Earth's Internal Structure: Crust, Mantle, Core

Crust, mantle, outer core, inner core — what each is made of, how deep it sits, and how seismic waves prove the mantle is solid.

Intro GeologyUni Year 1
⏱️ About 18 min
Earth's Internal Structure: Crust, Mantle, Core — illustration
Illustrative image (AI-generated).

You will never hold the Earth's core in your hand. The deepest hole ever drilled reaches barely 12 km down — and the centre of the Earth lies 6371 km beneath your feet. Yet we know the outer core is liquid iron, the inner core is a solid ball nearly as hot as the Sun's surface, and the mantle — the thickest layer of all — is solid rock. How can anyone know that? The answer is earthquakes: every quake rings the planet like a bell, and the way those vibrations travel lets us read the inside of a world we cannot visit.

💡
The big idea: Earth is built in concentric layers, sorted by density: a thin, light rocky crust; a thick, solid silicate mantle; a liquid iron-nickel outer core; and a solid iron inner core. Two kinds of seismic wave reveal this structure — P-waves pass through liquids, S-waves do not, so the S-wave shadow zone proves the outer core is liquid. And because the light crust floats on the denser mantle, it rides in buoyant balance (isostasy) that rises and falls as ice sheets, mountains, and erosion load or unload it.
🎯 By the end, you'll be able to
  • Name Earth's four main layers (crust, mantle, outer core, inner core), state the approximate depth of each boundary, and say whether each is solid or liquid
  • Explain how seismic wave behaviour — especially the S-wave shadow zone — provides the evidence for a liquid outer core and a solid inner core
  • Correct the misconception that the mantle is liquid magma, explaining that it is solid rock that flows only very slowly over geologic time
  • Describe how temperature and pressure rise with depth, and define the geothermal gradient and lithostatic pressure
  • Explain isostasy (crustal buoyancy) and use the Airy model to predict how loading and unloading raise or lower the crust
📎 Helpful to know first

A planet in layers

The previous lesson ended with a differentiated Earth: dense metal sunk to the core, lighter silicates floating up to the mantle and crust. Those layers are still there today, and geologists divide the solid Earth into four main ones by composition and physical state:

  • Crust — the thin, light, rocky outer skin. Two kinds: thin, dense oceanic crust (about 5–10 km thick) and thicker, lighter continental crust (about 30–70 km, thickest under mountain ranges).
  • Mantle — the thickest layer, a solid silicate shell reaching from the base of the crust down to about 2890 km depth.
  • Outer core — a liquid layer of molten iron and nickel, from 2890 km down to 5150 km.
  • Inner core — a solid ball of iron and nickel at the very centre, from 5150 km to 6371 km (Earth's centre).
🎮 Earth Interior Explorer LIVE
Predict first: Click any layer to reveal its depth, composition, physical state, temperature, and the seismic evidence for the boundary above it.

Interactive cross-section of Earth's interior showing four layers from outside in: the thin crust (0–~35 km), the thick solid mantle (down to 2890 km), the liquid outer core (2890–5150 km), and the solid inner core (5150–6371 km). Key boundaries: the Mohorovičić discontinuity (Moho) at the crust–mantle boundary, the Gutenberg discontinuity at the core–mantle boundary (~2890 km), and the Lehmann discontinuity at the inner–outer core boundary (~5150 km).

Earth Interior Explorer — an interactive cross-section. Click each layer to read its depth, what it is made of, whether it is solid or liquid, its approximate temperature, and the seismic discontinuity (Moho, Gutenberg, or Lehmann) that marks the boundary.

How we see through solid rock: seismic waves

Geologists mapped these layers not by drilling — no drill reaches them — but by listening to earthquakes. A large quake sends two body waves racing through the planet:

  • P-waves (primary, push-pull) are fast and can travel through solids and liquids.
  • S-waves (secondary, side-to-side shear) are slower and can travel only through solids — they cannot pass through liquid.

After big quakes, there is a wide region on the far side of Earth where seismometers record P-waves but no S-waves. That S-wave shadow zone is the smoking gun: the S-waves were blocked by a liquid layer — the outer core. (We treat the waves in full in Module 8.)

🔑 Boundaries have names
Each major change in properties is a discontinuity named for its discoverer. The Moho (Mohorovičić) marks the crust–mantle boundary (~5–10 km under oceans, ~30–70 km under continents). The Gutenberg discontinuity marks the core–mantle boundary at ~2890 km. And the Lehmann discontinuity at ~5150 km — found by Inge Lehmann in 1936 — marks the boundary between the liquid outer core and the solid inner core.
Earth's interior to scale Earth's interior — cutaway to scale (centre at left) Crust — thin rocky skin (0–~35 km; Moho at its base) Mantle — solid silicate, down to 2890 km Outer core — liquid Fe–Ni (2890–5150 km) Inner core — solid Fe (5150–6371 km) Gutenberg discontinuity core–mantle boundary, ~2890 km Lehmann discontinuity (~5150 km) S-waves stop at the liquid outer core ✕
Earth's interior to scale, with the named seismic discontinuities. Note how thin the crust is relative to the mantle — drawn honestly here, not exaggerated.
⚠️ The mantle is NOT liquid magma
The wrong belief: many people picture the mantle as a churning ocean of liquid magma, with the crust floating on it like a boat. The correction: the mantle is solid rock. You could hold a piece of it and it would look and feel like stone — its temperature is high, but the enormous pressure keeps it from melting. Only the outer core is a truly liquid layer — small pockets of partly molten rock can exist locally in the upper mantle, and it is those melts that rise to feed volcanoes. What is true is that over geologic time, the solid mantle flows very slowly — a few centimetres per year — the way cold glacier ice slowly deforms. Solid yet flowing: that is how the mantle drives plate tectonics. We revisit this exact misconception in Module 2, where it is central.

Density-stratified, not uniform

Notice the pattern: each layer is denser than the one above it. Continental crust (~2.7 g/cm³) sits on mantle peridotite (~3.3 g/cm³ in the upper mantle, rising with pressure), which overlies the liquid outer core (~9–12 g/cm³) and the solid inner core (~13 g/cm³). The densest material is at the centre — exactly what differentiation would produce, and the opposite of a uniform lump.

⚠️ Earth's interior is not uniform
The wrong belief: that the Earth is more or less the same stuff all the way through, just getting hotter. The correction: the Earth is strongly density-stratified — a light rocky crust over a denser solid silicate mantle, over a much denser liquid iron core, over the densest solid iron inner core. Seismic wave speeds jump sharply at each boundary precisely because the composition and state change, not because the rock merely gets warmer.

Hotter and more squeezed with depth

Two trends matter as you descend. Temperature climbs steadily — in the upper crust by roughly 25 °C per kilometre on average (the geothermal gradient) — reaching perhaps 500–1000 °C at the base of the crust, thousands of degrees at the core, and ~5200–6000 °C at the very centre. Pressure climbs too, because each layer bears the weight of everything above it: this lithostatic pressure reaches about 1.4 million atmospheres at the core–mantle boundary and about 3.6 million atmospheres at the very centre of the Earth.

This pressure–temperature balance explains a seeming paradox: the inner core is the hottest part of Earth yet it is solid, while the cooler outer core above it is liquid. The crushing pressure at the centre squeezes the iron into a solid despite the heat.

\[ P \approx \rho\, g\, h \]
Lithostatic pressure at a shallow depth h is approximately the overlying rock density ρ times gravity g times depth. (This treats density and g as roughly constant over the upper crust — a good first estimate, not an exact value for the deep Earth.)

The crust floats: isostasy

Because the light crust sits on the denser mantle, it behaves like a block floating in water — most of it is submerged, with a thick hidden root below. This buoyant balance is called isostasy. A tall mountain range stands high precisely because it has a deep crustal root beneath it, the way a tall iceberg rides lower in the water and extends deeper below.

The simplest version, the Airy model, treats the crust as one fixed density floating on a denser mantle. The taller the crust stands above a reference level, the deeper its root extends below it.

Airy isostasy — crust floats like icebergs Mantle (ρₘ ≈ 3.3 g/cm³) — the denser fluid the crust floats in reference level Mountain (thick crust) Plain (thin crust) deep root shallow root h b height root b ≈ ρ꜀ ⁄ (ρₘ−ρ꜀) · h ≈ 5.6 × h Post-glacial rebound ice sheet ice melts → crust rises
Airy isostasy: a high mountain rides on a deep crustal root, like an iceberg. Add or remove a load (an ice sheet, erosion) and the crust slowly sinks or rises to restore balance.
\[ b = \dfrac{\rho_{c}}{\rho_{m}-\rho_{c}}\, h \]
Airy isostasy: a mountain of height h above the reference level has a crustal root of depth b beneath it, where ρ꜀ is crustal density and ρₘ is mantle density. With ρ꜀ ≈ 2.8 and ρₘ ≈ 3.3 g/cm³, the root is about 5–6 times the mountain's height.
📝 Worked example: Using the Airy model, how deep is the crustal root beneath a mountain range that stands 5 km above the reference continental elevation? Take ρ꜀ = 2.8 and ρₘ = 3.3 g/cm³.
  1. Write the Airy root formula: b = ρ꜀ ÷ (ρₘ − ρ꜀) × h.
  2. The density factor = 2.8 ÷ (3.3 − 2.8) = 2.8 ÷ 0.5 = 5.6.
  3. Multiply by the height: b = 5.6 × 5 km = 28 km.
  4. So a 5 km mountain rides on a ~28 km root — meaning the crust under the range is far thicker than under a low plain.
✓ About 28 km of crustal root beneath the 5 km mountain.
✏️ Practice: A high plateau stands 4 km above the reference continental elevation. Using the Airy model with ρ꜀ = 2.8 and ρₘ = 3.3 g/cm³, what is the depth of its crustal root? (Answer in km.)
km
Solution
  1. b = ρ꜀ ÷ (ρₘ − ρ꜀) × h.
  2. Density factor = 2.8 ÷ (3.3 − 2.8) = 2.8 ÷ 0.5 = 5.6.
  3. b = 5.6 × 4 km = 22.4 km.
  4. The plateau's crust is ~22 km thicker at its root than at the reference level — which is why continents 'float' higher where they are thicker.
✏️ Practice: Estimate the lithostatic pressure at the bottom of 10 km of crust of average density 2900 kg/m³. Use P = ρgh with g = 9.8 m/s². Give your answer in GPa (1 GPa = 10⁹ Pa).
GPa
Solution
  1. P = ρ × g × h.
  2. = 2900 kg/m³ × 9.8 m/s² × 10 000 m.
  3. = 284 200 000 Pa = 2.842 × 10⁸ Pa.
  4. = 0.28 GPa (about 2800 times atmospheric pressure). This is the squeeze that, far deeper down, keeps the inner core solid.
✨ Post-glacial rebound: the crust is still adjusting
Isostasy is not just theory — you can watch it happen. During the last ice age, kilometre-thick ice sheets weighed down Scandinavia and Hudson Bay. When the ice melted ~10 000 years ago, the unloaded crust began to rise, and it is still rebounding today — up to about a centimetre per year. That slow rebound is direct evidence that the crust floats in buoyant balance on the mantle. We will return to this in Module 9 (glaciers).

Check your understanding

1. Many people believe the mantle is liquid magma. What is the reality?
The mantle is solid — high pressure keeps it from melting, even though it is hot. It flows only very slowly (centimetres per year). The only truly liquid layer is the outer core, proved by the S-wave shadow zone.
2. How do we know the outer core is liquid?
S-waves travel only through solids. A broad zone on the far side of large earthquakes receives no S-waves, meaning they were blocked by a liquid layer — the outer core.
3. Under the Airy model, how deep is the crustal root beneath a 5 km mountain (ρ꜀ = 2.8, ρₘ = 3.3)?
b = ρ꜀/(ρₘ−ρ꜀) × h = 2.8/0.5 × 5 = 5.6 × 5 ≈ 28 km. High mountains stand high because they have deep roots — crustal buoyancy, or isostasy.
🔎 Related lessons
Previous: Earth in the Solar System From Atoms to Minerals to Rocks What Drives the Plates? (Module 2)
✅ Key takeaways
  • Earth has four main layers: a thin crust, a thick solid mantle, a liquid outer core, and a solid inner core — each denser than the one above.
  • Key boundaries are the Moho (~5–70 km), the Gutenberg (core–mantle, ~2890 km), and the Lehmann (inner–outer core, ~5150 km).
  • S-waves cannot pass through liquid, so the S-wave shadow zone proves the outer core is liquid; the inner core is solid because extreme pressure overpowers the heat.
  • The mantle is SOLID rock (not liquid magma) — it flows only centimetres per year; the interior is density-stratified, not uniform.
  • Temperature and pressure both rise with depth (geothermal gradient ~25 °C/km in the upper crust; lithostatic pressure P ≈ ρgh).
  • Isostasy means the light crust floats on the denser mantle; Airy's model gives a root depth b = ρ꜀/(ρₘ−ρ꜀)·h, ~5–6 times a mountain's height, and explains post-glacial rebound.
➡️ We know what Earth is made of in bulk — layers of rock and metal. But what, exactly, is a rock? And how is it different from a mineral or an element? Before we can talk about the rock cycle, we need those definitions nailed down.
Want to test yourself on this? Try the Science practice tests →
🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of earth as a planet & geologic materials.

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