Crust (geology)

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 crusts of Earth, Moon, Mercury, Venus, Mars, Io, 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.

Earth poster
The internal structure of Earth

Types of crust

Planetary geologists divide crust into three categories, based on how and when they 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 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 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 our Solar System with plate tectonics.

Earth's crust

Structure

Plates tect2 en
Plates in the crust of Earth

The crust is a thin shell on the outside of the Earth, accounting for less than 1% of Earth's volume. It is the top component of 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 the Earth into space.

The crust lies on top of the mantle, a configuration that is stable because the upper mantle is made of peridotite and so is significantly denser than the crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity.

World geologic provinces
Geologic provinces of the world (USGS)

The crust of the Earth is of two distinctive types:

  1. Oceanic: 5 km (3 mi) to 10 km (6 mi) thick[10] and composed primarily of denser, more mafic rocks, such as basalt, diabase, and gabbro.
  2. Continental: 30 km (20 mi) to 50 km (30 mi) thick and mostly composed of less dense, more felsic rocks, such as granite.

Because both continental and oceanic crust are less dense than the mantle below, both types of crust "float" on the mantle. This is isostasy, and it's also one of the reasons continental crust is higher than oceanic: continental is less dense and so "floats" higher. As a result, water pools in above the oceanic crust, forming the oceans.

The temperature of the crust increases with depth,[11] 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 temperature increases by as much as 30 °C (54 °F) for every kilometer locally in the upper part of the crust, but the geothermal gradient is smaller in deeper crust.[12]

Composition

Elemental abundances
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 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.[13] The most abundant minerals in Earth's continental crust are feldspars, which make up about 41% of the crust by weight, followed by quartz at 12%, and pyroxenes at 11%.[14] 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 on Earth, it contains 20% to 70% of the incompatible elements.

Most Abundant Elements of Earth's Crust Approximate % by weight
O 46.6
Si 27.7
Al 8.1
Fe 5.0
Ca 3.6
Na 2.8
K 2.6
Mg 1.5
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.[15]

Formation and evolution

Earth formed approximately 4.6 billion years ago from a disk of dust and gas orbiting the newly formed Sun. It formed via accretion, where planetesimals and other smaller rocky bodies collided and stuck, gradually growing into a planet. This process generated an enormous amount of heat, which caused early Earth to melt completely. As planetary accretion slowed, Earth began to cool, forming its first crust, called a primary or primordial crust.[16] This crust was likely repeatedly destroyed by large impacts, then reformed from the magma ocean left by the impact. None of Earth's primary crust has survived to today; all was destroyed by erosion, impacts, and plate tectonics over the past several billion years.

Since then, Earth has been forming secondary and tertiary crust. Secondary crust forms at mid-ocean spreading centers, where partial-melting of the underlying mantle yields basaltic magmas and new ocean crust forms. This "ridge push" is one of the driving forces of plate tectonics, and it is constantly creating new ocean crust. That means that old crust must be destroyed somewhere, so, opposite a spreading center, there is usually a subduction zone: a trench where an ocean plate is being shoved back into the mantle. This constant process of creating new ocean crust and destroying old ocean crust means that the oldest ocean crust on Earth today is only about 200 million years old.

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 [17][18] 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.[19] 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 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.

Moon's crust

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.[20] 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.[21]

See also

References

  1. ^ 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.
  2. ^ 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.
  3. ^ 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.
  4. ^ Earth's oldest rocks. Van Kranendonk, Martin., Smithies, R. H., Bennett, Vickie C. (1st ed.). Amsterdam: Elsevier. 2007. ISBN 9780080552477. OCLC 228148014.
  5. ^ a b c 1925–, Taylor, Stuart Ross (2009). Planetary crusts : their composition, origin and evolution. McLennan, Scott M. Cambridge, UK: Cambridge University Press. ISBN 978-0521841863. OCLC 666900567.
  6. ^ 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.
  7. ^ 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.
  8. ^ 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.
  9. ^ Robinson, Eugene C. (January 14, 2011). "The Interior of the Earth". U.S. Geological Survey. Retrieved August 30, 2013.
  10. ^ Structure of the Earth. The Encyclopedia of Earth. March 3, 2010
  11. ^ Wikisource Peele, Robert (1911). "Boring" . In Chisholm, Hugh. Encyclopædia Britannica. 4 (11th ed.). Cambridge University Press. p. 251.
  12. ^ Earth. Channel4.com. Retrieved on 2011-12-13.
  13. ^ 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
  14. ^ Anderson, Robert S.; Anderson, Suzanne P. (2010). Geomorphology: The Mechanics and Chemistry of Landscapes. Cambridge University Press. p. 187. ISBN 978-1-139-78870-0.
  15. ^ "Structure and composition of the Earth". Australian Museum Online. Retrieved 2007-09-14.
  16. ^ Erickson, Jon (2014). Historical Geology: Understanding Our Planet's Past. Infobase Publishing. p. 8. ISBN 978-1438109640. Retrieved 28 September 2017.
  17. ^ "Team finds Earth's 'oldest rocks'". BBC News. 2008-09-26. Retrieved 2010-03-27.
  18. ^ P. J. Patchett and S. D. Samson, 2003, Ages and Growth of 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
  19. ^ A. I. S. Kemp and C. J. Hawkesworth, 2003, Granitic Perspectives on the Generation and Secular Evolution of the Continental 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
  20. ^ 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
  21. ^ Herald Hiesinger and James W. Head III (2006). "New views of Lunar geoscience: An introduction and overview" (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.

External links

C. P. Rajendran

Chittenipattu Puthenveettil Rajendran, also known among his peers as CP (Malayalam: സീ പീ രാജേന്ദ്രന്‍) (born 29 May 1955, Ottapalam, Palakkad Kerala India) is an Indian geologist who has worked mainly in paleoseismology and Indian geology.

Crystal mush

A crystal mush is a magmatic body which contains a significant amount of crystals (up to 50% of the volume) suspended in the liquid phase (melt). As the crystal fraction makes up less than half of the volume, there is no rigid large-scale three-dimensional network as in solids. As such, their rheological behavior mirrors that of absolute liquids. Within a single crystal mush, there is grading to a higher solid fraction towards the margins of the pluton while the liquid fraction increases towards the uppermost portions, forming a liquid lens at the top. Furthermore, depending on depth of placement crystal mushes are likely to contain a larger portion of crystals at greater depth in the crust than at shallower depth, as melting occurs from the adiabatic decompression of the magma as it rises, this is particularly the case for mid-oceanic ridges.Seismic investigation offers strong evidence for the existence of crystal mushes rather than fully liquid magmatic bodies.Crystal mushes can have a wide range of chemical and mineralogical compositions, from mafic (SiO2-poor, MgO-rich) to felsic (SiO2-rich, MgO-poor).

Ecosphere (planetary)

An ecosphere is a planetary closed ecological system. In this global ecosystem, the various forms of energy and matter that constitute a given planet interact on a continual basis. The forces of the four Fundamental interactions cause the various forms of matter to settle into identifiable layers. These layers are referred to as component spheres with the type and extent of each component sphere varying significantly from one particular ecosphere to another. Component spheres that represent a significant portion of an ecosphere are referred to as a primary component spheres. For instance, Earth's ecosphere consists of five primary component spheres which are the Geosphere, Hydrosphere, Biosphere, Atmosphere, and Magnetosphere.

Eoarchean geology

Eoarchean geology is the study of the oldest preserved crustal fragments of Earth during the Eoarchean era from 4 to 3.6 billion years ago. Major well-preserved rock units dated Eoarchean are known from three localities, the Isua Greenstone Belt in Southwest Greenland, the Acasta Gneiss in the Slave Craton in Canada, and the Nuvvuagittuq Greenstone Belt in the eastern coast of Hudson Bay in Quebec. From the dating of rocks in these three regions scientists suggest that plate tectonics could go back as early as Eoarchean.

All 3 regions contain an abundance of Archean felsic volcanic rocks, including tonalite, trondhjemite and granodiorite (TTG) series rocks, with minor granulite to amphibolite facies gneiss complexes, which means that the original characters of the rocks has been disturbed by at least one ductile deformation at deep crustal conditions.Eoarchean geology is important in investigating earth's tectonic history. It is because the earth had just undergone an transformation to the present-day-similar convective mode and lithosphere from a magma ocean in Hadean Eon, to either a protoplate tectonics or an unstable stagnant lithosphere lid at its infant stages. The earth's condition during Archean to Proterozoic (including Eoarchean era) serves as a crucial linkage between Hadean magma ocean to present-day plate tectonics. Various interpretations have been suggested to explain the prevalent tectonic style corresponding to Eoarchean geology. However it can be, in general, classified into two tectonic models, which are vertical tectonics and plate tectonics.Explanation on the release of large amount of mantle heat is the prominent concern. Most of the evidences shows a probability that pre-plate tectonics dominantly involved intense surface volcanism, active magmatism and crustal recycling.

Hadean zircon

Hadean zircon is the oldest-surviving crustal material from the Earth's earliest geological time period, the Hadean eon, about 4 billion years ago. Zircon is a mineral that is commonly used for radiometric dating because it is highly resistant to chemical changes and appears in the form of small crystals or grains in most igneous and metamorphic host rocks.

Hadean zircon has very low abundance around the globe because of recycling of material by plate tectonics. When the rock at the surface is buried deep in the Earth it is heated and can recrystallise or melt. In the Jack Hills, Australia, scientists obtained a relatively comprehensive record of Hadean zircon crystals in contrast to other locations. The Jack Hills rocks are from the Archean eon, about 3.6 billion years old. However, the zircon crystals there are older than the rocks that contain them. Many investigations have been carried out to find the absolute age and properties of zircon, for example the isotope ratios, mineral inclusions, and geochemistry of zircon. The characteristics of Hadean zircons show early Earth history and the mechanism of Earth's processes in the past. Based on the properties of these zircon crystals, many different geological models were proposed.

Osmium

Osmium (from Greek ὀσμή osme, "smell") is a chemical element with symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mostly in platinum ores. Osmium is the densest naturally occurring element, with an experimentally measured (using x-ray crystallography) density of 22.59 g/cm3. Manufacturers use its alloys with platinum, iridium, and other platinum-group metals to make fountain pen nib tipping, electrical contacts, and in other applications that require extreme durability and hardness. The element's abundance in the Earth's crust is among the rarest.

Outline of Earth sciences

The following outline is provided as an overview of and topical guide to Earth science:

Earth science – all-embracing term for the sciences related to the planet Earth. It is also known as geoscience, the geosciences or the Earth sciences, and is arguably a special case in planetary science, the Earth being the only known life-bearing planet.

Earth science is a branch of the physical sciences which is a part of the natural sciences. It in turn has many branches.

Shallow water marine environment

Shallow water marine environment refers to the area between the shore and deeper water, such as a reef wall or a shelf break. This environment is characterized by oceanic, geological and biological conditions, as described below. The water in this environment is shallow and clear, allowing the formation of different sedimentary structures, carbonate rocks, coral reefs, and allowing certain organisms to survive and become fossils.

Slave Craton

The Slave Craton is an Archaean craton in the north-western Canadian Shield, in Northwest Territories and Nunavut. The Slave Craton includes the 4.03 Ga-old Acasta Gneiss which is one of the oldest dated rocks on Earth.

Covering about 300,000 km2 (120,000 sq mi), it is a relatively small but well-exposed craton dominated by ~2.73–2.63 Ga (billion years-old) greenstones and turbidite sequences and ~2.72–2.58 Ga plutonic rocks, with large parts of the craton underlain by older gneiss and granitoid units.

The Slave Craton is one of the blocks that compose the Precambrian core of North America, also known as the palaeocontinent Laurentia.The exposed portion of the craton, called the Slave Province, comprises 172,500 km2 (66,600 sq mi) and has an elliptical shape that stretches 680 km (420 mi) NNE from Gros Cap on the Great Slave Lake to Cape Barrow on the Coronation Gulf and 460 km (290 mi) EW along latitude 64°N. It covers about 700 km × 500 km (430 mi × 310 mi) and is bounded by Palaeoproterozoic belts to the south, east, and west, while younger rocks cover it to the north.The Slave Craton is divided into a west-central basement complex, the Central Slave Basement Complex, and an eastern province, named the Hackett River Terrane or the Eastern Slave Province. These two domains are separated by a 2.7 Ga-old suture defined by two isotopic boundaries running north to south over the craton.

Solid earth

Solid earth refers to "the earth beneath our feet" or terra firma, the planet's solid surface and its interior. It contrasts with the Earth's fluid envelopes, the atmosphere and hydrosphere (but includes the ocean basin), as well as the biosphere and interactions with the Sun. It includes the liquid core.Solid-earth science refers to the corresponding methods of study, a subset of Earth sciences, predominantly geophysics and geology, excluding aeronomy, atmospheric sciences, oceanography, hydrology, and ecology.

Zirconian

The Zirconian is the second Era within the Hadean Eon in a proposed revision of the Precambrian time scale. It lasted 373 million years from the end of the Chaotian Era 4,404 million years ago to the beginning of Eoarchean Era 4,031 million years ago. The Zirconian follows the Chaotian Era and its beginning is chronometrically set at 4.404 ± 0.008 Gya. This corresponds to the age of the first occurrence of Hadean zircons in the Jack Hills in Western Australia (Yilgarn craton). The end of the Zirconian Era and the transition to the Acastan Period (the earliest period of the Archean Eon and Eoarchean Era) occurred with the appearance of the oldest rock at 4.031 ± 0.003 Gya.

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