An isopycnic surface is a surface of constant density inside a

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...

. Isopycnic surfaces contrast with

isobaric Isobar may refer to: * Isobar (meteorology), a line connecting points of equal atmospheric pressure reduced to sea level on the maps. * Isobaric process, a process taking place at constant pressure * Isobar (nuclide) Isobars are atoms (nucl ...

isothermal In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and ...

surfaces, which describe surfaces of constant pressure and constant temperature respectively. Isopycnic surfaces are sometimes referred to as "iso-density" surfaces, although this is strictly incorrect. Isopycnic typically describes surfaces, not processes. Unless there is a flux of mass into or out of a control volume, a process which occurs at a constant density also occurs at a constant volume and is called an isochoric process and not an isopycnic process. The term "isopycnic" is commonly encountered in the fluid dynamics of compressible fluids, such as in meteorology and geophysical fluid dynamics, astrophysics, or the fluid dynamics of explosions or high Mach number flows. It may also be applied to other situations where a continuous medium has smoothly varying density, such as in the case of an inhomogeneous colloidal suspension. In general isopycnic surfaces will occur in fluids in hydrostatic equilibrium coinciding with equipotential surfaces formed by gravity. The term "isopycnic" is also encountered in biophysical chemistry, usually in reference to a process of separating particles, subcellular

organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...

s, or other substances on the basis of their density. Isopycnic centrifugation refers to a method wherein a density gradient is either pre-formed or forms during high speed centrifugation. After this gradient is formed particles move within the gradient to the position having a density matching their own (this is in fact an incorrect description of the exact physical process but does describe the result in a meaningful way). This technique is extremely powerful. In

geology Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...

, isopycnic surfaces occur especially in connection with

craton A craton (, , or ; from grc-gre, κράτος "strength") is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and ...

s which are very old geologic formations at the core of the

continent A continent is any of several large landmasses. Generally identified by convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continents. Ordered from largest in area to smallest, these seven ...

s, little affected by

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

events. These formations are often known as

shields A shield is a piece of personal armour held in the hand, which may or may not be strapped to the wrist or forearm. Shields are used to intercept specific attacks, whether from close-ranged weaponry or projectiles such as arrows, by means of a ...

or platforms. These formations are, relative to other

lithospheric A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years o ...

formations, cooler and less dense but much more isopycnic.

References

See also