The weird and chaotic world of salt domes in the Earth’s subsurface
How do salt domes form?
Salt domes require large deposits of salts and evaporites (including halite). In restricted sedimentary marine-basins, large influxes of salt water from the sea will cause the basin to increase in salinity. The restricted basin stops the salty water from flowing away and means all the water is lost to evaporation. An example of such a basin would be the Mediterranean Salinity Crisis where the Mediterranean basin was cut off from the Atlantic. Salty seawater would occasionally flow into the basin due to the rise and fall of sea-level and would get evaporated. This cyclic process creates large deposits of salt which in time will get covered by younger sediments. As more and more sediments are deposited on top of the salt layers, the overburden increases, compacting the sediments below.
Salt, unlike most other sediments, changes its physical properties when put under certain pressures. Salt at the surface is brittle, however, as it reaches deeper depths it starts to become more plastic. To develop a salt dome, the pressure on the salt must be high enough to push it upwards into the overlying compacted layers. One of the ways this can happen is by gravity driven forces. Once there is enough pressure from overlying rock deposits, it will cause the salt, which is less dense than the surrounding rocks, to become gravitationally unstable. This instability will cause the salt to rise and exploit any areas of weakness in the overburden. Examples of such weaknesses are fractures, faults and anticlinal folds.
Tectonics play a crucial role in the formation of salt domes. Divergent plate boundaries stretch the Earth’s crust and cause thinning. Extensional basins around these areas will destabilize rock columns allowing salt to intrude upwards. Furthermore, in extensional regimes, large tectonic faults and fractures will act as a pathway for salt movement. In areas like the Dutch North Sea, salt domes pierce the Cenozoic stratigraphy along fault planes. This area has been exposed to many tectonic events which include episodes of basin inversion (doming of the basin), regional thermal subsidence (sagging of basins) and basin extension. The salt domes in the North Sea are Permian in age, they have risen 3-4km to reach the Cenozoic stratigraphy and are a prime example of how salt domes occur.If salt domes rise to the surface, they create a salt glacier. These are rare and occur in a few places around the globe. The Zagros Mountains in Iran are home to some of the largest salt glaciers on the planet. The image above shows one of the salt glaciers on the Zagross Mountains flowing down to valleys either side of the ridge. The lack of water stops the salt from dissolving causing the sat to building up and flow under the influence of gravity. The process of moving salt, like all geological processes, takes a long time to occur.