A buoyant mass of ductile rock or sediment that has pierced, or appears to have pierced, overlying rock, known as overburden. The overburden can yield by ductile processes or by brittle faulting (see illus.). Diapirs form by lateral and vertical intrusion of buoyant or nonbuoyant rock. Emplacement of a diapir, known as diapirism, involves piercement of overlying rocks.