Seismic measurements show that the core is divided into two parts, a solid inner core with a radius of ~1,220 km[2] and a liquid outer core extending beyond it to a radius of ~3,400 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 (4.5×109) 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.[3][4] 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, 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.[5][6] 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 force exerted by Earth's gravity can be used to calculate its mass, and by estimating the volume of the planet, its average density can be calculated. Astronomers can also calculate Earth's mass from its orbit and effects on nearby planetary bodies. Observations of rocks, bodies of water and atmosphere allow estimation of the mass, volume and density of rocks to a certain depth, so the remaining mass must be in the deeper layers.

The interior structure of the Earth is layered in spherical shells, like an onion.

Earth has an outer silicate solid crust, a highly viscous mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core.

Chemically, Earth can be divided into the crust, upper mantle, lower mantle, outer core, and inner core.