A diapir is region of lower density material (in this case salt) intruding into the overlying material.

Salt Diapirs are formed through the slow process of evaporation, sedimentation, and compaction.

1. A region experiences frequent flooding
2. Subsequent evaporation will deposit salt
3. The area is covered by later deposition where the overlying material will compact the deeper material. The density of the deeper sediment will increase while the density of salt is less affected by the pressure because of its crystal structure (remains less dense).
4. The salt is less dense than the overlying material so it will experience a strong upward buoyant force. The salt will then flow to intrude at shallower depths. This uplift will deform surrounding sediment.

Salt diapirs are particularly important to petroleum engineering because the deformation of the country rock can trap oil and natural gases. However, there are other important applications of diapirism. For example, diapirism produced through compositional and thermal differences within ice shell is believed to be related to the topographic domes and chaos terrain seen on Europa.

Credit: NASA/JPL/ASU