Editing Crust (geology) (section)

== Types of crust ==
{{See also|Abundance of elements in Earth's crust}}
Planetary geologists divide crust into three categories based on how and when it formed.<ref name="Hargitai 2014 1–8">{{Cite book|title=Encyclopedia of Planetary Landforms|last=Hargitai|first=Henrik|date=2014|publisher=Springer New York|isbn=9781461492139|pages=1–8|language=en|doi=10.1007/978-1-4614-9213-9_90-1|chapter = Crust (Type)}}</ref>

=== Primary crust / primordial crust ===
This is a planet's "original" crust. It forms from solidification of a magma ocean. Toward the end of [[Formation and evolution of the Solar System|planetary accretion]], the terrestrial planets likely had surfaces that were magma oceans. As these cooled, they solidified into crust.<ref>{{Cite journal|last=Chambers |first=John E. |title=Planetary accretion in the inner Solar System |journal=[[Earth and Planetary Science Letters]] |volume=223 |issue=3–4|pages=241–252 |doi=10.1016/j.epsl.2004.04.031 |bibcode=2004E&PSL.223..241C |year=2004}}</ref> This crust was likely destroyed by large impacts and re-formed many times as the [[Formation and evolution of the Solar System|Era of Heavy Bombardment]] drew to a close.<ref>{{Cite journal |last=Taylor |first=Stuart Ross |title=Growth of planetary crusts |journal=[[Tectonophysics (journal)|Tectonophysics]] |volume=161 |issue=3–4 |pages=147–156 |doi=10.1016/0040-1951(89)90151-0 |bibcode=1989Tectp.161..147T |year=1989}}</ref> 

The nature of primary crust is still debated: its chemical, mineralogic, and physical properties are unknown, as are the igneous mechanisms that formed them. This is because it is difficult to study: none of Earth's primary crust has survived to today.<ref>{{Cite book|title=Earth's oldest rocks |date=2007 |publisher=[[Elsevier]] |last1=Van Kranendonk |first1=Martin |last2=Smithies |first2=R. H. |last3=Bennett |first3=Vickie C. |isbn=9780080552477 |edition=1st |location=Amsterdam |oclc=228148014}}</ref> Earth's high rates of erosion and crustal recycling from plate tectonics has destroyed [[Oldest dated rocks|all rocks older than about 4 billion years]], including whatever primary crust Earth once had. 

However, geologists can glean information about primary crust by studying it on other terrestrial planets. Mercury's highlands might represent primary crust, though this is debated.<ref name="Taylor-2009">{{Cite book|title=Planetary crusts : their composition, origin and evolution |last1=Taylor |first1=Stuart Ross |date=2009 |publisher=[[Cambridge University Press]] |last2=McLennan |first2=Scott M. |isbn=978-0521841863 |location=Cambridge, UK |oclc=666900567}}</ref> The [[anorthosite]] [[Geology of the Moon|highlands]] of the Moon are primary crust, formed as [[plagioclase]] crystallized out of the Moon's initial magma ocean and floated to the top;<ref>{{Cite journal|last=Taylor |first=G. J. |date=2009-02-01 |title=Ancient Lunar Crust: Origin, Composition, and Implications |journal=[[Elements (journal)|Elements]] |language=en |volume=5 |issue=1 |pages=17–22 |doi=10.2113/gselements.5.1.17 |bibcode=2009Eleme...5...17T |s2cid=17684919 |issn=1811-5209}}</ref> however, it is unlikely that Earth followed a similar pattern, as the Moon was a water-less system and Earth had water.<ref>{{Cite journal|last1=Albarède |first1=Francis |last2=Blichert-Toft |first2=Janne |author2-link=Janne Blichert-Toft |title=The split fate of the early Earth, Mars, Venus, and Moon |journal=Comptes Rendus Geoscience |volume=339 |issue=14–15 |pages=917–927 |doi=10.1016/j.crte.2007.09.006 |bibcode=2007CRGeo.339..917A |year=2007|url=https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/j.crte.2007.09.006/ }}</ref> The [[Martian meteorite]] [[Allan Hills 84001|ALH84001]] might represent primary crust of Mars; however, again, this is debated.<ref name="Taylor-2009" /> Like Earth, Venus lacks primary crust, as the entire planet has been repeatedly resurfaced and modified.<ref>{{Cite book|title=Venus II—geology, geophysics, atmosphere, and solar wind environment |date=1997 |publisher=[[University of Arizona Press]] |others=Bougher, S. W. (Stephen Wesley), 1955–, Hunten, Donald M., Phillips, R. J. (Roger J.), 1940– |isbn=9780816518302 |location=Tucson, Ariz. |oclc=37315367}}</ref>

=== Secondary crust ===
Secondary crust is formed by [[partial melting]] of mostly [[Silicate mineral|silicate]] materials in the mantle, and so is usually [[basaltic]] in composition.<ref name="Hargitai 2014 1–8"/>

This is the most common type of crust in the Solar System. Most of the surfaces of Mercury, Venus, Earth, and Mars comprise secondary crust, as do the [[Lunar mare|lunar maria]]. On Earth secondary crust forms primarily at [[Mid-ocean ridge|mid-ocean spreading centers]], where the [[Adiabatic process|adiabatic]] rise of mantle causes partial melting.

=== Tertiary crust ===
Tertiary crust is more chemically-modified than either primary or secondary. It can form in several ways:
* Igneous processes: partial-melting of secondary crust, coupled with differentiation or dehydration<ref name="Taylor-2009" />
* Erosion and sedimentation: sediments derived from primary, secondary, or tertiary crust
The only known example of tertiary crust is the continental crust of the Earth. It is unknown whether other terrestrial planets can be said to have tertiary crust, though the evidence so far suggests that they do not. This is likely because plate tectonics is needed to create tertiary crust, and Earth is the only planet in the Solar System with plate tectonics.