What Are the Layers of Earth?
Peel back our planet like an onion — four layers from the thin crust to the super-hot inner core.
A Planet with Layers
If you could slice Earth in half like an apple, you wouldn't find solid rock all the way through. Instead, you'd see distinct layers, each with different temperatures, thicknesses, and compositions. Scientists have identified four main layers: the crust, the mantle, the outer core, and the inner core. Each layer plays a role in making Earth the dynamic, living planet it is.
The Crust — Where We Live
The crust is Earth's outermost layer — the thin, hard shell we walk on. Compared to the rest of the planet, it's incredibly thin: oceanic crust (under the oceans) is only about 5–10 kilometers thick, while continental crust (under land) averages 30–50 kilometers. Relative to Earth's total diameter of 12,742 kilometers, the crust is thinner than the skin on an apple. It's made mostly of rock: basalt under the oceans and granite under the continents.
The Mantle — Slowly Moving Rock
Below the crust lies the mantle, which makes up about 84% of Earth's total volume. It extends roughly 2,900 kilometers deep and is made of silicate rock rich in iron and magnesium. The upper mantle is somewhat rigid, but deeper down, the rock is so hot (up to about 3,700°C) that it behaves like an extremely thick, slow-moving fluid — more like taffy than liquid. These slow-motion currents in the mantle are called convection currents, and they're the driving force behind the movement of tectonic plates on the surface.
The Outer Core — Liquid Metal
Beneath the mantle is the outer core, a layer of liquid iron and nickel about 2,200 kilometers thick. Temperatures here range from about 4,400°C to 6,100°C — hot enough to keep metal in a molten state. The outer core is critically important because the churning motion of this liquid metal generates Earth's magnetic field. Without it, we'd have no protective magnetosphere, compass needles wouldn't work, and harmful solar radiation would strip away our atmosphere over time.
The Inner Core — A Solid Metal Ball
At the very center of Earth sits the inner core — a solid ball of iron and nickel about 1,220 kilometers in radius (roughly the size of the Moon's diameter). Despite being the hottest part of Earth at approximately 5,400°C, it's solid rather than liquid because the pressure at the center is so immense — about 3.6 million times the air pressure at the surface. That extreme pressure forces the atoms so tightly together that they can't flow as a liquid.
How Do We Know What's Inside?
No one has ever drilled to the mantle, let alone the core. The deepest hole ever drilled (the Kola Superdeep Borehole in Russia) reached only about 12.2 kilometers — barely scratching the crust. Instead, scientists study Earth's interior using seismic waves — vibrations from earthquakes that travel through the planet. Different types of waves speed up, slow down, or bend when they pass through materials of different densities and states (solid vs. liquid). By analyzing these wave patterns at stations worldwide, scientists can map the boundaries between layers with remarkable precision.
Earth's inner core is growing — very slowly. As the outer core gradually cools over millions of years, liquid iron crystallizes onto the surface of the inner core, making it about 1 millimeter larger in diameter per year. The inner core is estimated to have started forming between 1 and 1.5 billion years ago, making it one of the younger features of our 4.5-billion-year-old planet.
Last reviewed: April 2026