Inside the Earth

How the Earth formed, its layers (crust, mantle, outer core and inner core), the lithosphere and asthenosphere, and how earthquake waves let scientists see inside the planet.

The deepest hole ever drilled, the Kola Superdeep Borehole in Russia, took about 20 years to reach 12.3 km. That is less than 0.2% of the way to the centre of the Earth. So how do we know what's down there?

How the Earth formed

The Earth formed about 4.54 billion years ago, along with the Sun and the other planets, from a spinning disc of gas and dust. Dust grains stuck together into rocks, rocks into larger bodies, and these crashed together to build the planets. The energy of all those collisions, plus heat from radioactive elements, kept the young Earth largely molten.

While it was molten, the heavy iron and nickel sank to the centre and lighter rocky material floated towards the top, a bit like oil separating from vinegar in salad dressing. That is why the Earth is made of layers. As the surface cooled, a thin rocky crust formed. Water vapour released by volcanoes, and water carried in by icy asteroids and comets, condensed as the planet cooled and filled the first oceans.

You may read that the Earth was "a piece flung out by the Big Bang". It wasn't: the Big Bang happened about 13.8 billion years ago, more than nine billion years before the Sun and Earth formed.

The Earth's layers

The Earth cut in half. The layers are drawn to scale except the crust, which would be thinner than a line.

Crust. The thin, solid, rocky outer layer we live on. It is 5–10 km thick under the oceans and 25–70 km thick under the continents, thickest beneath big mountain ranges. Oceans cover about 71% of the Earth's surface, so most of the crust is under water.
Mantle. About 2900 km of hot rock, making up roughly 84% of the Earth's volume. It is solid, but it is so hot (from about 500 °C at the top to nearly 4000 °C at the bottom) that over millions of years it flows very slowly, like extremely stiff toffee. Hot rock rises and cooler rock sinks in giant convection currents, moving a few centimetres a year.
Outer core. About 2260 km of liquid iron and nickel at around 4000–5000 °C. The swirling of this liquid metal produces the Earth's magnetic field, which makes compasses work and shields us from harmful particles from the Sun.
Inner core. A solid ball of iron and nickel with a radius of about 1220 km, at roughly 5200 °C: about as hot as the surface of the Sun. It doesn't melt because the pressure there, more than three million times the air pressure at the surface, squeezes the atoms into a solid.

Where does all this heat come from? Partly it is heat left over from the Earth's formation, and partly it comes from the slow decay of radioactive elements such as uranium, thorium and potassium in the rock. The Earth is still cooling down, very slowly.

InteractiveDrill to the centre of the Earth

Try this

Drill down slowly. At what depth do you pass the deepest hole ever drilled? How hot is it there?
Where does the temperature rise fastest: in the crust or in the mantle?
Switch to earthquake waves. Which kind of wave never reaches the far side of the Earth, and what does that tell us about the outer core?

Crust, lithosphere and asthenosphere

Geologists also divide the outer Earth by how stiff the rock is, not just what it's made of. The crust and the top part of the mantle are cold enough to behave as one rigid layer, about 100 km thick, called the lithosphere (from the Greek lithos, stone). Below it is the asthenosphere ("weak layer"), where the mantle rock is hot and soft enough to flow slowly.

The lithosphere is broken into giant pieces called tectonic plates, which slide around on the asthenosphere at between about 1 and 10 cm a year: roughly the speed your fingernails grow. You'll find out what happens at their edges in the next topic.

A slice through the outer Earth. Oceanic crust is thin and made of dense basalt; continental crust is thicker and made of lighter, granite-like rock. Both sit on top of the rigid upper mantle, and together they form the lithosphere.

How do we know?

No one has ever seen the mantle or core. Scientists have pieced together the inside of the Earth from several kinds of indirect evidence:

Earthquake waves. Earthquakes send waves through the whole planet, and thousands of instruments called seismometers record when they arrive. P waves (push–pull waves) travel through solids and liquids; S waves (side-to-side waves) travel only through solids. S waves never reach the far side of the Earth, which shows the outer core is liquid. In 1936 the Danish seismologist Inge Lehmann noticed faint P waves arriving where none were expected and worked out that there must be a solid inner core.
Density. Measurements of gravity show the Earth as a whole is about twice as dense as the rocks at the surface, so the middle must be made of something much heavier, such as iron.
Meteorites. Many meteorites are pieces of broken-up early planets. Some are rocky and some are almost pure iron and nickel, matching what we think the mantle and core are made of.
The magnetic field needs moving, electrically conducting liquid deep inside, which fits a liquid iron outer core.
Lab experiments squeeze tiny samples of rock and iron between diamonds and heat them with lasers to copy the conditions deep inside the Earth.

Test yourself

Which layer of the Earth is liquid?
The crust is thickest
The lithosphere is made of
Why is the inner core solid even though it is about 5200 °C?
S waves cannot pass through the outer core because
Tectonic plates move at about

Label the layers of the Earth.

Match each fact to the layer.

The rigid outer shell of the Earth is the . It is broken into tectonic . Below it is the soft, slowly flowing . Instruments that record earthquake waves are called . Much of the Earth's internal heat comes from decay.

4 marks

No one has ever drilled deeper than about 12 km. Explain how scientists know that the Earth has a liquid outer core and a solid inner core.

Your answer