What is the Name of Earth’s Innermost Layer

What is the Name of Earth’s Innermost Layer.

Innermost part of Earth, a solid ball of iron-nickel blend

The internal structure of Earth

Earth’s inner core
is the innermost geologic layer of planet Earth. Information technology is primarily a solid ball with a radius of well-nigh 1,220 km (760 mi), which is nigh 20% of World’s radius or 70% of the Moon’south radius.[1]
[2]

In that location are no samples of Earth’s core accessible for direct measurement, equally there are for Globe’due south drapery.[3]
Data almost World’s cadre mostly comes from analysis of seismic waves and Earth’s magnetic field.[four]
The inner core is believed to be equanimous of an iron–nickel blend with some other elements. The temperature at the inner core’s surface is estimated to exist approximately v,700 K (5,430 °C; 9,800 °F), which is almost the temperature at the surface of the Sunday.[5]

Scientific history

[edit]

Earth was discovered to accept a solid inner cadre distinct from its molten outer core in 1936, by the Danish seismologist Inge Lehmann,[vi]
[7]
who deduced its presence by studying seismograms from earthquakes in New Zealand. She observed that the seismic waves reverberate off the boundary of the inner core and can exist detected past sensitive seismographs on the Earth’s surface. She inferred a radius of 1400 km for the inner core, not far from the currently accepted value of 1221 km.[8]
[9]
[10]
In 1938, Beno Gutenberg and Charles Richter analyzed a more extensive ready of data and estimated the thickness of the outer core as 1950 km with a steep just continuous 300 km thick transition to the inner cadre; implying a radius betwixt 1230 and 1530 km for the inner core.[11]

: p.372

A few years later, in 1940, it was hypothesized that this inner core was made of solid iron. In 1952, Francis Birch published a detailed analysis of the available data and ended that the inner core was probably crystalline fe.[12]

The purlieus betwixt the inner and outer cores is sometimes called the “Lehmann discontinuity”,[thirteen]
although the name usually refers to another discontinuity. The proper noun “Bullen” or “Lehmann-Bullen aperture”, after Keith Edward Bullen has been proposed,[xiv]
but its utilise seems to be rare. The rigidity of the inner core was confirmed in 1971.[15]

Adam Dziewonski and James Freeman Gilbert established that measurements of normal modes of vibration of Earth caused by big earthquakes were consequent with a liquid outer core.[16]
In 2005, shear waves were detected passing through the inner core; these claims were initially controversial, simply are now gaining acceptance.[17]

Data sources

[edit]

Seismic waves

[edit]

Almost all straight measurements that scientists have about the physical properties of the inner cadre are the seismic waves that laissez passer through it. The virtually informative waves are generated by deep earthquakes, 30 km or more beneath the surface of the World (where the pall is relatively more homogeneous) and recorded past seismographs as they reach the surface, all over the globe.[
citation needed
]

Seismic waves include “P” (primary or pressure level) waves, compressional waves that tin travel through solid or liquid materials, and “South” (secondary or shear) shear waves that can just propagate through rigid rubberband solids. The two waves have different velocities and are damped at dissimilar rates as they travel through the same fabric.

Of particular involvement are the so-called “PKiKP” waves—force per unit area waves (P) that start near the surface, cross the drape-core boundary, travel through the cadre (K), are reflected at the inner cadre boundary (i), cross again the liquid core (K), cross back into the mantle, and are detected every bit pressure level waves (P) at the surface. Also of interest are the “PKIKP” waves, that travel through the inner core (I) instead of being reflected at its surface (i). Those signals are easier to translate when the path from source to detector is close to a direct line—namely, when the receiver is just higher up the source for the reflected PKiKP waves, and converse to it for the transmitted PKIKP waves.[18]

While S waves cannot reach or go out the inner core every bit such, P waves can be converted into S waves, and vice versa, as they striking the boundary between the inner and outer core at an oblique angle. The “PKJKP” waves are similar to the PKIKP waves, merely are converted into South waves when they enter the inner core, travel through it equally S waves (J), and are converted again into P waves when they exit the inner core. Cheers to this miracle, it is known that the inner core can propagate S waves, and therefore must exist solid.

Other sources

[edit]

Other sources of information about the inner core include

  • The magnetic field of the Earth. While information technology seems to be generated more often than not by fluid and electric currents in the outer cadre, those currents are strongly affected by the presence of the solid inner cadre and past the heat that flows out of it. (Although made of fe, the cadre is not ferromagnetic, due to being above the Curie temperature.)
  • The Earth’southward mass, its gravitational field, and its angular inertia. These are all affected past the density and dimensions of the inner layers.[19]
  • The natural oscillation frequencies and modes of the whole Earth oscillations, when large earthquakes make the planet “ring” like a bell. These oscillations also depend strongly on the density, size, and shape of the inner layers.[xx]

Concrete backdrop

[edit]

Seismic wave velocity

[edit]

The velocity of the Southward waves in the core varies smoothly from about 3.seven km/s at the center to nearly iii.5 km/s at the surface. That is considerably less than the velocity of Southward waves in the lower crust (almost 4.5 km/south) and less than half the velocity in the deep curtain, just in a higher place the outer core (well-nigh 7.3 km/south).[five]

: fig.2

The velocity of the P-waves in the core also varies smoothly through the inner cadre, from nearly 11.4 km/s at the center to about xi.1 km/south at the surface. So the speed drops abruptly at the inner-outer core purlieus to almost 10.4 km/due south.[5]

: fig.2

Size and shape

[edit]

On the footing of the seismic information, the inner core is estimated to be nearly 1221 km in radius (2442 km in diameter),[v]
which is about 19% of the radius of the Earth and seventy% of the radius of the Moon.

Its volume is about vii.six billion cubic km (7.6 × 1018
chiliad3
), which is well-nigh

i146

(0.69%) of the book of the whole Earth.

Its shape is believed to be shut to an oblate ellipsoid of revolution, like the surface of the Earth, only that more spherical: The flattening
f
is estimated to exist betwixt

1400

and

1416
;[19]

: f.2

meaning that the radius along the Earth’due south axis is estimated to be about 3 km shorter than the radius at the equator. In comparison, the flattening of the Globe as a whole is shut to

one300
, and the polar radius is 21 km shorter than the equatorial one.

Pressure and gravity

[edit]

The force per unit area in the Earth’s inner core is slightly college than it is at the boundary betwixt the outer and inner cores: Information technology ranges from about 330 to 360 gigapascals (iii,300,000 to 3,600,000 atm).[five]
[21]
[22]

The acceleration of gravity at the surface of the inner core can be computed to exist four.three m/south2;[23]
which is less than half the value at the surface of the Earth (9.eight m/due south2).

Density and mass

[edit]

The density of the inner core is believed to vary smoothly from about 13.0 kg/L (= thou/cmthree
= t/thousandiii) at the centre to nigh 12.8 kg/L at the surface. As it happens with other material properties, the density drops all of a sudden at that surface: The liquid just above the inner core is believed to be significantly less dumbo, at about 12.1 kg/L.[5]
For comparison, the average density in the upper 100 km of the Earth is almost iii.4 kg/L.

That density implies a mass of about ten23
kg for the inner cadre, which is

ane60

(1.7%) of the mass of the whole Earth.

Temperature

[edit]

The temperature of the inner core can be estimated from the melting temperature of impure iron at the force per unit area which iron is under at the boundary of the inner core (most 330 GPa). From these considerations, in 2002 D. Alfè and others estimated its temperature as between 5,400 Chiliad (5,100 °C; ix,300 °F) and 5,700 Yard (v,400 °C; ix,800 °F).[5]
Notwithstanding, in 2013 Southward. Anzellini and others obtained experimentally a substantially higher temperature for the melting betoken of atomic number 26, 6230 ± 500 K.[24]

Iron tin be solid at such high temperatures only because its melting temperature increases dramatically at pressures of that magnitude (encounter the Clausius–Clapeyron relation).[25]
[26]

Magnetic field

[edit]

In 2010, Bruce Buffett determined that the average magnetic field in the liquid outer core is about 2.five milliteslas (25 gauss), which is about 40 times the maximum force at the surface. He started from the known fact that the Moon and Sun cause tides in the liquid outer core, just as they do on the oceans on the surface. He observed that movement of the liquid through the local magnetic field creates electric currents, that dissipate energy as rut co-ordinate to Ohm’south law. This dissipation, in turn, damps the tidal motions and explains previously detected anomalies in Globe’southward nutation. From the magnitude of the latter effect he could calculate the magnetic field.[27]
The field inside the inner cadre presumably has a similar force. While indirect, this measurement does not depend significantly on any assumptions about the evolution of the Earth or the composition of the core.

Viscosity

[edit]

Although seismic waves propagate through the core equally if it was solid, the measurements cannot distinguish betwixt a solid material from an extremely viscous one. Some scientists have therefore considered whether there may be wearisome convection in the inner core (equally is believed to exist in the curtain). That could exist an explanation for the anisotropy detected in seismic studies. In 2009, B. Buffett estimated the viscosity of the inner cadre at 10xviii Pa·due south,[28]
which is a sextillion times the viscosity of water, and more a billion times that of pitch.

Composition

[edit]

In that location is still no directly prove well-nigh the composition of the inner core. However, based on the relative prevalence of various chemical elements in the Solar Organization, the theory of planetary formation, and constraints imposed or implied by the chemistry of the remainder of the Earth’s volume, the inner cadre is believed to consist primarily of an atomic number 26–nickel alloy.

At the known pressures and estimated temperatures of the core, information technology is predicted that pure iron could be solid, but its density would exceed the known density of the core by approximately 3%. That consequence implies the presence of lighter elements in the core, such as silicon, oxygen, or sulfur, in addition to the probable presence of nickel.[29]
Contempo estimates (2007) allow for up to ten% nickel and two–3% of unidentified lighter elements.[v]

According to computations by D. Alfè and others, the liquid outer core contains viii–xiii% of oxygen, merely as the iron crystallizes out to form the inner cadre the oxygen is mostly left in the liquid.[5]

Laboratory experiments and assay of seismic moving ridge velocities seem to indicate that the inner core consists specifically of ε-atomic number 26, a crystalline form of the metal with the hexagonal shut-packed (
HCP
) structure. That construction tin can nonetheless admit the inclusion of small amounts of nickel and other elements.[18]
[30]

Likewise, if the inner core grows by precipitation of frozen particles falling onto its surface, then some liquid can besides be trapped in the pore spaces. In that case, some of this rest fluid may nonetheless persist to some pocket-sized caste in much of its interior.[
commendation needed
]

Structure

[edit]

Many scientists had initially expected that the inner core would be found to be homogeneous, because that aforementioned process should have proceeded uniformly during its entire formation. Information technology was even suggested that Earth’s inner core might exist a single crystal of iron.[31]

Axis-aligned anisotropy

[edit]

In 1983, G. Poupinet and others observed that the travel fourth dimension of PKIKP waves (P waves that travel through the inner core) was almost two seconds less for straight north–south paths than straight paths on the equatorial plane.[32]
Even taking into business relationship the flattening of the Globe at the poles (most 0.33% for the whole Earth, 0.25% for the inner core) and crust and upper mantle heterogeneities, this difference implied that P waves (of a broad range of wavelengths) travel through the inner cadre about 1% faster in the north–s direction than along directions perpendicular to that.[33]

This P wave speed anisotropy has been confirmed past afterwards studies, including more seismic data[18]
and study of the complimentary oscillations of the whole Earth.[20]
Some authors accept claimed higher values for the divergence, up to 4.8%; withal, in 2017 D. Frost and B. Romanowicz confirmed that the value is between 0.5% and 1.v%.[34]

Non-axial anisotropy

[edit]

Some authors have claimed that P wave speed is faster in directions that are oblique or perpendicular to the North−S axis, at least in some regions of the inner core.[35]
However, these claims take been disputed past D. Frost and B. Romanowicz, who instead claim that the direction of maximum speed is as close to the Earth’s rotation centrality every bit can exist determined.[36]

Causes of anisotropy

[edit]

Laboratory information and theoretical computations indicate that the propagation of force per unit area waves in the

HCP

crystals of ε-iron are strongly anisotropic, too, with one “fast” axis and two equally “slow” ones. A preference for the crystals in the core to align in the due north–south direction could account for the observed seismic anomaly.[eighteen]

One phenomenon that could cause such partial alignment is slow flow (“creep”) inside the inner core, from the equator towards the poles or vice versa. That menstruation would cause the crystals to partially reorient themselves according to the direction of the flow. In 1996, Southward. Yoshida and others proposed that such a flow could be caused past higher rate of freezing at the equator than at polar latitudes. An equator-to-pole flow then would gear up in the inner core, disposed to restore the isostatic equilibrium of its surface.[37]
[30]

Others suggested that the required menstruation could be acquired past tiresome thermal convection within the inner core. T. Yukutake claimed in 1998 that such convective motions were unlikely.[38]
All the same, B. Buffet in 2009 estimated the viscosity of the inner core and found that such convection could have happened, especially when the core was smaller.[28]

On the other hand, Chiliad. Bergman in 1997 proposed that the anisotropy was due to an observed trend of iron crystals to grow faster when their crystallographic axes are aligned with the management of the cooling rut menstruation. He, therefore, proposed that the heat flow out of the inner cadre would be biased towards the radial direction.[39]

In 1998, S. Karato proposed that changes in the magnetic field might also deform the inner core slowly over time.[xl]

Multiple layers

[edit]

In 2002, Thousand. Ishii and A. Dziewoński presented evidence that the solid inner core contained an “innermost inner cadre” (IMIC) with somewhat dissimilar properties than the shell effectually it. The nature of the differences and radius of the IMIC are still unresolved equally of 2019, with proposals for the latter ranging from 300 km to 750 km.[41]
[42]
[43]
[36]

A. Wang and X. Song proposed, in 2018, a three-layer model, with an “inner inner core” (IIC) with almost 500 km radius, an “outer inner core” (OIC) layer well-nigh 600 km thick, and an isotropic shell 100 km thick. In this model, the “faster P wave” management would be parallel to the Earth’s axis in the OIC, but perpendicular to that axis in the IIC.[35]
Nevertheless, the conclusion has been disputed by claims that at that place demand not exist sharp discontinuities in the inner core, only a gradual alter of properties with depth.[36]

Lateral variation

[edit]

In 1997, South. Tanaka and H. Hamaguchi claimed, on the basis of seismic data, that the anisotropy of the inner core cloth, while oriented N−S, was more pronounced in “eastern” hemisphere of the inner core (at about 110 °E longitude, roughly nether Borneo) than in the “western” hemisphere (at about seventy °West, roughly under Republic of colombia).[44]

: fg.9

Alboussère and others proposed that this asymmetry could be due to melting in the Eastern hemisphere and re-crystallization in the Western one.[45]
C. Finlay conjectured that this procedure could explain the asymmetry in the Globe’due south magnetic field.[46]

However, in 2017 D. Frost and B. Romanowicz disputed those earlier inferences, claiming that the data shows only a weak anisotropy, with the speed in the N−S direction being just 0.v% to 1.5% faster than in equatorial directions, and no clear signs of E−W variation.[34]

Other structure

[edit]

Other researchers merits that the properties of the inner core’due south surface vary from identify to place across distances as small as 1 km. This variation is surprising since lateral temperature variations forth the inner-cadre boundary are known to be extremely small (this conclusion is confidently constrained by magnetic field observations).[
citation needed
]

Growth

[edit]

Schematic of the World’s inner core and outer cadre motility and the magnetic field it generates.

The World’s inner core is thought to be slowly growing as the liquid outer core at the purlieus with the inner core cools and solidifies due to the gradual cooling of the Earth’s interior (about 100 degrees Celsius per billion years).[47]

Co-ordinate to calculations past Alfé and others, equally the iron crystallizes onto the inner cadre, the liquid simply above it becomes enriched in oxygen, and therefore less dense than the residue of the outer core. This process creates convection currents in the outer cadre, which are thought to be the prime driver for the currents that create the Globe’due south magnetic field.[5]

The existence of the inner core also affects the dynamic motions of liquid in the outer core, and thus may help fix the magnetic field.[
citation needed
]

Dynamics

[edit]

Considering the inner core is not rigidly connected to the Earth’s solid drapery, the possibility that it rotates slightly more than quickly or slowly than the rest of Globe has long been entertained.[48]
[49]
In the 1990s, seismologists made various claims about detecting this kind of super-rotation by observing changes in the characteristics of seismic waves passing through the inner core over several decades, using the same property that information technology transmits waves more than quickly in some directions. In 1996, X. Song and P. Richards estimated this “super-rotation” of the inner cadre relative to the pall as about one degree per year.[50]
[51]
In 2005, they and J. Zhang compared recordings of “seismic doublets” (recordings by the same station of earthquakes occurring in the same location on the opposite side of the Earth, years apart), and revised that judge to 0.3 to 0.5 degree per year.[52]

In 1999, Thou. Greff-Lefftz and H. Legros noted that the gravitational fields of the Dominicus and Moon that are responsible for bounding main tides also apply torques to the Earth, affecting its axis of rotation and a slowing down of its rotation charge per unit. Those torques are felt mainly past the crust and drapery, so that their rotation centrality and speed may differ from overall rotation of the fluid in the outer core and the rotation of the inner core. The dynamics is complicated because of the currents and magnetic fields in the inner core. They find that the centrality of the inner cadre wobbles (nutates) slightly with a period of near i day. With some assumptions on the development of the Earth, they conclude that the fluid motions in the outer cadre would have entered resonance with the tidal forces at several times in the past (three.0, ane.eight, and 0.3 billion years ago). During those epochs, which lasted 200–300 million years each, the extra heat generated by stronger fluid motions might accept stopped the growth of the inner core.[53]

Age

[edit]

Theories most the historic period of the cadre are necessarily part of theories of the history of Globe as a whole. This has been a long-debated topic and is still under word at the present time. Information technology is widely believed that the Globe’south solid inner cadre formed out of an initially completely liquid cadre as the Earth cooled downwardly. However, in that location is still no firm prove well-nigh the time when this procedure started.[iv]

Age estimates from
different studies and methods

T
= thermodynamic modeling
P
= paleomagnetism analysis
(R)
= with radioactive elements
(N)
= without them
Date Authors Historic period Method
2001 Labrosse et al.[54] 1±0.5 T(N)
2003 Labrosse[55] ~2 T(R)
2011 Smirnov et al.[56] ii–3.5 P
2014 Driscoll and Bercovici[57] 0.65 T
2015 Labrosse[58] < 0.seven T
2015 Biggin et al.[59] 1–one.5 P
2016 Ohta et al.[60] < 0.vii T
2016 Konôpková et al.[61] < iv.2 T
2019 Bono et al.[62] 0.5 P

Two main approaches have been used to infer the age of the inner cadre: thermodynamic modeling of the cooling of the Earth, and analysis of paleomagnetic evidence. The estimates yielded by these methods still vary over a big range, from 0.5 to ii billion years quondam.

Thermodynamic evidence

[edit]

Heat menstruation of the inner earth, co-ordinate to S.T. Dye[63]
and R. Arevalo.[64]

One of the ways to estimate the age of the inner core is by modeling the cooling of the World, constrained by a minimum value for the estrus flux at the core–drape purlieus (CMB). That estimate is based on the prevailing theory that the Earth’s magnetic field is primarily triggered by convection currents in the liquid part of the core, and the fact that a minimum heat flux is required to sustain those currents. The heat flux at the CMB now time can be reliably estimated because it is related to the measured heat flux at Earth’s surface and to the measured rate of mantle convection.[65]
[54]

In 2001, Southward. Labrosse and others, assuming that there were no radioactive elements in the core, gave an guess of ane±0.5 billion years for the age of the inner core — considerably less than the estimated age of the World and of its liquid cadre (about 4.v billion years)[54]
In 2003, the aforementioned group concluded that, if the core independent a reasonable amount of radioactive elements, the inner core’s age could be a few hundred one thousand thousand years older.[55]

In 2012, theoretical computations past K. Pozzo and others indicated that the conductivity of iron and other hypothetical core materials, at the loftier pressures and temperatures expected at that place, were ii or three times higher than causeless in previous research.[66]
These predictions were confirmed in 2013 by measurements by Gomi and others.[67]
The higher values for electrical conductivity led to increased estimates of the thermal conductivity, to 90 W/m·K; which, in turn, lowered estimates of its historic period to less than 700 million years old.[58]
[60]

However, in 2016 Konôpková and others directly measured the thermal conductivity of solid iron at inner cadre conditions, and obtained a much lower value, 18–44 W/thou·K. With those values, they obtained an upper bound of four.two billion years for the historic period of the inner core, uniform with the paleomagnetic evidence.[61]

In 2014, Driscoll and Bercovici published a thermal history of the Earth that avoided the and then-chosen curtain
thermal catastrophe
and
new core paradox
by invoking three TW of radiogenic heating by the decay of


40


K





in the cadre. Such loftier abundances of K in the cadre are not supported past experimental division studies, so such a thermal history remains highly debatable.[57]

Paleomagnetic evidence

[edit]

Some other fashion to estimate the age of the Earth is to clarify changes in the magnetic field of Earth during its history, as trapped in rocks that formed at various times (the “paleomagnetic tape”). The presence or absenteeism of the solid inner cadre could result in dissimilar dynamic processes in the core that could lead to noticeable changes in the magnetic field.[68]

In 2011, Smirnov and others published an analysis of the paleomagnetism in a big sample of rocks that formed in the Neoarchean (2.viii–two.five billion years ago) and the Proterozoic (2.5–0.541 billion). They establish that the geomagnetic field was closer to that of a magnetic dipole during the Neoarchean than afterward it. They interpreted that change as evidence that the dynamo effect was more securely seated in the core during that epoch, whereas in the later on time currents closer to the core-mantle boundary grew in importance. They further speculate that the change may take been due to growth of the solid inner core between 3.v–2.0 billion years agone.[56]

In 2015, Biggin and others published the analysis of an extensive and carefully selected set up of Precambrian samples and observed a prominent increase in the Earth’s magnetic field strength and variance around i.0–1.5 billion years ago. This modify had not been noticed earlier due to the lack of sufficient robust measurements. They speculated that the change could be due to the birth of Earth’s solid inner core. From their age estimate they derived a rather modest value for the thermal conductivity of the outer core, that allowed for simpler models of the Earth’south thermal development.[59]

In 2016, P. Driscoll published a numerical
evolving dynamo
model that made a detailed prediction of the paleomagnetic field evolution over 0.0–2.0 Ga. The
evolving dynamo
model was driving by time-variable boundary weather condition produced by the thermal history solution in Driscoll and Bercovici (2014). The
evolving dynamo
model predicted a strong-field dynamo prior to one.7 Ga that is multipolar, a strong-field dynamo from 1.0–ane.seven Ga that is predominantly dipolar, a weak-field dynamo from 0.six–one.0 Ga that is a non-axial dipole, and a strong-field dynamo after inner core nucleation from 0.0–0.6 Ga that is predominantly dipolar.[69]

An analysis of stone samples from the Ediacaran epoch (formed about 565 million years agone), published by Bono and others in 2019, revealed unusually low intensity and two distinct directions for the geomagnetic field during that fourth dimension that provides back up for the predictions by Driscoll (2016). Considering other evidence of loftier frequency of magnetic field reversals around that time, they speculate that those anomalies could be due to the onset of germination of the inner core, which would then be 0.five billion years former.[62]
A
News and Views
by P. Driscoll summarizes the state of the field post-obit the Bono results.[70]

Run across also

[edit]

  • Geodynamics
  • Atomic number 26 meteorite
  • Structure of the Earth
  • Travel to the Earth’s eye
  • Thermal history of the Earth

References

[edit]


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Further reading

[edit]

  • Tkalčić, Hrvoje (March 2015). “Complex inner cadre of the Earth: The last frontier of global seismology”.
    Reviews of Geophysics.
    53
    (1): 59–94. Bibcode:2015RvGeo..53…59T. doi:10.1002/2014RG000469.



What is the Name of Earth’s Innermost Layer

Source: https://en.wikipedia.org/wiki/Earth%27s_inner_core