Large shield volcanoes grow not only by adding material to their flanks as erupted lava, but also by spreading laterally at their bases. As a volcano grows in size, the stress field underneath the volcano changes from compressional to extensional. A subterranean rift may develop at the base of the volcano, causing the underlying crust to spread apart. If the volcano rests on sediments containing mechanically weak layers (e.g., beds of water-saturated clay), detachment zones (decollements) may develop in the weak layers. The extensional stresses in the detachment zones can produce giant landslides and normal faults on the volcano's flanks, leading to the formation of a basal escarpment. Further from the volcano, these detachment zones can express themselves as a succession of overlapping, gravity driven thrust faults. This mechanism has long been cited as an explanation of the Olympus Mons aureole deposits (discussed below).