<P> Many rocks now making up Earth's crust formed less than 100 million (1 × 10) years ago; however, the oldest known mineral grains are about 4.4 billion (4.4 × 10) years old, indicating that Earth has had a solid crust for at least 4.4 billion years . </P> <P> Earth's mantle extends to a depth of 2,890 km, making it the thickest layer of Earth . The mantle is divided into upper and lower mantle . The upper and lower mantle are separated by the transition zone . The lowest part of the mantle next to the core - mantle boundary is known as the D" (pronounced dee - double - prime) layer . The pressure at the bottom of the mantle is ≈ 140 G Pa (1.4 M atm). The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust . Although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales . Convection of the mantle is expressed at the surface through the motions of tectonic plates . As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of the mantle flows less easily than does the upper mantle (chemical changes within the mantle may also be important). The viscosity of the mantle ranges between 10 and 10 Pa s, depending on depth . In comparison, the viscosity of water is approximately 10 Pa s and that of pitch is 10 Pa s . The source of heat that drives plate tectonics is the primordial heat left over from the planet's formation as well as the radioactive decay of uranium, thorium, and potassium in Earth's crust and mantle . </P> <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>

In what part of the earths internal structure can rocks melt