What are Exfoliation Joints ?
Exfoliation joints are a type of fracture that forms due to exhumation. The overlying rocks are removed by erosion which decreases the vertical stress leading to tension at 90 degrees to the surface.
They are common in eroded granite bodies such as those in SW England. Here, in the Al Hajar mountains in northern Oman, they occur in carbonates (Fig. 1). The joints are parallel to the surface, quite different to tectonic fractures, which in this case are perpendicular to the bedding (Fig. 2).
Joints form perpendicular to the least principle stress (𝛔3). Valley floors and mountain sides can rotate the principle stresses - our example of extensional joints in Fig.1 is placed by the arrow in Fig. 3. Therefore, we need to rotate 𝛔3 so that it is perpendicular to the slope to account for the orientation of the exfoliation joints. The local stresses on the valley floor are a full 90 degree rotation from regional stress, as shown on Fig. 3.
Reference
Fossen. H. 2016. Structural Geology, Cambridge University Press. 2nd Ed.