Internal structure of Earth

Earth's average density can be determined through gravimetric experiments, which have historically involved pendulums.

[19] Earth's mantle extends to a depth of 2,890 km (1,800 mi), making it the planet's thickest layer.

[25] Although solid, the mantle's extremely hot silicate material can flow over very long timescales.

The source of heat that drives this motion is the decay of radioactive isotopes in Earth's crust and mantle combined with the initial heat from the planet's formation[27] (from the potential energy released by collapsing a large amount of matter into a gravity well, and the kinetic energy of accreted matter).

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.

[38][39] Under laboratory conditions a sample of iron–nickel alloy was subjected to the core-like pressure 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.

[40][41] The composition of Earth bears strong similarities to that of certain chondrite meteorites, and even to some elements in the outer portion of the Sun.

This ignores the less abundant enstatite chondrites, which formed under 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.

[citation needed] Dynamo theory suggests that convection in the outer core, combined with the Coriolis effect, gives rise to Earth's magnetic field.

[44] The magnetic field generated by core flow is essential to protect life from interplanetary radiation and prevent the atmosphere from dissipating in the solar wind.

[46][better source needed] The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes.

The changes in seismic velocity between different layers causes refraction owing to Snell's law, like light bending as it passes through a prism.