Earth's mantle
Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01×1024 kg (8.84×1024 lb) and thus makes up 67% of the mass of Earth.[1] It has a thickness of 2,900 kilometers (1,800 mi)[1] making up about 46% of Earth's radius and 84% of Earth's volume. It is predominantly solid but, on geologic time scales, it behaves as a viscous fluid, sometimes described as having the consistency of caramel.[2][3] Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.[4]
Structure
Rheology
Earth's mantle is divided into two major
The Earth's mantle is divided into three major layers defined by sudden changes in seismic velocity:[6]
- the upper mantle (starting at the Moho, or base of the crust around 7 to 35 km [4.3 to 21.7 mi] downward to 410 km [250 mi])[7]
- the transition zone (approximately 410–660 km [250–410 mi]), in which wadsleyite (≈ 410–520 km [250–320 mi]) and ringwoodite(≈ 525–660 km [326–410 mi]) are stable
- the bridgmanite (≈ 660–2,685 km [410–1,668 mi]) and post-perovskite(≈ 2,685–2,891 km [1,668–1,796 mi]) are stable
The lower ~200 km of the lower mantle constitutes the D" (
Mineralogical structure
The top of the mantle is defined by a sudden increase in seismic velocity, which was first noted by Andrija Mohorovičić in 1909; this boundary is now referred to as the Mohorovičić discontinuity or "Moho".[8][9]
The upper mantle is dominantly
At the top of the transition zone, olivine undergoes isochemical phase transitions to
The lower mantle is composed primarily of bridgmanite and ferropericlase, with minor amounts of calcium perovskite, calcium-ferrite structured oxide, and stishovite. In the lowermost ~200 km (120 mi) of the mantle, bridgmanite isochemically transforms into post-perovskite.[14]
Possible remnants of Theia collision
Seismic images of Earth’s interior have revealed in the lowermost mantle two continent-sized anomalies with low seismic velocities. These zones are denser and likely compositionally different from the surrounding mantle. These anomalies may represent buried relics of Theia mantle material remaining after the Moon-forming giant impact. [15]
Composition
The chemical composition of the mantle is difficult to determine with a high degree of certainty because it is largely inaccessible. Rare exposures of mantle rocks occur in ophiolites, where sections of oceanic lithosphere have been obducted onto a continent. Mantle rocks are also sampled as xenoliths within basalts or kimberlites.
Compound | Mass percent |
---|---|
SiO2 | 44.71 |
MgO | 38.73 |
FeO | 8.18 |
Al2O3 | 3.98 |
CaO | 3.17 |
Cr2O3 | 0.57 |
NiO | 0.24 |
MnO | 0.13 |
Na2O | 0.13 |
TiO2 | 0.13 |
P2O5 | 0.019 |
K2O | 0.006 |
Most estimates of the mantle composition are based on rocks that sample only the uppermost mantle. There is debate as to whether the rest of the mantle, especially the lower mantle, has the same bulk composition.[18] The mantle's composition has changed through the Earth's history due to the extraction of magma that solidified to form oceanic crust and continental crust.
It has also been proposed in a 2018 study that an exotic form of water known as ice VII can form from supercritical water in the mantle when diamonds containing pressurized water bubbles move upward, cooling the water to the conditions needed for ice VII to form.[19]
Temperature and pressure
In the mantle, temperatures range from approximately 500 K (230 °C; 440 °F) at the upper boundary with the crust to approximately 4,200 K (3,900 °C; 7,100 °F) at the
The pressure in the mantle increases from a few hundred megapascals at the Moho to 139
Movement
Because of the temperature difference between the Earth's surface and outer core and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a
The convection of the Earth's mantle is a chaotic process (in the sense of fluid dynamics), which is thought to be an integral part of the motion of plates. Plate motion should not be confused with continental drift which applies purely to the movement of the crustal components of the continents. The movements of the lithosphere and the underlying mantle are coupled since descending lithosphere is an essential component of convection in the mantle. The observed continental drift is a complicated relationship between the forces causing oceanic lithosphere to sink and the movements within Earth's mantle.
Although there is a tendency to larger viscosity at greater depth, this relation is far from linear and shows layers with dramatically decreased viscosity, in particular in the upper mantle and at the boundary with the core. that descended and came to rest at the core–mantle boundary or from a new mineral polymorph discovered in perovskite called post-perovskite.
Earthquakes at shallow depths are a result of faulting; however, below about 50 km (31 mi) the hot, high pressure conditions ought to inhibit further seismicity. The mantle is considered to be viscous and incapable of brittle faulting. However, in subduction zones, earthquakes are observed down to 670 km (420 mi). A number of mechanisms have been proposed to explain this phenomenon, including dehydration, thermal runaway, and phase change. The geothermal gradient can be lowered where cool material from the surface sinks downward, increasing the strength of the surrounding mantle, and allowing earthquakes to occur down to a depth of between 400 km (250 mi) and 670 km (420 mi).[26]
The pressure at the bottom of the mantle is ~136 GPa (19,700,000 psi; 1,340,000 atm).
Exploration
Exploration of the mantle is generally conducted at the seabed rather than on land because of the relative thinness of the oceanic crust as compared to the significantly thicker continental crust.
The first attempt at mantle exploration, known as Project Mohole, was abandoned in 1966 after repeated failures and cost over-runs. The deepest penetration was approximately 180 m (590 ft). In 2005 an oceanic borehole reached 1,416 metres (4,646 ft) below the sea floor from the ocean drilling vessel JOIDES Resolution.
More successful was the
On 5 March 2007, a team of scientists on board the
A novel method of exploring the uppermost few hundred kilometres of the Earth was proposed in 2005, consisting of a small, dense, heat-generating probe which melts its way down through the crust and mantle while its position and progress are tracked by acoustic signals generated in the rocks.[33] The probe consists of an outer sphere of tungsten about one metre in diameter with a cobalt-60 interior acting as a radioactive heat source. It was calculated that such a probe will reach the oceanic Moho in less than 6 months and attain minimum depths of well over 100 km (62 mi) in a few decades beneath both oceanic and continental lithosphere.[34]
Exploration can also be aided through computer simulations of the evolution of the mantle. In 2009, a supercomputer application provided new insight into the distribution of mineral deposits, especially isotopes of iron, from when the mantle developed 4.5 billion years ago.[35]
In 2023, JOIDES Resolution recovered cores of what appeared to be rock from the upper mantle after drilling only a few hundred meters into the Atlantis Massif. The borehole reached a maximum depth of 1,268 meters and recovered 886 meters of rock samples consisting of primarily peridotite. There is debate over the extent to which the samples represent the upper mantle with some arguing the effects of seawater on the samples situates them as examples of deep lower crust. However, the samples offer a much closer analogue to mantle rock than magmatic xenoliths as the sampled rock never melted into magma or recrystallized.[36]
See also
References
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- ^ "In Depth | Earth". NASA Solar System Exploration. Archived from the original on 2021-02-12. Retrieved 2021-01-29.
- ^ "What is the Earth's Mantle Made Of? - Universe Today". Universe Today. 2016-03-26. Retrieved 2018-11-24.
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- ^ The location of the base of the crust varies from approximately 10 to 70 km (6.2 to 43.5 mi). Oceanic crust is generally less than 10 km (6.2 mi) thick. "Standard" continental crust is around 35 km (22 mi) thick, and the large crustal root under the Tibetan Plateau is approximately 70 km (43 mi) thick.
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- ^ Yuan, Q., Li, M., Desch, S.J. et al. Moon-forming impactor as a source of Earth’s basal mantle anomalies. Nature 623, 95–99 (2023). https://doi.org/10.1038/s41586-023-06589-1
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- ^ Netburn, Deborah. "What scientists found trapped in a diamond: a type of ice not known on Earth". Los Angeles Times. Archived from the original on 12 March 2018. Retrieved 12 March 2018.
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- ^ Louie, J. (1996). "Earth's Interior". University of Nevada, Reno. Archived from the original on 2011-07-20. Retrieved 2007-12-24.
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- ^ a b Walzer, Uwe; Hendel, Roland and Baumgardner, John. Mantle Viscosity and the Thickness of the Convective Downwellings. igw.uni-jena.de
- ^ Alden, Andrew. "The End of D-Double-Prime Time?". About.com. Archived from the original on 2008-10-06. Retrieved 2007-12-25.
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Burns, Roger George (1993). Mineralogical Applications of Crystal Field Theory. Cambridge University Press. p. 354. ISBN 978-0-521-43077-7. Retrieved 2007-12-26.
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- ^ "About DSDP". Deep Sea Drilling Project.
- ^ Than, Ker (2007-03-01). "Scientists to study gash on Atlantic seafloor". NBC News. Retrieved 2008-03-16.
A team of scientists will embark on a voyage next week to study an "open wound" on the Atlantic seafloor where the Earth's deep interior lies exposed without any crust covering.
- Science Daily. 2007-03-02. Retrieved 2008-03-16.
Cardiff University scientists will shortly set sail (March 5) to investigate a startling discovery in the depths of the Atlantic.
- PhysOrg.com. 2005-12-15. Archived from the originalon 2005-12-19. Retrieved 2008-03-16.
An ambitious Japanese-led project to dig deeper into the Earth's surface than ever before will be a breakthrough in detecting earthquakes including Tokyo's dreaded "Big One," officials said Thursday.
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External links
- The Biggest Dig: Japan builds a ship to drill to the earth's mantle – Scientific American (September 2005)
- Information on the Mohole Project