The first indication of Earth-core complexity came with Lehmann’s 1936 discovery of the inner core.The core is approximately half the radius of the Earth and is about twice as dense as the mantle. It represents 32 percent of the mass of the Earth. Iron crystals are the main constituents of the solid inner core.The actual presence of the solid inner core inside Earth was proven after the great Alaska earthquake of 28 March 1964 and is no longer questioned today. Estimating properties of this inner core, however, remains a major challenge.The crystals may have grown with a preferred orientation when the inner core was formed, or processes at work in the inner core may have aligned them over time.The dynamics of the core is critically influenced by the combined effects of rotation and magnetic fields.The core has some additional impurities; about 6% of Ni and about 10% of lighter elements such as H, O, S, and C. The light elements are believed to exist preferentially in the fluid outer core as compared with the solid inner core.Earth in its entirety can be considered a slow nuclear reactor with its solid ”inner core” providing a major contribution to the total energy output. Since radionic heat is generated in the entire volume and cooling can only occur at the surface, the highest temperature inside Earth occurs at the center of the inner core. Overheating the center of the inner core reactor due to the so-called greenhouse effect on the surface of Earth may cause a meltdown condition, an enrichment of nuclear fuel and a gigantic atomic explosion.In order to understand better the origin of the seismic anisotropy observed in the inner core, experiments and theories have focussed on the mechanical behavior of crystalline iron at the extreme conditions of the inner core estimated to have temperature of about
5000K and pressure of 330GPa.
The primary dynamics of the Earth’s fluid core is controlled by rapid rotation, small viscosity, thermal or compositional convection, and a self-generatedmagnetic field.
Several facts about the core, however, can be established directly from the seismic data with quite considerable ncertainty:
The ”inner core” (the part inside the 1220 km radius) is a solid, because it transmits shear waves and only a solid can do this.
The ”outer core” that surrounds the solid inner core appears to be a fluid - due to the absence of shear waves there.
To a seismologist, who performs observations with respect to the surface of Earth, the solid inner core seems slightly anisotropic, a few %, and this anisotropy seems "spinning" inside the planet about 4% faster than the planet spins around its own axis.
In contrast to the outer core, the inner core is denser than pure iron, so it is thought to be iron-nickel without a light element. Thus it is chemically different from the outer core, as well as being solid rather