Inside the Earth

Nobody has been inside the Earth. The deepest mine goes about 4 km down, and the deepest drill hole, on the Kola Peninsula in Arctic Russia, stopped at 12,262 m. That is 0.2% of the way to the centre. Yet geologists can tell you the depth of the core to within a few kilometres. This page is about how.

The drawing in the textbook is wrong

Most diagrams of the Earth give the crust a thick, friendly band so you can see it. At true scale it almost disappears. Under the continents the crust is about 30 km thick on average, and under the oceans only 5 to 10 km. Shrink the Earth to the size of a football and the crust would be about half a millimetre: thinner than the cover of a book.

Measured by volume, the mantle is about 82% of the planet, the outer core about 16%, and the crust and the inner core less than 2% between them. (Our own sums, from the depths on this page.)

Listening to earthquakes

When a large earthquake happens, it sends waves through the whole planet, and seismometers around the world record when they arrive. Two kinds matter here. P-waves push and pull the rock, like a sound wave, and travel through solids and liquids. S-waves shake it from side to side, and only travel through solids, because a liquid cannot spring back from being sheared.

Their speed changes with what they pass through, so they bend as they go, just as light bends in water. Time enough arrivals at enough places and you can work out what the inside must be like for the waves to arrive when they did.

What the shadows tell us

Seismometers more than about 104° away from an earthquake, measured around the Earth from its centre, record no S-waves at all. Something liquid must be blocking them. That is the outer core. Its top is 2,891 km down.

P-waves get through the core, but they bend sharply as they enter and leave it. That leaves a band between about 104° and 140° from the earthquake where no direct P-waves arrive. In 1936 the Danish seismologist Inge Lehmann noticed faint P-waves turning up inside that band where none should be. She explained them with a separate inner core that deflects some of the waves into the band. Later measurements showed it is solid. Its edge is 5,150 km down.

The same method found the base of the crust. In 1909 Andrija Mohorovičić saw that waves from an earthquake near Zagreb reached far-off stations sooner than they should. They had dipped into faster rock below. That boundary, the Moho, is named after him.

Solid, liquid, solid

It seems odd that the inner core is solid when it is hotter than the liquid outer core. Estimates put its edge between about 5,200 and 6,000 °C, close to the temperature of the Sun's surface. The reason is pressure: at the inner core it is about 3.3 million times the pressure of the air at sea level, and that squeeze raises the melting point of iron faster than the temperature rises.

The mantle is solid too, even though most of it is thousands of degrees. Over millions of years it creeps, a few centimetres a year, which is what carries the plates around. Most of it is one mineral, bridgmanite. It is the most common mineral on Earth, but it falls apart at the surface, and the first natural grains were only found in 2014, inside a meteorite.

Take the whole fall yourself, at true scale: Surface to Core.