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Earth’s Crust

The earth’s crust is its outermost layer, right above the mantle. The crust is mostly made of silicon and oxygen, but it also contains a variety of metals. Its makeup differs slightly between its two types: oceanic and continental crust. Most importantly, the crust is always in motion. This movement is the basis for both continental drift and plate tectonics

An illustration of the different layers of the Earth. The crust is the thinnest outer layer.

The structure of the earth. The crust is the top layer

What is the Earth’s Crust Made of?

The crust is massive, so there’s no one thing that makes it up. Instead, it’s composed of a bunch of different minerals which vary with depth. For example, there are more magnesium-containing minerals in the oceanic crust than continental crust. This is because oceanic crust is lower into the earth. However, as we will see, the two types are largely the same in terms of their composition.
 

Most Abundant Element in Earth’s Crust

Throughout the crust, the most abundant element is oxygen. This might come as a surprise. While oxygen is vital to life, it seems to take up no space at all! So, how could it make-up our dense earth?  
 

Well, we really don’t just find pure oxygen underground. Instead, we find oxygen in the form of minerals and ores. It combines with other elements to form solid crystals.

A macro image of a crystal structure.

This crystal is made mostly out of oxygen. Who knew air could look so beautiful?

Most Abundant Metal in Earth’s Crust

The most abundant of these elements, or the second most abundant element in the earth, is silicon. Silicon isn’t a metal. But, aluminum, the third most common element in the earth’s crust, is. Notably, this aluminum is actually much more concentrated in continental than oceanic crust. Still, it’s not hard to find at all until we hit the mantle.

An image of bauxite, an aluminum ore.

This is aluminum ore. It makes up much of the crust.

And how far do we have to go to get there? We can only estimate because no human has been below the crust. Turns out, it’s really difficult to dig through solid rock! Yet, scientists can still figure out a lot from the surface. Here’s what they’ve found.
 

How Thick is the Earth’s Crust?

The most important thing about the earth’s crust is that its thickness varies. While continental and oceanic crust end at the same depth, they start at different heights. The resulting difference in thickness gives them separate properties. So, we’ll consider them in turn.
 

Continental Crust

On average, continental crust is 20 to 30 miles thick. However, the actual depth varies considerably. It’s largely based on what landforms are on top of the crust. To give an example, the crust under mountains can be over 50 miles thick. This leads it to dip into the mantle, something which is really important to the concept of isostasy.
 

Oceanic Crust

Next, oceanic crust is about 3 to 6 miles deep. Clearly, this is way thinner than continental crust. Oceanic crust is by far the younger kind of crust. It’s recycled back into the crust by subduction before it gets too old. At the same time, it’s constantly being made at ocean ridges.
 

How do we know?

Now, I’ve just said a lot of pretty significant things about our world. You might be wondering: How do we know all of this?
 
The answer: lots and lots of scientific discoveries. Some of these are pretty complicated, so I’ll only discuss them briefly. But, you can also learn more about them in the following video as well as the resources below.
 

With that, much of what we know about our planet comes from seismic wave data. Sudden plate motion, like the shaking of an earthquake, releases what are called seismic waves. These waves travel through the earth in an arc-like shape.

An image of a seismograph showing readings.

An illustration of a seismograph, what we use to measure seismic waves.

Their travel speed depends upon what they’re moving through. If the crust were a separate layer with its own composition, we would expect seismic wave speed to change as it leaves the crust. That’s exactly what we see. Likewise, we see different wave speeds for the continental and oceanic crust. This tells us their composition is slightly different.

Other Great Resources:

Trying to Reach the Mantle: https://www.smithsonianmag.com/science-nature/decades-long-quest-drill-earths-mantle-may-soon-hit-pay-dirt-180957908/

How Do We Know about Earth’s Thickness?: http://scienceline.ucsb.edu/getkey.php?key=3249

More on Seismic Waves: http://www.indiana.edu/~geol105/images/gaia_chapter_3/seismic.htm

NatGeo Kids on the Crust: https://www.natgeokids.com/au/discover/geography/physical-geography/structure-of-the-earth/

Written by: Noah Louis-Ferdinand