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.
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.
Dynamics of Liquid Solidification Thermal Resistance of Contact Layer - Ghent University Library
Thanks to this phenomeon, it is known that the inner core can propagate S waves, and therefore must be solid. The velocity of the S-waves in the core varies smoothly from about 3.
That is considerably less than the velocity of S-waves in the lower crust about 4. The velocity of the P-waves in the core also varies smoothly through the inner core, from about Then the speed drops abruptly at the inner-outer core boundary to about Its volume is about 7.
The acceleration of gravity at the surface of the inner core can be computed to be 4. The density of the inner core is believed to vary smoothly from about 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 From these considerations, in D.
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.
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In , B. Buffet determined that the average magnetic field in the liquid outer core is about 2. 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. While indirect, this measurement does not depend significantly on any assumptions about the evolution of the Earth or the composition of the core. 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. 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.
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. According to computations by D. That structure can still admit the inclusion of small amounts of nickel and other elements.
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. 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. In , 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. This P-wave speed anisotropy has been confirmed by later studies, including more seismic data  and study of the free oscillations of the whole Earth.
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Frost and B. Romanowicz confirmed that the value is between 0. Some authors have claim that P-wave speed is faster in directions that are oblique or perpendicular to the N-S axis, at least in some regons of the inner core. Romanowicz, who instead claim that the direction of maximum speed is as close to the Earth's rotation axis as can be determined.
A preference for the crystals in the core to align in the north-south direction could account for the observed seismic anomaly. 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 accoding to the direction of the flow. In , 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.
Others suggested that the required flow could be caused by slow thermal convection inside the inner core. Yukutake claimed in that such convective motions were unliley. Buffet in estimated the viscosity of the inner core and found that such convection could have happened, especially when the core was smaller. On the other hand, M. Bergman in 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. Karato proposed that changes in the magnetic field might also deform the inner core slowly over time. In , M. Ishii and A. Wang and X. Tanaka and H. Finlay conjectured that this process could explain the asymmetry in the Earth's magnetic field. However, in D. 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. 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.
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 degrees Celsius per billion years. 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.
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. 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. In , X. Song and P. Richards estimated this "super-rotation" of the inner core relative to the mantle as about one degree per year.
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. 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.
During those epochs, which lasted — million years each, the extra heat generated by stronger fluid motions might have stopped the growth of the inner core.
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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. 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.
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. In , 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.
With those values, they obtained an upper bound of 4. 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".