Geografia

Structure of the Earth: A Simple Summary of the Layers with 5 Solved Exercises

The internal structure of the Earth made simple: crust, mantle, outer and inner core, the discontinuities and seismic waves, with 5 solved exercises and 3 FAQs for quick revision.

Recommended for: Grade 6 · Grade 7 · Grade 8

The Earth is made of concentric layers, like an onion: from the outside towards the centre we find the crust, the mantle and the core, the last of which is divided into an outer core (liquid) and an inner core (solid). Each layer has a different thickness, temperature and material.

The four main layers

Imagine cutting the Earth in half. You would see four stacked bands: a very thin outer skin, an extremely thick rocky layer and, at the centre, a metal sphere split in two. These are the crust, the mantle, the outer core and the inner core.

The crust: the thin shell

The crust is the outermost and thinnest layer, the one we live on. Under the continents it is on average 30-40 km thick, under the oceans just 5-10 km. Compared with the whole Earth it is extremely thin: if the planet were an apple, the crust would be thinner than its skin.

The mantle: the thickest layer

Below the crust is the mantle, which reaches down to about 2900 km and is the thickest layer: on its own it makes up about 84% of the Earth’s volume. It is made of solid rock, but in its upper part it is partly plastic and flows extremely slowly: it is these movements that push the tectonic plates and cause earthquakes and volcanoes.

The core: outer and inner

At the centre of the Earth is the core, made mostly of iron and nickel. It is divided into two parts. The outer core is liquid, and its motion generates the Earth’s magnetic field, the one that makes a compass work. The inner core, on the other hand, is solid: although it is the hottest part (over 5000 °C, like the surface of the Sun), the enormous pressure keeps it compact.

How do we know? Seismic waves

Nobody has ever gone down more than a few kilometres: the deepest borehole reaches about 12 km. So how do we know the inside of the planet? Thanks to the seismic waves from earthquakes, which travel through the Earth changing speed and direction depending on the material. The points where the waves change sharply are called discontinuities: the Moho separates crust and mantle, the Gutenberg separates mantle and core, and the Lehmann separates outer and inner core.

Lithosphere and asthenosphere

The layers can also be classified by how they behave, not just by their composition. The lithosphere is the rigid, outer part (crust + upper mantle) and it is broken into the tectonic plates. Beneath it lies the asthenosphere, a hotter, partly plastic band of the mantle on which the plates “float” and slide.

In the exercises below you will find the layers to put in order, a true/false, the discontinuities to match, a calculation about the size of the core and a reasoning question about seismic waves: the same path explained on this page.

Solved exercises

1. Put the four main layers of the Earth in order, starting from the surface and going down to the centre: mantle, inner core, crust, outer core. base

Show solution
  1. I start from the outermost layer, the one we walk on: the crust.
  2. Right below the crust is the mantle, the thickest layer of rock.
  3. Below the mantle the core begins, split into two parts: first the outer core (liquid).
  4. At the exact centre of the Earth lies the inner core (solid).

Answer: Crust → mantle → outer core → inner core

2. True or false? Correct the false statements. a) The crust is the thickest layer of the Earth. b) The outer core is liquid. c) The inner core is solid even though it is the hottest part. d) The mantle is made mostly of gas. base

Show solution
  1. a) False: the crust is the thinnest layer; the thickest layer is the mantle, which alone makes up about 84% of the Earth's volume.
  2. b) True: the outer core is made of molten iron and nickel, that is, in a liquid state.
  3. c) True: the inner core is solid even though it is hotter than 5000 °C, because the enormous pressure at the centre keeps it compressed and solid.
  4. d) False: the mantle is made of solid, partly plastic rock (which flows slowly), not gas.

Answer: a) False (the thickest is the mantle); b) True; c) True; d) False (it is rock, not gas)

3. Match each discontinuity to the boundary between layers that it marks: Moho, Gutenberg, Lehmann. intermedio

Show solution
  1. The Moho (Mohorovičić) discontinuity marks the boundary between the crust and the mantle.
  2. The Gutenberg discontinuity marks the boundary between the mantle and the outer core, at about 2900 km depth.
  3. The Lehmann discontinuity marks the boundary between the outer core (liquid) and the inner core (solid).

Answer: Moho = crust/mantle; Gutenberg = mantle/outer core; Lehmann = outer core/inner core

4. The inner core has a radius of about 1220 km, while the mean radius of the Earth is about 6371 km. What percentage of the Earth's radius does the inner core take up? Round to the nearest whole number. intermedio

Show solution
  1. To find the percentage I divide the inner core's radius by the total radius: 1220 ÷ 6371.
  2. Calculation: 1220 ÷ 6371 = about 0.191.
  3. I turn it into a percentage by multiplying by 100: 0.191 × 100 = 19.1%.
  4. Check: 6371 × 0.19 = 1210 km, very close to 1220 km. The result holds.

Answer: About 19% of the Earth's radius

5. If we cannot dig all the way to the centre of the Earth, how do we know that the outer core is liquid? Explain the reasoning using seismic waves. avanzato

Show solution
  1. Earthquakes produce two main types of waves that travel through the Earth's interior: P-waves (which compress the material) and S-waves (which make it shear from side to side).
  2. S-waves cannot travel through liquids, while P-waves can (although they slow down and change direction).
  3. By recording earthquakes all over the world, seismologists found that behind the core there is a 'shadow zone' where S-waves do not arrive.
  4. If the S-waves are blocked by the outer core, then that material must be liquid: this is how we worked out the state of the outer core without digging.

Answer: Because seismic S-waves do not pass through liquids: they create a shadow zone behind the core, showing that the outer core is liquid.

FAQ

Why is the inner core solid if it is the hottest part of the Earth?

Because at the centre of the Earth the pressure is enormous: it squeezes the iron so much that it raises its melting point above the actual temperature. As a result, even though it is extremely hot, the inner core stays solid.

How do scientists know about the Earth's interior without digging all the way down?

Mainly by studying the seismic waves from earthquakes: their speed and path change depending on the material they cross, which lets us 'x-ray' the inside of the planet. The deepest borehole ever drilled (Kola, in Russia) reaches only about 12 km, barely a scratch in the crust.

What is the difference between the crust and the lithosphere?

The crust is the outermost layer defined by chemical composition. The lithosphere, instead, is a rigid layer defined by mechanical behaviour: it includes the crust plus the uppermost rigid part of the mantle, and it is broken into the tectonic plates that move slowly.