<P> The average density of Earth is 7000551499999999999 ♠ 5.515 g / cm . Because the average density of surface material is only around 7000300000000000000 ♠ 3.0 g / cm, we must conclude that denser materials exist within Earth's core . This result has been known since the Schiehallion experiment, performed in the 1770s . Charles Hutton in his 1778 report concluded that the mean density of the Earth must be about 9 5 (\ displaystyle (\ tfrac (9) (5))) that of surface rock, concluding that the interior of the Earth must be metallic . Hutton estimated this metallic portion to occupy some 65% of the diameter of the Earth . Hutton's estimate on the mean density of the Earth was still about 20% too low, at 7000450000000000000 ♠ 4.5 g / cm Henry Cavendish in his torsion balance experiment of 1798 found a value of 7000545000000000000 ♠ 5.45 g / cm, within 1% of the modern value . Seismic measurements show that the core is divided into two parts, a "solid" inner core with a radius of ≈ 1,220 km and a liquid outer core extending beyond it to a radius of ≈ 3,400 km . The densities are between 9,900 and 12,200 kg / m in the outer core and 12,600--13,000 kg / m in the inner core . </P> <P> The inner core was discovered in 1936 by Inge Lehmann and is generally believed to be composed primarily of iron and some nickel . Since this layer is able to transmit shear waves (transverse seismic waves), it must be solid . Experimental evidence has at times been critical of crystal models of the core . Other experimental studies show a discrepancy under high pressure: diamond anvil (static) studies at core pressures yield melting temperatures that are approximately 2000 K below those from shock laser (dynamic) studies . The laser studies create plasma, and the results are suggestive that constraining inner core conditions will depend on whether the inner core is a solid or is a plasma with the density of a solid . This is an area of active research . </P> <P> In early stages of Earth's formation about 4.6 billion years ago, melting would have caused denser substances to sink toward the center in a process called planetary differentiation (see also the iron catastrophe), while less - dense materials would have migrated to the crust . The core is thus believed to largely be composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust (see felsic materials). Some have argued that the inner core may be in the form of a single iron crystal . </P> <P> Under laboratory conditions a sample of iron--nickel alloy was subjected to the corelike pressures by gripping it in a vise between 2 diamond tips (diamond anvil cell), and then heating to approximately 4000 K . The sample was observed with x-rays, and strongly supported the theory that Earth's inner core was made of giant crystals running north to south . </P>

The study of earth's composition structure and history is called