In geology, the crust is the outermost solid shell of a rockyplanet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase (solid crust vs. liquid mantle).
Most terrestrial planets 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.
Planetary geologists divide crust into three categories based on how and when it formed.[1]
Primary crust / primordial crust
This is a planet's "original" crust. It forms from solidification of a magma ocean. Toward the end of planetary accretion, the terrestrial planets likely had surfaces that were magma oceans. As these cooled, they solidified into crust.[2] This crust was likely destroyed by large impacts and re-formed many times as the Era of Heavy Bombardment drew to a close.[3]
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.[4] Earth's high rates of erosion and crustal recycling from plate tectonics has destroyed 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.[5] The anorthositehighlands of the Moon are primary crust, formed as plagioclase crystallized out of the Moon's initial magma ocean and floated to the top;[6] however, it is unlikely that Earth followed a similar pattern, as the Moon was a water-less system and Earth had water.[7] The Martian meteoriteALH84001 might represent primary crust of Mars; however, again, this is debated.[5] Like Earth, Venus lacks primary crust, as the entire planet has been repeatedly resurfaced and modified.[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 maria. On Earth, we see secondary crust forming primarily at mid-ocean spreading centers, where the 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[5]
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 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.[9] The lithosphere is broken into tectonic plates that move, allowing heat to escape from the interior of Earth into space.[10]
A theoretical protoplanet named "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." Plagioclasefeldspar crystallized in large amounts from this magma ocean and floated toward the surface. The cumulate rocks 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 pyroxenes and olivine, but even that lower part probably averages about 78% plagioclase.[11] The underlying mantle is denser and olivine-rich.
The thickness of the crust ranges between about 20 and 120 km. Crust on the far side of the Moon averages about 12 km thicker than that on the near side. Estimates of average thickness fall in the range from about 50 to 60 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 basalts 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.[12]
^ abHargitai, Henrik (2014). "Crust (Type)". Encyclopedia of Planetary Landforms. Springer New York. pp. 1–8. doi:10.1007/978-1-4614-9213-9_90-1. ISBN9781461492139.
^Chambers, John E. (2004). "Planetary accretion in the inner Solar System". Earth and Planetary Science Letters. 223 (3–4): 241–252. Bibcode:2004E&PSL.223..241C. doi:10.1016/j.epsl.2004.04.031.
^Taylor, Stuart Ross (1989). "Growth of planetary crusts". Tectonophysics. 161 (3–4): 147–156. Bibcode:1989Tectp.161..147T. doi:10.1016/0040-1951(89)90151-0.
^Van Kranendonk, Martin; Smithies, R. H.; Bennett, Vickie C. (2007). Earth's oldest rocks (1st ed.). Amsterdam: Elsevier. ISBN9780080552477. OCLC 228148014.
^Taylor, G. J. (2009-02-01). "Ancient Lunar Crust: Origin, Composition, and Implications". Elements. 5 (1): 17–22. doi:10.2113/gselements.5.1.17. ISSN 1811-5209.
^Albarède, Francis; Blichert-Toft, Janne (2007). "The split fate of the early Earth, Mars, Venus, and Moon". Comptes Rendus Geoscience. 339 (14–15): 917–927. Bibcode:2007CRGeo.339..917A. doi:10.1016/j.crte.2007.09.006.
^Venus II—geology, geophysics, atmosphere, and solar wind environment. Bougher, S. W. (Stephen Wesley), 1955–, Hunten, Donald M., Phillips, R. J. (Roger J.), 1940–. Tucson, Ariz.: University of Arizona Press. 1997. ISBN9780816518302. OCLC 37315367.{{cite book}}: CS1 maint: others (link)
^Robinson, Eugene C. (January 14, 2011). "The Interior of the Earth". U.S. Geological Survey. Retrieved August 30, 2013.
^Wieczorek, M. A. & Zuber, M. T. (2001), "The composition and origin of the lunar crust: Constraints from central peaks and crustal thickness modeling", Geophysical Research Letters, 28 (21): 4023–4026, Bibcode:2001GeoRL..28.4023W, doi:10.1029/2001GL012918, S2CID 28776724
^Herald Hiesinger and James W. Head III (2006). (PDF). Reviews in Mineralogy & Geochemistry. 60 (1): 1–81. Bibcode:2006RvMG...60....1H. doi:10.2138/rmg.2006.60.1. Archived from the original (PDF) on 2012-02-24.
Condie, Kent C. (1989). "Origin of the Earth's Crust". Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section). 75 (1–2): 57–81. Bibcode:1989PPP....75...57C. doi:10.1016/0031-0182(89)90184-3.
External links
The Wikibook Historical Geology has a page on the topic of: Structure of the Earth
crust, geology, other, crust, disambiguation, geology, crust, outermost, solid, shell, rocky, planet, dwarf, planet, natural, satellite, usually, distinguished, from, underlying, mantle, chemical, makeup, however, case, satellites, distinguished, based, phase,. For any other use see Crust disambiguation In geology the crust is the outermost solid shell of a rocky planet dwarf planet or natural satellite It is usually distinguished from the underlying mantle by its chemical makeup however in the case of icy satellites it may be distinguished based on its phase solid crust vs liquid mantle The internal structure of Earth The crusts of Earth Mercury Venus Mars Io the Moon and other planetary bodies formed via igneous processes and were later modified by erosion impact cratering volcanism and sedimentation Most terrestrial planets 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 Contents 1 Types of crust 1 1 Primary crust primordial crust 1 2 Secondary crust 1 3 Tertiary crust 2 Earth s crust 3 Moon s crust 4 See also 5 References 6 External linksTypes of crustSee also Abundance of elements in Earth s crust Planetary geologists divide crust into three categories based on how and when it formed 1 Primary crust primordial crust This is a planet s original crust It forms from solidification of a magma ocean Toward the end of planetary accretion the terrestrial planets likely had surfaces that were magma oceans As these cooled they solidified into crust 2 This crust was likely destroyed by large impacts and re formed many times as the Era of Heavy Bombardment drew to a close 3 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 4 Earth s high rates of erosion and crustal recycling from plate tectonics has destroyed 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 5 The anorthosite highlands of the Moon are primary crust formed as plagioclase crystallized out of the Moon s initial magma ocean and floated to the top 6 however it is unlikely that Earth followed a similar pattern as the Moon was a water less system and Earth had water 7 The Martian meteorite ALH84001 might represent primary crust of Mars however again this is debated 5 Like Earth Venus lacks primary crust as the entire planet has been repeatedly resurfaced and modified 8 Secondary crust Secondary crust is formed by partial melting of mostly silicate materials in the mantle and so is usually basaltic in composition 1 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 maria On Earth we see secondary crust forming primarily at mid ocean spreading centers where the 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 5 Erosion and sedimentation sediments derived from primary secondary or tertiary crustThe 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 crustMain article Earth s crust 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 9 The lithosphere is broken into tectonic plates that move allowing heat to escape from the interior of Earth into space 10 Moon s crustFurther information Geology of the Moon A theoretical protoplanet named 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 rocks 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 pyroxenes and olivine but even that lower part probably averages about 78 plagioclase 11 The underlying mantle is denser and olivine rich The thickness of the crust ranges between about 20 and 120 km Crust on the far side of the Moon averages about 12 km thicker than that on the near side Estimates of average thickness fall in the range from about 50 to 60 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 basalts 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 12 See also Geology portalEductionReferences a b Hargitai Henrik 2014 Crust Type Encyclopedia of Planetary Landforms Springer New York pp 1 8 doi 10 1007 978 1 4614 9213 9 90 1 ISBN 9781461492139 Chambers John E 2004 Planetary accretion in the inner Solar System Earth and Planetary Science Letters 223 3 4 241 252 Bibcode 2004E amp PSL 223 241C doi 10 1016 j epsl 2004 04 031 Taylor Stuart Ross 1989 Growth of planetary crusts Tectonophysics 161 3 4 147 156 Bibcode 1989Tectp 161 147T doi 10 1016 0040 1951 89 90151 0 Van Kranendonk Martin Smithies R H Bennett Vickie C 2007 Earth s oldest rocks 1st ed Amsterdam Elsevier ISBN 9780080552477 OCLC 228148014 a b c Taylor Stuart Ross McLennan Scott M 2009 Planetary crusts their composition origin and evolution Cambridge UK Cambridge University Press ISBN 978 0521841863 OCLC 666900567 Taylor G J 2009 02 01 Ancient Lunar Crust Origin Composition and Implications Elements 5 1 17 22 doi 10 2113 gselements 5 1 17 ISSN 1811 5209 Albarede Francis Blichert Toft Janne 2007 The split fate of the early Earth Mars Venus and Moon Comptes Rendus Geoscience 339 14 15 917 927 Bibcode 2007CRGeo 339 917A doi 10 1016 j crte 2007 09 006 Venus II geology geophysics atmosphere and solar wind environment Bougher S W Stephen Wesley 1955 Hunten Donald M Phillips R J Roger J 1940 Tucson Ariz University of Arizona Press 1997 ISBN 9780816518302 OCLC 37315367 a href Template Cite book html title Template Cite book cite book a CS1 maint others link Robinson Eugene C January 14 2011 The Interior of the Earth U S Geological Survey Retrieved August 30 2013 Earth s internal heat Wieczorek M A amp Zuber M T 2001 The composition and origin of the lunar crust Constraints from central peaks and crustal thickness modeling Geophysical Research Letters 28 21 4023 4026 Bibcode 2001GeoRL 28 4023W doi 10 1029 2001GL012918 S2CID 28776724 Herald Hiesinger and James W Head III 2006 New views of Lunar geoscience An introduction and overview PDF Reviews in Mineralogy amp Geochemistry 60 1 1 81 Bibcode 2006RvMG 60 1H doi 10 2138 rmg 2006 60 1 Archived from the original PDF on 2012 02 24 Condie Kent C 1989 Origin of the Earth s Crust Palaeogeography Palaeoclimatology Palaeoecology Global and Planetary Change Section 75 1 2 57 81 Bibcode 1989PPP 75 57C doi 10 1016 0031 0182 89 90184 3 External links The Wikibook Historical Geology has a page on the topic of Structure of the Earth USGS Crustal Thickness Map Geikie Archibald 1911 Geology Encyclopaedia Britannica Vol 11 11th ed pp 638 674 Crust of the Earth Encyclopedia Americana 1920 Retrieved from https en wikipedia org w index php title Crust geology amp oldid 1137350201, wikipedia, wiki, book, books, library,
