Forceful, upward intrusion of a rock mass into overlying rock. In the case of an igneous diapir, the intruding rock may be magma or a crystal-rich mush, either of which is less dense than the surrounding rock.
A diapir ( http://www.bartleby.com/61/93/D0199300.html) (French, from Greek diapeirein, to pierce through) is an intrusion caused by buoyancy and pressure differentials. Diapirs may include igneous structures, but are more commonly applied to non-igneous, relatively cold materials, such as salt domes and mud diapirs; diapirism is thought to occur on Enceladus. A diapir is any relatively mobile mass that intrudes into preexisting strata.