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Earth's Structure and Plates » Inside the Earth: Core, Mantle and Crust

What you'll learn this session

Study time: 30 minutes

Cambridge spec: 4.1.1

The four main layers of the Earth and what each one is like
How oceanic crust and continental crust are different
What the lithosphere and asthenosphere are
How scientists know what is inside the Earth

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A journey to the centre of the Earth

Earthquakes and volcanoes are caused by things happening deep below our feet. To understand them, you first need a picture of what the Earth is made of. Think of a peach. It has a thin skin, a thick fleshy part and a hard stone in the middle. The Earth also has layers, and each one is different in temperature, thickness and state (solid, liquid or slowly flowing rock).

From the centre outwards, the layers are the inner core, the outer core, the mantle and the crust. The distance from the surface to the centre of the Earth is about 6,400 km.

Key terms:

Core: the centre of the Earth, made mostly of iron and nickel. It has two parts: the inner core and the outer core.
Mantle: the thick layer of hot rock between the core and the crust.
Crust: the thin, hard, outer layer of the Earth, where we live.
Lithosphere: the crust together with the rigid top part of the mantle.
Asthenosphere: the part of the upper mantle just below the lithosphere, where the rock is hot and can flow very slowly.

The layers one by one

All figures below are rounded textbook figures. Use the word "about" when you write them in an exam.

🔥 Inner core

Solid, made of iron and nickel. It is about 1,200 km thick and is the hottest part of the Earth, at about 5,000 to 6,000 °C. It stays solid because the pressure on it is so enormous.

🌊 Outer core

Liquid, also iron and nickel. It is about 2,200 km thick and about 4,000 to 5,000 °C. The metal can flow here because the pressure is lower than in the inner core.

🌋 Mantle

Mostly solid rock, about 2,900 km thick, and by far the biggest layer. It is about 1,000 to 3,500 °C, hotter with depth. The rock flows very slowly, a few centimetres a year, like thick toffee.

🌎 Crust

The thinnest layer, a thin skin on the outside. It is solid rock and ranges from a few kilometres to about 70 km thick. It is cool at the surface and gets hotter with depth, up to about 1,000 °C at the base.

Do not mix up the mantle and the outer core

The outer core is a liquid metal. The mantle is solid rock that flows very slowly. It is not a sea of runny lava. Lava is molten rock that reaches the surface.

Two kinds of crust

The crust is not all the same. There are two types, and the difference matters in the next lessons. Here they are side by side.

Feature	Oceanic crust	Continental crust
Where	Under the oceans	Makes up the continents
Thickness	Thin: about 5 to 10 km	Thicker: about 30 to 70 km
Density	Dense (heavier)	Less dense (lighter)
Main rock	Basalt	Granite
Age	Younger: less than about 200 million years	Older: some parts about 4 billion years

A good way to remember it: oceanic is thin, dense and young. Continental is thick, light and old.

The lithosphere and the asthenosphere

Geographers often group layers by how they behave, not just by what they are made of.

The lithosphere is the crust plus the top, rigid part of the mantle. It is about 100 km thick on average.
The lithosphere is cold and brittle. It is broken into giant pieces called plates.
Below it is the asthenosphere, part of the upper mantle. It is hotter and softer and can flow very slowly.
The rigid plates sit on top of the flowing asthenosphere, a bit like cracked ice floating on thick syrup.

The next lessons look at the names of the plates and how they move.

How do we know what is inside the Earth?

Nobody has ever been to the centre of the Earth. The deepest hole ever drilled, in Russia, is only about 12 km deep. That is not even through the crust in many places, and the centre is about 6,400 km down. So how do we know about the layers?

The answer is seismic waves. These are shock waves from earthquakes that travel through the Earth. Scientists record them at stations all over the world. Waves change speed and direction when they pass from one layer to another, so they act like a scan of the inside of the Earth. There are two main types:

P waves can travel through solids and liquids.
S waves can travel only through solids.

S waves do not arrive on the far side of the Earth after passing through the middle. This shows there is a liquid layer in the centre, the outer core. How earthquake waves behave is explained in a later lesson.

Common mistakes

1. Saying the whole core is liquid. Only the outer core is liquid. The inner core is solid.
2. Saying the mantle is molten (runny). It is mostly solid and flows very slowly.
3. Saying the crust is the same thing as the lithosphere. The lithosphere also includes the top of the mantle.
4. Mixing up oceanic and continental crust. Remember: oceanic is thin and dense (basalt); continental is thick and light (granite).

Exam-style question

Fig. 1.1 shows made-up data for four layers of the Earth.

Layer	Thickness (km)	State	Temperature (°C)
Layer P	30 to 70	Solid	Up to about 1,000
Layer Q	About 2,900	Mostly solid, flows slowly	About 1,000 to 3,500
Layer R	About 2,200	Liquid	About 4,000 to 5,000
Layer S	About 1,200	Solid	About 5,000 to 6,000

(a) Identify layer R and layer S. [2 marks]

(b) State two ways in which oceanic crust is different from continental crust. [2 marks]

(d) Describe the characteristics of layer Q and explain why it is important for the movement of the plates. [6 marks]

Model answer

(a) Layer R is the outer core (1). Layer S is the inner core (1).
(b) Any two from: oceanic crust is thinner (1); oceanic crust is denser (1); oceanic crust is made of basalt, continental is granite (1); oceanic crust is younger (1).
(c) Seismic waves from earthquakes are recorded around the world (1). S waves cannot travel through liquids (1). They do not reach the far side of the Earth, which shows there is a liquid layer in the middle (1).
(d) Top level (5 to 6 marks): Layer Q is the mantle, the thickest layer at about 2,900 km (1). It lies between the crust and the core (1). It is mostly solid rock and is hot, about 1,000 to 3,500 °C (1). The rock in the upper part, the asthenosphere, is soft and can flow very slowly (1). The lithosphere above it is rigid and broken into plates (1). The plates sit on the asthenosphere, so they can move as the mantle flows slowly beneath them (1).

Exam tip

For a "Describe the characteristics" question, go through thickness, state (solid or liquid), temperature and material. Data like Fig. 1.1 is there to be quoted, so use the numbers in your answer.

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