introduction to Fracture and joints

1. University of Quid-I-Azam
Department of Earth Sciences
Assignment #1
Subject: Structural Geology and Tectonics
Topic: Fracture and Joints
Submitted by:
Abdul Bari Qanit
PhD. Geology
2nd
Semester

2. Fracture and Joints
Joints and fractures are the most abundant signs of strain and deformation
in rocks. They are ubiquitous and may part rocks in various regular or irregular sizes
as small as a fraction of an inch.
Fractures, Faults and joints are form of brittle deformations and brittle
deformations are 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. When rocks break in response to
stress, the resulting break is called a fracture. If rocks on one side of the break shift
relative to rocks on the other side, then the fracture is a fault. If there is no movement
of one side relative to the other, and if there are many other fractures with the same
orientation, then the fractures are called joints. Joints with a common orientation
make up a joint set (figure-1).
Figure-1 Joint sets have broken these siltstone and shale beds into long rectangular planks.
Fracture:
The term fracture is general and includes any break in rocks. 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.
There are four principal classes of fractures: joints, faults (including shears),
cleavage, and small irregular breaks. Joints are the prime consideration herein, but
other forms of fractures are considered to the extent that genetic, spatial, or

3. transitional relationships exist. Fractures caused by weathering, such as exfoliation
and spheroidal spalling, are not described.
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.
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.
Causes for Fractures:
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:
First, there are three modes of fractures that occur (regardless of mechanism):
 Mode I crack – Opening mode (a tensile stress normal to the plane of the
crack)
 Mode II crack – Sliding mode (a shear stress acting parallel to the plane of
the crack and perpendicular to the crack front)
 Mode III crack – Tearing mode (a shear stress acting parallel to the plane of
the crack and parallel to the crack front)
Figure-2. Cracks in rock are a mechanism
of brittle deformation in response to stress.

4. Joints:
A joint is a fracture without significant relative displacement of the walls,
which is a member of a group of fractures spatially extensive in three dimensions
generally, or within the bounds of a given rock body. Joints are more or less regular
groups of fractures paralleled by little or no movement or orientation of rock
components. Fractures paralleled by movement are, of course, faults, and those
paralleled by considerable or pervasive orientation of minerals or other rock
components are cleavage of one sort or another.
Most joints form when the overall stress regime is one of tension (pulling apart)
rather than compression. The tension can be from a rock contracting, such as during
the cooling of volcanic rock. It can also be from a body of rock
expanding. Exfoliation joints, which make the rock appear to be flaking off in
sheets(Figure-3), occur when a body of rock expands in response to reduced pressure,
such as when overlying rocks have been removed by erosion.

5. Figure-3 Half Dome at Yosemite National Park is an exposed granite batholith that displays
exfoliation joints, causing sheets of rock to break off.
Nevertheless, it is possible for joints to develop
where the overall regime is one of compression. Joints can
develop where rocks are being folded, because the hinge
zone of the fold is under tension as it stretches to
accommodate the bending (Figure-4).
Joints can also develop in a rock a rock under
compression as a way to accommodate the change in shape
(Figure-5). The joints accommodate the larger compression
stress (larger red arrows) by allowing the rock to stretch in
the up-down direction (along the green arrows).
Figure-4 Joints developed in the hinge zone
of folded rocks.
Figure-5 Joints developing to
accommodate the larger horizontal
component of compression (large red
arrows).

6. Joint sets and systems:
Joints are ubiquitous features of rock exposures and often form families of straight
to curviplanar fractures typically perpendicular to the layer boundaries in
sedimentary rocks. A set is a group of joints with similar orientation and
morphology. Several sets usually occur at the same place with no apparent
interaction, giving exposures a blocky or fragmented appearance. Two or more sets
of joints present together in an exposure compose a joint system. Joint sets in
systems commonly intersect at constant dihedral angles. They are conjugate for
dihedral angles from 30 to 60°, orthogonal when the dihedral angle is nearly 90°.
F i g u r e - 6 (a) Traces of various types of joint arrays on a bedding surface. (b) Idealized
arrangement of joint arrays with respect to fold symmetry axes. The “hk0” label for joints that cut
diagonally across the fold-hinge is based on the Miller indices from mineralogy; they refer to the
intersections of the joints with the symmetry axes of the fold.

7. Geometry of Joints
The geometry of joint systems refers to the orientation (plotted on
stereonets and rose-diagrams), the scale, the shapes and trajectories, the spacing,
the aperture, the intersections and terminations of the studied joints. The mean
orientation and orientation distribution, spacing and relative chronology are
general characters used to define joint sets. In this respect, a three-dimensional
observation is essential to avoid skewed sampling measurements due to simple
geometrical reasons.
 Bedding-contained joints: terminate at the top and bottom of beds.
 Systematic joints: are characterized by a roughly planar geometry; they have
relatively long traces and typically form sets of approximately parallel and
almost equally spaced joints.
 Non-systematic joints: are usually short, curved and irregularly spaced. They
generally terminate against systematic joints.
Spacing:
The sizes and spacing (the average orthogonal distance between neighboring
fracture planes) are essential characteristics of joint sets. In isotropic rocks (e.g.
granite) joint spacing follows an approximately log-normal frequency (the number
of joints occurring within a unit length) distribution. In anisotropic (layered) rocks,
joint spacing differs according to several parameters.
Bed thickness: For the same lithology, joints are more closely spaced in thinner
beds. This is because the formation of joints relieves tensile stress in the layer over
a lateral distance proportional to the joint length. Since joints end at layer
boundaries, which are rock discontinuities, the longer joints in thicker layers need
to be spaced less frequently.
Lithology: Stronger, more brittle rocks have more closely spaced joints than
weaker rocks. Similarly, rocks with low tensile strength show more joints than

8. stiffer lithologies, because the strain is the same along layers of different types. Yet,
higher stresses are required to achieve the same amount of strain in the stronger
layers. Therefore, strong layers fracture more frequently. However, this response
is particularly sensitive to local pore fluid pressure.
Structural position and strain: The structural position (particularly within folds)
and the magnitude of extensional strain also control joint spacing.