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(krŭst)

1. 1. The hard outer portion or surface area of bread.
   2. A piece of bread consisting mostly of the hard outer portion.
   3. A piece of bread that has become hard and dry.
2. A pastry shell, as of a pie or tart.
3. A hard crisp covering or surface: snow with a firm crust.
4. A hard deposit formed on the interior of a wine bottle as the wine matures.
5. Geology.
   1. The exterior portion of the earth that lies above the Mohorovičić discontinuity.
   2. The outermost solid layer of a planet or moon.
6. The hard outer covering or integument of certain plants and animals, such as lichens and crustaceans.
7. Pathology. An outer layer or coating formed by the drying of a bodily exudate such as pus or blood; a scab.
8. Informal. Insolence; audacity; gall.

1. To cover with a crust.
2. To form into a crust.

1. To become covered with a crust.
2. To harden into a crust.

[Middle English cruste, from Old French crouste, from Latin crūsta.]

For more information on crust, visit Britannica.com.

The low-density outermost layer of the Earth above the Mohorovičić discontinuity (the Moho), a global boundary that is defined as the depth in the Earth where the compressional-wave seismic velocity increases rapidly or discontinuously to a value in excess of 4.7 mi/s (7.6 km/s; the upper mantle). The crust is also the cold, upper portion of the Earth's lithosphere, which in terms of plate tectonics is the mobile, outer layer that is underlain by the hot, convecting asthenosphere. See also Asthenosphere; Lithosphere; Moho (Mohorovičić discontinuity); Plate tectonics.

Continental crust

The Earth's continental crust has evolved over the past 4 billion years, and is highly variable in geologic composition and internal structure. The worldwide mean thickness of continental crust is 24 mi (40 km), with a standard deviation of 5.4 mi (9 km). The thinnest continental crust (found in the Afar Triangle, northeast Africa) is about 9 mi (15 km) thick, and the thickest crust (the Himalayan Mountains in China) is about 47 mi (75 km) thick. Ninety-five percent of all continental crust has a thickness within two standard deviations of the mean thickness, between 13 mi (22 km) and 37 mi (58 km). The Antarctic continent has a crustal thickness of 24 mi (40 km) in the ancient, stable (cratonic) region of East Antarctica, and about 12 mi (20 km) in the recently stretched (extended) crust of West Antarctica. Continental margins, which mark the transition from oceanic to continental crust, range in thickness from about 9 mi (15 km) to 18 mi (30 km). See also Continental margin.

Despite its geologic complexity, the continental crust may generally be divided into four layers: an uppermost sedimentary layer, and an upper, middle, and lower crust composed of crystalline rocks. The sedimentary cover of the continental crust is an important source of natural resources. This cover averages 0.6 mi (1 km) in thickness, and varies in thickness from zero (for example, on shields) to more than 9 mi (15 km) in deep basins. In stable continental crust of average thickness (25 mi or 40 km), the crystalline upper crust is commonly 6–9 mi (10–15 km) thick and has an average composition equivalent to a granite. The middle crust is 3–9 mi (5–15 km) thick and has a composition equivalent to a diorite; and the lower crust is 3–12 mi (5–20 km) thick and has a composition equivalent to a gabbro. Due to increasing temperature and pressure with depth, the metamorphic grade of rocks increases with depth, and the rocks within the deep continental crust generally are metamorphic rocks, even if they originated as sedimentary or igneous rocks. See also Diorite; Gabbro; Granite; Metamorphic rocks.

Crustal properties vary systematically with geologic setting, which may be divided into six groups: orogens (mountain belts), shields and platforms, island arcs (volcanic arcs), continental magmatic arcs, rifts, extended (stretched) crust, and forearcs. Orogens are typified by thick crust [average thickness is 29 mi (46 km), but the maximum thickness is as much as 47 mi (75 km) in the Himalayas]. Shields and platforms, such as the Canadian Shield and the Russian Platform, commonly have an approximately 26-mi-thick (42-km) crust, including a 3–6 mi-thick (5–10 km) lower crust. In comparison with shields, island arcs (such as Japan) have thinner crusts and significantly shallower middle and lower crustal layers due to the intrusion of mafic (that is, low silica content) plutons. Continental magmatic arcs, such as the Cascades volcanoes of the northwestern United States, intrude preexisting continental crust, and therefore they are generally 3–9 mi (10–15 km) thicker than island arcs. Continental rifts, such as the East African and Rio Grande rifts, have an average crustal thickness of about 22 mi (36 km). Extended continental crust, such as the Basin and Range Province of the western United States, averages 18 mi (30 km) in thickness. Forearcs are regions that were formed oceanward of volcanic arcs, such as much of the west coast of North America. They typically have thin crust, about 15 mi (25 km), and have a thick (9 mi or 15 km) upper crustal section that consists of relatively low-density metasedimentary rocks. See also North America; Oceanic islands; Pluton; Rift valley; Sedimentary rocks; Volcano.

At least three processes provide new continental crust. The first is the accretion and consolidation of island arcs, such as Japan or the Aleutian Islands, onto a continental margin. The second process is the tectonic underplating of oceanic crust at active subduction zones. In this process, the continental crust grows from below as oceanic crust is welded to the base of the continental margin, either when subduction stops or when subduction steps oceanward and a new trench is formed. This process has been identified in western Canada and southern Alaska. The third process is the magmatic inflation of the crust at continental arcs, rifts, and regions of crustal extension. This process has been identified in many regions. See also Geodynamics.

Oceanic crust

The surface of the ocean crust, except for some locally high volcanoes and plateaus, resides some 1–3 mi (2–5 km) below sea level, and about another kilometer below the average level of the continents. The ocean crust represents the youngest and geologically most dynamic portion of the Earth's surface. Most of it was produced at mid-ocean ridges during the process of sea-floor spreading. The ridges define the trailing edges, or accreting boundaries, of the major lithospheric plates that are moving about the surface of the Earth at present. Thus, the oldest rocks of the ocean crust date back no earlier than the rifting episodes that created most of these plates and initiated the most recent phase of continental drift, the Pangaean breakup, in Late Jurassic times. See also Jurassic; Mid-Oceanic Ridge.

There are fault slices of types of ocean crust on land, known as ophiolites, where nearly or entirely complete cross sections through the crust can be mapped and sampled. These strongly indicate that the ocean crust consists in downward sequence of submarine extrusives (usually pillow basalts), feeder dikes (often vertically sheeted), or sills, gabbros, and peridotites. There is much uncertainty, however, about the extent to which typical ophiolites, most of which formed in island-arc or backarc environments, can represent abyssal ocean crust, which is produced at the major accreting plate boundaries. Moreover, the physical correspondence of the rocks in ophiolites to ocean crust is often complicated by their complex structure and extent of alteration and metamorphism, particularly in the ultramafic sections.

This multipurpose word has many meanings, including the hardened outer layer of a cooked food such as bread; a thin layer of pastry covering a pie, pâté, etc.; and the sediment of organic salts deposited in a bottle of aged red wine.

noun

The state or quality of being impudent or arrogantly self-confident: assumption, audaciousness, audacity, boldness, brashness, brazenness, cheek, cheekiness, chutzpah, discourtesy, disrespect, effrontery, face, familiarity, forwardness, gall, impertinence, impudence, impudency, incivility, insolence, nerve, nerviness, overconfidence, pertness, presumptuousness, pushiness, rudeness, sassiness, sauciness. Informal brass, sauce, uppishness, uppityness. See attitude/good attitude/bad attitude/neutral attitude, courtesy/discourtesy.

Another name for the sediment thrown off by red wines as they age in bottles. The use of the term crust is generally associated with vintage port (see port).

The outermost layer of a planet or moon, above the mantle.

In geology, the outermost layer of the Earth. It overlies the mantle.

• Materials, Formations, and Substances - crust: thin outer layer of Earth’s surface, composed of sial and sima layers

Geologic provinces of the world (USGS)

Shield   Platform   Orogen   Basin   Large igneous province   Extended crust

Oceanic crust:   0–20 Ma   20–65 Ma   >65 Ma

In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle. The crusts of Earth, our Moon, Mercury, Venus, Mars, Io, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in incompatible elements than their respective mantles.

Contents 1 Earth's crust and mantle 1.1 Composition 2 Moon's crust 3 See also 4 References 5 External links

Earth's crust and mantle

Earth cutaway from core to exosphere

The crust of the Earth is composed of a great variety of igneous, metamorphic, and sedimentary rocks. The crust is underlain by the mantle. The upper part of the mantle is composed mostly of peridotite, a rock denser than rocks common in the overlying crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity. The crust occupies less than 1% of Earth's volume.

The oceanic crust of the sheet is different from its continental crust. The oceanic crust is 5 km (3 mi) to 10 km (6 mi) thick^[1] and is composed primarily of basalt, diabase, and gabbro. The continental crust is typically from 30 km (20 mi) to 50 km (30 mi) thick and is mostly composed of slightly less dense rocks than those of the oceanic crust. Some of these less dense rocks, such as granite, are common in the continental crust but rare to absent in the oceanic crust. Both the continental and oceanic crust "float" on the mantle. Because the continental crust is thicker, it extends both above and below the oceanic crust. The slightly lighter density of felsic continental rock compared to basaltic ocean rock contributes to the higher relative elevation of the top of the continental crust. Because the top of the continental crust is above that of the oceanic, water runs off the continents and collects above the oceanic crust. The continental crust and the oceanic crust are sometimes called sial and sima respectively. Because of the change in velocity of seismic waves it is believed that on continents at a certain depth sial becomes close in its physical properties to sima, and the dividing line is called the Conrad discontinuity.

The temperature of the crust increases with depth, reaching values typically in the range from about 200 °C (392 °F) to 400 °C (752 °F) at the boundary with the underlying mantle. The crust and underlying relatively rigid uppermost mantle make up the lithosphere. Because of convection in the underlying plastic (although non-molten) upper mantle and asthenosphere, the lithosphere is broken into tectonic plates that move. The temperature increases by as much as 30 °C (about 50 °F) for every kilometer locally in the upper part of the crust, but the geothermal gradient is smaller in deeper crust.^[2]

Plates in the crust of the earth, according to the plate tectonics theory

Partly by analogy to what is known about our Moon, Earth is considered to have differentiated from an aggregate of planetesimals into its core, mantle and crust within about 100 million years of the formation of the planet, 4.6 billion years ago. The primordial crust was very thin and was probably recycled by much more vigorous plate tectonics and destroyed by significant asteroid impacts, which were much more common in the early stages of the solar system.

The Earth has probably always had some form of basaltic crust, but the age of the oldest oceanic crust today is only about 200 million years. In contrast, the bulk of the continental crust is much older. The oldest continental crustal rocks on Earth have ages in the range from about 3.7 to 4.28 billion years ^[3][4] and have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on other cratonic regions such as those on the Fennoscandian Shield. Some zircon with age as great as 4.3 billion years has been found in the Narryer Gneiss Terrane.

The average age of the current Earth's continental crust has been estimated to be about 2.0 billion years.^[5] Most crustal rocks formed before 2.5 billion years ago are located in cratons. Such old continental crust and the underlying mantle asthenosphere are less dense than elsewhere in the earth and so are not readily destroyed by subduction. Formation of new continental crust is linked to periods of intense orogeny; these periods coincide with the formation of the supercontinents such as Rodinia, Pangaea and Gondwana. The crust forms in part by aggregation of island arcs including granite and metamorphic fold belts, and it is preserved in part by depletion of the underlying mantle to form buoyant lithospheric mantle.

Composition

Main articles: Abundance of elements in Earth's crust and Goldschmidt classification

Abundance (atom fraction) of the chemical elements in Earth's upper continental crust as a function of atomic number. The rarest elements in the crust (shown in yellow) are not the heaviest, but are rather the siderophile (iron-loving) elements in the Goldschmidt classification of elements. These have been depleted by being relocated deeper into the Earth's core. Their abundance in meteoroid materials is higher. Additionally, tellurium and selenium have been depleted from the crust due to formation of volatile hydrides.

The continental crust has an average composition similar to that of andesite.^[6] Continental crust is enriched in incompatible elements compared to the basaltic ocean crust and much enriched compared to the underlying mantle. Although the continental crust comprises only about 0.6 weight percent of the silicate Earth, it contains 20% to 70% of the incompatible elements.

Oxide	Percent
SiO2	60.6
Al2O3	15.9
CaO	6.4
MgO	4.7
Na2O	3.1
Fe as FeO	6.7
K2O	1.8
TiO2	0.7
P2O5	0.1

All the other constituents except water occur only in very small quantities and total less than 1%. Estimates of average density for the upper crust range between 2.69 and 2.74 g/cm³ and for lower crust between 3.0 and 3.25 g/cm³.^[7]

Moon's crust

Main article: 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.^[8] 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.^[9]

See also

• Eduction
• Continental crust
• Oceanic crust

References

1. ^ Structure of the Earth. The Encyclopedia of Earth. March 3, 2010
2. ^ Earth. Channel4.com. Retrieved on 2011-12-13.
3. ^ "Team finds Earth's 'oldest rocks'". BBC News. 2008-09-26. http://news.bbc.co.uk/1/hi/sci/tech/7639024.stm. Retrieved 2010-03-27. 
4. ^ P. J. Patchett and S. D. Samson, 2003, Ages and Growth ot the Continental Crust from Radiogenic Isotopes. In The Crust (ed. R. L. Rudnick) volume 3, pp. 321–348 of Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergamon, Oxford ISBN 0-08-043751-6
5. ^ A. I. S. Kemp and C. J. Hawkesworth, 2003, Granitic Perspectives on the Generation and Secular Evolution of the Contnental Crust. In The Crust (ed. R. L. Rudnick) volume 3, pp. 349–410 of Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergamon, Oxford ISBN 0-08-043751-6
6. ^ R. L. Rudnick and S. Gao, 2003, Composition of the Continental Crust. In The Crust (ed. R. L. Rudnick) volume 3, pp. 1–64 of Treatise on Geochemistry (eds. H. D. Holland and K. K. Turekian), Elsevier-Pergamon, Oxford ISBN 0-08-043751-6
7. ^ "Structure and composition of the Earth". Australian Museum Online. http://australianmuseum.net.au/Structure-and-composition-of-the-Earth/. Retrieved 2007-09-14. 
8. ^ 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 
9. ^ Herald Hiesinger and James W. Head III (2006). "New views of Lunar geoscience: An introduction and overview". Reviews in Mineralogy & Geochemistry 60: 1–81. doi:10.2138/rmg.2006.60.1. http://www.planetary.brown.edu/pdfs/2961_proof.pdf. 

•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. 
• Kent C. Condie, Origin of the Earth's Cruist, Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), 75:57–81 1989, doi:10.1016/0031-0182(89)90184-3

External links

• USGS Crust Thickness Map

v t e Structure of the Earth Shells Crust · Lithosphere · Asthenosphere · Mesosphere · Mantle · Outer core · Inner core Discontinuities Conrad (continental crust)  · Mohorovičić (crust–mantle) · Lehmann (upper mantle) · Gutenberg (mantle–core)  · Lehmann (core) · Arguments Plate tectonics

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - skorpe, tørt brødstykke, job, frækhed, dejlåg, aflejring, sårskorpe, bundfald, hård facade
v. tr. - dække med skorpe, danne skorpe af
v. intr. - dækkes med skorpe

idioms:

• crust of the earth    jordskorpe

Nederlands (Dutch)
korst (brood/wond etc.), schaal van schaaldier, levensonderhoud, bovenlaag(je), wijnsteen, brutaliteit

Français (French)
n. - (lit, fig) croûte, croûte (de sang, de boue), dépôt (de vin, de cristaux de tartre), (US) culot
v. tr. - se couvrir d'une croûte
v. intr. - former une croûte, s'encroûter

idioms:

• crust of the earth    l'écorce terrestre

Deutsch (German)
n. - Kruste
v. - verkrusten, überkrusten, eine Kruste bilden

idioms:

• crust of the earth    Erdkruste

Ελληνική (Greek)
n. - κρούστα, κόρα (ψωμιού κ.λπ.), φύλλο πίτας κ.λπ., επίστρωμα, επίπαγος, τσίπα, πέτσα, κέλυφος, καύκαλο, κακάδι πληγής, (γεωλ.) φλοιός (της γης κ.λπ.), θράσος

idioms:

• crust of the earth    φλοιός της γης

Italiano (Italian)
crosta

idioms:

• earth's crust    crosta terrestre

Português (Portuguese)
n. - crosta (f)

idioms:

• earth's crust    crosta (f) da terra (Geol.)
• upper crust    crosta (f) superior (Geol.)

Русский (Russian)
корка

idioms:

• earth's crust    земная кора
• upper crust    элита

Español (Spanish)
n. - corteza
v. tr. - encostrar, incrustar
v. intr. - encostrarse

idioms:

• crust of the earth    corteza terrestre

Svenska (Swedish)
n. - skorpa, sårskorpa, jordskorpa, skare, skal, avsättning, fräckhet (sl.)

中文（简体）(Chinese (Simplified))
外壳, 面包皮, 坚硬的外壳, 盖以硬皮, 结硬皮

idioms:

• crust of the earth    地壳

中文（繁體）(Chinese (Traditional))
n. - 外殼, 面包皮, 堅硬的外殼
v. tr. - 蓋以硬皮
v. intr. - 結硬皮

idioms:

• crust of the earth    地殼

한국어 (Korean)
n. - 빵 껍질, 딱딱한 표면, 겉보기
v. tr. - 겉껍질로 덮다
v. intr. - 껍질이 생기다

日本語 (Japanese)
n. - パンの皮, パイの皮, 堅い表面, 外皮
v. - 覆う

idioms:

• earth's crust    地殻

العربيه (Arabic)
‏(الاسم) قشرة الخبز, القشرة الأرضيه‏

עברית (Hebrew)
n. - ‮קרום, קליפה, שכבה קשה, שכבת הסלע החיצונה של כדור-הארץ‬
v. tr. - ‮הקרים‬
v. intr. - ‮הקרים, קרם‬

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