Layers of the Earth: the crust, mantle, outer core and inner core, how thick each one is (from the US Geological Survey: crust about 5 km under the oceans and 30 km under the continents, mantle about 2,900 km, outer core 2,200 km of liquid, inner core 1,250 km of solid), what each is made of, the lithosphere and the asthenosphere that plate tectonics needs, how we know what is inside a planet nobody has drilled, and a printable labelled diagram

The Earth is about 12,750 km across and nobody has been more than about 12 km into it, yet its inside is known in some detail: three main layers of different material, one of them split into a liquid and a solid part, with a mechanical layering on top that explains why continents move. "Layers of the Earth" is searched about 40,000 times a month in the United States, mostly by children in Grades 4 to 8 asked to label a diagram; the figures below are the US Geological Survey's, the diagram is on the printable, and the two different ways of slicing the planet (by material and by behaviour) are separated, because that is where the marks are lost.

Download the layers of the Earth diagram and fact sheet (PDF) (the labelled cross-section with the USGS thicknesses and the composition of each layer on page 1, the mechanical layers, the evidence and a practice set with answers on page 2; two pages, free).

The three main layers (by what they are made of)

The core as a whole is nearly twice as dense as the mantle because it is metal rather than stone. The egg comparison used by the USGS works for shell, white and yolk, with one difference: the yolk is one thing, the core is two.

The mechanical layers (by how they behave)

The second way to slice the Earth is by stiffness, and it is the one plate tectonics needs.

• Lithosphere: the crust plus the cool, rigid uppermost mantle, behaving as one stiff layer. The USGS puts it at least 80 km thick on average, thinnest under the oceans and under volcanically active continental regions such as the western United States. It is broken into the plates that carry the continents and oceans.
• Asthenosphere: a relatively narrow, mobile zone of the mantle below the lithosphere, hot and semi-solid, able to soften and flow over geologic time. The rigid plates "float" on it and move about.
• Below that, the rest of the mantle, then the outer and inner core.

The exam trap: "crust" and "lithosphere" are not the same word, and neither are "mantle" and "asthenosphere". The crust is a kind of rock; the lithosphere is the crust and some mantle acting as one rigid slab. A diagram labelled by composition (crust, mantle, core) and one labelled by behaviour (lithosphere, asthenosphere, mesosphere, outer core, inner core) are both right; mixing the two sets on one diagram is the error.

How we know

No drill reaches the mantle. The structure comes from earthquakes. A large earthquake sends waves through the whole planet, and seismographs on the other side record when they arrive. P waves (pressure waves) travel through solids and liquids; S waves (shear waves) travel only through solids. S waves never arrive on the far side of the Earth from an earthquake, so something liquid is in the way: the outer core. P waves bend and speed up as they enter denser material, and the depths where their speed jumps mark the boundaries between crust and mantle and between mantle and core. The inner core's solidity shows up in the way P waves pass through it. The rest, the composition, comes from the Earth's overall density, from meteorites (left over from the material the planets formed from, iron-nickel ones included) and from rocks the mantle sends up in volcanoes.

Worked questions

1. Which layer is the thickest? The mantle, about 2,900 km: the crust is tens of km, the outer core 2,200 km, the inner core 1,250 km.
2. Which layer is liquid? The outer core.
3. Why is the inner core solid when it is at least as hot as the outer core? Pressure: at the centre the pressure is high enough to keep the iron-nickel solid.
4. Where does the magnetic field come from? The spinning liquid outer core.
5. A child labels the top layer "lithosphere" on a diagram whose other labels are mantle, outer core and inner core. Mixed sets: with those three labels the top one should be crust; lithosphere goes with asthenosphere.
6. Which is thicker, oceanic or continental crust? Continental (about 30 km against about 5 km), and continental crust is the lighter rock, which is why continents stand high.

The mistakes that lose marks

• Labelling the layers by colour from a picture without the names: the marks are for crust, mantle, outer core, inner core.
• Writing "the core is liquid": only the outer core is; the inner core is solid.
• Using crust and lithosphere as synonyms, or mantle and asthenosphere.
• Giving the crust a single thickness: it is about 5 km under oceans and about 30 km under continents, which is often the question.
• Saying scientists "drilled" or "saw" the layers: the evidence is earthquake waves.

Where to get help

If the method is not sticking from a page, a 1-on-1 online class fixes it faster than another worksheet: one tutor with a master's degree, one child, on a shared whiteboard, in your time zone (7 am before school, after 4 pm and weekend mornings, US time; after-school slots for the UK, Canada, Australia and the UAE), for about US$10 to $13 an hour. The first 30-minute class is free: how 1-on-1 online tutoring works.

Related pages

• Earth's four spheres: geosphere, hydrosphere, atmosphere, biosphere
• Monomers and polymers of the four macromolecules
• Net force and free-body diagrams

Questions parents ask

See how we teach this, 1-on-1 online →