Editing Crust (geology)

{{Short description|Outermost solid shell of astronomical bodies}}
{{For|any other use|Crust (disambiguation)}}
{{pp-semi-indef}}
[[File:Earth Internal Structure.svg|thumb|300x300px|The internal structure of Earth]]
In [[geology]], the '''crust''' is the outermost solid shell of a [[planet]], [[dwarf planet]], or [[natural satellite]]. It is usually distinguished from the underlying [[mantle (geology)|mantle]] by its chemical makeup; however, in the case of icy satellites, it may be defined based on its phase (solid crust vs. liquid mantle).

The crusts of [[Earth]], [[Mercury (planet)|Mercury]], [[Venus]], [[Mars]], [[Io (moon)|Io]], the [[Moon]] and other [[planetary body|planetary bodies]] formed via [[igneous rock|igneous]] processes and were later modified by [[erosion]], [[impact crater|impact cratering]], volcanism, and sedimentation.

Most [[terrestrial planet]]s have fairly uniform crusts. Earth, however, has two distinct types: [[continental crust]] and [[oceanic crust]]. These two types have different chemical compositions and physical properties and were formed by different geological processes.

== 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.

==Earth's crust==
{{Main|Earth's crust}}

[[Image:Plates tect2 en.svg|thumb|350px|Plates in the crust of Earth]]

[[Earth's crust]] is a thin shell on the outside of Earth, accounting for less than 1% of Earth's volume. It is the top component of the [[lithosphere]], a division of Earth's layers that includes the crust and the upper part of the [[mantle (geology)|mantle]].<ref>{{cite web|url=http://pubs.usgs.gov/gip/interior/|title=The Interior of the Earth|last1=Robinson|first1=Eugene C.|date=January 14, 2011|publisher=[[U.S. Geological Survey]]|access-date=August 30, 2013}}</ref> The lithosphere is broken into tectonic plates that move, allowing heat to escape from the interior of Earth into space.<ref>{{Cite web|url=https://ugc.berkeley.edu/background-content/earths-internal-heat/#heading1|title = Earth's internal heat}}</ref>

== Moon's crust ==
{{further|Geology of the Moon}}
A theoretical [[protoplanet]] named "[[Theia (planet)|Theia]]" is thought to have collided with the forming Earth, and part of the material ejected into space by the collision accreted to form the Moon. As the Moon formed, the outer part of it is thought to have been molten, a "[[lunar magma ocean]]". [[Plagioclase]] [[feldspar]] crystallized in large amounts from this [[magma]] ocean and floated toward the surface. The [[cumulate rock]]s form much of the crust. The upper part of the crust probably averages about 88% plagioclase (near the lower limit of 90% defined for [[anorthosite]]): the lower part of the crust may contain a higher percentage of ferromagnesian minerals such as the [[pyroxene]]s and [[olivine]], but even that lower part probably averages about 78% plagioclase.<ref>{{Citation |last1=Wieczorek |first1=M. A. |name-list-style=amp |last2=Zuber |first2=M. T. |s2cid=28776724 |year=2001 |title=The composition and origin of the lunar crust: Constraints from central peaks and crustal thickness modeling |journal=Geophysical Research Letters |volume=28 |issue=21 |pages=4023–4026 |doi=10.1029/2001GL012918 |bibcode=2001GeoRL..28.4023W|doi-access=free }}</ref> The underlying mantle is denser and olivine-rich.

The thickness of the crust ranges between about 20 and 120&nbsp;km. Crust on the [[far side of the Moon]] averages about 12&nbsp;km thicker than that on the [[Near side of the Moon|near side]]. Estimates of average thickness fall in the range from about 50 to 60&nbsp;km. Most of this plagioclase-rich crust formed shortly after formation of the Moon, between about 4.5 and 4.3 billion years ago. Perhaps 10% or less of the crust consists of igneous rock added after the formation of the initial plagioclase-rich material. The best-characterized and most voluminous of these later additions are the mare [[basalt]]s formed between about 3.9 and 3.2 billion years ago. Minor volcanism continued after 3.2 billion years, perhaps as recently as 1 billion years ago. There is no evidence of [[plate tectonics]].

Study of the Moon has established that a crust can form on a rocky planetary body significantly smaller than Earth. Although the radius of the Moon is only about a quarter that of Earth, the lunar crust has a significantly greater average thickness. This thick crust formed almost immediately after formation of the Moon. Magmatism continued after the period of intense meteorite impacts ended about 3.9 billion years ago, but igneous rocks younger than 3.9 billion years make up only a minor part of the crust.<ref>{{cite journal|doi=10.2138/rmg.2006.60.1|author=Herald Hiesinger and James W. Head III|year=2006|url=http://www.planetary.brown.edu/pdfs/2961_proof.pdf|title=New views of Lunar geoscience: An introduction and overview|journal=Reviews in Mineralogy & Geochemistry|volume=60|issue=1|pages=1–81|url-status=dead|archive-url=https://web.archive.org/web/20120224232636/http://www.planetary.brown.edu/pdfs/2961_proof.pdf|archive-date=2012-02-24|bibcode=2006RvMG...60....1H}}</ref>

==See also==
{{Portal|Geology}}
* [[Eduction (geology)|Eduction]]

==References==
{{reflist|35em}}

* {{cite journal | last1 = Condie | first1 = Kent C. | year = 1989 | title = Origin of the Earth's Crust | journal = Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section) | volume = 75 | issue = 1–2| pages = 57–81 | doi = 10.1016/0031-0182(89)90184-3 | bibcode = 1989PPP....75...57C }}

==External links==
{{Wikibooks |Historical Geology|Structure of the Earth}}
*[https://earthquake.usgs.gov/data/crust/crust.php USGS Crustal Thickness Map]
*{{Cite EB1911|wstitle= Geology |volume= 11 |last= Geikie |first= Archibald |author-link= Archibald Geikie | pages = 638&ndash;674 |short= 1}}
*{{Cite Americana|wstitle=Crust of the Earth |short=x}}


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[[Category:Plate tectonics]]
[[Category:Structure of the Earth]]

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