» Distinguish from diaper. A diapir (French, from Greek diapeirein, to pierce through)'s a type of intrusion in which a more mobile and ductily-deformable material's forced into brittle overlying rocks. Depending on the tectonic environment, diapirs can range from idealized mushroom-shaped Rayleigh-Taylor instability-type structures in regions with low tectonic stress such as in the Gulf of Mexico to narrow dikes of material that move along tectonically-induced fractures in surrounding rock. Th… (More on Diapir)