Crust (geology)

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The internal structure of Earth Earth poster.svg
The internal structure of Earth

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).

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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.

Types of 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 secondary crust forms 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:

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

Plates in the crust of Earth Plates tect2 en.svg
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 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. [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

Related Research Articles

<span class="mw-page-title-main">Plate tectonics</span> Movement of Earths lithosphere

Plate tectonics is the scientific theory that Earth's lithosphere comprises a number of large tectonic plates, which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s.

<span class="mw-page-title-main">Basalt</span> Magnesium- and iron-rich extrusive igneous rock

Basalt is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron exposed at or very near the surface of a rocky planet or moon. More than 90% of all volcanic rock on Earth is basalt. Rapid-cooling, fine-grained basalt is chemically equivalent to slow-cooling, coarse-grained gabbro. The eruption of basalt lava is observed by geologists at about 20 volcanoes per year. Basalt is also an important rock type on other planetary bodies in the Solar System. For example, the bulk of the plains of Venus, which cover ~80% of the surface, are basaltic; the lunar maria are plains of flood-basaltic lava flows; and basalt is a common rock on the surface of Mars.

<span class="mw-page-title-main">Giant-impact hypothesis</span> Theory of the formation of the Moon

The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, suggests that the Moon was formed from the ejecta of a collision between the early Earth and a Mars-sized planet, approximately 4.5 billion years ago in the Hadean eon. The colliding body is sometimes called Theia, named after the mythical Greek Titan who was the mother of Selene, the goddess of the Moon. Analysis of lunar rocks published in a 2016 report suggests that the impact might have been a direct hit, causing a fragmentation and thorough mixing of both parent bodies.

<span class="mw-page-title-main">Andesite</span> Type of volcanic rock

Andesite is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predominantly of sodium-rich plagioclase plus pyroxene or hornblende.

<span class="mw-page-title-main">Anorthosite</span> Mafic intrusive igneous rock composed predominantly of plagioclase

Anorthosite is a phaneritic, intrusive igneous rock characterized by its composition: mostly plagioclase feldspar (90–100%), with a minimal mafic component (0–10%). Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.

<span class="mw-page-title-main">Tharsis</span> Volcanic plateau on Mars

<span class="mw-page-title-main">Oceanic crust</span> Uppermost layer of the oceanic portion of a tectonic plate

Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumulates. The crust overlies the rigid uppermost layer of the mantle. The crust and the rigid upper mantle layer together constitute oceanic lithosphere.

<span class="mw-page-title-main">Earth's crust</span> Earths outer shell of rock

Earth's crust is Earth's thick outer shell of rock, referring to less than 1% of Earth's radius and 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. The lithosphere is broken into tectonic plates whose motion allows heat to escape the interior of the Earth into space.

<span class="mw-page-title-main">Geology of the Moon</span> Structure and composition of the Moon

The geology of the Moon is quite different from that of Earth. The Moon lacks a true atmosphere, and the absence of free oxygen and water eliminates erosion due to weather. Instead, the surface is eroded much more slowly through the bombardment of the lunar surface by micrometeorites. It does not have any known form of plate tectonics, it has a lower gravity, and because of its small size, it cooled faster. In addition to impacts, the geomorphology of the lunar surface has been shaped by volcanism, which is now thought to have ended less than 50 million years ago. The Moon is a differentiated body, with a crust, mantle, and core.

<span class="mw-page-title-main">Geology of Venus</span> Geological structure and composition of the second planet from the Sun

The geology of Venus is the scientific study of the surface, crust, and interior of the planet Venus. Of all the other planets in the Solar System, it is the one nearest to Earth and most like it in terms of mass, but has no magnetic field or recognizable plate tectonic system. Much of the ground surface is exposed volcanic bedrock, some with thin and patchy layers of soil covering, in marked contrast with Earth, the Moon, and Mars. Some impact craters are present, but Venus is similar to Earth in that there are fewer craters than on the other rocky planets that are largely covered by them. This is due in part to the thickness of the Venusian atmosphere disrupting small impactors before they strike the ground, but the paucity of large craters may be due to volcanic re-surfacing, possibly of a catastrophic nature. Volcanism appears to be the dominant agent of geological change on Venus. Some of the volcanic landforms appear to be unique to the planet. There are shield and composite volcanoes similar to those found on Earth. Given that Venus has approximately the same size, density, and composition as Earth, it is plausible that volcanism may be continuing on the planet today, as demonstrated by recent studies.

<span class="mw-page-title-main">Rock cycle</span> Transitional concept of geologic time

The rock cycle is a basic concept in geology that describes transitions through geologic time among the three main rock types: sedimentary, metamorphic, and igneous. Each rock type is altered when it is forced out of its equilibrium conditions. For example, an igneous rock such as basalt may break down and dissolve when exposed to the atmosphere, or melt as it is subducted under a continent. Due to the driving forces of the rock cycle, plate tectonics and the water cycle, rocks do not remain in equilibrium and change as they encounter new environments. The rock cycle explains how the three rock types are related to each other, and how processes change from one type to another over time. This cyclical aspect makes rock change a geologic cycle and, on planets containing life, a biogeochemical cycle.

<span class="mw-page-title-main">Lunar magma ocean</span> Theorized historical geological layer on the Moon

The Lunar Magma Ocean (LMO) is the layer of molten rock that is theorized to have been present on the surface of the Moon. The Lunar Magma Ocean was likely present on the Moon from the time of the Moon's formation to tens or hundreds of millions of years after that time. It is a thermodynamic consequence of the Moon's relatively rapid formation in the aftermath of a giant impact between the proto-Earth and another planetary body. As the Moon accreted from the debris from the giant impact, gravitational potential energy was converted to thermal energy. Due to the rapid accretion of the Moon, thermal energy was trapped since it did not have sufficient time to thermally radiate away energy through the lunar surface. The subsequent thermochemical evolution of the Lunar Magma Ocean explains the Moon's largely anorthositic crust, europium anomaly, and KREEP material.

<span class="mw-page-title-main">Internal structure of the Moon</span>

Having a mean density of 3,346.4 kg/m3, the Moon is a differentiated body, being composed of a geochemically distinct crust, mantle, and planetary core. This structure is believed to have resulted from the fractional crystallization of a magma ocean shortly after its formation about 4.5 billion years ago. The energy required to melt the outer portion of the Moon is commonly attributed to a giant impact event that is postulated to have formed the Earth-Moon system, and the subsequent reaccretion of material in Earth orbit. Crystallization of this magma ocean would have given rise to a mafic mantle and a plagioclase-rich crust.

<span class="mw-page-title-main">Geology of solar terrestrial planets</span> Geology of Mercury, Venus, Earth, Mars and Ceres

The geology of solar terrestrial planets mainly deals with the geological aspects of the four terrestrial planets of the Solar System – Mercury, Venus, Earth, and Mars – and one terrestrial dwarf planet: Ceres. Earth is the only terrestrial planet known to have an active hydrosphere.

<span class="mw-page-title-main">Volcanism on Mars</span> Overview of volcanism in the geological history of Mars

Volcanic activity, or volcanism, has played a significant role in the geologic evolution of Mars. Scientists have known since the Mariner 9 mission in 1972 that volcanic features cover large portions of the Martian surface. These features include extensive lava flows, vast lava plains, and the largest known volcanoes in the Solar System. Martian volcanic features range in age from Noachian to late Amazonian, indicating that the planet has been volcanically active throughout its history, and some speculate it probably still is so today. Both Earth and Mars are large, differentiated planets built from similar chondritic materials. Many of the same magmatic processes that occur on Earth also occurred on Mars, and both planets are similar enough compositionally that the same names can be applied to their igneous rocks and minerals.

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

<span class="mw-page-title-main">Warren B. Hamilton</span> American geologist

Warren B. Hamilton was an American geologist known for integrating observed geology and geophysics into planetary-scale syntheses describing the dynamic and petrologic evolution of Earth's crust and mantle. His primary career (1952–1995) was as a research scientist with the US Geological Survey (USGS) in geologic, then geophysical, branches. After retirement, he became a Distinguished Senior Scientist in the Department of Geophysics, Colorado School of Mines (CSM). He was a member of the National Academy of Sciences, and a holder of the Penrose Medal, highest honor of the Geological Society of America (GSA). Hamilton served in the US Navy from 1943 to 1946, completed a bachelor's degree at the University of California, Los Angeles (UCLA) in a Navy training program in 1945, and was a commissioned officer on the aircraft carrier USS Tarawa. After returning to civilian life, he earned an MSc in geology from the University of Southern California in 1949, and a PhD in geology from UCLA in 1951. He married Alicita V. Koenig (1926–2015) in 1947. Hamilton died in October 2018 at the age of 93; until the last few weeks he was working on new research. His final paper, "Toward a myth-free geodynamic history of Earth and its neighbors," was published posthumously (2019) in Earth-Science Reviews. In 2022 the Geological Society of America published an edited volume in his honor, with 33 papers: In the Footsteps of Warren B. Hamilton: New Ideas in Earth Science. The first chapter of this book describes how Hamilton's last paper was written; the second applies Thomas Kuhn's model of scientific change to interpreting Hamilton's career.

<span class="mw-page-title-main">Magma ocean</span>

Magma oceans exist during periods of Earth's or any planet's or some natural satellite's accretion when the planet or the natural satellite is completely or partly molten.

<span class="mw-page-title-main">Hadean zircon</span> Oldest-surviving crustal material from the Earths earliest geological time period

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.

<span class="mw-page-title-main">Earth's crustal evolution</span>

Earth's crustal evolution involves the formation, destruction and renewal of the rocky outer shell at that planet's surface.

References

  1. 1 2 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. ↑ Van Kranendonk, Martin; Smithies, R. H.; Bennett, Vickie C. (2007). Earth's oldest rocks (1st ed.). Amsterdam: Elsevier. ISBN   9780080552477. OCLC   228148014.
  5. 1 2 3 Taylor, Stuart Ross; McLennan, Scott M. (2009). Planetary crusts : their composition, origin and evolution. 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. S2CID   17684919.
  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.{{cite book}}: CS1 maint: others (link)
  9. ↑ Robinson, Eugene C. (January 14, 2011). "The Interior of the Earth". U.S. Geological Survey . Retrieved August 30, 2013.
  10. ↑ "Earth's internal heat".
  11. ↑ 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
  12. ↑ 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.


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