Here's why:
* Joints are fractures in rocks where there has been no movement or displacement of the rock on either side of the fracture. They are often formed by the cooling and contraction of magma as it solidifies, or by tectonic forces that stress the rock.
* Faults are fractures in rocks where there has been movement or displacement of the rock on either side of the fracture.
* Folds are bends or curves in rock layers that are caused by tectonic forces.
While faults and folds can also be zones of weakness, joints are specifically associated with the cooling and contraction of igneous rocks, making them particularly common in these structures.
