DEFINITION OF FRACTURE

A general name for a break in a rock or other

body that may or may not have any observable displacement.

Fractures include joints, faults, and cracks formed under brittle

deformation conditions and are a form of permanent

(nonelastic) strain. Brittle deformation processes generally

involve the growth of fractures or sliding along existing fractures.

Frictional sliding involves the sliding on preexisting

fracture surfaces, whereas cataclastic flow includes grainscale

fracturing and frictional sliding producing macroscopic

ductile flow over a band of finite width. Tensile cracking

involves the propagation of cracks into unfractured material

under tensile stress perpendicular to the maximum compres

sive stress, whereas shear rupture refers to the initiation of

fracture at an angle to the maximum principal stress.

Fractures may propagate in one of three principal modes.

Mode I refers to fracture growth by incremental extension

perpendicular to the plane of the fracture at the tip. Mode II

propagation is where the fracture grows by incremental shear

parallel to the plane of the fracture at the tip, in the direction

of fracture propagation. Mode III is when the fracture grows

by incremental shear parallel to the plane of the fracture at

the tip, perpendicular to the direction of propagation.

Joints are fractures with no observable displacement parallel

to the fracture surface. They generally occur in subparallel

joint sets, and several sets often occur together in a

consistent geometric pattern forming a joint system. Joints

are sometimes classified into extension joints or conjugate

sets of shear joints, a subdivision based on the angular relationships

between joints. Most joints are continuous for only

short distances, but in many regions master joints may run

for very long distances and control geomorphology or form

air photo lineaments. Microfractures or joints are visible only

under the microscope and only affect a single grain.

Many joints are contained within individual beds and

have a characteristic joint spacing, measured perpendicular to

the joints. This is determined by the relative strength of individual

beds or rock types, the thickness of the jointed layer,

and structural position, and is very important for determining

the porosity and permeability of the unit. In many regions,

fractures control groundwater flow, the location of aquifers,

and the migration and storage of petroleum and gas.

Joints and fractures are found in all kinds of environments

and may form by a variety of mechanisms. Desiccation cracks

and columnar joints form by the contraction of materials. Bedding

plane fissility, characterized by fracturing parallel to bedding,

may be produced by mineral changes during diagenesis

that lead to volume changes in the layer. Unloading joints form

by stress release, such as during uplift, ice sheet withdrawal, or

quarrying operations. Exfoliation joints and domes may form

by mineral changes, including volumetric changes during

weathering, or by diurnal temperature variations. Most joints

have tectonic origins, typically forming in response to the last

phase of tectonic movements in an area. Other joints seem to

be related to regional doming, folding, and faulting.

Many fractures and joints exhibit striated or ridged surfaces

known as plumose structures, since they vaguely

resemble feathers. Plumose structures develop in response to

local variations in propagation velocity and the stress field.

The origin is the point at which the fracture originated, the

mist is the small ridging on the surface, and the plume axis is

the line that starts at the axis and from which individual

barbs propagate. The twist hackle is the steps at the edge of

the fracture plane along which the fracture has split into a

set of smaller fractures.

E. M. Anderson elegantly explained the geometry and

orientation of some fracture sets in a now classic work published

in 1951. In Andersonian theory, the attitude of a fracture

plane tells a lot about the orientation of the stress field

that operated when the fracture formed. Fractures are

assumed to form as shear fractures in a conjugate set, with

the maximum compressive stress bisecting an acute (60°)

angle between the two fractures. In most situations, the surface

of the Earth may be the maximum, minimum, or intermediate

principal stress, since the surface can transmit no

shear stress. If the maximum compressive stress is vertical,

two fracture sets will form, each dipping 60° toward each

other and intersecting along a horizontal line parallel to the

intermediate stress. If the intermediate stress is vertical, two

vertical fractures will form, with the maximum compressive

stress bisecting the acute angle between the fractures. If the

least compressive stress is vertical, two gently dipping fractures

will form, and their intersection will be parallel to the

intermediate principal stress.

Other interpretations of fractures and joints include modifications

of Andersonian geometries that include volume

changes and deviations of principal stresses from the vertical.

Many joints show relationships to regional structures such as

folds, with some developing parallel to the axial surfaces of

folds and others crossing axial surfaces. Other features on

joint surfaces may be used to interpret their mode of formation.

For instance, plumose structures typically indicate Mode

I or extensional types of formation, whereas the development

of fault striations (known as slickensides) indicate Mode II or

Mode III propagation. Observations of these surface features,

the fractures’ relationships to bedding, structures such as folds

and faults, and their regional orientation and distribution can

lead to a clear understanding of their origin and significance.

See also DEFORMATION OF ROCKS; STRUCTURAL GEOLOGY.

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