The terms "fracture" and "fissure" are used in my geotechnical engineering reports for Central Texas geology (i.e. limestone formations typically). These are two different terms and not synonyms. Both are "discontinuities" in a rock mass. Discontinuities can be categorized as:
- Fractures
- Joints
- Bedding planes
- Cleavage or foliation
- Faults
When a crack forms along a rock cleavage plane, some prefer not to associate the term fracture with this type of discontinuity. But joints are also fractures.
A fracture develops mechanically in rock due to a failure of structure under tensile stress (the initial stage of a crack) or stress-strain conditions due to environmental or man-made conditions. When shear stresses parallel to the plane of the crack are out of equilibrium, sliding may occur (i.e. fault).
A fissure is a specific shape of fracture, one that is long and narrow. It is also a discontinuity dividing an otherwise continuous rock material without separation in 2 of 3 dimensions (i.e. no sliding has yet occurred; separation in 1 dimension). A fissure may be void or may have been filled in. A fracture or a joint with a visible opening (aperture) and a long or deep alignment can be called a fissure.
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