Earth's inner core is the innermost geologic layer of the Earth. It is primarily a solid ball with a radius of about 1,220 kilometres (760 miles), which is about 20% of the Earth's radius or 70% of the Moon's radius.^[1]^[2]

There are no samples of the Earth's core available for direct measurement, as there are for the Earth's mantle. Information about the Earth's core mostly comes from analysis of seismic waves and the magnetic field.^[3] The inner core is believed to be composed of an iron–nickel alloy with some other elements. The temperature at the inner core's surface is estimated to be approximately 5700 K (5430 °C or 9806 °F), which is about the temperature at the surface of the Sun.^[4]

The Core Within Earth's Inner Core

A study published in early 2015 revealed that Earth possesses a second inner Core. A team led by seismologists Tao Wang from Nanjing University and Xiaodong Song from the University of Illinois showed that Earth’s inner core is divided into two layers distinguished only by the polarity differences of the iron crystals found within them. The polarity of the iron crystals of the innermost layer, the “inner-inner core” or IIC, is oriented in an east-west direction, whereas that of the outermost layer, the “outer-inner core” or OIC, is oriented north-south.

Earth's coreA second inner core, the inner-inner core, which spans 1,180 km (about 733 mi) in diameter, was announced by scientists who were studying the abrupt changes in the polarity of iron crystals within Earth's core.Encyclopædia Britannica, Inc.
anisotropy within Earth's coreEarth's inner-inner core was discovered after scientists found out that the whole of Earth's core is elastically anisotropic; in other words, seismic waves were found to travel at different speeds along different paths (or radii) toward Earth's centre. The anisotropy has been found to change with hemisphere and with radius. The dominant polarity (or magnetic orientation) of the iron crystals in each region can be represented by lines of increasing length along a given axis, while areas of stronger anisotropy (indicating abrupt changes in polarity) can be represented by lines of decreasing length.Source: Xinlei Sun and Xiaodong Song, "The Inner Inner Core of the Earth: Texturing of Iron Crystals from Three-Dimensional Seismic Anisotropy," Earth and Planetary Science Letters (2008). Redrawn by © Encyclopædia Britannica, Inc.

Earth differentiated into an iron core and a rocky mantle early in its formation under the influence of gravity. The outer core is liquid because of its high temperature, but at Earth’s centre an inner core formed and gradually grew as the planet cooled and the liquid iron solidified under tremendous pressure some three million times greater than the atmospheric pressure. The convection generated by the fluid outer core has generated electric currents that have maintained Earth’s magnetic field for some three billion years. The size of the inner core (1,220 km [760 mi] in radius) is slightly smaller than that of the Moon. Because of the inaccessibility of Earth’s interior, seismic waves from earthquakes have been a primary source for studying it, much as X-rays are used in medical imaging.

Earth’s core was discovered in 1906 by British geologist Richard Oldham, and its solid inner core was discovered in 1936 by Danish seismologist Inge Lehmann soon after the invention of sufficiently sensitive seismometers. Using seismic observations, Columbia University seismologists Xiaodong Song and Paul Richards reported in 1996 that the inner core rotates relative to the mantle, which is likely driven by the interaction between the geomagnetic field and the conducting inner core.

Anisotropy of the Inner Core.

Anisotropy is the quality of exhibiting a property that has different values when measured along different axes. Seismic waves in an anisotropic medium travel at different speeds depending on both their polarization (vibration) and their propagation direction. Earth’s inner core was long thought to have been featureless; in the late 1980s and early 1990s, however, it was found to possess strong seismic anisotropy. Seismic compressional waves travel through the inner core on average about 2% faster along the north-south direction (parallel to Earth’s spin axis) than along the east-west direction (parallel to the Equator). The seismic anisotropy comes from the way anisotropic iron crystals composing the solid inner core tend to align along a certain direction.

Subsequent studies using seismic waves have shown a complex three-dimensional structure of the inner-core anisotropy. The outermost part of the inner core is nearly isotropic (that is, exhibiting a property that has a similar value when measured along different axes). The thickness of that weak anisotropic layer varies from the upper 100–250 km (62–155 mi) in the quasi-western inner core (which extends roughly from longitude 40° E westward to longitude 160° E) to the upper 400 km (approximately 250 mi) or more in the quasi-eastern inner core (roughly from longitude 40° E eastward to longitude 160° E). The transition from isotropy in the upper inner core to strong anisotropy in the lower inner core can be sharp in some places, and the structure also varies laterally with longitudes, on all scales, from a hemisphere to a few kilometres. At intermediate depths the quasi-western hemisphere is strongly anisotropic but the quasi-eastern hemisphere is nearly isotropic.

An IIC toward Earth’s centre was proposed in 2002 by Harvard University researchers Miaki Ishii and Adam Dziewonski; they showed that the IIC has a distinct form of anisotropy, with the slow direction at 45° from the spin axis rather than near the equatorial plane, as it occurs in the OIC. Later studies showed considerable uncertainty regarding the IIC’s existence and characteristics. In one such study University of Illinois researchers Xinlei Sun and Xiaodong Song in 2008 confirmed the change in the form of anisotropy and found the radius of the inner sphere, which they called the IIC, to be almost half the radius of the inner core as a whole.

An Inner-Inner Core with Different Crystal Alignment.

All the previous studies assumed a cylindrical anisotropy of the inner core in which the fast axis (that is, the axis in which wave velocities are greatest) was parallel to Earth’s spin axis. Early in 2015 Wang, Song, and others reported that the IIC has a different fast axis, which is near the equatorial plane through Central America and Southeast Asia, in contrast to the north-south fast direction in the OIC. The result indicated that the iron crystals in the IIC are aligned at nearly right angles to those in the OIC.

The finding was based on a new seismic-imaging technique called seismic interferometry or coda-wave cross-correlation. Instead of relying on seismic waves generated by earthquakes, the technique uses the “echoes” from large earthquakes (occurring some 3–12 hours after the event), which manifest as underground reverberations and scatterings producing the coda waves. In traditional analyses small ground disturbances such as the “coda” energy (that is, energy coming from the backscattering of the movement of surface and body waves) occurring after an earthquake and ambient noise (random fluctuations, here largely from ocean waves, that accompany and tend to obscure meaningful signals) are typically discarded. By enhancing the coherent signals in those sources, however, in the past decade this technique has revolutionized the seismic imaging of Earth’s lithosphere with unprecedented resolution. It first became available in 2005 and has since been used routinely with hundreds of research papers published annually with the method.

The study published in 2015 used a signal-processing technique called autocorrelation that allows the detection of repeated signals. That technique is analogous to the process of detecting beats in music to determine a song’s tempo. The stacking of the autocorrelations of the coda-energy readings after major earthquakes at a cluster of close stations (a station array) greatly enhanced the signatures of the waves passing through Earth’s inner core—which was never observed from direct seismic waves caused by the largest earthquakes, let alone smaller ones. That new type of data made it possible to sample the very centre of Earth. Song and colleagues used seismic station arrays at different latitudes and longitudes to examine how waves changed as a function of direction through the inner core. Measurements indicated that seismic waves passing through the inner core along the fast axis of the IIC are as fast as those that travel along the spin axis. The inner core can be separated into an OIC of variable anisotropy, with the fast axis running in parallel to the spin axis, and an IIC spanning half the inner-core radius, with the fast axis running near the equatorial plane.

What’s Next?

There remains much to learn about Earth’s deep interior, and surprises continue to occur. Earth scientists hope to be able to reveal its deepest mysteries. The inner core is small and remote; the very centre of Earth is even harder to sample, yet Earth scientists expect that new technologies will provide the means to extract a completely new set of samples, a feat not possible earlier.

The inner core has taken more than one billion years to grow to its present size, and it has thus preserved a long geologic history at the heart of Earth. By obtaining clearer images of the structure, Earth scientists hope to reveal how the inner core (and the planet itself) has evolved, how it interacts with the magnetic field generated in the fluid outer core, and perhaps how it affects convection in the solid mantle.

Several basic questions remain. What is the main phase of iron in the inner core; is it solid or liquid? What caused crystals in the inner core to align in certain directions and in variable concentrations? Could mantle convection affect the convection of the fluid core and the growth and deformation of the inner core? What caused the difference in crystal alignment between the IIC and the OIC? Earth’s magnetic field is thought to contain a significant equatorial component starting during the Ediacaran Period (about 600 million years ago), which is roughly the age of origin of the IIC. Could those two observations be related? Any answers to such questions will need to be determined by future multidisciplinary and interdisciplinary investigations that include contributions from seismology, geodynamics, and mineral physics.

CITATION INFORMATION

ARTICLE TITLE: The Core Within Earth's Inner Core

WEBSITE NAME: Encyclopaedia Britannica

PUBLISHER: Encyclopaedia Britannica, Inc.

DATE PUBLISHED: 11 December 2015

URL: https://www.britannica.com/topic/Core-Within-Earths-Inner-Core-The-2046720

ACCESS DATE: April 29, 2020

Though the seismic waves from earthquakes are best known for their destructive capabilities, in the hands of geologists, they can be powerful tools of discovery. A research team at the University of Illinois has just used the rumbles from quakes to more closely examine the inner core of our planet, and what they found there was quite a surprise. It seems there's another core inside the inner core that measures about half its diameter.

What demarcates this "inner-inner core" is that the iron crystals it contains are oriented on an east-west axis, unlike the iron crystals in the "outer-inner core" which organize along a north-south axis.

"The fact that we have two regions that are distinctly different may tell us something about how the inner core has been evolving," Xiaodong Song, a professor of geology at UI who worked on the project with visiting postdoctoral researcher Tao Wang, said in a University of Illinois report about the findings. "For example, over the history of the Earth, the inner core might have had a very dramatic change in its deformation regime. It might hold the key to how the planet has evolved."

While multiple components of the inner core have been suggested before, this is the first time the difference in polarity has been noted. "Indeed, the layering of the inner core has been suggested more than 10 years ago, at shallow depths of the inner core and at deeper parts of the inner core as well," Song told Crave. "Everyone assumed before the crystal alignment was north-south. But here we found alignment in the inner-inner core to be nearly east-west."

If all this inner and inner-inner talk sounds confusing, perhaps a quick geology refresher is in order. The Earth consists of three layers: the crust where we live; the mantle, a layer of scalding-hot liquid rock; and the core. The core consists of a liquid outer core containing mainly nickel and iron and a solid inner core made up mostly of iron. Even though the inner core is even hotter than its surroundings, the intense pressure at the Earth's center means the inner core is unable to melt and remains solid, according to a National Geographic entry about the topic.

And now we can add another layer to our Earth's composition: the inner-inner core, which is still mostly solid iron, but has a different polarity than the substance surrounding it.

In "unearthing" the inner-inner core, the research team relied on seismic sensors that pick up the waves that penetrate the planet after an earthquake hits, known as the quake's coda. "The earthquake is like a hammer striking a bell; much like a listener hears the clear tone that resonates after the bell strike, seismic sensors collect a coherent signal in the earthquake's coda," the report says.

"It turns out the coherent signal enhanced by the technology is clearer than the ring itself," said Song. "The basic idea of the method has been around for a while, and people have used it for other kinds of studies near the surface. But we are looking all the way through the center of the Earth."

The researchers' findings were published in the journal Nature on Monday.

The solid inner core (Fig. 1) is the most remote and enigmatic part of our planet, and, next to the crust, is the smallest “official” subdivision of Earth's interior. It was discovered in 1936 (1), and by 1972 it was established that it was solid, albeit with a very small rigidity (2–4). By 1993 it had been established that it was crystalline (5). The inner core is isolated from the rest of Earth by the low-viscosity fluid outer core, and it can rotate, nod, wobble, precess, oscillate, and even flip over, being only loosely constrained by the surrounding shells. Its existence, size, and properties constrain the temperature and mineralogy near the center of the Earth. Among its anomalous characteristics are low rigidity and viscosity (compared with other solids), bulk attenuation, extreme anisotropy, and superrotation (or deformation; refs. 5–8). From seismic velocities and cosmic abundances, we know that it is composed mainly of iron-nickel crystals, and the crystals must exhibit a large degree of common orientation. The inner core is predicted to have very high thermal and electrical conductivity, a nonspherical shape, and frequency-dependent properties; also, it may be partially molten. It may be essential for the existence of the magnetic field and for polarity reversals of this field (D. Gubbin, D. Alfe, G. Masters, D. Price, and M. Gillan, unpublished work). Freezing of the inner core and expulsion of impurities is likely responsible for powering the geodynamo. Yet, the inner core represents less than 1% of the volume of Earth, and only a few seismic waves ever reach it and return to the surface. The inner core is a small target for seismologists, and seismic waves are distorted by passing through the entire Earth before reaching it. Conditions near the center of the Earth are so extreme that both theoreticians and experimenters have difficulty in duplicating its environment. Nevertheless, there has been a recent flurry of activity about the inner core by seismologists, geochemists, dynamicists, materials scientists, and geodynamo theoreticians. Almost everything known or inferred about the inner core from seismology or from indirect inference is controversial. In this issue of PNAS, Ishii and Dziewoński (8) add further intrigue and complication to phenomena near the center of the Earth, and they suggest a complex history for this small object.

Almost everything known or inferred about the inner core, from seismology or indirect inference, is controversial.

Fig 1.

View of the Earth's interior. The volumetric relation of the various regions of the core to the whole Earth is shown: outer core (pale blue) occupies 15%, the inner core (pink) occupies less than 1%, and the innermost inner core (red) constitutes only 0.01% of the Earth's volume. The Earth's core lies beneath 3,000-km thick, heterogeneous mantle (anomalies with higher than average seismic speed are shown in blue and those with lower than average speed are shown in red), making investigations of core properties challenging.

Planets differentiate as they accrete and gain gravitational energy. Timing of this differentiation is a long-standing goal of Earth science (9–13). Density stratification explains the locations of the crust, mantle, and core. The inner core is likely also the result of chemical stratification, although the effect of pressure on the melting point would generate a solid inner core even if it were chemically identical to the outer core. Low-density materials are excluded when solidification is slow, so the inner core may be purer and denser than the outer core. As the inner core crystallizes and the outer core cools, the material held in solution and suspension will plate out, or settle, at the core mantle boundary and may be incorporated into the lowermost mantle. The mantle is usually treated as a chemically homogeneous layer, but this is unlikely. Denser silicates, possibly silicon- and iron-rich, also gravitate toward the lower parts of the mantle. Crustal and shallow mantle materials were sweated out of the Earth as it accreted, and some were apparently never in equilibrium with core material. The effect of pressure on physical properties implies that the mantle and core probably stratified irreversibly upon accretion, that only the outer shells of the mantle participate in surface processes such as volcanism and plate tectonics, and that only the deeper layers currently interact with the core.

The crust, upper mantle, lower mantle, core, and inner core are the textbook subdivisions of the Earth's interior. Seismic tomography is used to map large-scale lateral variations in these major subdivisions. Higher resolution seismic techniques have been used to discover and map small-scale features at the top and bottom of the core (14–16). The classical boundaries inside the Earth (6) were all discovered in the early part of the last century. In the 1960s, boundaries internal to the mantle were discovered at depths of 400 and 650 km and were attributed to solid–solid phase changes (17), in contrast to the others which are chemical or solidification boundaries. More recently, a probable chemical discontinuity was found deep in the mantle (16), and another one was inferred near 900 km (18). Seismic discontinuities are conventionally found by the reflection and refraction of seismic waves, but recently factors such as anisotropy, attenuation, scattering, spectral density, and statistical decorrelations have been used to find the more subtle features. The new region deep in the inner core represents a change in character of the anisotropy pattern (8) and may represent a fundamentally different phenomenon.

The long-standing controversy regarding a drawn-out (100 million years) vs. a rapid (≈1 million year) terrestrial accretion seems to be resolving itself in favor of the shorter time scales and a high-temperature origin. Geophysical data require rapid accretion of Earth and early formation of the core (9). Until recently, rapid accretion has been at odds with accretional theory and isotopic data, but now, these disciplines are also favoring a contracted time scale. A variety of isotopes have confirmed short time intervals between the formation of the solar system and planetary differentiation processes (10–13). This finding has bearing on the age of the inner core and its cooling history.

There are three quite different mechanisms for making a planetary core. In the homogeneous accretion hypothesis, the silicates and the metals accrete together but, as the Earth heats up, the heavy metals percolate downwards, eventually forming large dense accumulations that sink rapidly toward the center, taking the siderophile elements with them. In the heterogenous accretion hypothesis, the refractory condensates (including iron and nickel) from a cooling nebula start to form the nucleus of a planet before the bulk of the silicates and volatiles are available. The late veneer contributes low-temperature condensates and gases, including water, from the far reaches of the solar system. Finally, large late impacts can efficiently and rapidly inject their metallic cores to the center of the impacted planet and trigger additional separation of iron from the mantle. The Moon is a byproduct of one of these late impacts. The material in the core may, therefore, have multiple origins and a complex history. Other issues regarding the inner core involve its age, growth rate, density, temperature, texture, and internal energy sources (refs. 8 and 19–21, and D. Gubbin, D. Alfe, G. Masters, D. Price, and M. Gillan, unpublished work).

The outer core is usually considered to be completely molten because of its low viscosity and inability to transmit shear waves. However, it could contain more than 50% suspended crystals and still behave as a fluid. The boundary of the inner core then could represent the crossing of the geotherm with the melting curve (the conventional explanation) or a compaction boundary where the particle density of the slurry exceeds a threshold. It is usually assumed that the outer core is homogeneous, entirely fluid, and convects turbulently. The inner core also may contain a substantial melt fraction, particularly if there is a large interval between the solidus and the liquidus. It has also been proposed that the inner core is a viscous fluid or a metallic glass (19). The new results on anisotropy make this unlikely. The low, inferred viscosity of the inner core means that it can deform and convect from the influence of tidal and rotational stresses and outer core motions as well as from internally generated stresses. The inner core is one of the few places in the interior where one might expect to see changes on a human timescale. It may exhibit semirigid differential rotation with respect to the mantle but also, and more likely, nonrigid or plastic deformation. Anisotropy is one indicator of such deformation or convection.

Crystals are anisotropic and tend to be oriented by sedimentation, freezing, recrystallization, deformation, and flow. Therefore, we expect the solid portions of the Earth to be anisotropic to the propagation of seismic waves and other material properties. Despite these expectations, seismology proceeded and flourished with the assumption of isotropy until the 1960s. At this point, the theory of seismic anisotropy was worked out and observations verified the expectations (see references in ref. 6). Nevertheless, most seismologists ignored anisotropy until fairly recently in the progress of seismology. Not only is anisotropy a useful tool for determining composition, mineralogy, and deformation from seismology, but Earth models based on isotropy can be completely wrong. Anisotropy is not simply a small perturbation to an essentially isotropic Earth. The variation of seismic wave speeds as a function of direction can be greater than those caused by temperature and composition. In the case of the inner inner core (8), the penetrating seismic waves travel almost radially, so very little information is extractable, except the variation of travel time with azimuth, e.g., equatorial vs. polar paths, or with waves propagating in different directions in the equatorial plane. The size of the Fresnel zone also limits the seismic resolution of the innermost core. Fortunately, high-pressure iron crystals have a large anisotropy (21, 22); otherwise, little could be said about heterogeneity or rotation/deformation of the inner core.

The shape and fabric of the inner core are affected by gravitational forces from the mantle, electromagnetic and viscous stresses from the outer core, and rotational and tidal stresses. These stresses cause irreversible plastic flow, crystal alignment, and recrystallization. Seismic anisotropy is one result.

The inner core is subjected to a variety of external stresses involving variations in orbital and rotational parameters, tides, gravitational tugs from the mantle, viscous drag of the outer core, and electromagnetic forces. It also may generate internal stresses by thermal and chemical variations, anisotropy and cooling, and respond to these by porous flow, differential rotation, convection, and deformation and creation of material anisotropy. Anisotropy can also form by freezing of the inner core and sedimentation on its surface. Small-scale heterogeneity, for example, can melt channels or exsolution fabric and can also generate apparent anisotropy.

The conventional explanation of the formation of the solid inner core involves slow cooling and crystallization. Because the melting temperature increases with pressure, the core will solidify from the center outwards. But this effect also means that as pressure increases because of accretion, the core can pressure-freeze when the Earth reaches a critical size, unless there is a large amount of superheat. Although we know that the magnetic field is ancient and that a solid and growing inner core may be essential to its existence, it is possible that catastrophic events such as the Moon-forming impact may have caused the inner core to reform one or more times. Initial superheat and episodic growth will possibly resolve some of the current energy problems (ref. 20, and D. Gubbin, D. Alfe, G. Masters, D. Price, and M. Gillan, unpublished work). A growing inner core is needed to power the current dynamo, but rapid cooling may have powered the ancient dynamo (D. Gubbin, D. Alfe, G. Masters, D. Price, and M. Gillan, unpublished work). The inner core may, therefore, be much younger than the Earth. The heterogeneity and anisotropy of the inner core may help constrain its apparently complex history.

The inner core has bearing on a wide variety of geophysical, geochemical (23), magnetic field, and planetary problems. Anisotropy is not only an important parameter bearing on core dynamics, but it also makes it possible to characterize and monitor the inner core. Anisotropy has become an indispensable tool to seismologists, rather than the bother it was once considered. And the prospect of finding differences the next time we look offers an excitement unusual in most routine mapping endeavors.

A humongous hunk of iron — that’s how scientists have long thought of Earth’s solid inner core. But new research suggests there’s more to it than that: namely, that the inner part of the inner core may have different physical properties than the outer part. In addition to revealing a new feature in Earth’s layer-cake internal structure, the discovery may shed light on the planet’s formation, say the authors of the study, published in Nature Geoscience.

Inner Core

The inner core is a hot, dense ball of (mostly) iron. It has a radius of about 1,220 kilometers (758 miles). Temperature in the inner core is about 5,200° Celsius (9,392° Fahrenheit). The pressure is nearly 3.6 million atmosphere (atm).

The temperature of the inner core is far above the melting point of iron. However, unlike the outer core, the inner core is not liquid or even molten. The inner core’s intense pressure—the entire rest of the planet and its atmosphere—prevents the iron from melting. The pressure and density are simply too great for the iron atoms to move into a liquid state. Because of this unusual set of circumstances, some geophysicists prefer to interpret the inner core not as a solid, but as a plasma behaving as a solid.

The liquid outer core separates the inner core from the rest of the Earth, and as a result, the inner core rotates a little differently than the rest of the planet. It rotates eastward, like the surface, but it’s a little faster, making an extra rotation about every 1,000 years.

Geoscientists think that the iron crystals in the inner core are arranged in an “hcp” (hexagonal close-packed) pattern. The crystals align north-south, along with Earth’s axis of rotation and magnetic field.

The orientation of the crystal structure means that seismic waves—the most reliable way to study the core—travel faster when going north-south than when going east-west. Seismic waves travel four seconds faster pole-to-pole than through the Equator.

Growth in the Inner Core

As the entire Earth slowly cools, the inner core grows by about a millimeter every year. The inner core grows as bits of the liquid outer core solidify or crystallize. Another word for this is “freezing,” although it’s important to remember that iron’s freezing point more than 1,000° Celsius (1,832° Fahrenheit).

The growth of the inner core is not uniform. It occurs in lumps and bunches, and is influenced by activity in the mantle.

Growth is more concentrated around subduction zones—regions where tectonic plates are slipping from the lithosphere into the mantle, thousands of kilometers above the core. Subducted plates draw heat from the core and cool the surrounding area, causing increased instances of solidification.

Growth is less concentrated around “superplumes” or LLSVPs. These ballooning masses of superheated mantle rock likely influence “hot spot” volcanism in the lithosphere, and contribute to a more liquid outer core.

The core will never “freeze over.” The crystallization process is very slow, and the constant radioactive decay of Earth’s interior slows it even further. Scientists estimate it would take about 91 billion years for the core to completely solidify—but the sun will burn out in a fraction of that time (about 5 billion years).

Core Hemispheres

Just like the lithosphere, the inner core is divided into eastern and western hemispheres. These hemispheres don’t melt evenly, and have distinct crystalline structures.

The western hemisphere seems to be crystallizing more quickly than the eastern hemisphere. In fact, the eastern hemisphere of the inner core may actually be melting.

Inner Inner Core

Geoscientists recently discovered that the inner core itself has a core—the inner inner core. This strange feature differs from the inner core in much the same way the inner core differs from the outer core. Scientists think that a radical geologic change about 500 million years ago caused this inner inner core to develop.

The crystals of the inner inner core are oriented east-west instead of north-south. This orientation is not aligned with either Earth’s rotational axis or magnetic field. Scientists think the iron crystals may even have a completely different structure (not hcp), or exist at a different phase.

Magnetism

Earth’s magnetic field is created in the swirling outer core. Magnetism in the outer core is about 50 times stronger than it is on the surface.

It might be easy to think that Earth’s magnetism is caused by the big ball of solid iron in the middle. But in the inner core, the temperature is so high the magnetism of iron is altered. Once this temperature, called the Curie point, is reached, the atoms of a substance can no longer align to a magnetic point.

Dynamo Theory

Some geoscientists describe the outer core as Earth’s “geodynamo.” For a planet to have a geodynamo, it must rotate, it must have a fluid medium in its interior, the fluid must be able to conduct electricity, and it must have an internal energy supply that drives convection in the liquid.

Variations in rotation, conductivity, and heat impact the magnetic field of a geodynamo. Mars, for instance, has a totally solid core and a weak magnetic field. Venus has a liquid core, but rotates too slowly to churn significant convection currents. It, too, has a weak magnetic field. Jupiter, on the other hand, has a liquid core that is constantly swirling due to the planet’s rapid rotation.

Earth is the “Goldilocks” geodynamo. It rotates steadily, at a brisk 1,675 kilometers per hour (1,040 miles per hour) at the Equator. Coriolis forces, an artifact of Earth’s rotation, cause convection currents to be spiral. The liquid iron in the outer core is an excellent electrical conductor, and creates the electrical currents that drive the magnetic field.

The energy supply that drives convection in the outer core is provided as droplets of liquid iron freeze onto the solid inner core. Solidification releases heat energy. This heat, in turn, makes the remaining liquid iron more buoyant. Warmer liquids spiral upward, while cooler solids spiral downward under intense pressure: convection.

Earth’s Magnetic Field

Earth’s magnetic field is crucial to life on our planet. It protects the planet from the charged particles of the solar wind. Without the shield of the magnetic field, the solar wind would strip Earth’s atmosphere of the ozone layer that protects life from harmful ultraviolet radiation.

Although Earth’s magnetic field is generally stable, it fluctuates constantly. As the liquid outer core moves, for instance, it can change the location of the magnetic North and South Poles. The magnetic North Pole moves up to 64 kilometers (40 miles) every year.

Fluctuations in the core can cause Earth’s magnetic field to change even more dramatically. Geomagnetic pole reversals, for instance, happen about every 200,000 to 300,000 years. Geomagnetic pole reversals are just what they sound like: a change in the planet’s magnetic poles, so that the magnetic North and South Poles are reversed. These “pole flips” are not catastrophic—scientists have noted no real changes in plant or animal life, glacial activity, or volcanic eruptions during previous geomagnetic pole reversals.

Studying the Core

Geoscientists cannot study the core directly. All information about the core has come from sophisticated reading of seismic data, analysis of meteorites, lab experiments with temperature and pressure, and computer modeling.

Most core research has been conducted by measuring seismic waves, the shock waves released by earthquakes at or near the surface. The velocity and frequency of seismic body waves changes with pressure, temperature, and rock composition.

In fact, seismic waves helped geoscientists identify the structure of the core itself. In the late 19th century, scientists noted a “shadow zone” deep in the Earth, where a type of body wave called an s-wave either stopped entirely or was altered. S-waves are unable to transmit through fluids or gases. The sudden “shadow” where s-waves disappeared indicated that Earth had a liquid layer.

In the 20th century, geoscientists discovered an increase in the velocity of p-waves, another type of body wave, at about 5,150 kilometers (3,200 miles) below the surface. The increase in velocity corresponded to a change from a liquid or molten medium to a solid. This proved the existence of a solid inner core.

Meteorites, space rocks that crash to Earth, also provide clues about Earth’s core. Most meteorites are fragments of asteroids, rocky bodies that orbit the sun between Mars and Jupiter. Asteroids formed about the same time, and from about the same material, as Earth. By studying iron-rich chondrite meteorites, geoscientists can get a peek into the early formation of our solar system and Earth’s early core.

In the lab, the most valuable tool for studying forces and reactions at the core is the diamond anvil cell. Diamond anvil cells use the hardest substance on Earth (diamonds) to simulate the incredibly high pressure at the core. The device uses an x-ray laser to simulate the core’s temperature. The laser is beamed through two diamonds squeezing a sample between them.

Complex computer modeling has also allowed scientists to study the core. In the 1990s, for instance, modeling beautifully illustrated the geodynamo—complete with pole flips.

The core is the hottest, densest part of the Earth.

Illustration by Chuck Carter

Buried Treasure

Although the inner core is mostly NiFe, the iron catastrophe also drove heavy siderophile elements to the center of the Earth. In fact, one geoscientist calculated that there are 1.6 quadrillion tons of gold in the core—that’s enough to gild the entire surface of the planet half-a-meter (1.5 feet) thick.

Geoneutrinos

One of the most bizarre ways geoscientists study the core is through “geoneutrinos.” Geoneutrinos are neutrinos, the lightest subatomic particle, released by the natural radioactive decay of potassium, thorium, and uranium in Earth’s interior. By studying geoneutrinos, scientists can better understand the composition and spatial distribution of materials in the mantle and core.

Subterranean Fiction

“Subterranean fiction” describes adventure stories taking place deep below the surface of the Earth. Jules Verne’s Journey to the Center of the Earth is probably the most well-known piece of subterranean fiction. Other examples include Dante Alighieri’s Divine Comedy, in which the deepest center of Earth is Hell itself; the movie Ice Age: Dawn of the Dinosaurs, in which an underground world allows dinosaurs to survive into the present day; and the rabbit hole of Alice’s Adventures in Wonderland—which was originally titled Alice’s Adventures Under Ground.

Inge Lehman

Inge Lehman, who called herself “the only Danish seismologist” working in the 1930s, was a pioneering figure in the study of Earth’s interior. Lehman was the first to identify Earth’s solid inner core, and became a leading expert in the structure of the upper mantle as well. She was the first woman to receive the prestigious William Bowie Medal, the highest honor awarded by the American Geophysical Union. In 1997, the AGU created the Inge Lehman Medal, recognizing a scientist’s “outstanding contributions to the understanding of the structure, composition, and dynamics of the Earth's mantle and core.”

Planetary Cores
All known planets have metal cores. Even the gas giants of our solar system, such as Jupiter and Saturn, have iron and nickel at their cores.

Scientists have used echoes produced by earthquakes to gain new insights into what’s going on right at the centre of the Earth, and suggest that the central core is actually split into two sections.

Conventional wisdom says that the Earth is made of of several layers - like an onion - consisting of the solid, outer silicate crust; the viscous mantle; the liquid outer core; and the inner core, made up of a solid ball of iron. If we wanted to see what was going on right down there in the core, we'd have to drill at least 5,000 kilometres into the Earth, which isn't exactly feasible - the deepest we’ve ever gone is 12.262 km, which is a mere 0.2 percent of the distance down to the inner core. So scientists have to use more indirect methods to figure out what’s down there.

By looking at how earthquake echoes change as they travel back and forth from one side of the Earth to the other, scientists now suspect that the iron crystals in the centre and on the outside of the inner core have very different structures. So much so, that it looks like the newly proposed 'inner inner core’ takes up about half the diameter of the entire inner core, with its iron crystals pointing in an east-to-west direction. The iron crystals in the outer inner core, on the other hand, appear to be pointing north-to-south.

And not only are the iron crystals in the outer and inner inner core aligned differently, but they also appear to behave differently too, the researchers report, which means that these two different core layers could be made up of different types of crystals. Perhaps, they say, these different structures and behaviours can tell us more about the turbulent period of formation that the Earth underwent 4.6 billion years ago, and what the inner core is going to do next. We know that it’s growing about 0.05 mm each year, but how will this change things on a larger scale in the future?

"The fact that we have two regions that are distinctly different may tell us something about how the inner core has been evolving," one of the team, geologist Xiaodong Song from the University of Illinois in the US, said in a press release. "For example, over the history of the Earth, the inner core might have had a very dramatic change in its deformation regime. It might hold the key to how the planet has evolved. We are right in the centre - literally, the centre of the Earth.”

The results have been published in the journal Nature Geoscience.

"People have noticed differences in the way seismic waves travel through the outer parts of the inner core and its innermost reaches before, but never before have they suggested that the alignment of crystalline iron that makes up this region is completely askew compared to the outermost parts," geologist Simon Redfern from the University of Cambridge in the UK, who was not involved in the research, told Rebecca Morelle at BBC News. "If this is true, it would imply that something very substantial happened to flip the orientation of the core to turn the alignment of crystals in the inner core north-south as is seen today in its outer parts."

Discovery[edit]

The Earth was discovered to have a solid inner core distinct from its molten outer core in 1936, by the Danish seismologist Inge Lehmann,^[5]^[6] who deduced its presence by studying seismograms from earthquakes in New Zealand. She observed that the seismic waves reflect off the boundary of the inner core and can be detected by sensitive seismographs on the Earth's surface. She inferred a radius of 1400 km for the inner core, not very far from the currently accepted value of 1221 km.^[7]^[8]^[9] In 1938, B. Gutenberg and C. Richter analyzed a more extensive set of data and estimated the thickness of the outer core as 1950 km with a steep but continuous 300 km thick transition to the inner core; implying a radius between 1230 and 1530 km for the inner core.^[10]^:p.372

A few years later, in 1940, it was hypothesized that this inner core was made of solid iron.^{[citation needed]} In 1952, F. Birch published a detailed analysis of the available data and concluded that the inner core was probably crystalline iron.^[11]

The boundary between the inner and outer cores is sometimes called the "Lehmann discontinuity",^[12] although the name usually refers to another discontinuity. The name "Bullen" or "Lehmann-Bullen discontinuity", after K. Bullen has been proposed,^{[citation needed]} but its use seems to be rare. The rigidity of the inner core was confirmed in 1971.^[13]

Dziewoński and Gilbert established that measurements of normal modes of vibration of Earth caused by large earthquakes were consistent with a liquid outer core.^[14] In 2005, shear waves were detected passing through the inner core; these claims were initially controversial, but are now gaining acceptance.^[15]

Data sources[edit]

Seismic waves[edit]

Almost all direct measurements that we have about the physical properties of the inner core are the seismic waves that pass through it. The most informative waves are generated by deep earthquakes, 30 km or more below the surface of the Earth (where the mantle is relatively more homogeneous) and recorded by seismographs as they reach the surface, all over the globe.

Seismic waves include "P" (primary or pressure) waves, compressional waves that can travel through solid or liquid materials, and "S" (secondary or shear) shear waves that can only propagate through rigid elastic solids. The two waves have different velocities and are damped at different rates as they travel through the same material.

Of particular interest are the so-called "PKiKP" waves—pressure waves (P) that start near the surface, cross the mantle-core boundary, travel through the core (K), are reflected at the inner core boundary (i), cross again the liquid core (K), cross back into the mantle, and are detected as pressure 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 interpret when the path from source to detector is close to a straight line—namely, when the receiver is just above the source for the reflected PKiKP waves, and antipodal to it for the transmitted PKIKP waves.^[16]

While S waves cannot reach or leave the inner core as such, P waves can be converted into S waves, and vice-versa, as they hit the boundary between the inner and outer core at an oblique angle. The "PKJKP" waves are similar to the PKIKP waves, but are converted into S-waves when they enter the inner core, travel through it as S-waves (J), and are converted again into P waves when they exit the inner core. Thanks to this phenomenon, it is known that the inner core can propagate S waves, and therefore must be solid.

Other sources[edit]

Other sources of information about the inner core include

The magnetic field of the Earth. While it seems to be generated mostly by fluid and electric currents in the outer core, those currents are strongly affected by the presence of the solid inner core and by the heat that flows out of it. (Although made of iron, the core is apparently not ferromagnetic, due to its extremely high temperature.)^{[citation needed]}
The Earth's mass, its gravitational field, and its angular inertia. These are all affected by the density and dimensions of the inner layers.^[17]
The natural oscillation frequencies and modes of the whole Earth oscillations, when large earthquakes make the planet "ring" like a bell. These oscillations too depend strongly on the density, size, and shape of the inner layers.^[18]

Physical properties[edit]

Seismic wave velocity[edit]

The velocity of the S-waves in the core varies smoothly from about 3.7 km/s at the center to about 3.5 km/s at the surface. That is considerably less than the velocity of S-waves in the lower crust (about 4.5 km/s) and less than half the velocity in the deep mantle, just above the outer core (about 7.3 km/s).^[4]^:fig.2

The velocity of the P-waves in the core also varies smoothly through the inner core, from about 11.4 km/s at the center to about 11.1 km/s at the surface. Then the speed drops abruptly at the inner-outer core boundary to about 10.4 km/s.^[4]^:fig.2

Size and shape[edit]

On the basis of the seismic data, the inner core is estimated to be about 1221 km in radius (2442 km in diameter);,^[4] which is about 19% of the radius of the Earth and 70% of the radius of the Moon.

Its volume is about 7.6 billion cubic km (7.6 × 10¹⁸ m³), which is about ¹⁄₁₄₀ (0.7%) of the volume of the whole Earth.

Its shape is believed to be very close to an oblate ellipsoid of revolution, like the surface of the Earth, only that more spherical: The flattening f is estimated to be between ¹⁄₄₀₀ and ¹⁄₄₁₆;^[17]^:f.2 meaning that the radius along the Earth's axis is estimated to be about 3 km shorter than the radius at the equator. In comparison, the flattening of the Earth as a whole is very close to ¹⁄₃₀₀, and the polar radius is 21 km shorter than the equatorial one.

Pressure and gravity[edit]

The pressure in the Earth's inner core is slightly higher than it is at the boundary between the outer and inner cores: it ranges from about 330 to 360 gigapascals (3,300,000 to 3,600,000 atm).^[4]^[19]^[20]

The acceleration of gravity at the surface of the inner core can be computed to be 4.3 m/s²;^[21] which is less than half the value at the surface of the Earth (9.8 m/s²).

Density and mass[edit]

The density of the inner core is believed to vary smoothly from about 13.0 kg/L (= g/cm³ = t/m³) at the center to about 12.8 kg/L at the surface. As it happens with other material properties, the density drops suddenly at that surface: the liquid just above the inner core is believed to be significantly less dense, at about 12.1 kg/L.^[4] For comparison, the average density in the upper 100 km of the Earth is about 3.4 kg/L.

That density implies a mass of about 10²³ kg for the inner core, which is 1/60 (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 pressure which iron is under at the boundary of the inner core (about 330 GPa). From these considerations, in 2002 D. Alfè and other estimated its temperature as between 5,400 K (5,100 °C; 9,300 °F) and 5,700 K (5,400 °C; 9,800 °F).^[4] However, in 2013 S. Anzellini and others obtained experimentally a substantially higher temperature for the melting point of iron, 6230 ± 500 K.^[22]

Iron can be solid at such high temperatures only because its melting temperature increases dramatically at pressures of that magnitude (see the Clausius–Clapeyron relation).^[23]^[24]

Magnetic field[edit]

In 2010, B. Buffet determined that the average magnetic field in the liquid outer core is about 2.5 milliteslas (25 gauss), which is about 40 times the maximum strength 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 motion of the liquid through the local magnetic field creates electric currents, that dissipate energy as heat according to Ohm's law. This dissipation, in turn, dampens the tidal motions and explains previously detected anomalies in Earth's nutation. From the magnitude of the latter effect he could calculate the magnetic field.^[25] The field inside the inner core presumably has a similar strength. 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 as if it was solid, the measurements cannot distinguish between a perfectly solid material from an extremely viscous one. Some scientists have therefore considered whether there may be slow convection in the inner core (as is believed to exist in the mantle). That could be an explanation for the anisotropy detected in seismic studies. In 2009, B. Buffett estimated the viscosity of the inner core at 10¹⁸ Pa·s;^[26] which is a sextillion times the viscosity of water, and more than a billion times that of pitch.

Composition[edit]

There is still no direct evidence about the composition of the inner core. However, based on the relative prevalence of various chemical elements in the Solar System, the theory of planetary formation, and constraints imposed or implied by the chemistry of the rest of the Earth's volume, the inner core is believed to consist primarily of an iron–nickel alloy.

At the known pressures and estimated temperatures of the core, it is predicted that pure iron could be solid, but its density would exceed the known density of the core by approximately 3%. That result implies the presence of lighter elements in the core, such as silicon, oxygen, or sulfur, in addition to the probable presence of nickel.^[27] Recent estimates (2007) allow for up to 10% nickel and 2–3% of unidentified lighter elements.^[4]

According to computations by D. Alfè and others, the liquid outer core contains 8–13% of oxygen, but as the iron crystallizes out to form the inner core the oxygen is mostly left in the liquid.^[4]

Laboratory experiments and analysis of seismic wave velocities seem to indicate that the inner core consists specifically of ε-iron, a crystalline form of the metal with the hexagonal close-packed (hcp) structure. That structure can still admit the inclusion of small amounts of nickel and other elements.^[16]^[28]

Also, if the inner core grows by precipitation of frozen particles falling onto its surface, then some liquid can also be trapped in the pore spaces. In that case, some of this residual fluid may still persist to some small degree in much of its interior.^{[citation needed]}

Structure[edit]

Many scientists had initially expected that the inner core would be found to be homogeneous, because that same process should have proceeded uniformly during its entire formation. It was even suggested that Earth's inner core might be a single crystal of iron.^[29]

Axis-aligned anisotropy[edit]

In 1983, G. Poupinet and others observed that the travel time of PKIKP waves (P-waves that travel through the inner core) was about 2 seconds less for straight north-south paths than straight paths on the equatorial plane.^[30] Even taking into account the flattening of the Earth at the poles (about 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 core about 1% faster in the north-south direction than along directions perpendicular to that.^[31]

This P-wave speed anisotropy has been confirmed by later studies, including more seismic data^[16] and study of the free oscillations of the whole Earth.^[18] Some authors have claimed higher values for the difference, up to 4.8%; however, in 2017 D. Frost and B. Romanowicz confirmed that the value is between 0.5% and 1.5%.^[32]

Non-axial anisotropy[edit]

Some authors have claimed that P-wave speed is faster in directions that are oblique or perpendicular to the N-S axis, at least in some regions of the inner core.^[33] However, these claims have been disputed by D. Frost and B. Romanowicz, who instead claim that the direction of maximum speed is as close to the Earth's rotation axis as can be determined.^[34]

Causes of anisotropy[edit]

Laboratory data and theoretical computations indicate that the propagation of pressure 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 north-south direction could account for the observed seismic anomaly.^[16]

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 flow would cause the crystals to partially reorient themselves according to the direction of the flow. In 1996, S. Yoshida and others proposed that such a flow could be caused by higher rate of freezing at the equator than at polar latitudes. An equator-to-pole flow then would set up in the inner core, tending to restore the isostatic equilibrium of its surface.^[35]^[28]

Others suggested that the required flow could be caused by slow thermal convection inside the inner core.T. Yukutake claimed in 1998 that such convective motions were unlikely.^[36]However, 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.^[26]

On the other hand, M. Bergman in 1997 proposed that the anisotropy was due to an observed tendency of iron crystals to grow faster when their crystallographic axes are aligned with the direction of the cooling heat flow. He, therefore, proposed that the heat flow out of the inner core would be biased towards the radial direction.^[37]

In 1998, S. Karato proposed that changes in the magnetic field might also deform the inner core slowly over time.^[38]

Multiple layers[edit]

In 2002, M. Ishii and A. Dziewoński presented evidence that the solid inner core contained an "innermost inner core" (IMIC) with somewhat different properties than the shell around it. The nature of the differences and radius of the IMIC are still unresolved as of 2019, with proposals for the latter ranging from 300 km to 750 km.^[39]^[40]^[41]^[34]

A. Wang and X. Song recently proposed a three-layer model, with an "inner inner core" (IIC) with about 500 km radius, an "outer inner core" (OIC) layer about 600 km thick, and an isotropic shell 100 km thick. In this model, the "faster P-wave" direction would be parallel to the Earth's axis in the OIC, but perpendicular to that axis in the IIC.^[33] However, conclusion has been disputed by claims that there need not be sharp discontinuities in the inner core, only a gradual change of properties with depth.^[34]

Lateral variation[edit]

In 1997, S. Tanaka and H. Hamaguchi claimed, on the basis of seismic data, that the anisotropy of the inner core material, while oriented N-S, was more pronounced in "eastern" hemisphere of the inner core (at about 110 °E longitude, roughly under Borneo) than in the "western" hemisphere (at about 70 °W, roughly under Colombia.^[42]^: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.^[43] C. Finlay conjectured that this process could explain the asymmetry in the Earth's magnetic field.^[44]

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 only 0.5 to 1.5% faster than in equatorial directions, and no clear signs of E-W variation.^[32]

Other structure[edit]

Other researchers claim that the properties of the inner core's surface vary from place to place across distances as small as 1 km. This variation is surprising since lateral temperature variations along the inner-core boundary are known to be extremely small (this conclusion is confidently constrained by magnetic field observations).^{[citation needed]}

Growth[edit]

Schematic of the Earth's inner core and outer core motion and the magnetic field it generates.

The Earth's inner core is thought to be slowly growing as the liquid outer core at the boundary with the inner core cools and solidifies due to the gradual cooling of the Earth's interior (about 100 degrees Celsius per billion years).^[45]

According to calculations by Alfé and others, as the iron crystallizes onto the inner core, the liquid just above it becomes enriched in oxygen, and therefore less dense than the rest of the outer core. This process creates convection currents in the outer core, which are thought to be the prime driver for the currents that create the Earth's magnetic field.^[4]

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]

Because the inner core is not rigidly connected to the Earth's solid mantle, the possibility that it rotates slightly more quickly or slowly than the rest of Earth has long been entertained.^[46]^[47] 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 aforementioned property that it transmits waves more quickly in some directions. In 1996, X. Song and P. Richards estimated this "super-rotation" of the inner core relative to the mantle as about one degree per year.^[48]^[49] 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 estimate to 0.3 to 0.5 degree per year.^[50]

In 1999, M. Greff-Lefftz and H. Legros noted that the gravitational fields of the Sun and Moon that are responsible for ocean tides also apply torques to the Earth, affecting its axis of rotation and a slowing down of its rotation rate. Those torques are felt mainly by the crust and mantle, so that their rotation axis 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 axis of the inner core wobbles (nutates) slightly with a period of about 1 day. With some assumptions on the evolution of the Earth, they conclude that the fluid motions in the outer core would have entered resonance with the tidal forces at several times in the past (3.0, 1.8, 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 have stopped the growth of the inner core.^[51]

Age[edit]

Theories about the age of the core are necessarily part of theories of the history of Earth as a whole. This has been a long debated topic and is still under discussion at the present time. It is widely believed that the Earth's solid inner core formed out of an initially completely liquid core as the Earth cooled down. However, there is still no firm evidence about the time when this process started.^[3]

Age estimates from
different studies and methods

T = thermodynamic modeling
P = paleomagnetism analysis
**(R)** = with radioactive elements
**(N)** = without them
Date	Authors	Age	Method
2001	Labrosse et al.^[52]	1±0.5	T(N)
2003	Labrosse^[53]	~2	T(R)
2011	Smirnov et al.^[54]	2–3.5	P
2014	Driscoll and Bercovici^[55]	0.65	T
2015	Labrosse^[56]	< 0.7	T
2015	Biggin et al.^[57]	1–1.5	P
2016	Ohta et al.^[58]	< 0.7	T
2016	Konôpková et al.^[59]	< 4.2	T
2019	Bono et al.^[60]	0.5	P

Two main approaches have been used to infer the age of the inner core: thermodynamic modeling of the cooling of the Earth, and analysis of paleomagnetic evidence. The estimates yielded by these methods still vary over a large range, from 0.5 to 2 billion years old.

Thermodynamic evidence[edit]

Heat flow of the inner earth, according to S. T. Dye^[61] and R. Arevalo.^[62]

One of the ways to estimate the age of the inner core is by modeling the cooling of the Earth, constrained by a minimum value for the heat flux at the core–mantle boundary (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 at present 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.^[63]^[52]

In 2001, S. Labrosse and others, assuming that there were no radioactive elements in the core, gave an estimate of 1±0.5 billion years for the age of the inner core — considerably less than the estimated age of the Earth and of its liquid core (about 4.5 billion years)^[52] In 2003, the same group concluded that, if the core contained a reasonable amount of radioactive elements, the inner core's age could be a few hundred million years older.^[53]

In 2012, theoretical computations by M. Pozzo and others indicated that the electrical conductivity of iron and other hypothetical core materials, at the high pressures and temperatures expected there, were two or three times higher than assumed in previous research.^[64] These predictions were confirmed in 2013 by measurements by Gomi and others.^[65] 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 age to less than 700 million years old.^[56]^[58]

However, in 2016 Konôpková and others directly measured the thermal conductivity of solid iron at inner core conditions, and obtained a much lower value, 18–44 W/m/K. With those values, they obtained an upper bound of 4.2 billion years for the age of the inner core, compatible with the paleomagnetic evidence.^[59]

In 2014, Driscoll and Bercovici published a thermal history of the Earth that avoided the so-called mantle thermal catastropheand new core paradox by invoking 3 TW of radiogenic heating by the decay of 40K in the core. Such high abundances of K in the core are not supported by experimental partitioning studies, so such a thermal history remains highly debatable.^[55]

Paleomagnetic evidence[edit]

Another way to estimate the age of the Earth is to analyze changes in the magnetic field of Earth during its history, as trapped in rocks that formed at various times (the "paleomagnetic record"). The presence or absence of the solid inner core could result in very different dynamic processes in the core which could lead to noticeable changes in the magnetic field.^[66]

In 2011, Smirnov and others published an analysis of the paleomagnetism in a large sample of rocks that formed in the Neoarchean (2.8 to 2.5 billion years ago) and the Proterozoic (2.5 to 0.541 billion). They found that the geomagnetic field was closer to that of a magnetic dipole during the Neoarchean than after it. They interpreted that change as evidence that the dynamo effect was more deeply seated in the core during that epoch, whereas in the later time currents closer to the core-mantle boundary grew in importance. They further speculate that the change may have been due to growth of the solid inner core between 3.5 an 2 billion years ago.^[54]

In 2015, Biggin and others published the analysis of an extensive and carefully selected set of Precambrian samples and observed a prominent increase in the Earth's magnetic field strength and variance around 1–1.5 billion years ago. This change had not been noticed before 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's thermal evolution.^[57]

In 2016, P. Driscoll published an evolving numerical dynamo model that made a detailed prediction of the paleomagnetic field evolution over 0-2 Ga. The evolving dynamo model was driving by time-variable boundary conditions produced by the thermal history solution in Driscoll and Bercovici (2014). The evolving dynamo model predicted a strong-field dynamo prior to 1.7 Ga that is multipolar, a strong-field dynamo from 1.0-1.7 Ga that is predominantly dipolar, a weak-field dynamo from 0.6-1.0 Ga that is a non-axial dipole, and a strong-field dynamo after inner core nucleation from 0-0.6 Ga that is predominantly dipolar.^[67]

An analysis of rock samples from the Ediacaran epoch (formed about 565 million years ago), published by Bono and others in 2019, revealed unusually low intensity and two distinct directions for the geomagnetic field during that time that provides support for the predictions by Driscoll (2016). Considering other evidence of high frequency of magnetic field reversals around that time, they speculate that those anomalies could be due to the onset of formation of the inner core, which would then be 0.5 billion years old.^[60] A News and Views by P. Driscoll summarizes the state of the field following the Bono results.^[68]

Earth's Interior

Earth’s core is the very hot, very dense center of our planet. The ball-shaped core lies beneath the cool, brittle crust and the mostly-solid mantle. Click through the gallery to learn more about the core and the rest of Earth's interior.

Earth’s core is the very hot, very dense center of our planet. The ball-shaped core lies beneath the cool, brittle crust and the mostly-solid mantle. The core is found about 2,900 kilometers (1,802 miles) below Earth’s surface, and has a radius of about 3,485 kilometers (2,165 miles).

Planet Earth is older than the core. When Earth was formed about 4.5 billion years ago, it was a uniform ball of hot rock. Radioactive decay and leftover heat from planetary formation (the collision, accretion, and compression of space rocks) caused the ball to get even hotter. Eventually, after about 500 million years, our young planet’s temperature heated to the melting point of iron—about 1,538° Celsius (2,800° Fahrenheit). This pivotal moment in Earth’s history is called the iron catastrophe.

The iron catastrophe allowed greater, more rapid movement of Earth’s molten, rocky material. Relatively buoyant material, such as silicates, water, and even air, stayed close to the planet’s exterior. These materials became the early mantle and crust. Droplets of iron, nickel, and other heavy metals gravitated to the center of Earth, becoming the early core. This important process is called planetary differentiation.

Earth’s core is the furnace of the geothermal gradient. The geothermal gradient measures the increase of heat and pressure in Earth’s interior. The geothermal gradient is about 25° Celsius per kilometer of depth (1° Fahrenheit per 70 feet). The primary contributors to heat in the core are the decay of radioactive elements, leftover heat from planetary formation, and heat released as the liquid outer core solidifies near its boundary with the inner core.

Unlike the mineral-rich crust and mantle, the core is made almost entirely of metal—specifically, iron and nickel. The shorthand used for the core’s iron-nickel alloys is simply the elements’ chemical symbols—NiFe.

Elements that dissolve in iron, called siderophiles, are also found in the core. Because these elements are found much more rarely on Earth’s crust, many siderophiles are classified as “precious metals.” Siderophile elements include gold, platinum, and cobalt.

Another key element in Earth’s core is sulfur—in fact 90% of the sulfur on Earth is found in the core. The confirmed discovery of such vast amounts of sulfur helped explain a geologic mystery: If the core was primarily NiFe, why wasn’t it heavier? Geoscientists speculated that lighter elements such as oxygen or silicon might have been present. The abundance of sulfur, another relatively light element, explained the conundrum.

Although we know that the core is the hottest part of our planet, its precise temperatures are difficult to determine. The fluctuating temperatures in the core depend on pressure, the rotation of the Earth, and the varying composition of core elements. In general, temperatures range from about 4,400° Celsius (7,952° Fahrenheit) to about 6,000° Celsius (10,800° Fahrenheit).

The core is made of two layers: the outer core, which borders the mantle, and the inner core. The boundary separating these regions is called the Bullen discontinuity.

Outer Core

The outer core, about 2,200 kilometers (1,367 miles) thick, is mostly composed of liquid iron and nickel. The NiFe alloy of the outer core is very hot, between 4,500° and 5,500° Celsius (8,132° and 9,932° Fahrenheit).

The liquid metal of the outer core has very low viscosity, meaning it is easily deformed and malleable. It is the site of violent convection. The churning metal of the outer core creates and sustains Earth’s magnetic field.

The hottest part of the core is actually the Bullen discontinuity, where temperatures reach 6,000° Celsius (10,800° Fahrenheit)—as hot as the surface of the sun.

Inner Core

The temperature of the inner core is far above the melting point of iron. However, unlike the outer core, the inner core is not liquid or even molten. The inner core’s intense pressure—the entire rest of the planet and its atmosphere—prevents the iron from melting. The pressure and density are simply too great for the iron atoms to move into a liquid state. Because of this unusual set of circumstances, some geophysicists prefer to interpret the inner core not as a solid, but as a plasma behaving as a solid.

Growth in the Inner Core

The growth of the inner core is not uniform. It occurs in lumps and bunches, and is influenced by activity in the mantle.

Core Hemispheres

Just like the lithosphere, the inner core is divided into eastern and western hemispheres. These hemispheres don’t melt evenly, and have distinct crystalline structures.

The western hemisphere seems to be crystallizing more quickly than the eastern hemisphere. In fact, the eastern hemisphere of the inner core may actually be melting.

Inner Inner Core

Magnetism

Earth’s magnetic field is created in the swirling outer core. Magnetism in the outer core is about 50 times stronger than it is on the surface.

Dynamo Theory

Earth’s Magnetic Field

Studying the Core

Complex computer modeling has also allowed scientists to study the core. In the 1990s, for instance, modeling beautifully illustrated the geodynamo—complete with pole flips.

core

The core is the hottest, densest part of the Earth.

Illustration by Chuck Carter

Buried Treasure

Geoneutrinos

Subterranean Fiction

“Subterranean fiction” describes adventure stories taking place deep below the surface of the Earth. Jules Verne’s Journey to the Center of the Earth is probably the most well-known piece of subterranean fiction. Other examples include Dante Alighieri’s Divine Comedy, in which the deepest center of Earth is Hell itself; the movie Ice Age: Dawn of the Dinosaurs, in which an underground world allows dinosaurs to survive into the present day; and the rabbit hole of Alice’s Adventures in Wonderland—which was originally titled Alice’s Adventures Under Ground.

Inge Lehman

Planetary Cores

All known planets have metal cores. Even the gas giants of our solar system, such as Jupiter and Saturn, have iron and nickel at their cores.

Why is the interior of the Earth hot?

The interior of Earth is very hot (the temperature of the core reaches more than 5,000 degrees Celsius) for two main reasons:

The heat from when the planet formed,
The heat from the decay of radioactive elements.

The Earth was formed by the process of accretion. After the creation of our solar system, meteorites gravitationally attracted each other and formed bigger objects, which attracted bigger masses, until our planets reach their current size. This process accumulated a lot of heat; when two objects collide, heat is generated. That is why your hands will get hot when you clap them for too long, or a nail gets very hot when you hammer it for a long time. This heat has not dissipated totally and represents about 10% of the total heat inside the Earth.

The main source of heat is the decay of radioactive elements. Radioactive decay is a natural process; unstable elements like 238U (Uranium) or 40K (Potassium) stabilise with time and produce what we call daughter products: 206P (Lead) for Uranium and 40Ar (Argon) for Potassium. This process produces heat, which represents about 90% of the total heat inside the Earth.

The behavior of Earth's core and the core's ingredients besides iron are major geological mysteries. Scientists can't exactly go take a sample. Yet understanding the core's exact makeup and conditions is a big deal for those who are trying to understand how our planet's complicated geophysical systems work together.

Not only is it likely the Earth's largely iron core plays a role in the movements of continentsover millions of years, it plays a major role in preserving life here: The roiling iron heart of our planet helps maintain the Earth's magnetic field, which helps shield life on the surface from damaging solar energy. In addition, it holds valuable clues about how the planet formed.

"Pinpointing the properties of iron is the gold standard — or, I guess, 'iron standard ' — for how the core behaves," Jennifer Jackson, assistant professor of mineral physics at Caltech, said in a statement. "That is where most discussions about the deep interior of the Earth begin. The temperature distribution, the formation of the planet — it all goes back to the core."

So how to study this inaccessible region lying roughly 1,860 miles (3,000 kilometers) below the planet's surface? Scientists at Caltech have used laboratory setups to put iron through the rigorous, high-pressure conditions inside the Earth to better understand its behavior there.

The researchers essentially sandwiched iron between small diamonds and squeezed until the pressure was 1.7 million times what we experience on the planet's surface. Then they put the compressed samples through tests to see how sound waves traveled through them, and compared the results with observations of how energy waves produced by earthquakes travel through the planet.

The work helped shed light on iron's density and behavior in such high-pressure conditions, and helped the team get a better idea of iron's melting point at the boundary between the Earth's liquid outer core and solid inner core: around 5,800 degrees Kelvin, or nearly 10,000 degrees Fahrenheit.

Jackson said the new data will help narrow down which light elements are inside the core and help fuel convection there — the process that helps maintain Earth's magnetic field.

Recent research at Carnegie Institution's Geophysical Laboratory indicated oxygen may not be one of the core's ingredients, but the Caltech study authors suggest that is still a possibility.

"There are a few candidate light elements for the core that everyone is always talking about — sulfur, silicon, oxygen, carbon and hydrogen, for instance," Caitlin Murphy, co-author on the study, said in a statement. "Silicon and oxygen are a few of the more popular, but they have not been studied in this great of detail yet. So that's where we will begin to expand our study."

The Kola Superdeep Borehole was just 9 inches in diameter, but at 40,230 feet (12,262 meters) reigns as the deepest hole. It took almost 20 years to reach that 7.5-mile depth—only half the distance or less to the mantle. Among the more interesting discoveries: microscopic plankton fossils found at four miles down. The Kola hole was abandoned in 1992 when drillers encountered higher-than-expected temperatures—356 degrees Fahrenheit, not the 212 degrees that had been mapped.

Travelling to the Earth's center is a popular theme in science fiction. Some subterranean fiction involves traveling to the Earth's center and finding either a Hollow Earth or Earth's molten core. Planetary scientist David J. Stevenson suggested sending a probe to the core as a thought experiment.^[1]^[2] Humans have drilled over 12 kilometers (7.67 miles) in the Sakhalin-I.^[3] In terms of depth below the surface, the Kola Superdeep Borehole SG-3 retains the world record at 12,262 metres (40,230 ft) in 1989 and still is the deepest artificial point on Earth.^[4]

Core of the Earth

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Scientists believe that deep down inside the Earth, there’s a huge ball of liquid and solid iron. This is the Earth’s core, and it protects us from the dangerous radiation of space.

When the Earth first formed, 4.6 billion years ago, it was a hot ball of molten rock and metal. And since it was mostly liquid, heavier elements like iron and nickel were able to sink down into the planet and accumulate at the core. The core is believed to have two parts: a solid inner core, with a radius of 1,220 km, and then a liquid outer core that extends to a radius of 3,400 km. The core is through to be 80% iron, as well as nickel and other dense elements like gold, platinum and uranium.

The inner core is solid, but the outer core is a hot liquid. Scientists think that movements of metal, like currents in the oceans, create the magnetic field that surrounds the Earth. This magnetic field extends out from the Earth for thousands of kilometers, and redirects the solar wind blowing from the Sun. Without this magnetic field, the solar wind would blow away the lightest parts of our atmosphere, and make our environment more like cold, dead Mars.

Although the Earth’s crust is cool, the inside of the Earth is hot. The mantle is only about 30 km beneath our feet, and it’s hot enough to melt rock. At the core of the Earth, temperatures are thought to rise to 3,000 to 5,000 Kelvin.

Since the core is thousands of kilometers beneath our feet, how can scientists know anything about it? One way is to just calculate. The average density of the Earth is 5.5 grams per cubic cm. The Earth’s surface is made of less dense materials, so the inside must have something much more dense than rock. The second part is through seismology. When earthquakes rock the surface of the Earth, the planet rings like a bell, and the shockwaves pass through the center of the Earth. Monitoring stations around the planet detect how the waves bounce, and scientists are able to use this to probe the interior of the Earth.

We have written many articles about the Earth for Universe Today. Here’s an article about how the Earth might actually have an inner, inner core.

The Core of Earth

The Core Within Earth's Inner Core

Anisotropy of the Inner Core.

An Inner-Inner Core with Different Crystal Alignment.

What’s Next?

Contents

Discovery[edit]

Data sources[edit]

Seismic waves[edit]

Other sources[edit]

Physical properties[edit]

Seismic wave velocity[edit]

Size and shape[edit]

Pressure and gravity[edit]

Density and mass[edit]

Temperature[edit]

Magnetic field[edit]

Viscosity[edit]

Composition[edit]

Structure[edit]

Axis-aligned anisotropy[edit]

Non-axial anisotropy[edit]

Causes of anisotropy[edit]

Multiple layers[edit]

Lateral variation[edit]

Other structure[edit]

Growth[edit]

Dynamics[edit]

Age[edit]

Thermodynamic evidence[edit]

Paleomagnetic evidence[edit]

Why is the interior of the Earth hot?

Core of the Earth

The Core of Earth

The Core Within Earth's Inner Core

Anisotropy of the Inner Core.

An Inner-Inner Core with Different Crystal Alignment.

What’s Next?

Contents

Discovery[edit]

Data sources[edit]

Seismic waves[edit]

Other sources[edit]

Physical properties[edit]

Seismic wave velocity[edit]

Size and shape[edit]

Pressure and gravity[edit]

Density and mass[edit]

Temperature[edit]

Magnetic field[edit]

Viscosity[edit]

Composition[edit]

Structure[edit]

Axis-aligned anisotropy[edit]

Non-axial anisotropy[edit]

Causes of anisotropy[edit]

Multiple layers[edit]

Lateral variation[edit]

Other structure[edit]

Growth[edit]

Dynamics[edit]

Age[edit]

Thermodynamic evidence[edit]

Paleomagnetic evidence[edit]

Why is the interior of the Earth hot?

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