Joints Terminology

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).

Systematic joints Roughly planar joints which occur as part of a set in which the joints parallel one another, and which are relatively evenly spaced from one another.

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.