Earth has layers. This is something kids get taught in science class, and it’s true. The Earth has a solid metal inner core, a liquid metal outer core, a semi-solid rocky mantle, and an approximately-solid rocky crust. But like a lot of science class, this picture is highly simplified.

The inner core

Dive as deep as you can into the solid mass of the earth, and you’ll find the inner core. It’s a solid mass of nickel and iron, at least as best as we can tell. These metallic atoms seem to be arranged in giant crystals (or potentially one single huge crystal) running north-south.

How the heck do we know all this if the inner core is so far down? We originally guessed the inner core was solid by calculating the temperature and pressure that should be down there, based on the total mass of the Earth.

This was confirmed by looking at data from Earthquakes. When an earthquake sends out waves in the solid matter of the Earth, there are two main kinds. P waves (“primary” or “pressure” waves) are caused by the material being pushed back and forth along the direction of the wave, while S waves (“secondary” or “shear” waves) are caused by the material being moved side to side. S waves can only travel through solid material, while P waves can also flow through liquids. P waves passing through the liquid outer core can sometimes transform into S-waves when hitting the inner core. So, we know it must be solid, or at least extremely viscous.¹

The outer core

The liquid metal outer core flows above the inner core. Turbulence in the outer core is suspected to be the main cause of the Earth’s magnetic field. For this to be true, the outer core must be electrically conductive, which means it’s probably metallic. When we look at metallic meteorites, which we think originated in the cores of large objects in the early solar system, we see a specific mix of nickel and iron. So, geologists suspect this mix makes up our inner and outer core as well.

Earth’s metal core is thought to have formed comparatively quickly, about 500 million years after the Earth coalesced, in a process called the “iron catastrophe”.² In this model, when the earth reached a critical temperature from the heat of radioactive elements inside it, its rocks would melt enough for the metals to split from the lighter rocks and start falling to the center. The heat from the friction as the metals fell would cause more iron to fall, in a fast growing, self-reinforcing cycle. “Iron Catastrophe” would also be a good name for a metal band made up of geologists.

Studying Earth’s magnetic field also allows us to see the interaction of Earth’s two metallic layers, including letting us guess at the crystal structure of the inner core. As the Earth continues to cool, the center of the outer core is freezing onto the inner core at a rate of about 1 millimeter per year.³

The mantle

You may have heard that the mantle is liquid. This is only kind of true. If we extracted a chunk of the mantle and somehow kept it under its original heat and pressure, it would appear solid and you could break off shards of it with a hammer. Mantle material is more than a million times more viscous than pitch, the substance in the famous “pitch drop” experiment, where it drips out of a container at a rate of about one drop per decade. But because geology happens on the scale of millions or billions of years and with such high pressures, even extremely slow flowing substances can behave like liquids.

A funnel of pitch in the middle of dripping extremely slowly.

The mantle also isn’t one homogeneous layer. The very bottom of the mantle, where it meets the inner core, has seismic properties different from the layer above it. Within this layer, called the D’’ (D double prime) layer, we find structures somewhat like continents, called “large, low-shear-velocity provinces.” It’s unclear why they exist, though one hypothesis is that they’re leftover bits of Theia, a lost planet that hit the early Earth, with the debris forming the Moon.⁴

Map of LLSVPs

Farther up in the mantle, we see normal mantle stuff happening – slow flowing rocks pushing around the continents in the crust above. The different layers are primarily defined by the different minerals they’re formed of, thanks to the changing heat and pressure as you go up. Again, most of the data we have comes from how earthquake waves move and resonate through the mantle’s bulk. There have been a couple of attempts to drill down to Earth’s mantle, but so far none have succeeded.  Though, we have found broken bits of mantle rocks that have been naturally uplifted through Earth’s crust.

At the very top of the mantle is the Mohorovičić discontinuity, or “Moho” for short.⁵ Here, seismic waves suddenly increase in speed from about 7 km/s to about 8 km/s in the span of about 500 meters. This marks the boundary between the mantle and the crust.

The crust

And we’re back on the outside, where we live. This is the part of the Earth split up into constantly shifting tectonic plates, giving us the earthquakes that are so destructive but provide a rare window into the thousands of miles of rock and metal below our feet.

Coming soon: The dying idealism of Peacecore