Earth’s Layers

There are four separate layers within the earth. These are the crust, the mantle, the outer core, and the inner core. They’re classified by the different types of rocks and minerals that make them up. Additionally, each of Earth’s layers has unique properties based on both their composition and depth.

An illustration of a cutaway of the earths layers.

Earth’s layers and their thickness.

Earth’s Crust

The earth’s crust is the top and thinnest layer. It’s divided into two types, oceanic and continental. The crust is unique in that it’s the only layer made entirely of moving plates. This motion is actually the basis for a major theory: plate tectonics.
 
Yet, for all that we study it, the crust is tiny! Let’s put things into perspective.
 

How Thick is the Earth’s Crust?

Relative to the rest of the earth, not very. First of all, its thickness varies, so we can’t just give one answer. For oceanic crust, the average thickness is between 3 and 6 miles. Continental crust is a good deal thicker, with an average thickness of about 22 miles.
 
And 22 miles is a huge distance. But the entire earth is MUCH bigger. Current estimates say that the distance from earth’s surface to its center is about 4,000 miles! This means that the crust doesn’t even cover 1% of the distance. 
 

What is the Earth’s Crust Made of?

The crust is made primarily of three elements. In order of prevalence, these are oxygen, silicon, and aluminum. They’re found mostly in the form of minerals, usually silica minerals and bauxite (an ore of aluminum). That said, a few other, rarer minerals are also found in the crust. You can find out more about them in this video:

Earth’s Mantle

The layer right below the crust is the mantle. The mantle is an interesting layer in that it has both solid and liquid parts. By far, most of it is liquid. But, the solid portion is a component of the lithosphere. The lithosphere is crucial to plate tectonics.
 

How Thick is the Earth’s Mantle?

The mantle is the largest layer within the earth, spanning about 1,800 miles. 
 

What is the Earth’s Mantle Made of?

The composition of the mantle isn’t that much different from that of the crust. The elements in it are largely the same, just with more magnesium and less aluminum and silicon. Even so, most of the mantle is fluid, not solid. This is because the deeper we go into the earth, the hotter it gets. The increasing heat melts rocks in the mantle, forming magma.

An image of magnesite, a mineral containing magnesium.

A mineral containing magnesium. Magnesium is common in the mantle, but not the crust.

Earth’s Core

The core of the earth splits into two layers, inner and outer. The outer core is fluid, like the mantle. Yet, the inner core is solid. This can’t be explained by composition as both parts of the core are composed of iron and nickel. Instead, it has to do with pressure.

An image of nickel ore.

The symbol for nickel. Nickel is one of the elements that make up the earth’s core.

Outer Core

How Thick is the Outer Core?

The outer core is the earth’s second thickest layer at 1,500 miles thick. 
 

Outer Core Temperature

The outer core is extremely hot at 8100 degrees Fahrenheit. This causes it to be a liquid.
 

Inner Core 

The earth’s final layer is the inner core. The inner core is not unlike the outer core in terms of composition. Still, its physical properties are considerably different.
 

How Thick is the Inner Core?

To start, the inner core is way thinner than the outer core. It’s only about 760 miles thick. But, being the earth’s inmost layer, it’s also a full sphere This sphere is almost as large as the moon!
 

Why is the Inner Core Solid?

We learned earlier that the earth’s heat increases with depth, and that this can melt rocks. And if that’s true, why isn’t the inner core liquid? The answer has to do with pressure. Sure, heat increases as we go deeper into the earth (the inner core’s temperature is 11,000 degrees Fahrenheit). But, so does pressure.

An image a row of pressure gauges.

Devices which measure pressure. There’s a lot of pressure deep into the earth.

 
In the earth’s inner core, there’s actually so much pressure that it prevents the rock from melting. It pushes the atoms together, maintaining a solid state.

Other Great Resources:

What Rocks Form the Crust?: http://www.classzone.com/vpg_ebooks/sci_sc_8/accessibility/sci_sc_8/page_80.pdf

Ducksters on the Earth’s Composition: https://www.ducksters.com/science/composition_of_the_earth.php

Fun Facts about the Earth’s Layers: http://easyscienceforkids.com/all-about-earths-layers/

‘Could I Dig a Hole Through the Earth?’: https://www.youtube.com/watch?v=oEW_Qwj6ZCE

Written by: Noah Louis-Ferdinand