|0–60||0–37||Lithosphere (locally varies between 5 and 200 km)|
|0–35||0–22||… Crust (locally varies between 5 and 70 km)|
|35–60||22–37||… Uppermost part of mantle|
|35–660||22–410||… Upper mantle|
|660–2890||410–1790||… Lower mantle|
The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes. The core does not allow shear waves to pass through it, while the speed of travel (seismic velocity) is different in the other layers. The changes in the seismic velocity between the different layers causes refraction owing to Snell's law. Reflections are caused by a large increase in seismic velocity and are similar to light reflecting from a mirror.
Seismic measurements show that the core is divided into two parts, a solid inner core with a radius of ~1220 km and a liquid outer core extending beyond it to a radius of ~3400 km. The solid inner core was discovered in 1936 by Inge Lehmann and is generally believed to be composed primarily of iron and some nickel.
In early stages of Earth's formation about 4.5 billion (4.5) 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.
The liquid outer core surrounds the inner core and is believed to be composed of iron mixed with nickel and trace amounts of lighter elements.
The matter that Earth is composed of is connected in fundamental ways to the matter of certain chondrite meteorites, and to the matter of the outer portion of the Sun . There is good reason to believe that Earth is, in the main, like a chondrite meteorite. Beginning as early as 1940, scientists, including Francis Birch, built geophysics upon the premise that Earth is like ordinary chondrites, the most common type of meteorite observed impacting Earth, while totally ignoring another, albeit less abundant type, called enstatite chondrites. The principal difference between the two meteorite types is that enstatite chondrites formed under circumstances of extremely limited available oxygen, leading to certain normally oxyphile elements existing either partially or wholly in the alloy portion that corresponds to the core of Earth.
Dynamo theory suggests that convection in the outer core, combined with the Coriolis effect, gives rise to the Earth's magnetic field. The solid inner core is too hot to hold a permanent magnetic field (see Curie temperature) but probably acts to stabilise the magnetic field generated by the liquid outer core.
Recent evidence has suggested that the inner core of Earth may rotate slightly faster than the rest of the planet. In August 2005 a team of geophysicists announced in the journal Science that, according to their estimates, Earth's inner core rotates approximately 0.3 to 0.5 degrees per year relative to the rotation of the surface.
The current scientific explanation for the Earth's temperature gradient is a combination of the heat left over from the planet's initial formation, the decay of radioactive elements, and the freezing of the inner core.
Earth's mantle extends to a depth of 2890 km, making it the largest layer of Earth. The pressure, at the bottom of the mantle, is ~140 GPa (1.4 Matm). 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. The melting point and viscosity of a substance depends on the pressure it is under. 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 1021 and 1024 Pa·s, depending on depth. In comparison, the viscosity of water is approximately 10-3 Pa·s and that of pitch 107 Pa·s. Thus, the mantle flows very slowly.
It has been also suggested that Earth's core consists of metallic hydrogen.