Joints tend to occur under conditions of decreasing stress. An example of this is commonly found in granites. In response to this reduction in stress, the granite expands and cracks into sheets. If the joints are planar and closely spaced, the process is called sheeting. If the joints are curved, concentric, parallel and very closely spaced, the process is called exfoliation.
Joints also form in gineous rocks in response to cooling.
When an igneous rock cools it contracts. If the confining pressure is small as in a lava flow, this contraction on cooling causes the rock to fracture and form polygonal shaped columns bounded by joints that are oriented more or less perpendicular to the surface of the flow. Such joints are called columnar joints. Faults Faults are fractures in rocks across which ther has been some relative movement. The figure at left shows some basic nomenclature of faults.