The Earth and its atmosphere · Model
Inside the Earth
Nobody has ever been more than twelve kilometres into the ground, and the centre is six thousand kilometres down. So how does anyone know what is there?
Start here
The deepest hole ever drilled took twenty years and reached 12.3 kilometres. The centre of the Earth is 6371 kilometres down.
The Kola borehole was abandoned because the rock at the bottom was 180 °C and behaving like plastic — it kept closing the hole. Twelve kilometres is two tenths of one per cent of the way to the middle. On a football-sized Earth, that hole would not break the skin of the leather.
So how do we know what the inside is made of?
By listening to earthquakes. Every large earthquake sends waves through the whole planet, and those waves bend, speed up, slow down and sometimes stop dead depending on what they are travelling through. Hundreds of seismometers around the world record when each wave arrives. From those arrival times — and nothing else — you can work out where the boundaries are and whether each layer is solid or liquid. The inside of the Earth was mapped by sound, not by digging.
The Earth is not one substance. It is a set of layers, each with its own composition, temperature and state — and the boundaries between them are sharp.
Everything you have ever seen, mined or built on is the crust: a skin so thin that on the scale of the whole planet it is barely there.
Your turn · four layers
Tap a layer. The widths are the real proportions — except the crust.
1 of 4 opened
Almost everything on this bar is drawn to scale. The crust is the exception, and it is the sliver at the very top.
Surface · 0 km
Centre · 6371 km
One honest exception: the crust is really only 0.5% of the way down, which is thinner than a line you could tap. It is drawn wider here so you can reach it — everything else is to scale.
Crust
0 – 35 km
State
Solid rock
Temperature
Up to about 400 °C
Made of
Silicon and oxygen compounds
Share of the depth
0.5%
Thinner under the oceans, thicker under mountains, and cracked into plates that move a few centimetres a year. Everything anyone has ever mined or built on is in this layer, and the deepest borehole reached only a third of the way through it.
Mantle
35 – 2900 km
State
Solid, but flows slowly
Temperature
500 – 4000 °C
Made of
Silicon, oxygen, iron, magnesium
Share of the depth
45.0%
By far the largest layer. It is solid rock, but hot enough and under enough pressure to creep like extremely stiff toffee. Those slow currents are what drag the plates above them around.
Outer core
2900 – 5150 km
State
Liquid
Temperature
4000 – 5000 °C
Made of
Iron and nickel
Share of the depth
35.3%
Molten metal, and the only genuinely liquid layer in the planet. It is known to be liquid because one type of earthquake wave cannot pass through it at all — and that same swirling liquid metal generates the Earth’s magnetic field.
Inner core
5150 – 6371 km
State
Solid
Temperature
About 5500 °C
Made of
Iron and nickel
Share of the depth
19.2%
Hotter than the liquid layer above it and solid anyway, because the pressure at the centre of the Earth is too great to let the atoms move apart. A ball of iron about two-thirds the width of the Moon, as hot as the surface of the Sun.
Look at how little of that bar is crust.
Every mine, every borehole, every fossil, every ocean and every mountain range is in the thin strip on the left. The crust is 0.5% of the 6371 km from the surface to the centre. Scale the Earth down to the size of an apple and it is thinner than the skin.
On the bar above it is drawn wider than that, because 0.5% of a screen is a hairline nobody could tap. Every other width on the bar is the real proportion.
Key fact
Crust, mantle, outer core, inner core. The mantle is solid rock that flows very slowly; the outer core is liquid iron; the inner core is solid iron, kept solid by the pressure at the centre.
Three questions · how we know
The evidence, not the picture in the textbook
0 of 3 answered
Commit before you read. Every one of these was worked out without anyone going down there.
One type of earthquake wave travels through the mantle but stops dead at 2900 km. What does that show?
A liquid layer. S-waves cannot travel through liquids at all, so the depth at which they vanish marks the top of the liquid outer core exactly. The wave that stops tells you as much as the wave that arrives.
The whole Earth is much denser than any rock found at the surface. What does that suggest?
Something far denser is inside. Surface rock averages about 2.7 g/cm³ and the whole planet averages 5.5, so the interior must be made of something heavy. Iron fits, and meteorites, which formed from the same material, are full of it.
The Earth has a magnetic field strong enough to swing a compass needle anywhere on the surface. What does that require?
Moving liquid metal. A permanent magnet cannot survive those temperatures — heat destroys magnetism. What does work is electric current, and a churning ocean of molten iron carries enormous currents. The field is evidence that part of the core is liquid and in motion.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“The mantle is a sea of molten lava, and volcanoes are holes that let it out.”
Lava does come out of volcanoes, and it comes from below. Commit before you read on.
The mantle is solid rock. It has to be: earthquake waves that cannot travel through liquid pass straight through it, which is how anyone knows. What makes it confusing is that solid rock under that much heat and pressure can flow — a few centimetres a year, like extremely stiff toffee. Solid and rigid are not the same word.
Magma exists only in small pockets, mostly where the pressure drops enough to let a little of the rock melt. It is the exception, not the ocean. If the mantle really were liquid, the crust would not be floating on it — it would have sunk into it long ago.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
Name the four layers of the Earth in order from the surface inwards.
Rung 2 · The one that catches people
The inner core is hotter than the outer core, yet it is solid and the outer core is liquid. Why?
Rung 3 · Explain
Explain how scientists know the Earth has a liquid layer inside it, when nobody has ever been deeper than 12 km.
Rung 4 · Take it somewhere new
Mars has no global magnetic field today, although its oldest rocks record one, and its atmosphere is very thin. Suggest how those facts might be connected, and say what evidence would test your idea.
Key note
The Earth has four layers: a thin rocky crust, a thick mantle of solid rock that flows very slowly, a liquid iron outer core and a solid iron inner core. The structure is known from the way earthquake waves travel through the planet, because different waves behave differently in solids and liquids.
Going further
The liquid outer core is the reason a compass works. Iron that hot cannot be a magnet in the ordinary sense — heat destroys magnetism — but the core is molten metal, it moves, and moving metal carries electric currents. Those currents generate the magnetic field that surrounds the planet, deflects the solar wind and keeps the atmosphere from being stripped away. Mars has no global field today: its oldest rocks record one that switched off around four billion years ago, whatever was stirring its core stopped, and almost none of its atmosphere is left.
The inner core is the strangest part. It is hotter than the liquid layer above it — around 5500 °C, roughly the surface temperature of the Sun — and it is solid anyway, because the pressure at the centre of the Earth is over three million times atmospheric pressure and will not let the atoms move apart. It is also growing: every year a little more of the outer core freezes onto it, and the energy released as it does helps drive the currents that make the magnetic field.
Before this lesson
At GCSE this becomes
- Plate tectonics driven by convection in the mantle, and using P-wave and S-wave shadow zones as evidence for a liquid outer core.
Where to next
- Next: Three ways to make a rock
- Previous: Ceramics, polymers and composites
Metals and materials
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