stress field but in opposite directions, creating a symmetrical pattern. These can provide insight into the direction and nature of the stress that formed them.

Exfoliation Joints

A type of unloading joint where rock peels off in concentric sheets parallel to the rock surface, often seen in granite domes and other igneous rock outcrops.

Key terms related to joints in geology

Systematic joints: These are planar, parallel joints that can be traced for some distance and occur at regular intervals. They are often caused by regional stresses, such as those associated with mountain building.

Arrest line An arcuate ridge on a joint surface, located at a distance from the origin, where the joint front stopped or paused during propagation of the joint; also rib marks.

Columnar joints Joints that break rock into generally hexagonal columns; they form during cooling and contraction in hypabyssal intrusions or lava flows.

Conjugate system Two sets of joints oriented such that the dihedral angle between the sets is approximately 60°.

Continuous joints Throughgoing joints that can be traced across an outcrop, and perhaps across the countryside.

Cross joints Discontinuous joints that cut across the rock between two systematic joints, and are oriented at a high angle to the systematic joints.

Cross-strike joints Joints that cut across the general trend of fold hinges in a region of folded rocks (i.e., the joints cut across regional bedding strike).

Dessication cracks Joints formed in a layer of mud when it dries and shrinks; dessication cracks (or mud cracks) break the layer into roughly hexagonal plates.

Discontinuous joints Short joints that terminate within an outcrop, generally at the intersection with another joint.

En echelon An arrangement of parallel planes in a zone of fairly constant width; the planes are inclined to the borders of the zone and terminate at the borders of the zone. In an en echelon array, the component planes are of roughly equal length.

Hackle zone The main part of a plumose structure, where the fracture surface is relatively rough due to microscopic irregularities in the joint surface formed when the crack surfaces get deflected in the neighborhood of grain-scale inclusions in the rock, or due to off-plane cracking (formation of small cracks adjacent to the main joint surface) as the fracture propagates.

Hooking The curving of one joint near its intersection with an earlier formed joint.

Inclusion A general term for any solid inhomogeneity (e.g., fossil, pebble, burrow, xenolith, amygdule, coarse grain, etc.) in a rock; inclusion may cause local stress concentrations.

Joint A natural, unfilled, planar or curviplanar fracture which forms by tensile loading (i.e., the walls of a joint move apart very slightly as the joint develops). Joint formation does not involve shear displacement.

Joint array Any group of joints (systematic or nonsystematic).

Joint density The surface area of joints per unit volume of rock (also referred to as joint intensity).

Joint origin The point on the joint (usually a flaw or inclusion) at which the fracture began to propagate; it is commonly marked by a dimple.

Joint set A group of systematic joints.

Joint stress shadow The region around a joint surface where joint-normal tensile stress is insufficient to cause new joints to form.

Joint system Two or more geometrically related sets of joints in a region.

Mirror region Portion of a joint surface adjacent to the joint origin where the surface is very smooth; mirrors do not occur if the rock contains many small-scale heterogeneities.

Mist region A portion of a joint surface surrounding the mirror where the fracture surface begins to roughen.

Nonsystematic joints A joint that is not necessarily planar, and is not parallel to nearby joints.

Orthogonal system Two sets of joints that are at right angles to one another.

Plume axis The axis of the plume in a plumose structure.

Plumose structure A subtle roughness on the surface of some joints (particularly those in fine-grained rocks) that macroscopically resembles the imprint of a feather.

Sheeting joints Joints formed near the ground surface that are roughly parallel to the ground surface; sheeting joints on domelike mountains make the mountains resemble delaminating onions; also exfoliation.

Strike-parallel joints Joints that parallel the general trend of fold-hinges in a region of folded strata (i.e., the joints parallel regional bedding strike).

Twist hackle One of a set of small en echelon joints formed along the edge of a larger joint; a twist hackle is not parallel to the larger joint, and forms when the fracture surface twists continuously into a different orientation and then breaks up into segments.