Inside Earth: Earth’s Crust, Mantle and Core
Let’s talk about digging a hole
Imagine a team of drillers who set out to drill a hole to the other side of Earth. Because who wouldn’t want to build a shortcut to the other side of the Earth, right?
So our team of drillers hire a brilliant engineer to design the strongest drill bit possible. After several designs, the engineer has the perfect drill to get the job done.
How far do you think the team of drillers make it?
We’ll get back to our team of drillers in just a second. But first let’s get to know inside our Earth.
What’s inside Earth?
All planets have layers. Earth has a core, mantle and crust.
Within all planets, the densest material is separated. While the lightest material is on the outer edge, the densest is in the center.
For example, the heaviest material like iron and zinc is in the core. Finally, lighter silicate rocks remained on top to form a crust.
Now, we know Earth density is highest in the core and lighter in the crust. What type of challenges would the team of drillers encounter first?
First, you see Earth’s crust
Earth’s crust is all around us. If you’re not in outer space right now, it’s the layer you live on.
In comparison to other layers, Earth’s crust is thin and rigid. It’s mostly made up of rocks with a density from 2.7 to 3.3 g/cm3.
On the outer shell, Earth’s crust forms the lithosphere, also known as the plate. The lithosphere sits on top of the asthenosphere like a rigid shell. Because the asthenosphere is soft and fluid, it drives plate tectonics motion.
The Earth’s crust has a continental and oceanic crust. However, both turn out to be very different from each other.
Oceanic crust is young geologically. It all forms the same way. At mid-oceanic ridges, long chains of underwater volcano are where plates move apart from each other. Because lava spews out to form new plates, it fills in the gaps creating young rocks from the asthenosphere beneath.
But continental crust is completely different than oceanic crust. Continental crust is thicker and less dense than oceanic crust. It’s too buoyant to sink compared to heavier mantle rock underneath. Because continental crust floats on the surface of the mantle, continents can have rocks over 4 billion years old.
After drilling through the crust, you see Earth’s mantle
On Earth, we have various climate types and ecosystems like jungles, deserts and tundra. But if you could see inside the Earth’s crust, you’d see even more variation.
In general, as we go down into the crust into the mantle, we get into denser and heavier rocks. And the further we go, the hotter it becomes.
Actually, it’s so hot that you would just see white. It would be glowing white with temperatures at intense levels. When you go about 100 km down into the Earth, the temperature is already at 1400℃ and pressure is remarkably intense.
Next, the mantle’s structure is mostly silicates with density ranging from 3.2 to 5.7 g/cm3. Because the mantle and crust are made of rock, the transfer of heat is through convection. The hotter, fluid mantle causes the less dense crust to rise which consequently results in transfer of heat.
But if you could drill a bit deeper into Earth, what would you see next?
Finally, you’d hit Earth’s iron core
If you go beneath the liquid mantle, you get a solid iron core.
Actually, the core of the Earth is both liquid and solid. While the outer core is liquid at 9.9 to 12.2 g/cm3, the inner core is solid with densities from 12.6 to 13.0 g/cm3.
At the center of the Earth, it’s about 5500°C. The pressure is remarkably intense. Because the core is made of iron which is a metal, electrical conduction transfers from the core to the mantle.
Lastly, seismologists suggests that the core is rotating faster than the mantle. This plays an important role in generating a magnetic field. Like a forcefield, it protects us with a never-ending stream of charged particles.
How do we know what’s inside Earth?
We can’t physically go inside the Earth. And unfortunately, light doesn’t travel through rock so we can’t see inside it either.
To overcome this, we use imaging and seismic tomography to see what’s inside Earth. During an earthquakes, seismic waves pulse through rocks in the crust and mantle. Overall, the speed at which the waves travel give you rock characteristics.
For example, waves propagate faster for cooler rocks than hotter rocks.
By understanding the time it takes for waves to travel, we can get a clearer picture of what’s inside Earth. Because of seismic waves, we now have images of inside the Earth.
Back to our team of drillers…
It turns out that a team of drillers is a bit of a true story. I don’t think they were trying to drill to the other side of the world, but a team of Russian drillers did try to dig the longest hole in 1970.
The hole they dug is called Kola Superdeep Borehole. And the deepest they were able to dig was about 12.7 kilometers into the Earth. But the Earth has a radius of 6,371 km from the center to surface. This means they’ve barely scratched the surface.
What happened? How come they couldn’t drill any deeper?
It turns out that because the conditions became so inhospitable, the drill bits don’t couldn’t withstand the pressure and heat inside Earth.
But because we can analyze seismic waves during earthquake events, we can begin to understand what’s really inside Earth.