The lower mantle, historically also known as the mesosphere, represents approximately 56% of Earth's total volume, and is the region from 660 to 2900 km below

Earth's surface Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surface ...

; between the transition zone and the

outer core Earth's outer core is a fluid layer about thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. The outer core begins approximately beneath Earth's surface at the core-mantle boundary and e ...

. The

preliminary reference Earth model The preliminary reference Earth model (PREM) plots the average of Earth's properties by depth. It includes a table of Earth properties, including elastic properties, attenuation, density, pressure, and gravity. PREM has been widely used as ...

(PREM) separates the lower mantle into three sections, the uppermost (660–770 km), mid-lower mantle (770–2700 km), and the D layer (2700–2900 km). Pressure and temperature in the lower mantle range from 24–127 GPa and 1900–2600 K. It has been proposed that the composition of the lower mantle is pyrolitic, containing three major phases of

bridgmanite Silicate perovskite is either (the magnesium end-member is called bridgmanite) or (calcium silicate known as davemaoite) when arranged in a perovskite structure. Silicate perovskites are not stable at Earth's surface, and mainly exist in the l ...

, ferropericlase, and calcium-silicate perovskite. The high pressure in the lower mantle has been shown to induce a spin transition of iron-bearing bridgmanite and ferropericlase, which may affect both

mantle plume A mantle plume is a proposed mechanism of convection within the Earth's mantle, hypothesized to explain anomalous volcanism. Because the plume head partially melts on reaching shallow depths, a plume is often invoked as the cause of volcanic hot ...

dynamics and lower mantle chemistry. The upper boundary is defined by the sharp increase in

seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic ener ...

velocities and

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematicall ...

at a depth of . At a depth of 660 km,

ringwoodite Ringwoodite is a high-pressure phase of Mg2SiO4 (magnesium silicate) formed at high temperatures and pressures of the Earth's mantle between depth. It may also contain iron and hydrogen. It is polymorphous with the olivine phase forsterite (a ...

() decomposes into Mg-Si perovskite and magnesiowüstite. This reaction marks the boundary between the

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appr ...

and lower mantle. This measurement is estimated from seismic data and high-pressure laboratory experiments. The base of the mesosphere includes the D″ zone which lies just above the mantle–core boundary at approximately . The base of the lower mantle is about 2700 km.

Physical properties

The lower mantle was initially labelled as the D-layer in Bullen's spherically symmetric model of the Earth. The PREM seismic model of the Earth's interior separated the D-layer into three distinctive layers defined by the discontinuity in

velocities: * 660–770 km: A discontinuity in compression wave velocity (6–11%) followed by a steep gradient is indicative of the transformation of the mineral

to bridgmanite and ferropericlase and the transition between the transition zone layer to the lower mantle. * 770–2700 km: A gradual increase in velocity indicative of the adiabatic compression of the mineral phases in the lower mantle. * 2700–2900 km: The

D-layer The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an i ...

is considered the transition from the lower mantle to the

. The temperature of the lower mantle ranges from at the topmost layer to at a depth of . Models of the temperature of the lower mantle approximate

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the c ...

as the primary heat transport contribution, while conduction and radiative heat transfer are considered negligible. As a result, the lower mantle's temperature gradient as a function of depth is approximately adiabatic. Calculation of the geothermal gradient observed a decrease from at the uppermost lower mantle to at .

Composition

The lower mantle is mainly composed of three components, bridgmanite, ferropericlase, and calcium-silicate perovskite (CaSiO₃-perovskite). The proportion of each component has been a subject of discussion historically where the bulk composition is suggested to be, * Pyrolitic: derived from petrological composition trends from

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...

suggesting homogeneity between the upper and lower mantle with a Mg/Si ratio of 1.27. This model implies that the lower mantle is composed of 75% bridgmanite, 17% ferropericlase, and 8% CaSiO₃-perovskite by volume. * Chondritic: suggests that the Earth's lower mantle was accreted from the composition of

chondritic meteorite A chondrite is a stony (non-metallic) meteorite that has not been modified, by either melting or differentiation of the parent body. They are formed when various types of dust and small grains in the early Solar System accreted to form primi ...

suggesting a Mg/Si ratio of approximately 1. This infers that bridgmanite and CaSiO₃-perovskites are major components. Laboratory multi-anvil compression experiments of pyrolite simulated conditions of the adiabatic

geotherm Geothermal gradient is the rate of temperature change with respect to increasing depth in Earth's interior. As a general rule, the crust temperature rises with depth due to the heat flow from the much hotter mantle; away from tectonic plate bo ...

and measured the density using ''in situ''

X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

. It was shown that the density profile along the geotherm is in agreement with the PREM model. The first principle calculation of the density and velocity profile across the lower mantle geotherm of varying bridgmanite and ferropericlase proportion observed a match to the PREM model at an 8:2 proportion. This proportion is consistent with the pyrolitic bulk composition at the lower mantle. Furthermore, shear wave velocity calculations of pyrolitic lower mantle compositions considering minor elements resulted in a match with the PREM shear velocity profile within 1%. On the other hand, Brillouin spectroscopic studies at relevant pressures and temperatures revealed that a lower mantle composed of greater than 93% bridgmanite phase has corresponding shear-wave velocities to measured seismic velocities. The suggested composition is consistent with a chondritic lower mantle. Thus, the bulk composition of the lower mantle is currently a subject of discussion.

Spin transition zone

The electronic environment of two iron-bearing minerals in the lower mantle (bridgmanite, ferropericlase) transitions from a high-spin (HS) to a low-spin (LS) state. Fe²⁺ in ferropericlase undergoes the transition between 50–90 GPa. Bridgmanite contains both Fe³⁺ and Fe²⁺ in the structure, the Fe²⁺ occupy the A-site and transition to a LS state at 120 GPa. While Fe³⁺ occupies both A- and B-sites, the B-site Fe³⁺ undergoes HS to LS transition at 30–70 GPa while the A-site Fe³⁺ exchanges with the B-site Al³⁺ cation and becomes LS. This spin transition of the iron cation results in the increase in

partition coefficient In the physical sciences, a partition coefficient (''P'') or distribution coefficient (''D'') is the ratio of concentrations of a compound in a mixture of two immiscible solvents at equilibrium. This ratio is therefore a comparison of the solub ...

between ferropericlase and bridgmanite to 10–14 depleting bridgmanite and enriching ferropericlase of Fe²⁺. The HS to LS transition are reported to affect the physical properties of the iron bearing minerals. For example, the density and incompressibility was reported to increase from HS to LS state in ferropericlase. The effects of the spin transition on the transport properties and

rheology Rheology (; ) is the study of the flow of matter, primarily in a fluid ( liquid or gas) 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 ap ...

of the lower mantle is currently being investigated and discussed using numerical simulations.

History

Mesosphere (not to be confused with

mesosphere The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...

, a layer of the

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

) is derived from "mesospheric shell", coined by

Reginald Aldworth Daly Reginald Aldworth Daly (March 18, 1871 – September 19, 1957) was a Canadian geologist. Biography Reginald Daly was educated at the University of Toronto, where geologist A.P. Coleman persuaded him away from teaching mathematics and into Earth ...

, a

Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of highe ...

geology Geology () is a branch of natural science concerned with Earth and other Astronomical object, astronomical objects, the features or rock (geology), rocks of which it is composed, and the processes by which they change over time. Modern geology ...

professor. In the pre-

plate tectonics Plate tectonics (from the la, label= Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of larg ...

era, Daly (1940) inferred that the outer Earth consisted of three

spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

layers: lithosphere (including the crust), asthenosphere, and mesospheric shell. Daly's hypothetical depths to the lithosphere-asthenosphere boundary ranged from , and the top of the mesospheric shell (base of the asthenosphere) were from . Thus, Daly's asthenosphere was inferred to be thick. According to Daly, the base of the solid Earth mesosphere could extend to the base of the mantle (and, thus, to the top of the

core Core or cores may refer to: Science and technology * Core (anatomy), everything except the appendages * Core (manufacturing), used in casting and molding * Core (optical fiber), the signal-carrying portion of an optical fiber * Core, the centra ...

). A derivative term,

mesoplates The term "mesoplates" has been applied in two different contexts within geology and geophysics. The first is applicable to much of the Earth's mantle, and the second to distinct layering within the Earth's crust. Mantle Rheological model In ...

, was introduced as a

heuristic A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be optimal, perfect, or rational, but is nevertheless sufficient for reaching an immediate ...

, based on a combination of "mesosphere" and "plate", for postulated reference frames in which mantle

hotspots Hotspot, Hot Spot or Hot spot may refer to: Places * Hot Spot, Kentucky, a community in the United States Arts, entertainment, and media Fictional entities * Hot Spot (comics), a name for the DC Comics character Isaiah Crockett * Hot Spot (Tra ...

exist.

References

{{Earthsinterior Earth's mantle Structure of the Earth

Physical properties

Composition

Spin transition zone

History

See also

References