This effect, called hydrolytic weakening , causes quartz to become ductile at much lower pressures and temperatures when water is present than when it is absent. This diagram illustrates the effect that the type of rocks present in the will have on the style of deformation. If the crust is made of weak rocks, deformation will be ductile, even at low stress, while crust made up of stronger rocks will require much higher stresses before it will flow ductiley. An interesting result of this simple relationship is that the weakest rocks in any given part of the crust will always record the last deformation episode, while stronger rocks will only record higher stress deformation. Joints are fractures in the rock where the rocks on either side of the fracture have not moved relative to one another.
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.