The Structure of the Earth (Years 6–9) — Geography

Earth is built in layers — crust, mantle, outer core and inner core — and we know this from clues, not digging.

Stamp your foot on the ground. It feels solid and still. Yet beneath your shoes lies a churning, glowing world thousands of kilometres deep, hotter at its centre than the surface of the Sun. If you could fall straight down through the Earth, you would pass through four very different layers before popping out the other side — and no human has ever been anywhere near the bottom.

Why this matters — The Earth's inner layers drive volcanoes, earthquakes, mountains and even the compass in your phone. Understanding the planet's structure is the first step to understanding almost every dramatic thing that happens on its surface.

The four main layers

Think of the Earth like a peach: a thin skin, a thick fleshy middle, and a hard stone at the centre. From the outside in:

A slice through the Earth — layers are not to scale; the crust is far thinner than shown.

Crust — the thin, solid rocky shell we live on. Under the oceans it is only about 5–10 km thick; under continents around 30–70 km. On the scale of the whole Earth it is thinner than an apple's skin.
Mantle — a huge layer of hot rock about 2,900 km thick. It is mostly solid but can flow very slowly, like extremely stiff treacle, over millions of years. This slow movement drags the crust around.
Outer core — a layer of liquid iron and nickel. As the Earth spins, this swirling molten metal generates the planet's magnetic field, which makes compasses point north and shields us from harmful space radiation.
Inner core — a ball of iron and nickel at the very centre, around 6,000 °C. Astonishingly it is solid, because the crushing pressure squeezes the metal so hard it cannot melt.

Layer	State	Made of	Rough depth
Crust	Solid	Rock	0–70 km
Mantle	Solid but flows	Hot rock	to ~2,900 km
Outer core	Liquid	Iron & nickel	to ~5,150 km
Inner core	Solid	Iron & nickel	to ~6,370 km

Two kinds of crust

Not all crust is the same. Oceanic crust (under the seas) is thinner, younger and denser, made mainly of a dark rock called basalt. Continental crust (the land) is thicker, older and less dense, made largely of granite. Because continental crust is lighter, it floats higher — which is why continents rise above the ocean floor.

Thin dense oceanic crust and thick lighter continental crust both float on the mantle.

How do we know? We can't dig there

The deepest hole humans have ever drilled reached only about 12 km — not even through the crust. So how do we know what lies beneath? Scientists act like detectives, reading clues: the shockwaves from earthquakes (seismic waves) change speed and bend as they pass through different layers, revealing where solid becomes liquid. Volcanoes bring up material from the mantle. Meteorites — leftovers from the same cloud that built the Earth — tell us what the core is likely made of.

Detective work: Some seismic waves cannot travel through liquid. When these waves vanish on the far side of the Earth after a big earthquake, that "shadow" is our best proof that the outer core is molten — discovered by carefully mapping where the waves did and did not arrive.

Waves bend around the liquid outer core, leaving a "shadow zone" with no direct waves — proof the core is molten.

Common mistake: The inner core is the hottest layer yet it is solid. Heat alone does not melt it — the enormous pressure at the centre keeps the iron locked solid.

Temperature climbs steadily with depth — the centre is hottest, yet pressure keeps the inner core solid.

Memory hook: "Cool Men Often Iceskate" — Crust, Mantle, Outer core, Inner core, from outside to centre.