Ya – you might think that learning about Earth’s interior is boring.
But it doesn’t have to be.
In fact, it shouldn’t be – because it’s the basis of all Earthly things awesome!
Most of this topic is an exercise in imagination, anyway.
And imagining is fun, bro!
Time to make this seemingly dull topic come ALIVE!
Let’s ROCK!
The Earth is Cool!
Yes, it is!
So imagine with me.
Come on!
Conjure up a sphere in your mind’s eye.
A pretty, shiny, colorful sphere!
What does it look like?
Maybe it’s psychedelic purple and orange and green!
Maybe it’s a dark, brooding blue with intuitive depths that hold secrets about your future.
Maybe it’s covered in tangled vines and rainforest leaves.
Maybe it’s like a dragon egg, surrounded by blood-red scales with golden, fiery edges.
With glitter?
No. That’s too much.
Okay, you can have glitter, too, jeez. If you insist.
Now. Hold your sphere in your hand, and look at it.
Just observe it for a moment.
Pleasing, isn’t it?
A perfect sphere, full of potential.
What could possibly be inside? (OMG, do you really think it’s full of glitter?)
Welp. Only one way to find out.
SMASH IT!
Yes, I’m serious.
Take a giant sledgehammer and smash it into a bunch of tiny pieces!!
Is it glittery in the middle?
Nah – didn’t think so.
Looks a whole lot like rocks to me, man.
But that’s somehow cooler than glitter…
Hot? Yes, of course, it’s hot!
This is the Earth we’re talking about!
Oh – lost you there?
Right.
That’s because you were having fun, isn’t it?
Your shiny, stubbornly glittery orb was the Earth this whole time!
Surprise!
And now, we get to examine what’s inside it!
Peel Back Those Layers! – The First Look
Okay.
Look at those broken pieces of globe on the floor.
The stuff that used to be in the very middle of the sphere?
That’s the core.
There are two layers of core – the solid inner core and the liquid outer core.
Yes. Liquid.
That’s why there are chunks of solid core floating in liquid all over the floor, dude.
Don’t touch it! It’s shiny cuz it’s molten rock!
Next layer up.
The one above the core.
That’s the mantle.
There are a couple of different sub-layers inside the mantle, too.
The more solid part of the mantle is closest to the liquid outer core.
Further up, you get the ooey-gooey, “still-technically-solid-but-definitely-not-rigid” layer of the mantle called the “asthenosphere.”
Then comes the layer made up of the crust and the very top part of the mantle.
That layer is called the “lithosphere.”
And the lithosphere is important.
Like, really important.
At least as far as we are concerned.
Because we live on it. And it constantly moves around, dragging us along with it!
Bam!
Interesting.
Okay – put all those imaginary, oozing pieces on the floor back together.
Go ahead! This is not a mess you clean up with a towel, you know!
Good.
Let’s look at this thing again. Closer – and from the outside in!
Peel back your sphere’s beautiful, imaginary outer layer.
Oh, come on – don’t be disappointed! The Earth contains all those things you imagined!
After all, the Earth is YOUR home, and YOU imagined those things! So they must exist, however imaginary, on the Earth!
(I told you! The Earth is awesome!)
Okay.
What do we see now?
We see the Earth.
With all of its water and air and lightning and volcanoes and dirt and rock and LIFE!
Now, zoom in.
Beneath the biosphere – but only just beneath – you see a layer that is vital to life.
You’ve reached our first destination: The Crust
The Crust
The crust is crusty.
It’s solid.
And it’s hard and crumbly.
Because the conditions at the surface of the Earth look NOTHING like they do in the depths of rocky Nirvana – the mantle.
No. Here on the surface of the Earth, rocks are exposed.
And they get worn down.
This “weathering” is driven by water, wind, people, animals, tree roots, and all sorts of other rock-disturbing factors.
But that’s not bad, necessarily. It helps the rock cycle along!
The Earth’s crust is a habitable place…although not necessarily supportive to humans.
The ocean floor is still the crust, for instance. (Good luck with that, fishes! I’ll stay topside!)
In fact, everything we can reach is part of the crust – drill or no drill.
All of the fossil fuels.
All of the aquifers.
All of the mines filled with Earthly treasures.
All of it.
Everything we live on or off of or next to or in spite of exists on the Earth’s very thin crust.
It’s only ~5-70 km thick, man. (~30-40 km average)
That’s 3-44 miles! NOTHIN’ compared to the rest of the Earth!
Earth’s radius at the equator? 6,378 km (3963 miles). And that only gets you to the center.
We live on a huge planet.
(Relative to us, anyway!)
The Earth is a dynamic place, though.
And not just the life bustling on top of the thinnest layer of the planet.
No.
This planet is active.
There is a crazy amount of heat coming from its insides.
Internal heat like that moves things around!
And that motion is transferred to the surface layers.
Time for our next Imagination Destination: The Lithosphere.
The Lithosphere
The lithosphere is ~100 km (62 mi) thick and includes the crust and very top portion of the mantle.
Why do we care about the lithosphere?
Because it moves us! (Literally!)
Have you ever heard of the tectonic plates?
They are the giant, puzzle piece-like portions that make up Earth’s surface.
And they are lithosphere!
The tectonic plates are moved around on a conveyor belt-like asthenosphere (that’s the plastic/flowing part of the mantle), and we are carried along with them.
They crash and bang into each other, slide past another, and eventually restart the rock cycle by shoving beneath one another and making their way back to their source – the mantle.
The topmost part of the mantle is contained in the lithosphere, just beneath the Earth’s crust.
And the mantle layer below the lithosphere is the asthenosphere.
The Asthenosphere
~100-410 km (62-255 mi) below the Earth’s surface, underneath the lithosphere, lies the asthenosphere.
The asthenosphere is malleable. It isn’t liquid, but it isn’t a rigid solid either.
Too much heat and pressure.
Or just the right amount…depending on how you look at it.
Rocks are ductile here and move around on heat currents – which act like conveyor belts to the above lithosphere.
Where does the heat come from?
It is thought to come from a combination of energy left over from the Earth’s formation and heat released by radioactive decay inside the Earth.
This heat creates currents.
Kinda like you see in a lava lamp.
At the base of the lava lamp, there is a heat source.
That heat source heats the lamp’s “lava,” and it rises to the top of the lamp.
Once at the top, the “lava” is far enough from the heat source to cool off a bit.
The cooled “lava” is denser and heavier than when it was hot.
So it starts to sink back down, toward the heat source – where it is again heated and rises to the top.
This cyclical motion is called convection.
Convection currents drive the motion of the asthenosphere.
And the asthenosphere carries the tectonic plates (the lithosphere) along with it!
So no place on Earth is sitting still.
And we’re not even considering the fact that our planet moves around the sun and spins like a top along that orbit!
The Earth itself is churning and moving from within.
And that moves the outer surface!
What a ride!
Time to look at the rest of the mantle.
The Mantle
The mantle is big.
In all, it is ~2,900 km thick. That’s 1802 miles down, dude!
All the way around the core – like the hugest donut imaginable!
We’ve already talked about the top two layers of the mantle.
The lithosphere is made up of the crust (which isn’t the mantle at all!) and the VERY top portion of the mantle.
And the asthenosphere.
That goopy, malleable, elastic, plastic, crazy layer on which the whole lithosphere – and biosphere – surfs!
But it’s still part of the mantle.
Why do we differentiate these layers within the mantle?
Because not all rock is chemically or physically identical.
And neither is the mantle!
The upper (lower lithosphere/asthenosphere) and the lower mantles are all made of rock, but their physical differences are significant enough to need sub-categories.
The lithosphere is rigid and brittle.
The asthenosphere is malleable, even “flexible.”
The lower mantle is more solid than the asthenosphere.
These are all physical differences, so far, right? Goopy vs. brittle.
Chemistry in the mantle varies some, but its basic chemical makeup (mostly mineral-forming silicates) is similar enough to group under one name (Mantle!).
The crust and the core, though? Their chemical (and physical) properties differ significantly from the mantle’s composition.
So they are entirely different layers!
How do we know the chemical makeup of the middle of the Earth?
That’s tricky since we can’t GO there.
But we can deduce the composition of the Earth’s interior. (Donning our Sherlock hats!)
We base our deductions on what we observe at the surface, seismic wave patterns we gather from earthquakes and other disruptive events, the average density of the Earth, and the composition ratio of meteorites.
Since the center of the Earth is inaccessible (do NOT try climbing into a volcano to do so, Otto!), we have to do the best we can with the evidence we have.
And always be looking for more!
Scientific understanding is constantly advancing.
We, therefore, have to be willing to shift our perceptions when new, solid, tested evidence comes to light, even if that evidence threatens our basic understanding of how the world works.
And therein lies the scariest part of science – and life!
Discovering something that challenges one’s worldview can be very scary.
But where would you be in life if you never asked any questions??
Learning is how we grow! As people – and as a species.
As I always say, it is much better to boldly seek and find what could be challenging answers than never to ask questions at all!
So channel your inner Miss Frizzle!
“Take chances! Make mistakes! Get messy!”
Okay. Let’s move deeper into the Earth!
What’s next?
The core!
The Liquid Outer Core
We’ve reached the core, man!
The core has two layers – and the outer core, where we are, is liquid!
How do we know this part is liquid?
Well, the short answer is, “because of seismic wave patterns.”
Seismic waves are ripples of motion – caused by earthquakes, explosions, eruptions, etc. – that move through the Earth in specific patterns.
And we record and interpret seismic patterns using specialized equipment to figure out what things are like below our feet.
There are different types of seismic waves, and each behaves differently, depending on the kind of material they move through.
By studying data received at seismic stations, we can deduce the chemical and physical (liquid vs. solid) characteristics of Earth’s interior.
Come back, now. I see stars in your eyes.
It’s pretty awesome, I know.
But back to the Earth’s core! (For now!)
Much like the mantle, the core is layered.
The entire core is made up of chemically-similar material (mostly nickel & iron), but the outer core is a liquid, and the inner core is solid.
The outer core starts around 2,900 km below the surface and continues, in a ring, to a depth of ~5,100 km (1802-3169 mi).
That’s a total thickness of 2200 km (1367 miles).
The seismic waves that can move through liquid (some can’t) can move through the liquid outer core and the solid inner core.
How do we know the inner core is solid?
The seismic waves that move through the liquid out core act differently upon reaching the solid inner core.
They bend and move like they do when they are moving through a solid!
The Solid Inner Core
The rest of the core – from ~5100 km (3169 mi) below the surface to the very center of the Earth at 6,378 km (3963 mi) – is a solid inner core.
This, like the liquid outer core, is made up of mostly nickel and iron.
How do we know that?
Well.
Based on the gravitational pull, orbit, and size of the moon, we can calculate the mass and density of the Earth.
But most of what makes up the planet is significantly less dense (~2.2-4.4 g/cm3).
So the core of the Earth has to be exponentially denser than the rest of it.
Knowing all of that, and using densities of elements, minerals, and rocks we see at Earth’s surface, scientists “mathed” their way to the answers:
Our core is most likely made of iron, nickel, and smallish amounts of sulfur.
But that’s not the only line of evidence for the makeup of the core.
We also look at the composition of meteorites.
Meteorites are spacey leftovers from all sorts of planetary objects.
The universe’s rubbish.
But they are full of important information!
5-10% of the meteorites have high percentages of nickel and iron.
And the other 90-95% are made of silicate minerals – similar to the rest of the Earth’s composition.
Meteorites aren’t exactly young, either.
A lot of them were part of other planets or moons, after all!
Some of them are very old.
Like, Earth-aged old.
So old that the Earth is believed to have been formed from planetary objects like meteorites.
Thus, it is widely accepted in the scientific community that the Earth’s composition is roughly comparable to that of the material found in meteorites.
If you smash and spin a bunch of meteorites together, which are roughly 90% silicates and 10% nickel and iron, it makes sense that the heavier objects would come together in the middle.
Presto – a nickel and iron core (with a few impurities thrown in)!
How cool is that?!
Conclusion
Phew! We made it!
The biggest takeaway here?
There are three primary layers of the Earth – the crust, the mantle, and the core.
But like anything, it’s more complicated – and way cooler – than that.
Complicated doesn’t mean boring.
It can mean INTERESTING!
And the Earth is definitely interesting!
It’s glittery, remember?
Yes – you can put that imaginary layer back on now.
Ooo, pretty!
Yup, that’s the Earth, alright.
She’s a beauty!