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8.01 Overview P. Olson, Johns Hopkins University, Baltimore, MD, USA © 2007 Elsevier B.V. All rights reserved. 8.01.1 A Scientific Journey to the Center of the Earth For as long as man has speculated about the interior of the Earth, it has been presumed that there exists a central core. Centuries before the rise of modern science, philosophers and theologians had concluded that the Earth has a hot region at its center with properties distinct from all other parts of the planet. For nearly as long a time it has been known that the Earth is also magnetic, but the cause of the Earth’s magnetism remained just as mysterious as the nature of the deep interior. Scientific inquiry about the core grew from early investigations of the properties of the geomagnetic field, which began during the era of global exploration. Although ancient Chinese deserve credit for discovering Earth’s magnetism, Gilbert (1600) was the first to demonstrate that the compass needle is controlled by a force originating within the Earth (Figure 1). He showed that the pattern of magnetic field lines on a uniformly magnetized sphere approximates known directions of the compass needle over the Earth's surface. Three hundred and fifty years later, Sidney Chapman characterized Gilbert’s demonstration as “the only successful experiment in the history of geomagnetism!” Later it was observed that Earth’s magnetic field changes slowly with time. In his famous explanation for this secular variation, Halley (1683, 1692) proposed that the geomagnetic field has its origin near the Earth’s center, in a region separated from the solid crust by a cavernous, fluid-filled shell. Halley envisioned that both the crust and the central region or core rotate in the prograde sense, but the core spins slightly slower, causing the magnetic field to drift systematically westward as seen at the surface. Thus, two important and long-lasting concepts were born: the basic three-layer model of Earth’s interior (solid crust and mantle, liquid outer and solid inner core), and the association between the westward geomagnetic drift and westward motion of the fluid outer core with respect to other parts of the Earth system. Halley’s model implicitly assumed that the magnetic field originated in a solid inner core (Evans, 1988), akin to Gilbert’s uniformly magnetized sphere. Subsequently, it was shown that Halley’s model is at variance with the ferromagnetic properties of Earth materials, which lose their permanent magnetization at the Curie temperature at depths of a few tens of kilometers beneath the surface (see Chapter 5.06). However, by then the physical connection between magnetic fields and electric currents had been established, providing an alternative explanation for the geomagnetic field that relied on free electric currents rather than permanent magnetization. The liquid (i.e., molten) state of the outer core was established during the early part of the twentieth century, but the roots of the idea can be traced back into antiquity. Several independent lines of scientific evidence appeared in the middle of the nineteenth century in favor of high temperatures in the Earth’s deep interior, including the steep geothermal gradient measured in deep mines and petrologic discoveries that indicated very high temperatures are needed to form most igneous rocks. However, early estimates of actual temperature variation through the deep Earth varied wildly, preventing any firm conclusion about the state of matter in the core. Toward the end of the twentieth century, two competing models of the state of Earth’s deep interior became prominent. One model assumed that the interior was solid (except for small melt regions below volcanoes) and also elastic, with a very high shear modulus, “as rigid as steel,” according to Kelvin’s famous prescription. This model was supported by observations of the amplitudes of the tides (Darwin, 1879) and the period of Earth’s free nutation, the Chandler wobble (Newcomb, 1892). The competing model held that the interior was largely fluid, an idea popular with geologists at that time, although it had prominent adherents within the physics community as well, for example, Ritter (1878), Poincaré (1885, 1994), Arrhenius (1900), and even earlier, Franklin (1793). The terms of this debate underwent a permanent shift with the publication by Wiechert (1897) of the first quantitative model of Earth structure. Wiechert’s Earth model was based on all available astronomical and geodetic data, featuring a central metallic core surrounded by a rocky mantle. Wiechert is often given credit for being the first to attribute both chemical and physical differences to the core and mantle, and to infer that the core–mantle boundary, the most significant discontinuity in the planet’s interior, represents a change from silicates to iron, as well as a density jump. In any case, there is little doubt that his work launched the era of seismic exploration of the core. Within a decade, Oldham identified seismic P- and S-waves (1899) and interpreted the P-wave shadow as low velocity in a central core (Oldham, 1906). Shortly thereafter, Gutenberg (1912) determined the location of the core–mantle boundary at depth of 2900 ± 20 km, consistent within his calculated uncertainty with the present-day value. Gutenberg’s original Earth model included rigidity throughout the core, in spite of the fact that seismic shear wave transmission through the core had never been confirmed, a testimony to the lasting influence of Kelvin’s ideas. Indeed, the fluidity of the core remained controversial for more than another decade. The issue was settled when Jeffreys (1926) showed that it was possible to reconcile seismic wave speeds with tidal and Chandler wobble observations using an Earth model with a liquid core of radius 3471 km, essentially the same as in the Gutenberg model. As Brush (1996) points out in his history of the exploration of the Earth’s interior, Jeffreys’ reputation became somewhat tarnished by his refusal to accept continental drift and the concept of mantle convection (Jeffreys, 1929). Ironically, Jeffreys made several fundamental contributions to the theory of convection in viscous fluids – the basic model for mantle convection – and he also contributed importantly to the acceptance of the geodynamo theory by demonstrating the outer core is liquid. The final piece of the main radial structure of the core was provided by Lehmann (1936; Figure 4), who discovered the inner-core boundary, which she placed at 4970 km depth, or 1400 km radius (the currently preferred radius is about 1220 km). Following Lehmann’s discovery, seismologists have succeeded in demonstrating the crystalline nature of the inner-core material. The study by Dziewonski and Gilbert (1971) of normal mode overtones excited by the 1964 Alaska earthquake provided an estimate of its average rigidity, and subsequent investigations have determined that the inner core is also anisotropic. Table 1 gives the chronology of important milestones in this scientific journey. 8.01.2 State of the Core Figure 3 Emil Wiechert (1861–1928) constructed the first quantitative Earth model with a core. A full review of the composition of the core is found in Chapter 2.05 of this treatise. Here we provide a brief summary of the major constituents of the core, for purposes of this chapter. Table 2 lists some of the well-known physical properties of the core, Table 3 lists some important thermodynamic and transport properties (which are generally less well-known), and Figure 5 shows its basic radial structure. The model of a predominantly iron core was firmly in place by the middle of the twentieth century, and was well-supported by evidence from seismology (Bullen, 1954) and mineral physics (Birch, 1952). One early argument often cited in support of an iron core was Birch’s law (Birch, 1964), a linear relationship between density and bulk sound velocity with a coefficient proportional to the mean atomic weight of the material. Applications of Birch’s law revealed that the mean atomic weight of the outer core and inner core is slightly less than iron, respectively, but are far too large to be explained by a phase transformation of lower mantle material. Although it is now recognized that the theoretical basis for Birch’s law is weak, it was historically important because it seemed to demand a metallic core rather than an oxide-rich or silica-rich one. Other metals might also be present in the core. The abundance of nickel in iron meteorites suggests that some amount of nickel may be present in the outer core as well. Figure 4 Inge Lehmann (1888–1993) discovered the inner core. Ключевые слова: e, r, o