Joints and Stresses

Global Processes

Igneous Joints

Outline

Joints in Compressive Stress Fields
In compression, joints develop in the conjugate shear directions (the orientation of symmetric fracture planes) making the lower angle with the major principal stress direction. Shear joints are often groooved, striated, polished or slickensided by even small amounts of shear displacement.
Joints in Tensile Stress Fields
In tension, joints develop by stretching normal to the tensile stress direction which is usually the minor principal stress. Tension joints are rough (unless subsequently weathered). In coarse grained rocks such surfaces may be very rough.
Joints and Folding
Several sets of joints may develop in response to folding. Conjugate shear joints oblique to the fold axis are develpoed by compression. Tension joints can develop due to bending, particularly in the vicinity of the fold hinge. Joints parallel to the strike of the fold axis are called strike joints; those parallel to the limb dip are known as dip joints. Such joints are generally tensile fractures.

The photograph shows a set of conjugate joints, infilled with a later mineral phase. Notice that there is very little evidence of movement since the intersections are not displaced.

Global Processes

Igneous Joints

Outline

(c) Department of Civil and Geological Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, SK, Canada, S7N 5A9