[STRUCT] Joints and Veins

Joints

are natural fractures in rock across which there has been no shear displacement

Veins

which are fractures filled with minerals that precipitated from a fluid

Fracture

It refers to any surface of discontinuity, meaning a surface across which the material is no longer bonded

Arrest Line

An arcuate ridge on a joint surface, located at a distance from the origin, where the joint front

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.

Exfoliation

See sheeting joints.

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.

Vein

A fracture that has filled with minerals precipitated from water solutions that passed through the fracture.

Vein array

A group of veins in a body of rock.

Planar systematic arrays

In a planar systematic array, the component veins are planar, are mutually parallel, and are regularly spaced.

Nonsystematic arrays

The veins in nonsystematic arrays tend to be nonplanar, and individual veins may vary in width.

En echelon arrays

An en echelon array consists of short parallel veins that lie between two parallel enveloping surfaces and are inclined at an angle to the surfaces. The veins may be sigmoidal in cross section and typically taper toward their terminations.

Stockwork veins

Stockwork veins comprise a cluster of irregularly shaped veins that occur in a pervasively fractured rock body. The veins are nonplanar arrays and occur in a range of orientations. In rock bodies with stockwork veins, as much as 40% or 50% of the outcrop is composed of vein material, and vein material may completely surround blocks of the host rock.