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The inner core of the Earth, its innermost hottest part as detected by seismological studies, is a primarily solid ball about 1,220 km (760 mi) in radius, or about 70% that of the Moon. It is believed to consist of an iron–nickel alloy, and may have a temperature similar to the Sun's surface, approximately 5700 K (5430 °C).
The existence of a solid inner core distinct from the liquid outer core was discovered in 1936 by seismologist Inge Lehmann using observations of earthquake-generated seismic waves that partly reflect from its boundary and can be detected by sensitive seismographs on the Earth's surface. This boundary is referred as Bullen discontinuity or sometimes Lehmann discontinuity.
Later (1940) it was conjectured that this inner core was solid iron, and its rigidity was confirmed in 1971.
The outer core was believed to be liquid due to its inability to transmit elastic shear waves; only compressional waves are observed to pass through it. The solidity of the inner core has been difficult to establish because the elastic shear waves that are expected to pass through it are very weak and difficult to detect because they also must travel through the outer core. Dziewonski and Gilbert established the consistency of this hypothesis using normal modes of vibration of Earth caused by large earthquakes. Recent claims of detections of inner core transmitted shear waves were initially controversial but are now gaining acceptance.
Based on the abundance of chemical elements in the solar system, the theory of planetary formation, and other chemical constraints regarding the remainder of Earth's volume, the inner core is composed primarily of a nickel–iron alloy referred to as Nife: 'Ni' for nickel, and 'Fe' for ferrum or iron. Because the inner core is more dense (12.8 ~ 13.1)g⁄cm³ than pure iron or nickel, even under heavy pressures, it's believed that the remaining part of the core is composed of gold, platinum and other siderophile elements in quantity enough to coat Earth's surface for 0.45 m (1.5 feet). The relative abundance of precious metals and other heavy elements in respect to Earth's crust is explained with the theory of iron catastrophe, an event which occurred before the first eon during the accretion of early Earth.
The temperature of the inner core can be estimated using experimental and theoretical constraints on the melting temperature of impure iron at the pressure (about 330 GPa) of the inner core boundary, yielding estimates of 5,700 K (5,430 °C; 9,800 °F). The pressure in Earth's inner core is between about 330 to 360 gigapascals (3,300,000 to 3,600,000 atm), and iron can only be solid at such high temperatures because its melting temperature increases dramatically at pressures on that scale (see the Clausius–Clapeyron relation).
J. A. Jacobs  was the first to suggest that the inner core is freezing and growing out of the liquid outer core due to the gradual cooling of Earth's interior (about 100 degrees Celsius per billion years). Prior to the inner core's formation, the entire core was molten, and the age of the inner core is thought to lie between 2–4 billion years. Because it is younger than the age of Earth (about 4.5 billion years), the inner core cannot be a primordial feature inherited during the formation of the solar system.
Little is known about how the inner core grows. Because it is slowly cooling, many scientists expected that the inner core would be homogeneous. It was even suggested that Earth's inner core might be a single crystal of iron; However, this is at odds with the observed degree of disorder inside the inner core. Seismologists have revealed that the inner core is not completely uniform and contains large-scale structures that seismic waves pass more rapidly through than others. The surface of the inner core exhibits rapid variations in properties at scales at least as small as 1 km. This is puzzling, since lateral temperature variations along the inner core boundary are known to be extremely small (this conclusion is confidently constrained by magnetic field observations). Recent discoveries suggest that the solid inner core itself is composed of layers, separated by a transition zone about 250 to 400 km thick. If the inner core grows by small frozen sediments falling onto its surface, then some liquid can also be trapped in the pore spaces and some of this residual fluid may still persist to some small degree in much of its interior.
Because the inner core is not rigidly connected to Earth's solid mantle, the possibility that it rotates slightly faster or slower than the rest of Earth has long been entertained. 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 faster in some directions. Estimates of this super-rotation are around one degree of extra rotation per year, although others have concluded it is rotating more slowly than the rest of Earth by a similar amount.
Growth of the inner core is thought to play an important role in the generation of Earth's magnetic field by dynamo action in the liquid outer core. This occurs mostly because it cannot dissolve the same amount of light elements as the outer core and therefore freezing at the inner core boundary produces a residual liquid that contains more light elements than the overlying liquid. This causes it to become buoyant and helps drive convection of the outer core. The existence of the inner core also changes the dynamic motions of liquid in the outer core as it grows and may help fix the magnetic field since it is expected to be a great deal more resistant to flow than the outer core liquid (which is expected to be turbulent).
Speculation also continues that the inner core might have exhibited a variety of internal deformation patterns. This may be necessary to explain why seismic waves pass more rapidly in some directions than in others. Because thermal convection alone appears to be improbable, any buoyant convection motions will have to be driven by variations in composition or abundance of liquid in its interior. S. Yoshida and colleagues proposed a novel mechanism whereby deformation of the inner core can be caused by a higher rate of freezing at the equator than at polar latitudes, and S. Karato proposed that changes in the magnetic field might also deform the inner core slowly over time.
There is an East–West asymmetry[clarification needed] in the inner core seismological data. There is a model which explains this by differences at the surface of the inner core – melting in one hemisphere and crystallization in the other.
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