What Is the Difference Between Joints and Faults?

With joints, there is no significant movement-related displacement between the two sides of rock, whereas faults are characterized by movement of the two sides of the rock relative to one another. Joint and faults are often caused by different forces. However, they can occur simultaneously.

Joints are brittle surface fractures in rock with little or no displacement. In other words, the opposite sides of the fracture do not move. Nearly all surface rocks have joints, although they are often not visible in unweathered rock. Rocks most commonly develop joints when erosion and uplift reduce compressive load by eliminating overlying rocks. This allows the rock to expand laterally and produces fractures. Tectonic movement is also a cause of joints; in these situations, joints are often co-occurring with faults.

Faults are discontinuities in rock resulting from earth movement. Geologists normally designate fault zones, which are complex deformations consisting of many fractures. Faults occur when friction and frigidity do not allow rocks to flow past each other. Built-up stress eventually exceeds the stress threshold. The strain releases accumulated potential energy into the fault. Faults in ductile rocks can release instantaneously when the strain rate is high, resulting in a process that produces earthquakes.