A diagram of Earth’s interior shows that our planet is built in concentric layers, each with a different composition, thickness, and physical state. From the thin outer crust to the solid inner core roughly 6,450 km below the surface, these layers differ in density, temperature, and behavior. Understanding what the diagram represents also means understanding how scientists figured it out, since no one has ever drilled more than a tiny fraction of the way down.
The Four Main Layers
Most diagrams divide Earth into four chemical layers based on what each one is made of. Starting from the surface and moving inward:
- Crust: The outermost shell, ranging from about 5 km thick under the oceans to an average of 30 km under continents. Beneath large mountain ranges like the Alps, it can extend as deep as 100 km. This is the only layer humans have directly sampled.
- Mantle: A dense, hot layer of semi-solid rock approximately 2,900 km thick. It makes up the bulk of Earth’s volume and is composed primarily of magnesium- and silicon-rich rock.
- Outer core: A 2,200 km-thick layer of liquid iron and nickel, starting at about 2,900 km depth.
- Inner core: A solid ball of iron and nickel roughly 1,250 km thick, sitting at the very center of the planet, about 5,200 to 6,450 km below the surface.
On most diagrams, these layers are color-coded and drawn to scale (or close to it), which makes it easy to see that the mantle is by far the thickest layer and the crust is vanishingly thin by comparison.
Oceanic Versus Continental Crust
If the diagram distinguishes between two types of crust, it’s highlighting an important difference. Oceanic crust is thin, averaging just 7 to 10 km, and relatively dense at about 3.0 grams per cubic centimeter. It’s made of dark, heavy volcanic rock called basalt. Continental crust is much thicker, ranging from 25 to 75 km with an average of about 35 km, but it’s lighter, with an average density of 2.7 g/cc. It’s composed of lighter-colored rocks like granite.
This density difference is why continents sit higher than ocean floors. Continental crust floats on the mantle the way a thick block of wood floats higher in water than a thin, dense one. Even though oceanic crust covers more of Earth’s surface area, continental crust accounts for about 70% of the total crustal volume because it’s three to eight times thicker.
Boundaries Between Layers
Diagrams often mark the boundaries between layers with labeled lines. These boundaries are real, detectable features called discontinuities, where the physical properties of rock change abruptly.
The Mohorovičić discontinuity (usually labeled “Moho”) sits between the crust and the mantle. It’s found at an average depth of about 35 km under continents and just 7 km under the oceans. Below this line, rock becomes denser and seismic waves speed up sharply.
The Gutenberg discontinuity, at roughly 2,900 km depth, marks where the solid mantle meets the liquid outer core. This is one of the most dramatic transitions inside the planet, because rock gives way to molten metal. The Lehmann discontinuity, deeper still at about 5,200 km, separates the liquid outer core from the solid inner core.
Solid, Liquid, and In Between
One of the most important things a diagram reveals is the physical state of each layer. The crust is solid and brittle. The mantle is technically solid, but over long timescales it flows like an extremely thick fluid, which is why diagrams often describe it as “semi-solid” or “plastic.” This slow flow is what drives the movement of tectonic plates at the surface.
The outer core is genuinely liquid: molten iron and nickel at extreme temperatures. The inner core, despite being even hotter, is solid because the pressure at the center of the Earth is so immense (about 3.6 million times atmospheric pressure) that the iron is squeezed into a solid state. Temperatures at the center reach approximately 6,150 degrees Kelvin, which is roughly as hot as the surface of the Sun.
How Scientists Know What’s Inside
No drill has come close to reaching even the mantle, so diagrams of Earth’s interior are built almost entirely from seismic data. When an earthquake occurs, it sends two main types of energy waves through the planet. Compressional waves (P-waves) can travel through both solids and liquids. Shear waves (S-waves) can only travel through solids, because liquids cannot be sheared.
This difference is the key evidence for a liquid outer core. When seismologists record earthquake waves on the far side of the planet, they find a “shadow zone” between 104 and 140 degrees from the earthquake’s origin where no direct P-waves arrive, and S-waves disappear entirely at certain angles. The S-waves vanish because they cannot pass through the liquid outer core. The P-waves bend and create the shadow zone because their speed changes dramatically at the core-mantle boundary.
The fact that S-waves do appear again after passing through the inner core is what proved it must be solid. These wave patterns, recorded at seismograph stations around the world, are how every boundary on the diagram was originally discovered.
The Mantle’s Hidden Complexity
Simple diagrams show the mantle as a single layer, but more detailed versions break it into an upper mantle, a transition zone, and a lower mantle. The transition zone sits between roughly 410 and 660 km depth. At these depths, the crushing pressure forces the minerals in the rock to rearrange their crystal structures into denser forms. The dominant mineral in the upper mantle transforms into progressively denser versions as depth increases, with another significant change occurring around 520 km.
Some diagrams also distinguish between the lithosphere and the asthenosphere. The lithosphere includes the crust plus the rigid uppermost mantle, forming the stiff plates that move across Earth’s surface. Directly beneath it, the asthenosphere is a weaker, more pliable zone in the upper mantle where rock flows more easily. This distinction is about mechanical behavior rather than chemical composition: the lithosphere and asthenosphere are both made of similar rock, but one is rigid and the other is soft enough to allow plate movement.
Why the Outer Core Matters for Life
The liquid outer core does something no other layer can: it generates Earth’s magnetic field. The molten iron is in a state of turbulent convection, heated by radioactive decay and chemical processes. As this electrically conducting liquid churns, it converts kinetic energy into electrical and magnetic energy through a self-sustaining feedback loop. The electric currents in the flowing iron produce a magnetic field, and that magnetic field in turn influences the flow of iron, keeping the whole system running as long as there’s enough heat to drive convection.
This magnetic field extends far out into space and shields the planet from charged particles streaming from the Sun. Without it, solar radiation would gradually strip away the atmosphere. So when a diagram shows that thin shell of liquid metal between the mantle and inner core, it’s showing the engine behind one of the most important protective systems on the planet.