Beneath every footstep lies a planet built in layers — a thin crust riding on slowly shifting rock, wrapped around a core hot enough to melt iron. This article explains what Earth is made of, why its surface keeps moving, and what happens when that movement releases suddenly.
Earth looks solid and still from the surface, but underneath it is built from distinct layers, and its outer shell is broken into pieces that drift a few centimetres every year. That slow motion shapes mountains, oceans, and continents — and every so often, releases enough stored energy to shake the ground beneath our feet. This article works from the surface down to the centre, then back up to explain plate tectonics and earthquakes.
Earth is made of four main layers, each different in thickness, composition, and state — solid, liquid, or something in between. From the outside in, they are the crust, the mantle, the outer core, and the inner core.
| Layer | Approx. Thickness | State |
|---|---|---|
| Crust | 5–70 km | Solid rock |
| Mantle | ~2,900 km | Solid rock that flows very slowly |
| Outer Core | ~2,200 km | Liquid iron and nickel |
| Inner Core | ~1,220 km radius | Solid iron and nickel |
If Earth were shrunk to the size of an apple, the crust would be no thicker than the apple's skin. Almost everything we build, farm, and walk on happens on that thin outer layer.
The crust is Earth's outermost, solid layer, and it comes in two distinct types:
Because oceanic crust is denser, it sits lower, which is a major reason oceans exist where they do. The crust and the rigid uppermost part of the mantle together form the lithosphere — the layer broken into the tectonic plates discussed later in this article.
Below the crust lies the mantle, Earth's thickest layer, making up roughly 84% of the planet's volume. The mantle is solid rock, but under immense heat and pressure it behaves like an extremely thick, slow-moving fluid over long timescales — flowing at only a few centimetres per year.
This slow flow is driven by convection: hotter, less dense rock near the core rises, cooler, denser rock sinks, creating giant looping currents that take tens of millions of years to complete a single cycle. These convection currents are the engine that drives the movement of the tectonic plates above.
Mantle convection is often compared to a pot of thick soup simmering on a stove — heat rising at the bottom pushes material upward, it cools near the top, then sinks back down to be reheated. It's the same basic principle, just acting on solid rock over millions of years instead of liquid over minutes.
At Earth's centre lies the core, made almost entirely of iron and nickel, split into two very different parts:
Earth's magnetic field, generated deep in the outer core, extends far into space and shields the planet from much of the Sun's charged particles — without it, Earth's atmosphere would be far more exposed to solar radiation.
Earth's lithosphere is broken into roughly a dozen major tectonic plates, which fit together like a cracked shell and slowly drift on top of the flowing mantle beneath them. Plate boundaries come in three types:
| Boundary Type | What Happens | Example |
|---|---|---|
| Divergent | Plates move apart, new crust forms | Mid-Atlantic Ridge |
| Convergent | Plates collide or one slides beneath another | Himalayas; Pacific "Ring of Fire" |
| Transform | Plates slide past each other sideways | San Andreas Fault |
Over millions of years, plate movement has reshaped Earth's surface entirely — continents have collided to form mountain ranges, split apart to open new oceans, and drifted thousands of kilometres from where they started.
Tectonic plates don't glide smoothly past each other — friction along their edges, called faults, allows stress to build up over years or decades. When that stress finally exceeds the friction holding the rock in place, the plates slip suddenly, releasing the stored energy as seismic waves. That sudden release is an earthquake.
The point underground where the slip begins is called the focus (or hypocentre); the point on the surface directly above it is the epicentre, which is usually where shaking is felt most strongly. Earthquake strength is measured using the moment magnitude scale, where each whole number increase represents roughly 32 times more energy released.
Most of the world's strongest earthquakes occur along the Pacific "Ring of Fire," a horseshoe-shaped zone of convergent plate boundaries circling the Pacific Ocean, which is also home to roughly 75% of the world's active volcanoes.
Earth's surface feels permanent, but it is really the thin, moving skin of a planet built in layers and driven by heat from deep within. Understanding those layers — and the slow-motion collisions and separations of the plates above them — explains everything from where mountains rise to why earthquakes strike where they do.
10 questions. Select an answer for each, then submit to see your score instantly.