Brittle deformation 

The permanent change that occurs in a solid material due to the growth of fractures and/or due to sliding on fractures. Brittle deformation only occurs when stresses exceed a critical value, and thus only after a rock has already undergone some elastic and/or plastic behavior.

Brittle fault zone 

A band of finite width in which slip is distributed among many smaller discrete brittle faults, and/or in which the fault surface is bordered by pervasively fractured rock.

Brittle fault

A single surface on which movement occurs specifically by brittle deformation mechanisms.

Cataclasis

A deformation process that involves distributed fracturing, crushing, and frictional sliding of grains or of rock fragments.

Crack

Verb: to break or snap apart. Noun: a fracture whose displacement does not involve shear displacement (i.e., a joint or microjoint).

Fault

Broad sense: a surface or zone across which there has been measurable sliding parallel to the surface. Narrow sense: a brittle fault. The narrow definition emphasizes the distinctions between faults, fault zones, and shear zones.

Fracture zone

A band in which there are many parallel or subparallel fractures. If the fractures are wavy, they may anastomose with one another. Note: The term has a somewhat different meaning in the context of ocean-floor tectonics.

Fracture

A general term for a surface in a material across which there has been loss of continuity and, therefore, strength. Fractures range in size from grain-scale to continent-scale.

Healed microcrack

A microcrack that has cemented back together. Under a microscope, it is defined by a plane containing many fluid inclusions. (Fluid inclusions are tiny bubbles of gas or fluid embedded in a solid).

Joint

A natural fracture which forms by tensile loading, i.e., the walls of the fracture move apart very slightly as the joint develops. Note: A minority of geologists argue that joints can form due to shear loading.

Microfracture

A very small fracture of any type. Microfractures range in size from the dimensions of a single grain to the dimensions of a thin section.

Microjoint

A microscopic joint; microjoints range in size from the dimensions of a single grain to the dimensions of a hand-specimen. Synonymous with microcrack.

Shear fracture

A macroscopic fracture that grows in association with a component of shear parallel to the fracture. Shear fracturing involves coalescence of microcracks.

Shear joint

A surface that originated as a joint but later became a surface of sliding. Note: A minority of geologists consider a shear joint to be a joint that initially formed in response to shear loading.

Shear rupture

A shear fracture.

Shear zone

A region of finite width in which shear strain is significantly greater than in the surrounding rock. Movement in shear zones represents ductile (or distributed) deformation, involving cataclasis, crystal plasticity, and diffusion.

Vein

A fracture filled with minerals precipitated from a water solution.