Editing Planetary differentiation (section)

==Physical differentiation==

===Gravitational separation===
High-[[density]] materials tend to sink through lighter materials. This tendency is affected by the relative structural strengths, but such strength is reduced at temperatures where both materials are plastic or molten. [[Iron]], the most common element that is likely to form a very dense molten metal phase, tends to congregate towards planetary interiors. With it, many [[Siderophile element|siderophile]] elements (i.e. materials that readily [[metal alloy|alloy]] with iron) also travel downward. However, not all heavy elements make this transition as some [[chalcophile|chalcophilic]] heavy elements bind into low-density silicate and oxide compounds, which differentiate in the opposite direction.

The main compositionally differentiated zones in the solid Earth are the very dense iron-rich metallic [[Planetary core|core]], the less dense [[talc|magnesium-silicate]]-rich [[Mantle (geology)|mantle]] and the relatively thin, light [[Crust (geology)|crust]] composed mainly of silicates of [[aluminium]], [[sodium]], [[calcium]] and [[potassium]]. Even lighter still are the watery liquid [[hydrosphere]] and the gaseous, nitrogen-rich [[atmosphere]].

Lighter materials tend to rise through material with a higher density. A light mineral such as [[plagioclase]] would rise. They may take on dome-shaped forms called [[diapir]]s when doing so. On Earth, [[salt dome]]s are [[salt]] diapirs in the crust which rise through surrounding rock. Diapirs of molten low-density silicate rocks such as [[granite]] are abundant in the Earth's upper crust. The hydrated, low-density [[serpentinite]] formed by alteration of [[Mantle (geology)|mantle]] material at [[subduction zone]]s can also rise to the surface as diapirs. Other materials do likewise: a low-temperature, near-surface example is provided by [[mud volcano|mud volcanoe]]s.