Engineering Geology Practice Test: Joints and Jointing — Flashcards

Joints and Jointing topics include:Joints terminology, joints classification, joints occurrence, joints origin and engineering considerations.

In geology, a joint is a fracture in a rock that has little to no displacement. Joints are found in nearly all surface rocks and can extend in various directions, usually more vertically than horizontally. 

Joints can be classified by their geometry:
Longitudinal joints: Roughly parallel to fold axes and often fan around the fold
Cross-joints: Approximately perpendicular to fold axes
Diagonal joints: Typically occur as conjugate joint sets that trend oblique to the fold axes 

Joints can also be classified as systematic or non-systematic:
Systematic joints: Nearly planar, but not perfectly so, and occur in sets with regular spacing and orientation
Non-systematic joints: Irregular or seemingly random geometry, spacing, and orientation 

Joints form when tensile stress exceeds its threshold. This can be caused by a rock contracting, such as during the cooling of volcanic rock, or by a body of rock expanding. 

Jointing does not extend to a very great depth in the Earth's crust, because at about 12 kilometers (7.5 miles) even rigid rocks tend to flow plastically in response to stress.