page_title: Salt deformation
section_title: Gravitational differential loading
hierarchical_section_title: Salt deformation / Salt dynamics / Underground salt structure / Gravitational differential loading
caption_attribution_description: English: Hydraulic head 3
context_page_description: Salt deformation is the change of shape of natural salt bodies in response to forces and mechanisms that controls salt flow. Such deformation can generate large salt structures such as underground salt layers, salt diapirs or salt sheets at the surface. Strictly speaking, salt structures are formed by rock salt that is composed of pure halite crystal. However, most halite in nature appears in impure form, therefore rock salt usually refers to all rocks that composed mainly of halite, sometimes also as a mixture with other evaporites such as gypsum and anhydrite. Earth's salt deformation generally involves such mixed materials.
  Due to the unique physical and chemical properties of rock salt such as its low density, high thermal conductivity and high solubility in water, it deforms distinctively in underground and surface environments compared with other rocks. Instability of rock salt is also given by its low viscosity, which allows rock salt to flow as a fluid. As the rock salt flows, a variety of salt structures are formed. Therefore, basins containing salt deform more easily than those lacking salt.
context_section_description: Gravitational differential loading is produced by a combination of gravitational forces acting on the overburden rocks and the underlying salt layer. The effect of gravitational loading on salt flow can simply be expressed by the concept of hydraulic head:
  Where h is the hydraulic head, z is the elevation head that counts from a datum to the top of the salt layer, P is pressure exerted on the salt layer by overburden, is the density of the salt and g is the gravity acceleration. Pressure head are expressed as P over . As P is also equal to  , where is the density of the overburden and t is its thickness.
  Therefore, the equation can be rewritten as:
  Note that the pressure head P is then expressed as .
  Assuming the ratio of density of the overburden rock to that of the salt layer remains unchanged in the following three cases:
caption: Constant elevation head with a varying thickness of the overburden. Modified from Hudec & Jackson, 2007.[3]
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