Kicking back with a good book, it’s easy to look around and imagine this beautiful planet as being a steady, stable hunk of rock under our feet. But one good natural disaster reminds us that that’s not entirely true.
I’m far from being the first person to imagine what really lies beneath the soles of my shoes. Jules Verne wrote about all manner of crazy creatures and fantastic adventures in Journey to the Center of the Earth. Hollywood has tried to put a face to these wild imaginings on the silver screen. But what is the truth? What really lies beneath our feet? Earth is a rather large planet, after all. So there must be something down there worth exploring.
It seems like you’re as curious as I am. Let’s go take a trip – to the center of the earth. You bring the snacks; I’ll call a cab.
An Overview of the Earth
You brought peaches? Good choice. While we’re waiting for our ride, let me take one of those from you and cut it in half. When I hand it back to you, notice that there are layers on the inside of the peach. Turns out that this is a fantastic example of what we’re about to see inside the earth. Just like this fruit, the Earth has layers: the crust, the mantle, and the core.
Now, that’s a very basic (and delicious!) map of what we’re about to see but it gives you a general idea. Here’s our elevator – hop on in! Let’s take a ride down through the Earth’s layers and I’ll give you the grand tour.
Stop 1: The Crust
Temperature: 25°C and sunny, slight chance of rain
Depth: Up to 70 km deep
The Earth’s crust wraps around the entire surface of the planet like the skin of the peach. This is where we work, eat, sleep and play. This is where our lives happen. But it makes up only a tiny percentage of the Earth’s total volume. 1% to be exact.
There are 2 types of crust:
This is where the thinner sections of the crust are usually found. It is typically made of basalt and is more dense than the continental crust. This causes it to ride lower (like a heavy boat) than the continental crust, hence one reason for its being under water.
This is where the thicker sections of the crust are found. It is made of more granitic materials, and is generally lighter than oceanic crust which gives it a higher elevation.
The crust is anywhere from 3 – 43 miles (5 – 70 km) thick. For you runners out there, the thinnest spot in Earth’s crust is the equivalent of a 5k trot down the trail. If you had a tunnel and didn’t have to worry about scorching heat and bone crushing pressure, you could probably nail it in under an hour. The thickest spot in the crust is just a quick zip down the highway to the next town. This isn’t even the distance between Colorado Springs and Denver. Following the posted speeds, of course, I can drive that in about a half hour. Take a look at your local map. What towns are this far away from your house? Does this put in perspective how short of a distance this really is?
The deepest that we humans have ever been able to drill through the crust is about 7.5 miles at the Kola Superdeep Borehole in Russia. As you go deeper, the heat and the pressure crank up exponentially and it becomes impossible for our fragile human bodies to survive. But thanks to modern technology and some rather brilliant scientists, we know what lies beyond!
How do we know? Hang on. I’ll get into all the technical stuff a little later because right now we’re coming up on the Lithosphere, and you don’t want to miss this!
Stop 2: The Upper Mantle: Lithosphere
Temperature: 300°C – 500°C
Depth: 32 – 100 km
If you’re watching closely, you won’t see much of a transition between the crust and the very top part of the upper mantle, but it’s there. It’s called the Mohorovicic Discontinuity or The Moho. It shows up on our instruments because seismic waves change direction when they bump into it, but it’s really a small transition between the crust and the tippy top of the mantle.
This zone (crust + Moho + top section of the upper mantle) is called the Lithosphere. This part of the mantle is still cool enough that it is very rigid rock. It is broken up into 15 major plates and many minor ones.
What’s a plate? Imagine you took a boiled egg and dropped it on the floor. When you pick it up, there are going to be cracks running through the shell. The shell has now broken into several different sections, but still cover the egg. These sections are similar to what plates look like on the Earth. They “float” (for lack of a better word) on top of the rest of the mantle and move around thanks to the convective currents deeper in the earth. This movement causes much of the tectonic activity we experience up here on the crust.
We’re about to dive deeper into the mantle. The entire mantle makes up 84% of Earth’s volume by itself. That is one juicy peach.
Stop 3: The Upper Mantle: Asthenosphere
Temperature: 1300°C – 1600°C
Depth: 100 km – 660km
As we move lower into the upper mantle, the rocks outside are becoming less brittle and are becoming more elastic. The rocks here are strange: they move almost as though they’re fluid, but they are still solid rock. As the pressure and temperature increases, they’ve become molten, and are called magma. When a volcano erupts and spews out hot lava, that “lava-ly” display started here.
The asthenosphere is full of convective currents. Convection happens when super hot magma from the lower reaches of the mantle rises and cooler magma from the upper mantle falls. This is not unlike your house on a winter day. Hot air from your fireplace is going to rise and keep your upstairs bedrooms nice and toasty, but don’t go down into the basement or you’ll freeze your toes off! At any rate, it’s a similar story here. The constant rising and falling of magma causes almost a wheel effect that moves the plates sitting on top of it around.
Check out the graphic below:
If you were to take a trip through the mantle, you may occasionally stumble across a mantle plume. These are superheated upwellings of magma that blast straight upward through the mantle. Think blowtorch.
But let’s avoid those for now. The last thing we need is for one to melt a hole in our deep-earth taxi cab.
Bathroom Break: The Transition Zone
Depth: 410 – 660 km
The lithosphere and asthenosphere together make up the entire upper mantle. Here, we come across a thin transition zone between the upper and lower mantles. Here the rocks don’t just flat out melt, they actually change crystalline structure and become much more dense than what we just ventured through. This zone creates a barrier between the two mantles and prevents a lot of material exchange. There also is a bunch of water here (in the form of hydroxide) – enough to fill all of the oceans on the surface of the earth!
Stop 4: The Lower Mantle
Temperature: 3870°C – 4400°C
Depth: 650 km – 2900km
This lower section of the mantle is much hotter and denser than the above layers, and the pressure here is getting pretty intense. It’s about 1.3 million times what it was on the surface. If we weren’t safe in our cab, we’d easily be smushed flatter than a piece of paper!
There is quite a bit of debate about the composition and nature of this section of the mantle. But as our technology progresses, we will certainly be able to tell more about it.
Stop 5: D” (D Double Prime)
This is a super thin boundary between the lower mantle and Earth’s outer core. Blink and you’ll miss it!
Stop 6: The Outer Core
Temperature: 4400°C – 6000°C
Depth: 2890km – 5190km
Finally! We’re reached the pit of the peach: Earth’s core. The entire core makes up the last 14% of the Earth’s volume. This upper section that we are floating through is called the outer core. It is made up primarily of iron, nickle, and sulfur, spinning around in liquid form.
What happens when you put a spin on iron and nickle? You get a magnetic charge. Our outer core is what gives Earth it’s magnetic field. This magnetic field extends 600,000 km (370,000 miles) out into space to protect us from the harmful radiation that the sun blasts our way. Without it there is no way we could survive and, besides, we wouldn’t have terrific auroras to watch at night.
Stop 7: The Inner Core
Depth: 5190km – 6372km
Keep going a little further and the liquid outer core suddenly gives way to solid nickel and iron that is spinning at an insane rate – even faster than the rest of the planet! You’ll notice the pressure and temperature down here are at an all time high. Seriously, it’s as hot as the surface of the sun! And if you thought the pressure was crushing up in the mantle, it’s now 3.6 million times what you’d experience sitting in your backyard.
All the fun starts here, bay-bee! The heat from the core is what keeps the mantle alive and churning, and in turn keeps life thriving on the surface of our Earth. Without all the heat and inner workings of our planet, we wouldn’t stand a chance. It’s the pumping heart that keeps the rest of it in motion.
I don’t know about you, but it’s getting a little hot and sweaty down here. I’m ready to hop back on up to the crust and chug a cold lemonade. How about you?
Stop 8: A Cold Lemonade
Here, have a cold drink. You’ve been on quite the trip. I’m glad to say it’s much cooler up here and I don’t feel like a giant hippo is sitting on my back anymore.
Okay, so how do you know that what I just showed you is not all made up in my crazy imagination? Back to the technology and the rather brilliant scientists. And a few very helpful earthquakes.
When an earthquake happens, they release seismic waves. This is sort of like throwing a rock in a pond. The waves ripple out in all directions, including down into the interior of the Earth. There are three types of waves that occur: Primary waves, secondary waves, and surface waves.
– Are the first to arrive at seismic stations
– Travel at speeds of 1.5 – 8 kilometers/second in the Earth’s crust
– They shake the ground in the direction that they are moving
– Can travel through liquids and gas
– Can travel through the Earth’s core
– Are second to arrive at seismic stations
– Are almost 2x slower than primary waves
– Shake the ground perpendicular to the direction in which they are traveling
– Cannot travel through liquid or gas.
– These travel only along the Earth’s crust.
When these waves plow through a different material or a different consistency, they change direction. For example: if a wave starts through solid rock and then hits a bunch of magma, it will change direction. As a wave hits a rock of a different material (say, switching from basalt to something granitic), it will also change direction.
There are a bunch of seismic stations around the world that can track seismic waves. If an earthquake happens in Japan, we can pick it up in the United States, England, Russia, Australia, etc. We can also tell how many times it has changed direction. We can also tell how much the amplitude of the waves has changed, and how often those changes have occurred (usually as it hits a different type of material).
We can also make observations about rocks that were formed in the mantle and now reside up here on the crust, and can perform experiments at high temperatures and pressures. These both help us to form theories about what lies underground. The occasional meteorite can be helpful as well, in that they tell us about the makeup of other planetary bodies. This can help us to make some educated conclusions about our own planet.
There you have it. This is a very basic overview of how the scientists can paint a picture of the Earth’s interior. Each layer of our planet is very dynamic and diverse and our scientists are just now scratching the surface of what lies hidden beneath our feet. But since there are currently no real tours down as far as we went today, it leaves plenty of room for exploration and imagination.
*Note: Depths and temperatures are approximate and vary dependent on where in the Earth (or in it) you’re standing.
** Peach image used with permission and designed by FreePik: http://www.freepik.com/free-vector/fruit-label-collection_1105018.htm’
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