A
diapir (;
French, from
Greek diapeirein, to pierce through) is a type of geologic
intrusion in which a more mobile and ductily deformable material is 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. The term was introduced by the
Romanian geologist Ludovic Mrazek, who was the first to understand the principle of salt intrusion and
plasticity. The term "diapir" may be applied to
igneous structures, but it is more commonly applied to non-igneous, relatively cold materials, such as
salt domes and
mud diapirs.