Fracture (geology)

A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity.

Brittle deformation

Fractures are forms of brittle deformation. There are two types of primary brittle deformation processes. Tensile fracturing results in joints. Shear fractures are the first initial breaks resulting from shear forces exceeding the cohesive strength in that plane.
After those two initial deformations, several other types of secondary brittle deformation can be observed, such as frictional sliding or cataclastic flow on reactivated joints or faults.
Most often, fracture profiles will look like either a blade, ellipsoid, or circle.

Causes

Fractures in rocks can be formed either due to compression or tension. Fractures due to compression include thrust faults. Fractures may also be a result from shear or tensile stress. Some of the primary mechanisms are discussed below.

Modes

First, there are three modes of fractures that occur :

Mode I crack – Opening mode
Mode II crack – Sliding mode
Mode III crack – Tearing mode

For more information on this, see fracture mechanics.

Tensile fractures

Rocks contain many pre-existing cracks where development of tensile fracture, or Mode I fracture, may be examined.
The first form is in axial stretching. In this case a remote tensile stress, σ_n, is applied, allowing microcracks to open slightly throughout the tensile region. As these cracks open up, the stresses at the crack tips intensify, eventually exceeding the rock strength and allowing the fracture to propagate. This can occur at times of rapid overburden erosion. Folding also can provide tension, such as along the top of an anticlinal fold axis. In this scenario the tensile forces associated with the stretching of the upper half of the layers during folding can induce tensile fractures parallel to the fold axis.
Another, similar tensile fracture mechanism is hydraulic fracturing. In a natural environment, this occurs when rapid sediment compaction, thermal fluid expansion, or fluid injection causes the pore fluid pressure, σ_p, to exceed the pressure of the least principal normal stress, σ_n. When this occurs, a tensile fracture opens perpendicular to the plane of least stress.^{Fracture #cite note-4|}
Tensile fracturing may also be induced by applied compressive loads, σ_n, along an axis such as in a Brazilian disk test. This applied compression force results in longitudinal splitting. In this situation, tiny tensile fractures form parallel to the loading axis while the load also forces any other microfractures closed. To picture this, imagine an envelope, with loading from the top. A load is applied on the top edge, the sides of the envelope open outward, even though nothing was pulling on them. Rapid deposition and compaction can sometimes induce these fractures.
Tensile fractures are almost always referred to as joints, which are fractures where no appreciable slip or shear is observed.
To fully understand the effects of applied tensile stress around a crack in a brittle material such a rock, fracture mechanics can be used. The concept of fracture mechanics was initially developed by A. A. Griffith during World War I. Griffith looked at the energy required to create new surfaces by breaking material bonds versus the elastic strain energy of the stretched bonds released. By analyzing a rod under uniform tension Griffith determined an expression for the critical stress at which a favorably orientated crack will grow. The critical stress at fracture is given by,
where γ = surface energy associated with broken bonds, E = Young's modulus, and a = half crack length. Fracture mechanics has generalized to that γ represents energy dissipated in fracture not just the energy associated with creation of new surfaces

Linear elastic fracture mechanics

Linear elastic fracture mechanics builds off the energy balance approach taken by Griffith but provides a more generalized approach for many crack problems. LEFM investigates the stress field near the crack tip and bases fracture criteria on stress field parameters. One important contribution of LEFM is the stress intensity factor, K, which is used to predict the stress at the crack tip. The stress field is given by
where is the stress intensity factor for Mode I, II, or III cracking and is a dimensionless quantity that varies with applied load and sample geometry. As the stress field gets close to the crack tip, i.e., becomes a fixed function of. With knowledge of the geometry of the crack and applied far field stresses, it is possible to predict the crack tip stresses, displacement, and growth. Energy release rate is defined to relate K to the Griffith energy balance as previously defined. In both LEFM and energy balance approaches, the crack is assumed to be cohesionless behind the crack tip. This provides a problem for geological applications such a fault, where friction exists all over a fault. Overcoming friction absorbs some of the energy that would otherwise go to crack growth. This means that for Modes II and III crack growth, LEFM and energy balances represent local stress fractures rather than global criteria.

Crack formation and propagation

Cracks in rock do not form smooth path like a crack in a car windshield or a highly ductile crack like a ripped plastic grocery bag. Rocks are a polycrystalline material so cracks grow through the coalescing of complex microcracks that occur in front of the crack tip. This area of microcracks is called the brittle process zone. Consider a simplified 2D shear crack as shown in the image on the right. The shear crack, shown in blue, propagates when tensile cracks, shown in red, grow perpendicular to the direction of the least principal stresses. The tensile cracks propagate a short distance then become stable, allowing the shear crack to propagate. This type of crack propagation should only be considered an example. Fracture in rock is a 3D process with cracks growing in all directions. It is also important to note that once the crack grows, the microcracks in the brittle process zone are left behind leaving a weakened section of rock. This weakened section is more susceptible to changes in pore pressure and dilatation or compaction. Note that this description of formation and propagation considers temperatures and pressures near the Earth's surface. Rocks deep within the earth are subject to very high temperatures and pressures. This causes them to behave in the semi-brittle and plastic regimes which result in significantly different fracture mechanisms. In the plastic regime cracks acts like a plastic bag being torn. In this case stress at crack tips goes to two mechanisms, one which will drive propagation of the crack and the other which will blunt the crack tip. In the brittle-ductile transition zone, material will exhibit both brittle and plastic traits with the gradual onset of plasticity in the polycrystalline rock. The main form of deformation is called cataclastic flow, which will cause fractures to fail and propagate due to a mixture of brittle-frictional and plastic deformations.

Joint types

Describing joints can be difficult, especially without visuals. The following are descriptions of typical natural fracture joint geometries that might be encountered in field studies:

Plumose Structures are fracture networks that form at a range of scales, and spread outward from a joint origin. The joint origin represents a point at which the fracture begins. The mirror zone is the joint morphology closest to the origin that results in very smooth surfaces. Mist zones exist on the fringe of mirror zones and represent the zone where the joint surface slightly roughens. Hackle zones predominate after mist zones, where the joint surface begins to get fairly rough. This hackle zone severity designates barbs, which are the curves away from the plume axis.
Orthogonal Joints occur when the joints within the system occur at mutually perpendicular angles to each other.
Conjugate Joints occur when the joints intersect each other at angles significantly less than ninety degrees.
Systematic Joints are joint systems in which all the joints are parallel or subparallel, and maintain roughly the same spacing from each other.
Columnar Joints are joints that cut the formation vertically in hexagonal columns. These tend to be a result of cooling and contraction in hypabyssal intrusions or lava flows.
Desiccation cracks are joints that form in a layer of mud when it dries and shrinks. Like columnar joints, these tend to be hexagonal in shape.
Sigmoidal Joints are joints that run parallel to each other, but are cut by sigmoidal joints in between.
Sheeting joints are joints that often form near surface, and as a result form parallel to the surface. These can also be recognized in exfoliation joints.
In joint systems where relatively long joints cut across the outcrop, the throughgoing joints act as master joints and the short joints that occur in between are cross joints.
Poisson effect is the creation of vertical contraction fractures that are a result of the relief of overburden over a formation.
Pinnate joints are joints that form immediately adjacent to and parallel to the shear face of a fault. These joints tend to merge with the faults at an angle between 35 and 45 degrees to the fault surface.
Release joints are tensile joints that form as a change in geologic shape results in the manifestation of local or regional tension that can create Mode I tensile fractures.
Concurrent joints that display a ladder pattern are interior regions with one set of joints that are fairly long, and the conjugate set of joints for the pattern remain relatively short, and terminate at the long joint.
Sometimes joints can also display grid patterns, which are fracture sets that have mutually crosscutting fractures.
An en echelon or stepped array represents a set of tensile fractures that form within a fault zone parallel to each other.