wave) are consistent with those through basalt (6.7–7.2 km/s), and below they are similar to those through peridotite or dunite (7.6–8.6 km/s). Second, in oceanic crust, there is a chemical discontinuity between ultramafic cumulates and tectonized harzburgites, which has been observed from deep parts of the oceanic crust that have been obducted onto the continental crust and preserved as ophiolite sequences. Many rocks making up Earth's crust formed less than 100 million years ago; however, the oldest known mineral grains are about 4.4 billion years old, indicating that Earth has had a solid crust for at least 4.4 billion years. Mantle Earth's mantle extends to a depth of , making it the planet's thickest layer. The mantle is divided into upper and lower mantle separated by a transition zone. The lowest part of the mantle next to the core-mantle boundary is known as the D″ (D-double-prime) layer. The pressure at the bottom of the mantle is ≈140 GPa (1.4 Matm). The mantle is composed of silicate rocks richer in iron and magnesium than the overlying crust. Although solid, the mantle's extremely hot silicate material can flow over very long timescales. Convection of the mantle propels the motion of the tectonic plates in the crust. The source of heat that drives this motion is the decay of radioactive isotopes in the Earth's crust and mantle combined with the initial heat from the planet's formation. Due to increasing pressure deeper in the mantle, the lower part flows less easily, though chemical changes within the mantle may also be important. The viscosity of the mantle ranges between 1021 and 1024 pascal-second, increasing with depth. In comparison, the viscosity of water at is 0.89 pascal-second and pitch is (2.3 ± 0.5)8 pascal-second. Core Earth's outer core is a fluid layer about thick and composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. Its outer boundary lies beneath Earth's surface. The transition between the inner core and outer core is located approximately beneath the Earth's surface. Earth's inner core is the innermost geologic layer of the planet Earth. It is primarily a solid ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius. The inner core was discovered in 1936 by Inge Lehmann and is generally composed primarily of iron and some nickel. Since this layer is