Faulting occurs when Earth's crust ruptures as a consequence of accumulated stress; it is the most common cause of earthquakes. Along with folds, faults are the most obvious results of deformation within the crust and constitute the field of structural geology. While folds are the result of ductile (plastic) deformation, faults indicate brittle behavior and occur only in the relatively cold surficial layers of Earth, generally down to a depth of about 70 km (kilometers; 40 miles). Although faulting may cause vertical uplift, the stresses that lead to rupture are usually horizontal, related to movements along the boundaries that define the plates that make up Earth's crust. Three types of movement predominate—extension, compression, and lateral shear—and lead to three major types of fault, commonly referred ...