Pollard and Aydin also discuss the interesting cases of two parallel, but non-coplanar fractures propagating from opposite directions. The propagation of a joint is controlled by the stress field near its tip. Breccia can have high permeability, and gouge tends to be impermeable. Observations of joints in sedimentary rocks suggest that joints have lengths on the order of tens of meters. Similar lengths obtain for small faults, although fault zones can be much longer.
Joints tend to be hairline in thickness, whereas faults are thicker.
Fracture sets comprise approximately parallel fractures. Joint spacing in a fracture set decreases with increasing total strain, increasing number of loading cycles, and decreasing strain rate. For intersecting joint sets, one joint set can cross an older joint set ONLY if the older joints were under compression, or were filled by minerals. Adjacent fractures in a fracture set may link with a hook geometry. This interconnectivity, which can affect both aqueous and magmatic permeability, decreases with increasing differential stress.