Diapir
A diapyrus (from the Greek διαπείρειν ‘through’) is a type of intrusion in which a more ductile, deformable and mobile material is forced through the brittle overlying rocks. The shape of the diapir depends on the tectonic environment: in regions of low tectonic stress, diapirs have mushroom-shaped structures of the Rayleigh-Taylor instability type, while in more tectonically active areas they are narrow dykes of material that move along along the fractures induced in the surrounding rock. The term was introduced by the Romanian geologist Ludovic Mrazek, who was the first to understand the principle of saline intrusion and plasticity. The term «diapyr» can be applied to igneous structures, but it is more commonly applied to non-igneous structures, relatively cold materials, such as the evaporites that form the so-called saline diapirs. Clay diapirs also occur.
Saline diapir
They are intrusive geological structures, formed by masses of evaporites (salts, anhydrite and gypsum) that, coming from very plastic stratigraphic levels (especially from the Keuper) under great pressure, ascend through the sedimentary layers of the earth's crust, crossing them and deforming them, in a slow process measurable in millions of years known as diapirism. They acquire the shape of a cylinder, mushroom or drop and are usually large (hundreds of meters to 3 km in diameter in horizontal section). A very didactic simile of diapirism can be seen in the so-called lava lamps used in decoration. The movements by plastic flow of saline rocks are called halokinetic.
Its genesis is quite complex and is caused by several factors: high plasticity of the salts increased by gypsum; they develop in deep faults through which the materials rise and where volcanic processes occur that convert the diapir into a paste with even more plasticity and much more perforating power.
Saline domes are called when the masses of salts deform the overlying sediments but do not manage to intrude them.
In Spain there are populations such as Cabezón de la Sal in Cantabria that settle on outcropping saline diapirs and that were commercially exploited in the past. Others, such as Peralta and Caparroso in Navarra, had to change their location to others with more stable soils due to the subsidence processes that produce this type of halokinetic structures.
Some diapirs continue their mining, such as Cabezo de Sal del Pinoso in Alicante, from which 700,000 tons of salt are extracted per year by dissolving the salt with water and subsequently sending the brine to the Torrevieja salt mine where the process ends. Others, after the mining exploitation has been abandoned, have a tourist attraction, such as the salt mountain of Cardona.
The most interesting diapir in Europe is in Poza de la Sal (Burgos). It is a circular valley 2.5 km in diameter that visually allows us to understand what diapirism is; selected as a point of geological interest by the Burgos Geoscientific Association.
Geological hazards
The geological risks associated with diapirism include those related to upward movement and the dissolution processes of salts and gypsum.
Dissolution processes can be fast. Prevention is vital, which involves knowing the geological structure of the area and preparing risk maps, as well as associating them with water. Land use planning is also necessary based on risk. To correct this risk, it is possible to install drainage so that the water comes out and the diapir salt does not dissolve, or fill in the ground to avoid collapse.
Oil Traps
Saline diapirs have enormous economic value, as they can act as oil traps. The ascent of the diapir deforms the informed layers, raising them around them, forming an anticlinal structure, a kind of cone in which the diapir occupies the central axis (the hinge). When there are impermeable layers, hydrocarbons (plus gases and water) accumulate below them, becoming trapped in the porous rocks below. The diapir acts by sealing the center of the structure.
In the sedimentary substrate of the Gulf of Mexico, oil deposits associated with this type of trap are very frequent. In the Zagros Mountains this morphology is also common.
Non-saline diapirs
The term diapir, or diapirism, can also be applied in the case of the intrusion of granitic masses (batholith type) in the earth's crust or also in the case of diagenetic structures similar to salt pans, but of much smaller dimensions, produced by the plastic displacement of mud and mud.