Earth's layers

The centre of the Earth is 6,371 km below your feet, and the rocky crust you live on is only about 35 km of that under a continent.

An egg with a metal yolk

The US Geological Survey compares the Earth to a boiled egg. The crust is the shell: thin, rigid and easy to crack. The mantle is the white, a thick layer of hot rock. The core is the yolk, except that this yolk is made of metal, mostly iron with some nickel, and it comes in two parts. The comparison works for proportions too. If the Earth were an egg, the crust would be thinner than the shell.

The model above uses the real radii. The Earth's radius is 6,371 km. The crust is about 5 to 10 km thick under the oceans and about 25 to 70 km under the continents, so in the model it is a line you can barely see. Below it, the mantle reaches down to about 2,890 km. Seismologists split it at a depth of about 660 km into an upper and a lower mantle, because the minerals change and earthquake waves speed up there. The liquid outer core runs from 2,890 km to about 5,150 km, and the solid inner core fills the last 1,220 km or so to the centre.

Hotter and denser with depth

Temperature rises as you go down. The geology textbook by Steven Earle gives about 1,000 °C at the base of the crust, about 3,500 °C at the base of the mantle and about 5,000 °C at the centre. Laboratory experiments at the European Synchrotron in 2013 put the boundary between the inner and outer core near 6,000 °C, give or take 500 degrees. Nobody has drilled anywhere near those depths, so these numbers come from melting experiments and models, and different studies disagree by several hundred degrees. That is why the readout gives ranges.

Heat alone would melt much of the deep Earth, but pressure pushes the other way. The mantle is hot enough to melt at the surface, yet it stays solid because of the weight of rock above it. It does flow, very slowly, the way a glacier flows, at rates of centimetres per year. The inner core is hotter still and also solid, for the same reason. Only the outer core, where the balance tips, is a true liquid. USGS describes it as spinning as the Earth rotates, and its moving metal makes the Earth's magnetic field.

Plates are not the same as the crust

There is a second way to slice the outer Earth, by how the rock behaves rather than what it is made of. The crust and the cool top of the upper mantle are stiff together. This rigid shell, the lithosphere, averages at least 80 km thick and is broken into the plates. Under it lies the asthenosphere, a hotter and weaker zone in the upper mantle. Earle's textbook notes that between about 100 and 250 km the rock is very close to its melting point. The plates glide over this soft layer.