Earth’s layers: what you’d dig through on the way
The crust, mantle, outer core and inner core, explained in the order you’d hit them if you tunneled straight through the planet.
If you dug straight down from your house, you’d pass through four main layers before reaching the center, 6,371 km below you. Then you’d pass through all of them again, in reverse, on the way up the other side.
Nobody has ever seen these layers. Almost everything we know about them comes from earthquake waves, which bend, speed up, slow down and sometimes vanish as they pass through materials of different kinds. (How that detective work works.)
Fun fact: Earth is slightly squashed, so a tunnel from pole to pole would be about 43 km shorter than one through the equator.
1. The crust (0 to about 35 km)
The crust is the thin, rocky shell we live on. It’s about 30 to 50 km thick under the continents and only 5 to 10 km thick under the oceans. Compared to the whole planet, it’s about as thin as the skin of an apple.
Even this first layer is already beyond human reach. The deepest hole ever drilled, Russia’s Kola Superdeep Borehole, went down 12.3 km and never got out of the crust.
In the upper crust, temperature rises by roughly 25 to 30 °C for every kilometer you descend.
2. The mantle (about 35 to 2,890 km)
In 1909 the Croatian scientist Andrija Mohorovičić noticed that earthquake waves suddenly sped up a few dozen kilometers down. He’d found the bottom of the crust, a boundary now called the Moho.
Below it is the mantle, which makes up about 84% of Earth’s volume. Mantle rock is solid, but it is so hot that over millions of years it slowly flows, like extremely stiff putty. That slow churning carries the tectonic plates along at a few centimeters a year, about as fast as your fingernails grow.
The mantle is usually split in two:
- The upper mantle runs down to about 660 km. Most diamonds form here, roughly 150 to 200 km down.
- The lower mantle runs from 660 km down to 2,890 km. Temperatures climb to roughly 3,500 to 4,000 °C near the bottom.
3. The outer core (2,890 to 5,150 km)
In 1914 Beno Gutenberg pinned down the depth of the core–mantle boundary, about 2,890 km below the surface. Below it, the rock ends and you hit an ocean of liquid iron and nickel more than 2,000 km deep.
How do we know it’s liquid? Earthquakes send out two main kinds of waves. S-waves can only travel through solids, and they never make it through the outer core. That leaves a “shadow” on the far side of the planet where they can’t be detected.
The churning liquid metal in the outer core acts like a giant electrical generator. It produces Earth’s magnetic field, which makes compasses work and shields us from the solar wind.
4. The inner core (5,150 to 6,371 km)
In 1936 the Danish seismologist Inge Lehmann noticed faint earthquake waves bouncing off something deep inside the liquid core. She had discovered a solid inner core.
The inner core is about 1,220 km in radius, roughly 70% the size of the Moon. It’s around 5,000–6,000 °C (estimates vary), about as hot as the surface of the Sun. It should be molten at that temperature, but the pressure is so enormous (about 3.6 million times the air pressure at the surface) that the iron is squeezed into a solid. (Why is it so hot down there?)
Then back up again
A tunnel through the center passes through every layer twice: inner core, outer core, mantle, crust, in reverse. That’s about 12,740 km in total.