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Fractures and faults are prominent in the upper part of the continental and oceanic
lithosphere, where they are often associated with earthquake activity. 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. The term fracture is general and
includes any break in rocks. Some fractures that look like joints are actually shear fractures, which
in effect are microfaults, instead of joints. Shear fractures do not form as the result of the
perpendicular opening of a fracture due to tensile stress, but through the shearing of fractures that
causes lateral movement of its faces. Shear fractures can be confused with joints, because the lateral
offset of the fracture faces is not visible at the scale of outcrop or hand specimen. Because of the
absence of diagnostic ornamentation or the lack of any discernible movement or offset, they can be
indistinguishable from joints.
Joints: Planar discontinuities involving no relative displacement of the adjacent blocks. Joints
develop during the exhumation of rocks following erosion of the overburden. Joints result from
contraction and expansion due to cooling and decompression respectively.
Fractures are discontinuities with limited displacement. They form when applied stress reaches the
yielding threshold, i.e. the stress at which rock fractures.
A fault is a fracture across which two blocks have slipped; the displacement of adjacent blocks is
parallel to the fault plane. Faulting corresponds to the brittle failure of an undeformed rock
formation or, alternatively, involves frictional sliding on a pre-existing fault plane. Faulting occurs
when the maximum differential stress (i.e., maximum stress !1 minus minimum stress !3) exceeds
the shear strength of an intact rock formation, or the frictional strength of a pre-existing fault.
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 .
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. Small, irregular, and inconsistently oriented