Inside Earth: The 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 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?
For example, the heaviest material like iron and zinc are in the core. Finally, lighter silicate rocks remain on top to form a crust.
Now, we know Earth density is highest in the core and lighter in the crust. Let’s start with the lightest which is the lithosphere.
1. Earth’s crust
On the outer shell, Earth’s crust is thin and rigid. The crust is all around us. Unless you’re not floating in outer space right now, it’s the layer you live on.
In comparison to other layers, the crust is mostly made up of rocks with a density from 2.7 to 3.3 g/cm3. The lithosphere is split between continental and oceanic crust. But both turn out to be very different from each other.
All oceanic crust forms in the same way. First, long chains of underwater volcanoes spew out lava. It’s at these mid-oceanic ridges where plates move apart from each other. The lava it ejects turns into oceanic crust. As a result, the youngest geological rocks on Earth are found under the ocean at oceanic crust.
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 the heavier mantle rock underneath. Because continental crust floats on the surface of the mantle, continents can have rocks over 4 billion years old.
2. Earth’s mantle
As we move down through the crust into the mantle, we get into denser and heavier rocks. It’s not only density. But the further we go, the hotter it becomes.
Similar to how temperature fluctuates in the air on our planet, the temperature in the mantle varies. But it turns out that variation is even more extreme deep inside Earth.
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.
The asthenosphere (averaging 80-200 km) lies beneath the lithosphere. Unlike the rigid and brittle properties of Earth’s crust, the asthenosphere behave soft and plasticky. In fact, this fluid property is what provides the necessary lubrication for plate tectonics. So, the crust sits on top of the asthenosphere, which is part of the upper mantle. Then, it’s carried enormous distances through a process called “continental drift”.
The upper mantle (35-670 km) contains the asthenosphere. When you go about 100 km down into the Earth, the temperature is already in the range of 450-900°C. Actually, it’s so hot that you would just see white and pressure is remarkably intense. The upper mantle has a density of 3.9 g/cm3. The upper mantle and crust (outermost layer) together, make up the lithosphere.
The lower mantle (670-2900 km) represents a significant amount of volume of Earth. It contains about 56% of the total volume filling in the transition zone and upper core. The lower mantle has a significantly higher density than the upper mantle. It averages about 5.0 g/cm3 of mostly solid rocks.
3. Earth’s iron core
If you dig beneath the mantle, you get a mix of liquid and solid in the core of Earth. The core is shaped like a ball with a radius of about 1,220 km. The pressure is remarkably intense with temperatures up to 5500°C.
Seismologists suggest that the core is rotating faster than the mantle. This plays an important role in generating a magnetic field. Like a force field, the magnetic field protects us with a never-ending stream of charged particles from the sun.
Earth’s outer core is liquid with a thickness of about 2,400 km. It’s composed mostly of nickel and iron with a density between 9.9 to 12.2 g/cm3. Because the core is made of metal, electrical conduction transfers from the core to the mantle.
The transition between the inner and outer core is 5,150 km beneath Earth’s surface. At the center of the Earth, it’s about 5500°C. The pressure is remarkably intense. Earth’s inner core has the highest density at 12.9 g/cm3.
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 earthquake, seismic waves pulse through rocks in the crust and mantle. Overall, the speed at which the waves travel gives 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 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.