What Is the Earth's Core Made of

Earth's Core

David Loper , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

III Evolution of the Core

III.A Initial Formation

The existence of stony and iron meteorites provides strong evidence that planetary cores, in general, and earth's core, in particular, formed by separation of less dense silicate phases and more dense metallic phases as they accreted during the formation of the solar system some 4.5 billion years ago. This was a strongly exothermic process; the gravitational potential energy released by this process is sufficient to heat earth by some 2000 °. It follows that earth likely was very hot soon after its formation and has been cooling since then.

III.B Formation and Growth of Inner Core

It is very likely that the inner core has grown by solidification from the outer core as earth has cooled during the past 4.5 billion years and that solidification and growth is continuing. The inner core may well be a relatively recent feature; in some models of the evolution of the core it begins to grow roughly 2 billion years ago.

The core is cooled by transfer of heat to the mantle, and the rate of cooling is largely controlled by the thermal structure of the lowermost mantle (the D′′ layer). The outer core is coolest at the top, near the CMB, but freezing proceeds from the center outward because the increase of the freezing (liquidus) temperature with pressure is greater than the adiabatic gradient:

$d T_{L} / d p d T_{A} / d p .$

As the inner core grows, both latent heat and buoyant material are released at the base of the outer core. These work in parallel to drive convective motions in the outer core.

Solidification of outer-core material at the ICB is similar to the metallurgical process of unidirectional solidification of molten metallic alloys; the mathematical model is called a Stefan problem. The simplest solution to the Stefan problem involves the steady advance of a planar solidification front into a quiescent liquid. This simple solution has two known forms of instability. If the freezing process involves a change of composition (see Fig. 2) and the material rejected by the solid phase is buoyant compared with the parent liquid, the static state is prone to a compositional convective instability. It is very likely that this instability occurs in the outer core and that the resulting convective motions participate in the dynamo process which sustains earth's magnetic field.

Solidification of an alloy at a planar interface is prone to a second, morphological instability. The material rejected by the solid phase accumulates on the liquid side of the freezing interface, depressing the liquidus and making that liquid compositionally (or constitutionally) supercooled. This causes the flat freezing interface to be unstable and become convoluted. These convolutions can become extreme, forming a so-called mushy zone. Again, it is very likely that this instability occurs in the core and that the inner core is, in fact, an intimate mixture of solid and liquid. Dynamic processes cause the fraction of liquid phase to be small, so that the inner core acts structurally as a solid even though, thermodynamically, it behaves as a solid-liquid mixture.

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Mineral Physics

L. Vočadlo , in Treatise on Geophysics (Second Edition), 2015

2.06.1 Introduction

Earth's core plays a fundamental role in the evolution of our planet. The dominant composition of Earth's core is that of iron alloyed with 5–15% nickel. As Earth cools, the inner core grows, crystallizing from the liquid outer core, containing ~ 5–10% light elements, to form a solid inner core, with ~ 2–3% light elements (Alfè et al., 2002a; Jephcoat and Olson, 1987; Poirier, 1994; Stixrude et al., 1997). The release of latent heat of fusion, together with chemical buoyancy arising from the enrichment of the outer core with light elements, provides driving forces for the fluid flow responsible for the geodynamo and, hence, for Earth's magnetic field. The heat released from the core helps drive mantle convection, which, in turn, leads to surface features such as volcanism and plate tectonics. Because the inner core is sitting within the liquid outer core, it is isolated from the rest of Earth; however, coupling with the outer core and mantle (e.g., geomagnetic and gravitational) prevents its motion from being entirely independent, allowing it to only super-rotate and wobble.

The only direct observations of Earth's core come from seismology; therefore, any credible mineralogical model has to match exactly the seismic observations. Increasingly accurate seismic observations have shown Earth's inner core, in particular, to be far more complex than had previously been thought. The current seismological models reveal an inner core that is anisotropic, layered, and laterally heterogeneous, but the origins of these characteristics are not yet fully understood. It is generally considered that the anisotropy reflects the preferred orientation of the crystals present, which could either have arisen during inner-core crystallization or developed over time through solid-state flow; these two mechanisms have vastly different implications for core evolution. The observed layering may have been caused by changes in chemical composition, crystal structure, preferred orientation, or some combination of all three. Again, these imply very different core processes and evolution. The lateral heterogeneity could be because of uneven crystallization processes or even translational motion of the inner core itself. The origin of the anisotropy, layering, and lateral inhomogeneity is fundamental to understanding and constraining evolutionary models for Earth's inner core, and the mineralogical model must reflect this complex structure.

Mineral physics also has an important role to play in our understanding of the processes going on in the outer core. Knowledge of the thermoelastic properties of candidate liquid iron-alloys can be compared with seismological observations and thus lead to models for the composition of the outer core. Estimates for the dynamic properties of liquids such as diffusivity and viscosity can be incorporated into the magnetohydrodynamic equations that quantify the magnetic field. Furthermore, mineral physics can provide values for thermodynamic quantities that can be put into thermal evolution models of core formation, leading to timescales for inner core growth and quantification of the heat budget of Earth.

In order to place fundamental constraints on the properties of Earth's core, it is essential to know the behavior of iron and iron alloys at core conditions. In particular, the key questions to resolve are the following: (1) What is the most stable phase(s) of iron present in the inner core? (2) What are the elastic properties of the stable phase(s) (at core pressures and temperatures)? (3) What are the rheological properties of these candidate solid phases? (4) Do the combined thermoelastic and rheological properties of iron alloys lead to a comprehensive model for inner core composition, evolution, anisotropy, and layering? In addition, (5) what is the temperature of Earth's core? (6) What are the thermodynamic properties of candidate liquid iron-alloy phases? (7) What is the thermal conductivity of the outer core? (8) What are the rheological properties of these phases? (9) What is the composition of the outer core? Notwithstanding the difficulties in answering any single one of these questions, the combined answers to them all should exactly match the models inferred from seismology.

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The Core

Kent C. Condie , in Earth as an Evolving Planetary System (Second Edition), 2011

Publisher Summary

Earth's core is responsible for the generation of Earth's magnetic field. The core contains information regarding the earliest history of accretion of the planet. When the core formed, some thermal and compositional features were established which controlled the subsequent evolution of the core. These features also influenced the evolution of the mantle, crust, and atmosphere. Seismic velocity data indicate that the radius of the core is 3485 km and that the outer core does not transmit S waves, interpreting it is in a liquid state as also supported radio astronomical measurements of Earth's normal modes of free oscillations. The inner core, with a radius of 1220 km, transmits S waves at very low velocities, suggesting that it is a solid near the melting point or partly molten. Detailed analysis of the travel times of seismic waves reflected from and transmitted through the core indicates that the outer liquid core is relatively homogeneous and well mixed. The core is composed chiefly of iron as indicted by three points: the internal geomagnetic field must be produced by a dynamo mechanism, which is only possible in a liquid-metal outer core; the calculated density and measured body wave velocities in the core are close to those of iron measured at appropriate pressures and temperatures; iron is the most abundant element in the solar system that has seismic properties resembling those of the core.

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The Core

Kent C. Condie , in Earth as an Evolving Planetary System (Third Edition), 2016

Introduction

Earth's core is important for three main reasons: (1) it is responsible for the generation of Earth's magnetic field; (2) it contains information regarding the earliest history of accretion of the planet; and (3) thermal and compositional features established when the core formed have largely controlled the subsequent evolution of the core and also influence the evolution of the mantle, crust, and atmosphere (Nimmo, 2007). Seismic velocity data indicate that the radius of the core is 3485 ± 3 km and that the outer core does not transmit S-waves (Jacobs, 1992; Jeanloz, 1990; Tkalcic & Kennett, 2008) (Figure 1.2). This latter observation is interpreted to mean that the outer core is in a liquid state. Supporting this interpretation are radio astronomical measurements of Earth's normal modes of free oscillations. The inner core, with a radius of 1220 km, transmits S-waves at very low velocities, suggesting that it is a solid near the melting point or partly molten. There is a sharp velocity discontinuity in P-wave velocity (0.8 km/s; Figure 1.2) at the inner core boundary, and a low velocity gradient at the base of the outer core. Results suggest the top of the inner core attenuates seismic energy more than the deeper part of the inner core. Detailed analysis of travel-times of seismic waves reflected from and transmitted through the core indicates that the outer liquid core is relatively homogeneous and well mixed, probably due to convection currents. Seismic data also suggest relief on the core-mantle interface is limited to about 5 km. The viscosity of the outer core is not well known but appears to range from about 100 Pa s at the top to 10¹¹ Pa s at the bottom.

Three lines of evidence indicate that the core is composed chiefly of iron. First, the internal geomagnetic field must be produced by a dynamo mechanism, which is only possible in a liquid metal outer core. Because the fluidity and electrical conductivity of the mantle are too low to produce Earth's magnetic field, the outer core must be liquid metal for the geodynamo to operate (Glatzmaier, 2002; Jeanloz, 1990). Second, the calculated density and measured body wave velocities in the core are close to those of iron measured at appropriate pressures and temperatures. Third, iron is by far the most abundant element in the solar system that has the seismic properties resembling those of the core.

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Mantle Dynamics

J.W. Hernlund , A.K. McNamara , in Treatise on Geophysics (Second Edition), 2015

Abstract

The Earth's core–mantle boundary (CMB) is the largest density discontinuity in our planet, separating the rocky oxide mantle from the liquid metal core. The structure and dynamics of the CMB region are central to major questions such as the depth extent of lithospheric subduction, the thermal and chemical evolution of the Earth, the mechanisms and energetics involved in the generation of a geomagnetic field, and the nucleation of deep-seated mantle plumes that rise up to trace out volcanic hot spot tracks at the surface. Owing to the proliferation of digital seismic networks and developments in high-pressure experimental and theoretical mineral physics, we are now learning more about the structure and physical properties of the CMB region and the myriad ways in which deep processes are linked to the surface environment. This chapter surveys recent developments in understanding the nature and dynamics of this critical region of the Earth's interior.

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Evolution of Earth and its Climate: Birth, Life and Death of Earth

O.G. Sorokhtin , ... N.O. Sorokhtin , in Developments in Earth and Environmental Sciences, 2011

2.5 Earth core

The Earth core is clearly identified by seismic data. The indications are a clear shadow on the depth–time curves of seismic waves refracted within the mantle, compression and shear waves reflected from core's surface, and a total decay of the shear waves within it. At that, the compression wave velocity significantly declines (by about the factor of 1.7; see Fig. 2.11).

An important conclusion from this is that the matter in the external shell of the Earth's core (the external core or layer E) is in the liquid state. On the other hand, the existence of converted waves transformed from the compression waves to the shear waves and back again discovered by Lehmann (1934, 1936) in the central areas of Earth indicates the presence in Earth of the internal, effectively rigid core (Bullen, 1978).

Radius of the rigid internal core (layer G) is approximately 1200–1250 km, the thickness of the transitional layer between the external and internal core (layer F) is about 300–400 km, and the liquid layer E has the radius of 3450–3500 km (and the respective depths of 2870–2920 km). The core matter density within the external core changes monotonously from 9.5–10.1 on its surface to 11.4–12.3 g/cm³ at the base (see Fig. 2.12). Density within the internal core increases by about 8–10% and reaches 13–14 g/cm³ in the Earth center. The core mass is in various models on the order of (1.85−1.91) × 10²⁷ g. Our estimate is М _с ≈ 1.94 × 10²⁷ g, which is 32.5% of the Earth's mass.

An interpretation of the core's seismic tomography (Morelli and Dziewonski, 1987) showed that the core surface is uneven, with noticeable deviations by up to ±(6–10) km from the equilibrium shape of an ellipsoid of the revolution (see Fig. 2.10). This core surface topography, most likely, is a reflection of the roots of ascending and descending convection flows in the lower mantle. We predicted this phenomenon (Sorokhtin, 1974) long before the discovery of this topography. Indeed, mantle lows (the roots of the descending flows indented into the core) must form underneath the descending convective flows (i.e., under heavier portions of the mantle), whereas the surface highs must be observed under the ascending flows.

The upper limit of viscosity value for the external (liquid) core layer may be estimated from the degree of absorption in this geosphere of compression seismic waves. It was determined that average matter viscosity in the liquid core portion is much lower than η _c = 10⁹ poise (P) and probably does not exceed 10³ P (Zharkov, 1983). Studying variable components of the Earth magnetic field and energy balance of the geomagnetic dynamo, Loper (1975) concluded that core's kinetic viscosity is close to that of the water and most likely is ν _c ≈ 4 × 10^{− 2} cm²/s. Then, the core matter dynamic viscosity is η _c ≈ 0.4 P. It is worth noticing here that the external core viscosity that low indicates its clear overheating or, which is the same, its low-melting temperature, and is a necessary condition for the generation of a geomagnetic field.

As already mentioned, the internal core with its mass of 1.1 × 10²⁶ g (or about 1.8% of Earth mass) is solid and, most likely, is different in its composition from the external core.

The fact that the oldest igneous rocks appeared only 600–800 MMY after Earth formation is a convincing testimony toward the "cold" origin. On the other hand, the Earth core formation at early stages of Earth evolution requires its hot and even melted state. It follows from this that the young Earth indeed did not have a high-density core. It is also supported by the lead isotope ratio in Earth rocks (see a detailed discussion in Section 5.2.)

It is ever clearer now that many important processes were initiated and controlled by the planetary process of Earth core separation. These processes included tectonic activity, Earth crust formation with its economic mineral deposits, geochemical evolution of the mantle, its degassing and genetically related processes of the ocean and atmosphere formation as well as the emergence of life on Earth. This planetary process is continuing for about 4 BY now but is still not completed although most of the core matter has already sunk into Earth crust. The development of the main planetary process of Earth global evolution is completely defined by the composition of the Earth matter and Earth core. As this is an exceptionally important issue, we will dwell in some detail on the determination of the chemical composition of the core matter.

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The Mantle and Core

J. Li , Y. Fei , in Treatise on Geochemistry (Second Edition), 2014

3.15.1 Introduction

The Earth's core was discovered in 1906, when Oldham (1906) inferred the existence of a low-velocity region inside the Earth from changes in the amplitude of compressional waves traveling through the Earth's interior. Over the last century, a wealth of knowledge on the nature and dynamics of the core has been obtained. The core extends from near 2900 km depth to the center, taking up more than half of the Earth's radius ( Figure 1 ). It occupies roughly one-sixth of the Earth's volume and accounts for nearly one-third of its mass. The mass fraction in the core is much higher than its volume fraction because the density jumps from 5.5 to 9.9 g cm^{− 3} across the core–mantle boundary (CMB) ( Figure 2 ). Analyses of seismic velocities and attenuation revealed that the core has a layered structure, consisting of a solid inner sphere surrounded by a molten outer shell. The pressure in the core ranges from 136 GPa (1360 kbar) at the CMB to 360 GPa (3600 kbar) at the center. In order to remain molten under such high pressures, the temperature in the core must be high as well, reaching 5000–7000 K at the inner–outer core boundary (ICB), as will be discussed in Section 3.15.3.1.2 .

Geochemical and seismological observations of the Earth, in combination with laboratory measurements on relevant materials, suggest that more than 80 wt% of the core is made of iron ( Figures 3 and 4 ). Other elements with significant concentrations include nickel (~ 5 wt%) and one or more elements that are lighter than iron (e.g., Birch, 1952). According to the latest chronometric measurements using the tungsten–hafnium systematics, most of the core–mantle segregation took place in less than 30 My (Kleine et al., 2002; Yin et al., 2002). In other words, the core is almost as old as the Earth itself, with core formation occurring as soon as the Earth accreted or simultaneously with accretion.

Despite its old age, the core is dynamically active, giving rise to the Earth's magnetic field through the convection of the electrically conductive liquid metal in the outer core known as the geodynamo (Buffett, 2000). The core serves as a major energy source for the planet, storing a large amount of energy acquired during core–mantle segregation and from radioactive decay of short-life isotopes, as well as producing heat through ongoing growth of the inner core. Moreover, the core may contain a significant amount of potassium, which is capable of generating heat over the history of the Earth (e.g., Hall and Murthy, 1971; Murthy et al., 2003). Anomalous seismic features of the D″ layer have been attributed to chemical reaction and dynamical coupling between the core and mantle (e.g., Garnero, 2000; Lay et al., 1998), whereas mysterious isotopic enrichments are thought to originate from chemical fractionation between the inner and outer cores (Brandon et al., 1998; Walker, 2000).

Hidden below 2900 km depth, the core is the least accessible layer of the Earth. While a spacecraft has reached the edge of the Solar System at several billion kilometers away from the Earth, the deepest hole drilled into the Earth is less than 14 km in depth, and volcanic eruptions are unlikely to excavate pristine samples of the core to the surface of the Earth. To date, the most direct observations of the core have come from seismological studies. Because of the complex structure of the Earth's crust and mantle, however, seismic investigations of the core require extensive data coverage, efficient data analysis methods, and sophisticated modeling of wave propagation. Equally challenging is the task of deciphering geochemical signatures of the core carried by mantle plumes to the Earth's surface. On the other hand, experimental and computational investigations of core properties and processes face technical difficulties in simulating extreme pressure and temperature conditions. For these reasons, many fundamental issues concerning the Earth's core remain controversial and poorly understood.

As a result of improvements in observational, experimental, and computational techniques, significant progress has been made during the past few decades, helping to address a wide range of important issues including the timing, duration, and mechanism of core formation; the identity and abundance of light elements in the core; the history of the core's thermal and chemical evolution; the role of radiogenic heating in the core; the nature of core–mantle interaction; the inner core's structure and dynamics; and the origin, structure, and evolution of the geomagnetic field. Building upon previous reviews (e.g., Hillgren et al., 2000; Jeanloz, 1990; Li and Fei, 2003, 2007; Poirier, 1994; Stevenson, 1981), this chapter aims to provide an updated summary of experimental constraints on the major and light elements in the core. Reviews on related topics are found elsewhere in this volume: Chapter 3.16, offers a comprehensive review of the cosmochemical and geochemical constraints on core composition, and Chapter 3.12, provides a detailed review of the partitioning behavior of siderophile elements between core-forming alloys and mantle silicates and oxides. In addition, many chapters in the Treatise of Geophysics address issues of the Earth's core (Vocadlo, 2007; Jackson and Finlay, 2007; Rubie et al., 2007; Nimmo, 2007).

This chapter starts with a description of general techniques used for investigating the composition of the Earth's core. Following a review of geophysical and geochemical evidence for iron being the most abundant element in the core, a summary of experimental data on the phase diagram, an equation of state (EOS), and physical properties of iron is provided and their implications for the core composition discussed. The coverage of nickel will be brief, as the number of experimental studies is limited. A large portion of the chapter will be devoted to reviewing constraints on the light element composition of the core, a highly controversial subject with direct bearings on the origin, evolution, and current state of the Earth. Finally, the latest experimental results on potassium, niobium, rhenium, and osmium, some of the interesting minor and trace elements in the core, are reviewed.

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Mineral Physics

R.J. Hemley , ... G. Shen , in Treatise on Geophysics (Second Edition), 2015

2.13.5.5 Iron and Iron Alloys

The Earth's core plays a central role in the evolution and dynamic processes within the planet. As the major constituents of the core, iron and its alloys hold the key to understanding the nature of this most enigmatic region of the planet (Hemley and Mao, 2001). Geophysical observations have uncovered surprising inner core properties, such as seismic anisotropy, super rotation, and magnetism (Glatzmaier and Roberts, 1996; Niu and Wen, 2001; Romanowicz et al., 1996; Song and Helmberger, 1998; Song and Richards, 1996; Su et al., 1996; Tromp, 2001). These observations are supplemented by geodynamic simulations (Buffett, 2000, 2003; Buffett and Wenk, 2001; Karato, 1999; Olson and Aurnou, 1999). Ab initio theoretical calculations have been applied to examine and predict melting, phase stabilities, elastic anisotropy, and magnetism of iron beyond experimental capabilities (Alfé et al., 1999, 2000; Belonoshko et al., 2003; Laio et al., 2000; Steinle-Neumann et al., 2001; Stixrude and Cohen, 1995; Vocadlo et al., 2003). Theory shows that the bcc phase is stabilized by magnetism. There had been much discussion of bcc as the possible structure for iron in the Earth's inner core, but calculations showed that bcc iron is mechanically unstable at high pressures due to the loss of magnetism with pressure (Stixrude and Cohen, 1995). On the other hand, the hexagonal phase (ε-Fe) is nonmagnetic. The reflectivity of iron decreases markedly across the bcc–hcp transition; measurements to 300 GPa showed that this low reflectivity continues to much higher pressure (Reichlin et al., unpublished).

Pressure effects on the valence band densities of states and magnetic properties of Fe are being measured with the new synchrotron x-ray techniques described in the preceding text. Large differences in DOS are predicted between bcc Fe and the two closed-packed phases (hcp and fcc). Spin-dependent K _β emission fine structure can be used to probe localized magnetic properties with XMCD. The element-specific nature of XES and XMCD will be particularly important in the study of transition metal and rare earth alloys. High P–T XAS and XRD are providing electronic and structure information for iron melt and crystals (Jackson et al., 1993; Sanloup et al., 2002a). High P–T NRFS is providing information on Mössbauer effect and magnetism (Jackson et al., to be published), and high P–T NRIXS coupled with hydrostatic equation of state data yields phonon densities of state, bulk longitudinal and shear wave velocities, heat capacity, entropy, Debye temperature, and Grüneisen parameter (Lübbers et al., 2000; Mao et al., 2001; Struzhkin et al., 2001).

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Deep Earth Seismology

A. Souriau , M. Calvet , in Treatise on Geophysics (Second Edition), 2015

1.23.1 Introduction

The Earth's core represents 16% of the volume of the Earth and more than 30% of its mass. It is composed of an iron alloy that incorporates light elements and nickel. At the center of this wide ocean of iron, the solid inner core, of about the size of the moon, represents only 4.3% of the volume of the core and < 1% of the volume of the Earth. It results from the freezing of the liquid core during the cooling of the Earth, with a strong depletion in light elements. The core plays a very important role in Earth physics and chemistry. However, despite its impact on many physical processes such as magnetic field and the Earth's rotation, which have been investigated for centuries, the core has been detected very late, in 1906, when the seismological observation of the deep Earth became possible. Since then, important advances have been made, combining observations, laboratory experiments, and theoretical modeling. But the core remains enigmatic. If some of the core properties are now firmly established, many others are still controversial. This chapter will attempt to give a survey of the present state of the art, trying to give a degree of confidence to the various results.

Before going through the details of the core structure, it is useful to recall some basic information concerning the core, which in addition shows its importance in the planet Earth system. The core is governed by complex systems of relations implying thermodynamics, hydrodynamics, geomagnetism, and geochemistry. Advances and results concerning these different disciplines may be found in other chapters of this treatise, in particular Chapters 2.06, 3.10, 8.02, 8.04–8.09 8.04 8.05 8.06 8.07 8.08 8.09 , 8.12, 8.13, 9.03, and 9.08.

The core forms very early in the history of our planet, as revealed by the analyses of lead and uranium mantle isotopes and other radionuclide chronometers and by the presence of a magnetic field in the oldest rocks of the Earth, 3.5 Ga old. It is generally accepted that the inner core formed by crystallization of the liquid core. Some thermodynamic arguments favor a rather young inner core, about 1 Ga old, but this is still open to discussion.

Convection in the liquid core is at the origin of the main part of the magnetic field. From this point of view, the core is essential to the life, as it protects the Earth from the solar wind. Numerical simulations of the Earth geodynamo show that the inner core has probably a stabilizing role for the magnetic field, as it prevents frequent geomagnetic reversals.

The boundary of the core with the silicate mantle, the core–mantle boundary (CMB), is a place of thermal, dynamic, and probably chemical exchanges. In particular, the exchange of angular momentum influences the Earth's rotation. At the inner core boundary (ICB), the freezing of the liquid iron alloy occurs with a depletion of the light elements that are present in the liquid core. The ICB is thus also an important place for chemical and energy exchanges. Heat sources linked to inner core growth, including latent heat and gravitational energy, are essential for powering the geodynamo and are important factors in the thermal history of the Earth.

In what follows, we will only consider the aspects relevant to the structure of the core, giving a particular importance to seismology, which has provided most of the results obtained today about its internal properties. After a brief historical review, we will present the most usual tools used in core seismology. The radial structure of the core will then be investigated, paying particular attention to the core boundaries. Then, a large section will concern the inner core, for which different properties will be considered: its rigidity; its anisotropy for both velocity and attenuation; its hemispherical dichotomy, which also concerns both velocity and attenuation; and its possible differential rotation or oscillation with respect to the mantle. We will end this chapter with the still open questions and future challenges.

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Earth structure

Charles J. Ammon , ... Terry C. Wallace , in Foundations of Modern Global Seismology (Second Edition), 2021

10.6 Structure of the core

The Earth's core was first discovered in 1906 when Oldham found a rapid decay of P waves beyond distances of $100^{\circ}$ , and he postulated that a low-velocity region in the interior produced a shadow zone. Gutenberg accurately estimated a depth to the core of 2900 km in 1912, and by 1926 Jeffreys showed that the absence of S waves traversing the core required it to be fluid. The core extends over half the radius of the planet and contains 30% of its mass. The boundary between the mantle and core is very sharp and is the largest compositional contrast in the interior, separating the molten core alloy from the silicate crystalline mantle. Seismologists have used reflections from the top side of the core–mantle boundary (PcP), underside reflections (PKKP), and transmitted and converted waves (SKS, PKP, and PKIKP) to determine topography on the boundary, which appears to be less than a few kilometers (Fig. 10.27). The contrast in density across the boundary is larger than that at the surface of the Earth; thus it is not surprising that little if any topography exists. The strong density contrast may be responsible for a concentration of chemical heterogeneities in the $D^{″}$ region composed of materials that are denser than average mantle but less dense than the core. The material properties of the core are quite uniform (Fig. 1.21), with a smoothly increasing velocity structure down to a depth of 5150 km, where a sharp boundary separates the outer core from the solid inner core. This 7–9% velocity increase boundary was discovered by the presence of refracted energy in the core shadow zone by Lehmann in 1936. It was not until the early 1970s that solidity of the inner core was demonstrated by the existence of finite rigidity affecting normal modes that sense the deep structure of the core.

The decrease in P velocity from values near 13.7 km/s at the base of the mantle to around 8 km/s at the top of the outer core profoundly affects seismic raypaths through the deep Earth. The principal P waves with paths through the core are shown in Fig. 10.27. As the takeoff angle from the source decreases from that of rays that just graze the core and diffract into the shadow zone, the PKP waves are deflected downward by the low velocities, being observed at distances of $188^{\circ}$ to $143^{\circ}$ ( ${PKP}_{AB}$ ) and then again at $143^{\circ}$ to $170^{\circ}$ ( ${PKP}_{BC}$ ) as the takeoff angle continues to decrease. Reflections from the inner-core boundary define the PKiKP (cd) branch (Fig. 10.28), after which the P wave penetrates the inner core as PKIKP, which is observed from $110^{\circ}$ to $180^{\circ}$ . Note that most of this complexity of the core travel-time curve stems from the velocity decrease, the spherical geometry of the core, and the increase in velocity in the inner core.

These core phases are readily identified on teleseismic short-period recordings (Fig. 10.28), and the vast numbers of travel times reported by the ISC allow for the travel times, ray parameters, and positions of the cusps of the travel-time data to be used to invert for models of the core. Since the outer core does not transmit S waves, we believe it to be fluid, but the fact that the S velocity at the base of the mantle is slightly lower than the P velocity at the top of the core (Fig. 1.21) means that the core is not a low-velocity zone for the converted phase SKS. As a result, the SKS phase and attendant underside reflections off the core–mantle boundary, such as SKKS, are also used to determine the velocity structure of the outer core, particularly in the outermost 800 km where PKP phases do not have turning points. Some evidence from SKKS phases suggests that the outermost 100 km of the core may have reduced seismic velocities relative to a uniform composition core, possibly representing a chemical boundary layer.

The inner core–outer core boundary has been extensively investigated for more than 50 years, in part due to the scattered arrivals preceding the B cusp, or caustic (Fig. 10.28). These arrivals were originally attributed to a complex transition zone at the top of the inner core, and a wide variety of models have been developed for this boundary. However, seismic arrays established that the PKP precursors are caused by scattering, probably in the $D^{″}$ region or at the core–mantle boundary. Fig. 10.29 shows an example of various P-wave velocity models for the inner core-outer core boundary.

Both the velocity and density contrasts and attenuation structure of the inner core have been studied using PKiKP reflections and PKIKP refractions. An example of waveform comparisons used to determine inner-core properties is shown in Fig. 10.28. The different branches of the core phases PKP (DF BC AB) are readily visible on a single recording of two earthquakes close to each other (doublets). The DF branch arrival is broadened relative to the BC arrival, indicating a lower Q in the inner core. Waveform modeling can be used to match observations like these by considering a suite of Earth models and finding models that match the relative timing, amplitude, and frequency content of the core phases. Recent studies have used the travel times between the different branches of PKP to map long term changes in velocity of the inner core, suggesting inner core rotation. Specifically, the differential travel time between ( ${PKP}_{B C}$ – ${PKP}_{D F}$ ) is relatively insensitive to source location uncertainty and to 3-dimensional heterogeneities of the crust and mantle along the ray paths, allowing for the analysis of the $B C - D F$ . By measuring $B C - D F$ and $A B - B C$ differential times by cross-correlation for a period of about 28, residuals between sources in the South Sandwich Islands recorded in seismic station COL in Alaska paths shows $B C - D F$ residuals increase from the years 1967–75, to 1980–85, and to 1992–95 (≈0.3 s) (Song and Richards, 1996). For this residual increase to occur, faster paths can be obtained by decreasing the angle between the inner core path and the axis of anisotropic symmetry. Since the ray path is essentially fixed, inner core rotation can be one mechanism to explain this observation (Fig. 10.30).

Mineral physics experiments demonstrate that the seismologically determined density of the core is lower than expected for pure iron, so the outer core is believed to have about 10% of a light alloying component such as Si, O, C, or S. The inner core may be almost pure iron, with the freezing process brought about by the geotherm dipping below the alloy melting temperature. The freezing process tends to concentrate the lighter component in the fluid. Rise of this buoyant material provides compositionally driven convection in the core, which is believed to sustain core dynamics that produce the Earth's magnetic field. Proximity to the solidus is implied by the existence of the inner core; thus the outer core may actually contain suspended particles, up to 30% by volume. It is not known whether these impart any effective rigidity to the outer core, but anomalous, unexplained core modes may require a complex mechanism. Because the core rotates, the polar regions of the outer core may have a separate flow regime from the spherical annulus of material along the equator. Thus, cylindrical symmetry may play a role in core characteristics, perhaps with varying degrees of suspended particles in the polar regions.

The inner core is a very small region inside the Earth but appears to have surprising internal structure. Seismic waves traversing the inner core on paths parallel to the spin axis travel faster than waves in the equatorial plane, indicating the existence of inner-core anisotropy. This has been detected by travel times of PKIKP waves as well as by innercore–sensitive normal modes. The travel-time variations have ∼1-s systematic differences with angle from the north-south axis. This axial symmetry may result from convective flow in the inner core that induces an alignment in weakly anisotropic crystals of solid iron. Thus, seismological measurements can reveal dynamic processes as deep as 6000 km into the Earth.

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What Is the Earth's Core Made of

Source: https://www.sciencedirect.com/topics/earth-and-planetary-sciences/earth-core

What Is the Earth's Core Made of

Earth's Core

III Evolution of the Core

III.A Initial Formation

III.B Formation and Growth of Inner Core

Mineral Physics

2.06.1 Introduction

The Core

Publisher Summary

The Core

Introduction

Mantle Dynamics

Abstract

Evolution of Earth and its Climate: Birth, Life and Death of Earth

2.5 Earth core

The Mantle and Core

3.15.1 Introduction

Mineral Physics

2.13.5.5 Iron and Iron Alloys

Deep Earth Seismology

1.23.1 Introduction

Earth structure

10.6 Structure of the core

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What Is the Earth's Core Made of

Earth's Core

III Evolution of the Core

III.A Initial Formation

III.B Formation and Growth of Inner Core

Mineral Physics

2.06.1 Introduction

The Core

Publisher Summary

The Core

Introduction

Mantle Dynamics

Abstract

Evolution of Earth and its Climate: Birth, Life and Death of Earth

2.5 Earth core

The Mantle and Core

3.15.1 Introduction

Mineral Physics

2.13.5.5 Iron and Iron Alloys

Deep Earth Seismology

1.23.1 Introduction

Earth structure

10.6 Structure of the core

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