Isopycnic means "of the same density." In particular, an isopycnic surface is a surface of constant density. This term is a bit more obscure than the similar terms isobaric
Isobaric may refer to:*in thermodynamics, an isobaric process, i.e. one that is carried out at constant pressure;...

 or isothermal surfaces, which describe surfaces of constant pressure and constant temperature respectively. It is common in conversational use to hear isopycnic surfaces referred to simply as "iso-density" surfaces, which while strictly incorrect, is nonetheless abundantly more clear.

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.

Isopycnic surfaces occur in geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

, especially in connection with cratons
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...

 which are very old geologic formations at the core of the continents
A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

, little affected by tectonic events. These formations are often known as shields
Shield (geology)
A shield is generally a large area of exposed Precambrian crystalline igneous and high-grade metamorphic rocks that form tectonically stable areas. In all cases, the age of these rocks is greater than 570 million years and sometimes dates back 2 to 3.5 billion years...

 or platforms
Platform (geology)
In geology, a platform is a continental area covered by relatively flat or gently tilted, mainly sedimentary strata, which overlie a basement of consolidated igneous or metamorphic rocks of an earlier deformation...

. These formations are, relative to other lithospheric
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 formations, cooler and less dense but much more isopycnic.

Compare also with isochoric
Isochoric may refer to:*cell-transitive, in geometry*isochoric process, in chemistry or thermodynamics...

. Note that one may discuss isobaric surfaces or isobaric processes; likewise one may discuss isothermal surfaces or isothermal processes. However, the use of "isopycnic" is typically reserved for surfaces and not processes. In particular, 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 also encountered in biophysical chemistry and usually in reference to a process of separating particles, sub cellular organelles, 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.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.