Layers of the Earth

When the Earth first formed, it was extremely hot — so hot that much of it was molten (melted rock and metal). Because of this, the materials inside the Earth could separate based on their density. This process is called chemical differentiation.

Because of early chemical differentiation, the Earth is made up of four main layers, kind of like the layers of an onion. Each layer has different materials, temperatures, and behaviors.

Seismic Waves and Earth's Structure

We can’t drill deep enough to see what’s inside Earth, but scientists have found a clever way to study it: by using seismic waves from earthquakes.

When an earthquake happens, it sends out energy in the form of seismic waves. These waves travel through Earth, and by recording them with instruments called seismographs, scientists can learn what materials they passed through.

Seismic waves work like a kind of “X-ray” for the planet. Even though we can’t see inside Earth directly, these waves give scientists a clear picture of its hidden structure.

By studying how seismic waves travel, bend, speed up and slow down, and sometimes vanish, scientists discovered that Earth is made of several layers:

Crust

The outermost layer of the Earth.
Thin and solid, like the skin of an apple.
Made of rock and divided into large sections called tectonic plates.
There are two types:
- Continental crust: thicker, made mostly of granite (land), less dense.
- Oceanic crust: thinner, made mostly of basalt (ocean floor), more dense.
Thickness: 3–40 miles (5–70 km).

2. Mantle

The thickest layer, right under the crust.
Made of hot, solid rock that can flow slowly, almost like thick putty.
Heat from the mantle causes convection currents, which move the tectonic plates above.
Thickness: 1,800 miles (2,900 km).

3. Outer Core

Made of liquid iron and nickel.
Extremely hot (over 7,000°F / 4,000°C).
Thickness: 1,400 miles (2,300 km).

The movement of the liquid metal outer core around the solid metal inner core creates Earth’s magnetic field, which protects us from harmful solar radiation.

4. Inner Core

A solid ball of mostly iron and nickel at the very center.
So much pressure here that even though it’s hotter than the surface of the Sun, the metals stay solid.
Radius: about 750 miles (1,200 km).

Quick Analogy

Think of the Earth like a chocolate-covered jawbreaker:

The crust is the thin candy shell.
The mantle is the thick chewy layer.
The outer core is the gooey liquid center.
The inner core is the hard candy center.

Lithosphere vs. Asthenosphere

Two additional layers that are sometimes included with the layers of the Earth are the lithosphere and the asthenosphere. These two layers are actually parts of the crust and mantle, so they are often overlooked but are extremely important to geology.

Lithosphere

The rigid outer layer of the Earth.
Includes all of the crust and the uppermost solid part of the mantle.
It’s hard and brittle—it can crack and break.
Broken into tectonic plates that “float” on the softer layer below.
Thickness: about 60–200 miles (100–300 km).

Asthenosphere

Found just below the lithosphere, in the upper mantle.
Hotter and softer rock—still solid, but it can flow slowly like taffy or putty.
This flow lets the tectonic plates of the lithosphere move on top of it.
Thickness: about 125 miles (200 km).

Easy Analogy

Think of it like a crusty brownie:

The lithosphere is the hard, crispy top crust of the brownie.
The asthenosphere is the gooey, softer brownie underneath that lets the top crack and shift.