Mantle (geology)

Mantle (geology)

Overview
The mantle is a part of a terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

 or other rocky body large enough to have differentiation
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...

 by density. The interior of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

 and the outer core
Outer core
The outer core of the Earth is a liquid layer about 2,266 kilometers thick composed of iron and nickel which lies above the Earth's solid inner core and below its mantle. Its outer boundary lies beneath the Earth's surface...

. Earth's mantle is a rocky shell about 2890 km (1,795.8 mi) thick that constitutes about 84% of Earth's volume. It is predominantly solid and encloses the iron-rich hot core, which occupies about 15% of Earth's volume.
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Encyclopedia
The mantle is a part of a terrestrial planet
Terrestrial planet
A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets are the inner planets closest to the Sun...

 or other rocky body large enough to have differentiation
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...

 by density. The interior of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust
Crust (geology)
In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

 and the outer core
Outer core
The outer core of the Earth is a liquid layer about 2,266 kilometers thick composed of iron and nickel which lies above the Earth's solid inner core and below its mantle. Its outer boundary lies beneath the Earth's surface...

. Earth's mantle is a rocky shell about 2890 km (1,795.8 mi) thick that constitutes about 84% of Earth's volume. It is predominantly solid and encloses the iron-rich hot core, which occupies about 15% of Earth's volume. Past episodes of melting and volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....

 at the shallower levels of the mantle have produced a thin crust of crystallized melt products near the surface, upon which we live. Information about structure and composition of the mantle either result from geophysical investigation or from direct geoscientific analyses on Earth mantle derived xenolith
Xenolith
A xenolith is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption...

s.

Two main zones are distinguished in the upper mantle: the inner asthenosphere
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...

 composed of plastic
Plasticity (physics)
In physics and materials science, plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the...

 flowing rock about 200 km thick, and the lowermost part of the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

 composed of rigid rock about 50 to 120 km thick. A thin crust, the upper part of the lithosphere, surrounds the mantle and is about 5 to 75 km thick.

Structure


The mantle is divided into sections which are based upon results from seismology
Seismology
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...

. These layers (and their depths) are the following: the upper mantle (starting at the Moho, or base of the crust
Mohorovičić discontinuity
The Mohorovičić discontinuity , usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust and continental crust from underlying mantle...

 around 7 to 35 km downward to 410 km), the transition zone (410–660 km), the lower mantle (660–2891 km), and anomalous core–mantle boundary
Core–mantle boundary
The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...

 with a variable thickness (on average ~200 km thick).

The top of the mantle is defined by a sudden increase in seismic velocity, which was first noted by Andrija Mohorovičić
Andrija Mohorovicic
Andrija Mohorovičić was a Croatian meteorologist and seismologist. He is best known for the eponymous Mohorovičić discontinuity and is considered a founder of modern seismology.-Early years:...

 in 1909; this boundary is now referred to as the "Mohorovičić discontinuity" or "Moho". The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere
Lithosphere
The lithosphere is the rigid outermost shell of a rocky planet. On Earth, it comprises the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater.- Earth's lithosphere :...

, an irregular layer with a maximum thickness of perhaps 200 km. Below the lithosphere the upper mantle becomes notably more plastic. In some regions below the lithosphere, the seismic velocity is reduced; this so-called low-velocity zone
Low-velocity zone
The low-velocity zone occurs close to the boundary between the lithosphere and the asthenosphere in the upper mantle. It is characterized by unusually low seismic shear wave velocity compared to the surrounding depth intervals. This range of depths also corresponds to anomalously high electrical...

 (LVZ) extends down to a depth of several hundred km. Inge Lehmann
Inge Lehmann
Inge Lehmann FRS , was a Danish seismologist who, in 1936, argued that the Earth's core is not one single molten sphere, but that an inner core exists which has physical properties that are different from those of the outer core.-Life:Inge Lehmann was born and grew up in Østerbro, a part of...

 discovered a seismic discontinuity at about 220 km depth; although this discontinuity has been found in other studies, it is not known whether the discontinuity is ubiquitous. The transition zone is an area of great complexity; it physically separates the upper and lower mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous. The D" layer at the core–mantle boundary
Core–mantle boundary
The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...

 separates the mantle from the core.

Characteristics


The mantle differs substantially from the crust in its mechanical properties which is the direct consequence of chemical composition change (expressed as different mineralogy). The distinction between crust and mantle is based on chemistry, rock types, rheology
Rheology
Rheology is the study of the flow of matter, primarily in the liquid state, but also as 'soft solids' or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force....

  and seismic characteristics. The crust is a solidification product of mantle derived melts, expressed as various degrees of partial melting products during geologic time. Partial melting of mantle material is believed to cause incompatible elements to separate from the mantle, with less dense material floating upward through pore spaces, cracks, or fissures, that would subsequently cool and solidify at the surface. Typical mantle rocks have a higher magnesium to iron ratio and a smaller proportion of silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 and aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

 than the crust. This behavior is also predicted by experiments that partly melt rocks thought to be representative of Earth's mantle.
Mantle rocks shallower than about 410 km depth consist mostly of olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

, pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...

s, spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...

-structure minerals, and garnet
Garnet
The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" may come from either the Middle English word gernet meaning 'dark red', or the Latin granatus , possibly a reference to the Punica granatum , a plant with red seeds...

; typical rock types are thought to be peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...

, dunite
Dunite
Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...

 (olivine-rich peridotite), and eclogite
Eclogite
Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...

. Between about 400 km and 650 km depth, olivine is not stable and is replaced by high pressure polymorphs
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...

 with approximately the same composition: one polymorph is wadsleyite
Wadsleyite
Wadsleyite is a high-pressure polymorph of olivine, an orthorhombic mineral found in the Peace River meteorite in Alberta, Canada. In the phase transformations from Mg2SiO4-Fe2SiO4 , olivine is transformed to wadsleyite β-Mg2SiO4 and then to a spinel-structured γ-Mg2SiO4 with increasing pressure...

 (also called beta-spinel type), and the other is ringwoodite
Ringwoodite
Ringwoodite is a high-pressure polymorph of olivine, and it is stable at high temperatures and pressures like those in the Earth's mantle near 600 km depth. This mineral was first identified in the Tenham Meteorites in 1969, and it is inferred to be present in large quantity in the earth’s...

 (a mineral with the gamma-spinel
Spinel
Spinel is the magnesium aluminium member of the larger spinel group of minerals. It has the formula MgAl2O4. Balas ruby is an old name for a rose-tinted variety.-Spinel group:...

structure). Below about 650 km, all of the minerals of the upper mantle begin to become unstable. The most abundant minerals present have structures (but not compositions) like that of the mineral perovskite followed by the magnesium/iron oxide ferropericlase
Ferropericlase
Ferropericlase or Magnesiowüstite is a magnesium/iron oxide that is interpreted to be one of the main constituents of the Earth's lower mantle together with a magnesium /iron silicate with a perovskite structure. Ferropericlase has been found as inclusions in a few natural diamonds...

. The changes in mineralogy at about 400 and 650 km yield distinctive signatures in seismic records of the Earth's interior, and like the moho, are readily detected using seismic waves. These changes in mineralogy may influence mantle convection
Mantle convection
Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere , is divided into a number of plates that are continuously being...

, as they result in density changes and they may absorb or release latent heat as well as depress or elevate the depth of the polymorphic phase transitions for regions of different temperatures. The changes in mineralogy with depth have been investigated by laboratory experiments that duplicate high mantle pressures, such as those using the diamond anvil.
Composition of Earth's mantle in weight percent
Element Amount   Compound Amount
O
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

44.8    
Si
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

21.5 SiO2
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...

46
Mg
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

22.8 MgO
Magnesium oxide
Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...

37.8
Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

5.8 FeO
Wüstite
Wüstite is a mineral form of iron oxide found with meteorites and native iron. It has a gray color with a greenish tint in reflected light. Wüstite crystallizes in the isometric - hexoctahedral crystal system in opaque to translucent metallic grains. It has a Mohs hardness of 5 to 5.5 and a...

7.5
Al
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

2.2 Al2O3
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...

4.2
Ca
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

2.3 CaO
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....

3.2
Na
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...

0.3 Na2O
Sodium oxide
Sodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses, though not in a raw form. Treatment with water affords sodium hydroxide....

0.4
K
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...

0.03 K2O
Potassium oxide
Potassium oxide is an ionic compound of potassium and oxygen. This pale yellow solid, the simplest oxide of potassium, is a rarely encountered, highly reactive compound...

0.04
Sum 99.7 Sum 99.1


The inner core is solid, the outer core is liquid, and the mantle solid/plastic. This is because of the relative melting points of the different layers (nickel-iron core, silicate crust and mantle) and the increase in temperature and pressure as depth increases. At the surface both nickel-iron alloys and silicates are sufficiently cool to be solid. In the upper mantle, the silicates are generally solid (localised regions with small amounts of melt exist); however, as the upper mantle is both hot and under relatively little pressure, the rock in the upper mantle has a relatively low viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

. In contrast, the lower mantle is under tremendous pressure and therefore has a higher viscosity than the upper mantle. The metallic nickel-iron outer core is liquid because of the high pressure and temperature. As the pressure exponentially increases, the nickel-iron inner core becomes solid because the melting point of iron increases dramatically at these high pressures.

Temperature


In the mantle, temperatures range between 500 to 900 °C (932 to 1,652 F) at the upper boundary with the crust; to over 4000 °C (7,232 °F) at the boundary with the core. Although the higher temperatures far exceed the melting point
Melting point
The melting point of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at standard atmospheric pressure...

s of the mantle rocks at the surface (about 1200 °C for representative peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...

), the mantle is almost exclusively solid. The enormous lithostatic pressure exerted on the mantle prevents melting
Melting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...

, because the temperature at which melting begins (the solidus
Solidus (chemistry)
In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid...

) increases with pressure.

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 convective
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 material circulation in the mantle. Hot material upwells
Mantle plume
A mantle plume is a hypothetical thermal diapir of abnormally hot rock that nucleates at the core-mantle boundary and rises through the Earth's mantle. Such plumes were invoked in 1971 to explain volcanic regions that were not thought to be explicable by the then-new theory of plate tectonics. Some...

, while cooler (and heavier) material sinks downward. Downward motion of material occurs at convergent plate boundaries called subduction zones. Locations on the surface that lie over plumes are predicted to have high elevation
Dynamic topography
The term dynamic topography is used in geodynamics and oceanography to refer to elevation differences caused by the flow within the Earth's mantle and the ocean water, respectively.-Geodynamics:...

 (because of the buoyancy of the hotter, less-dense plume beneath) and to exhibit hot spot
Hotspot (geology)
The places known as hotspots or hot spots in geology are volcanic regions thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. They may be on, near to, or far from tectonic plate boundaries. There are two hypotheses to explain them...

 volcanism
Volcanism
Volcanism is the phenomenon connected with volcanoes and volcanic activity. It includes all phenomena resulting from and causing magma within the crust or mantle of a planet to rise through the crust and form volcanic rocks on the surface....

. The volcanism often attributed to deep mantle plumes is alternatively explained by passive extension of the crust, permitting magma to leak to the surface (the "Plate" hypothesis).

The convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 of the Earth's mantle is a chaotic
Chaos theory
Chaos theory is a field of study in mathematics, with applications in several disciplines including physics, economics, biology, and philosophy. Chaos theory studies the behavior of dynamical systems that are highly sensitive to initial conditions, an effect which is popularly referred to as the...

 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
Continental drift
Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...

 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. The mantle within about 200 km above the core-mantle boundary
Core-mantle boundary
The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...

 appears to have distinctly different seismic properties than the mantle at slightly shallower depths; this unusual mantle region just above the core is called D″ ("D double-prime"), a nomenclature introduced over 50 years ago by the geophysicist Keith Bullen
Keith Edward Bullen
Keith Edward Bullen FRS was a New Zealand-born mathematician and geophysicist. He is noted for his seismological interpretation of the deep structure of the Earth's mantle and core...

. D″ may consist of material from subducted slabs
Slab (geology)
In geology, a slab is the portion of a tectonic plate that is being subducted.Slabs constitute an important part of the global plate tectonic system. They drive plate tectonics both by pulling along the lithosphere to which they are attached in a processes known as slab pull and by inciting...

 that descended and came to rest at the core-mantle boundary
Core-mantle boundary
The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...

 and/or from a new mineral polymorph discovered in perovskite called post-perovskite
Post-perovskite
Post-perovskite is a high-pressure phase of magnesium silicate . It is composed of the prime oxide constituents of the Earth's rocky mantle , and its pressure and temperature for stability imply that it is likely to occur in portions of the lowermost few hundred km of Earth's mantle.The...

.

Earthquakes at shallow depths are a result of stick-slip faulting; however, below about 50 km 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. 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 400 km and 670 km.

The pressure at the bottom of the mantle is ~136 GPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...

 (1.4 million atm
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

). Pressure increases as depth increases, since the material beneath has to support the weight of all the material above it. The entire mantle, however, is thought to deform like a fluid on long timescales, with permanent plastic deformation accommodated by the movement of point, line, and/or planar defects through the solid crystals comprising the mantle. Estimates for the viscosity of the upper mantle range between 1019 and 1024 Pa·s, depending on depth, temperature, composition, state of stress, and numerous other factors. Thus, the upper mantle can only flow very slowly. However, when large forces are applied to the uppermost mantle it can become weaker, and this effect is thought to be important in allowing the formation of tectonic plate boundaries.

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
Project Mohole
Project Mohole was an ambitious attempt to drill through the Earth's crust into the Mohorovičić discontinuity, and to provide an Earth science complement to the high profile Space Race. The project was initially led by the American Miscellaneous Society with funding from the National Science...

, was abandoned in 1966 after repeated failures and cost over-runs. The deepest penetration was approximately 180 m (590.6 ft). In 2005 an oceanic borehole reached 1416 metres (4,645.7 ft) below the sea floor from the ocean drilling vessel JOIDES Resolution
JOIDES Resolution
thumb|right|295px|Drillship JOIDES Resolution in 1988JOIDES Resolution is a scientific drilling ship once used by the Ocean Drilling Program, then by its successor, the Integrated Ocean Drilling Program. It is the successor of the Glomar Challenger.The ship was launched as Sedco/BP 471, an oil...

.

On 5 March 2007, a team of scientists on board the RRS James Cook
RRS James Cook
The RRS James Cook is a British Royal Research Ship operated by the Natural Environment Research Council . She was built in 2006 to replace the ageing RRS Charles Darwin with funds from Britain's NERC and the DTI's Large Scientific Facilities Fund...

 embarked on a voyage to an area of the Atlantic seafloor where the mantle lies exposed without any crust covering, mid-way between the Cape Verde Islands and the Caribbean Sea
Caribbean Sea
The Caribbean Sea is a sea of the Atlantic Ocean located in the tropics of the Western hemisphere. It is bounded by Mexico and Central America to the west and southwest, to the north by the Greater Antilles, and to the east by the Lesser Antilles....

. The exposed site lies approximately three kilometres beneath the ocean surface and covers thousands of square kilometres.
A relatively difficult attempt to retrieve samples from the Earth's mantle was scheduled for later in 2007. The Chikyu Hakken
Chikyu Hakken
is a Japanese scientific drilling ship built for the Integrated Ocean Drilling Program . The vessel is designed to ultimately drill seven kilometres beneath the seabed, where the Earth's crust is much thinner, and into the Earth's mantle, deeper than any other hole drilled in the ocean thus...

 mission attempted to use the Japanese vessel 'Chikyu' to drill up to 7000 m (22,965.9 ft) below the seabed. This is nearly three times as deep as preceding oceanic drillings.

A novel method of exploring the uppermost few hundred kilometres of the Earth was recently proposed, 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. The probe consists of an outer sphere of tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...

 about one metre in diameter with a cobalt-60
Cobalt-60
Cobalt-60, , is a synthetic radioactive isotope of cobalt. Due to its half-life of 5.27 years, is not found in nature. It is produced artificially by neutron activation of . decays by beta decay to the stable isotope nickel-60...

  interior acting as a radioactive heat source. It was calculated that such a probe will reach the oceanic Moho
Mohorovičić discontinuity
The Mohorovičić discontinuity , usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust and continental crust from underlying mantle...

 in less than 6 months and attain minimum depths of well over 100 km in a few decades beneath both oceanic and continental lithosphere.

Exploration can also be aided through computer simulations of the evolution of the mantle. In 2009, a supercomputer
Supercomputer
A supercomputer is a computer at the frontline of current processing capacity, particularly speed of calculation.Supercomputers are used for highly calculation-intensive tasks such as problems including quantum physics, weather forecasting, climate research, molecular modeling A supercomputer is a...

 application provided new insight into the distribution of mineral deposits, especially isotopes of iron, from when the mantle developed 4.5 billion years ago.

See also

  • Core–mantle boundary
    Core–mantle boundary
    The core–mantle boundary lies between the Earth's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2900 km of depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth...

  • Mohorovičić discontinuity
    Mohorovičić discontinuity
    The Mohorovičić discontinuity , usually referred to as the Moho, is the boundary between the Earth's crust and the mantle. Named after the pioneering Croatian seismologist Andrija Mohorovičić, the Moho separates both the oceanic crust and continental crust from underlying mantle...

  • Lehmann discontinuity
    Lehmann discontinuity
    The Lehmann discontinuity refers to an abrupt increase of P-wave and S-wave velocities in the vicinity of 220±30 km depth, discovered by seismologist Inge Lehmann. It appears beneath continents, but not usually beneath oceans, and does not readily appear in globally averaged studies...

  • Post-perovskite phase transition
    Post-perovskite
    Post-perovskite is a high-pressure phase of magnesium silicate . It is composed of the prime oxide constituents of the Earth's rocky mantle , and its pressure and temperature for stability imply that it is likely to occur in portions of the lowermost few hundred km of Earth's mantle.The...

  • Mantle convection
    Mantle convection
    Mantle convection is the slow creeping motion of Earth's rocky mantle caused by convection currents carrying heat from the interior of the Earth to the surface. The Earth's surface lithosphere, which rides atop the asthenosphere , is divided into a number of plates that are continuously being...

  • Mesosphere (mantle)
    Mesosphere (mantle)
    The mesosphere refers to the mantle in the region between the asthenosphere and the outer core. The upper boundary is defined as the sharp increase in seismic wave velocities and density at a depth of 660 km...

  • Mantle xenoliths


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