KS3 · Chemistry

Layers of the Earth: composition and atmosphere

You're standing on a thin, rocky skin. Beneath it lie thousands of kilometres of rock and metal nobody has ever seen. Let's travel down and find out what's there.

Journey to the centre

Travel down through the Earth

Tap each layer, from the ground under your feet down to the very centre. For each one, ask: what is it made of, and is it solid or liquid? (Every band here is drawn the same height. The real layers are very different thicknesses — you'll fix that in a minute.)

↑ The surface — where all life is

↓ The centre of the Earth

Zone 1 of 4

Crust

The rocky outer skin of the Earth, and the only layer where life is found — partly covered in water. It's the thinnest layer and the most brittle: it breaks rather than bends. It's between 5 and 70 kilometres thick: thickest under the land, thinnest under the oceans.

  • Outermost layer
  • Thinnest layer
  • 5–70 km thick

Notice the pattern as you go down: it gets hotter, and the pressure — the squeeze from everything above — gets greater, all the way to the centre. Radioactive processes in the core generate the heat.

The twist in the mantle

What is the mantle really like?

You've just travelled through the mantle — about 2,900 km of it, the thickest layer in the Earth.

Which of these is closest to what you picture right now?
How sure are you?

Predict, then check

Hotter usually means more likely to melt. So what's going on at the centre?

The inner core is the hottest part of the Earth — nearly 5,200 °C, hotter than the liquid outer core around it. So why is the inner core solid?

Build it to scale

Stack the layers from the centre outwards

This line is distance from the centre of the Earth, in kilometres. Build the Earth from the middle out by stacking each layer on the one below: the inner core is about 1,250 km, the outer core about 2,200 km, and the mantle about 2,900 km. Drag each marker to where that boundary sits.

How do we know?

How deep have we ever drilled?

The crust is up to 70 km thick under the land. How far down has the deepest hole ever drilled reached?

Your estimate

35km

0km70km

Model versus reality

Hard-boiled eggvsThe real Earth

Scientists use models to understand things they can't see, to make predictions and to share ideas. A hard-boiled egg is a classic model of the Earth. Where does it work, and where does it break down?

Focus

Layers

Hard-boiled egg

Three layers, with a thin shell on the outside

The real Earth

Three main layers — crust, mantle and core — with a thin crust on the outside

The insight

This is why the egg is a good model: the shell, white and yolk line up with the crust, mantle and core.

Shape

Hard-boiled egg

Egg-shaped, not a sphere

The real Earth

Spherical

Outer layer thickness

Hard-boiled egg

The shell is the same thickness all the way round

The real Earth

The crust varies from 5 km under the oceans to 70 km under the land

Where the middle is

Hard-boiled egg

The yolk sits off-centre

The real Earth

The core is at the centre

Liquid layer

Hard-boiled egg

No liquid layer around the yolk

The real Earth

A liquid outer core around a solid inner core

Temperature at the centre

Hard-boiled egg

The centre is not hot

The real Earth

The inner core is nearly 5,200 °C

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Rock on the outside, metal in the middle — and a centre that is squeezed solid.

What you need to know

  • The Earth is a spherical, rocky planet made of distinct layers — the crust, the mantle and the core — which differ because of their chemical composition.
  • The crust is the rocky, outermost layer where all life is found. It is the thinnest and most brittle layer, 5–70 km thick: thickest on land, thinnest under the oceans.
  • The mantle is dense rock with more iron, calcium and magnesium than the crust. It is the thickest layer, at about 2,900 km.
  • The core is thought to be mostly iron and nickel: a liquid outer core directly below the mantle, and a solid inner core at the centre.
  • Temperature and pressure increase towards the centre. The inner core is solid because of the intense pressure from the layers above it.
  • No one can observe the Earth's interior directly, so scientists use evidence, such as from earthquakes, to build a model of it.

The big picture

Earth is a spherical, rocky planet made of layers that differ because of what they are made of: a thin, brittle crust of rock; a thick mantle of dense rock; and a core that is thought to be mostly iron and nickel — a liquid outer core around a solid inner core. Temperature and pressure rise towards the centre, and the inner core stays solid because of the intense pressure of the layers above it. Nobody can see inside the Earth, so scientists build a model from evidence such as earthquakes.

Key points

1In order from the surface: crust → mantle → outer core → inner core.
2Rock on the outside (crust and mantle), iron and nickel metal on the inside (outer and inner core).
3The mantle is not molten lava: it is dense rock, and just less than 1% of it can flow like a very thick fluid, over thousands to millions of years.
4The inner core is estimated at nearly 5,200 °C, heated by radioactive processes in the core — yet it is solid because of pressure.
5A scale diagram is built from the centre outwards: inner core about 1,250 km, outer core about 2,200 km, then the mantle, then the crust.
6The deepest borehole, the Kola Superdeep Borehole, is 12,262 m deep and pierced only a small part of the crust.
7Models such as a hard-boiled egg help us understand, predict and communicate, but every model has limitations.

Worked example

Problem

You want to draw the Earth's layers to scale on paper, using a scale of 1 cm to 500 km. How far from the centre should you draw each boundary, and how thick will the crust be on your drawing?

⚠ Watch out

Mixing up the two parts of the core. The OUTER core is the liquid one; the INNER core, at the very centre, is solid. Both are iron and nickel — the difference is the pressure.

🧠

Memory hook

Rock on the outside, metal in the middle, and the centre is squeezed solid. For the order, going down: Can Monkeys Open Ice-lollies? — Crust, Mantle, Outer core, Inner core.

✓

Check yourself

A friend says: 'The centre of the Earth must be liquid, because it's the hottest place — and the mantle's molten lava anyway.' Spot both mistakes and put each one right in a sentence.

Flashcards

(16)
Name the Earth's layers in order, from the surface to the centre.
Crust, mantle, outer core, inner core.
What makes the crust, mantle and core different layers?
Their chemical composition — what each one is made of.
Describe the crust.
The rocky, outermost layer where all life is found — the thinnest and most brittle layer.
How thick is the crust, and where is it thickest?
5–70 km. Thickest under the land, thinnest under the oceans.
What is the mantle made of?
Dense rock, containing more iron, calcium and magnesium than the crust.
Which is the Earth's thickest layer, and how thick is it?
The mantle, at about 2,900 km.
Can any of the mantle move?
Just less than 1% of it can flow like a very thick fluid, over thousands to millions of years.
What is the outer core like?
Liquid iron and nickel, directly below the mantle.
What is the inner core like?
Solid iron and nickel at the centre, estimated at nearly 5,200 °C.
Why is the inner core solid, even though it is the hottest part?
Intense pressure caused by the layers above it.
What happens to temperature and pressure as you go towards the centre?
Both increase.
What generates heat in the core?
Radioactive processes in the core.
How do you build a scale diagram of the Earth?
Start at the centre and stack each layer on the one below: inner core about 1,250 km, outer core about 2,200 km, then the mantle, then the crust.
How deep is the deepest borehole ever drilled?
The Kola Superdeep Borehole: 12,262 m — only a small part of the crust.
How do scientists know what is inside the Earth?
They collect evidence, for instance from earthquakes, develop theories and build a model from that evidence.
Give three limitations of the hard-boiled egg model of the Earth.
Any three: it isn't spherical; its shell is the same thickness all round; its yolk is off-centre; it has no liquid outer core; its centre isn't hot.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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