Earth's Layers

Learning Objectives

Name the four main layers of the Earth: crust, mantle, outer core, and inner core.

Describe the basic composition and state (solid/liquid) of each layer.

Explain how scientists know about these layers without being able to see them.

Journey to the Center of the Earth

If you could drill a hole all the way to the center of the Earth, what would you find? While we haven't actually been there, scientists have used clever methods to figure out that our planet is not just a solid ball of rock. It's made of several distinct layers, like an onion.

The four main layers, from the outside in, are the crust, the mantle, the outer core, and the inner core.

1. The Crust

The crust is the outermost layer of the Earth. It's the thinnest layer, like the skin of an apple. It's the rocky surface we live on. There are two types of crust:

Continental Crust: This is the crust that makes up the continents. It is thicker (30-50 km on average) and is mostly made of less dense rocks like granite.

Oceanic Crust: This is the crust found beneath the oceans. It is thinner (5-10 km) and is made of denser rocks like basalt.

2. The Mantle

Below the crust is the mantle. It is the thickest layer of the Earth, making up about 84% of Earth's volume. It is composed mostly of hot, dense, silicate rocks. The mantle itself has two main parts:

The very top part of the mantle is cool and rigid, just like the crust. The crust and this rigid top part of the mantle together are called the lithosphere.

Below the lithosphere is a hotter, softer layer of the mantle called the asthenosphere. The rock here is partially melted and can flow very slowly, like thick tar. The rigid plates of the lithosphere float on this flowing asthenosphere.

3. The Outer Core

Deeper still is the outer core. This layer is made of liquid iron and nickel. The movement and churning of this liquid metal is what generates Earth's magnetic field, which protects us from harmful solar radiation. It's incredibly hot here, with temperatures over 4,000°C.

4. The Inner Core

At the very center of the Earth is the inner core. Like the outer core, it is also made of iron and nickel. However, despite being even hotter than the outer core, the inner core is solid. This is because it is under immense pressure from the weight of all the other layers pushing down on it. This extreme pressure forces the atoms together into a solid state.

How Do We Know?

We can't drill to the core, so how do scientists know all this? The main evidence comes from studying seismic waves from earthquakes. When an earthquake happens, it sends shockwaves through the entire planet. By measuring how these waves travel, how they speed up, slow down, and change direction, scientists can infer the density and state (solid or liquid) of the materials they pass through, creating a map of Earth's interior.

Key Terms

**Crust: The thin, rocky, outermost layer of the Earth.~|~Mantle: The thick layer of hot, solid rock between the Earth's crust and core.~|~Lithosphere: The rigid outer part of the earth, consisting of the crust and upper mantle.~|~Asthenosphere: The soft, plastic-like layer of the upper mantle on which the tectonic plates of the lithosphere float.~|~Outer Core: A layer of molten (liquid) iron and nickel that surrounds the inner core of Earth.~|~Inner Core: A dense sphere of solid iron and nickel at the center of Earth.~|~Seismic Waves: Vibrations that travel through Earth carrying the energy released during an earthquake.