6: Working Out the Core
Last episode, we learned how Earth survived the gauntlet of the early Solar System between 4.6 and 4.5 billion years ago, dodging bullets and growing quickly to avoid being sucked into the sun. As the Earth grew, radioactive heat helped separate metal iron from lighter minerals, like oil and vinegar.
Today, I’ll introduce you to the modern Earth’s interior- the mantle and yes, the core. We’ll learn what’s below our feet, and how we know it’s there- including an incredible woman who discovered the inner core in the 1930s. After this episode, we’ll have the context to continue our exploration of the Hadean Earth 4.5 billion years ago. It’s crunch time.
Part 1: Planet Waves
You will often hear that we know more about the moon’s surface than the bottom of the ocean. While true, there’s a realm that makes both moon and ocean sound as familiar as the back of your hand: the Earth’s interior. Twelve humans have walked on the moon, thirteen have traveled to the deepest ocean trench, but no one has been to Earth’s mantle or core- not even close.
The distance from you, the listener, to the center of the Earth is 6,500 kilometers, the distance from Paris to New Delhi. If you wanted to get as close as possible to the core- how deep could you go?
The top of the Kola Superdeep Borehole, northern Russia.
The deepest caves on Earth are in the country of Georgia, stretching more than 2 kilometers down, and the deepest gold mines of South Africa are slightly longer. Two kilometers is impressive for a mine, but it’s less than one tenth of one percent of the Earth’s radius. Back to our Paris analogy, 2 kilometers is the distance between the Eiffel Tower and the Arc de Triomphe- an average person could walk that distance in half an hour.
But humans have dug deeper and more greedily. In the 1970’s, the Soviet Union began drilling the Kola Superdeep Borehole near the Norwegian border, north of the Arctic circle. The goal: to drill as deep as possible into the Earth. As of 2022, the record still holds. The Kola Borehole is 12 kilometers, and remains the deepest point reached by humanity. Or more accurately, machinery- the drill was nine inches wide, so if you’re wandering around the Russian Arctic, you’re not going to fall down a 12 kilometer hole. If you did, you would only be 0.2% closer to the center of the Earth- the same distance as a 3 hour walk.
We truly have only scratched the surface of our world. And yet, scientists talk about distinct layers thousands of kilometers deep, where no human has ever visited.
How is this possible? Earthquakes.
Earthquakes have plagued humanity since our first days. One major goal of geology is to help identify and track earthquakes to mitigate future destruction. Scientists who do so are called seismologists, and there are seismology stations around the world, a network of ears pricked for any rumblings below the surface. One helpful side effect of this network is that each earthquake lets us eavesdrop on the secrets of Earth’s interior.
We’ll cover how an earthquake starts in a later episode. Right now, we’re interested in the aftermath - specifically, waves. Earthquakes shake the earth in three different ways, each producing a different type of wave through the planet.
The easiest waves to see are surface waves- ocean tsunamis are the best examples. But surface waves don’t tell us anything about the core. For that information, we turn to two different waves, and a different analogy. It’s time for another home experiment: the tin-can telephone.
A tin-can telephone is made of two open metal cans connected by a long string. To save time here, I’ll let you look up instructions online. If the string is held tight between the two cans, you should be able to whisper in one end and have your friend hear clearly through the other. The sound of your voice is being compressed through the string to the other end. The string itself should be barely moving, but this sound still counts as a wave- we call it a compression wave. Compression waves can travel through solids, liquids, and gas, just at different speeds- it’s why you can still hear noises through water and doors.
Say you want to tease your friend- you shake the can a little on your end. Now here’s a wave you can see- the string whips up and down as the wave travels forward (hopefully not too much). Because the wave is moving up and down and forward, we call this a transverse wave- it’s making two different motions at once. Once you’ve settled down, the string comes back to rest. Unlike sound, transverse waves can’t travel through liquids. To see why, let’s imagine our string is made of beads just barely held together. One whip, and the whole thing shatters and the wave disappears. This is why transverse waves can’t travel through liquids- the liquid can’t come back to the same position like the string does. Keep that idea in your back pocket.
Waves leaving an earthquake.
OK, so what does this tin-can experiment tell us about the Earth’s interior? When an earthquake shakes Los Angeles, it sends compression and transverse waves through the entire planet. It’s like an angry phone call on our telephone, when you shout and shake the wire at the same time. These waves travel through Earth’s different layers, reaching seismic stations around the world different times. We can use these time delays to calculate what material the wave was traveling through, like iron or minerals. Repeating these calculations with thousands of earthquakes around the world helps confirm these ideas.
I should note that no one is eagerly waiting for an earthquake to strike- they can be deadly and destructive. But as long as earthquakes continue, scientists can use the aftermath to better understand how our world works. On that note, let’s find out earthquakes tell us about Earth’s interior.
Part 2: Inner Space
The Earth is divided into four physical layers: the crust, mantle, outer and inner core. For the rest of the episode, we’re going to take a brief elevator ride to the center of the Earth, focusing on the mantle and core. We’ll save the thin crust for later, since most tales in this podcast will take place on Earth’s surface. Today is a subsurface day.
Mantle
Mantle convection model, Kronbichler et al., 2012
Below the crust is the mantle, 3000 km thick. The mantle has a strange physical texture, best described like caramel- it is a solid structure made from minerals, but over millions of years it slowly flows like a fluid. This flow is driven by heat- the mantle is squeezed between the piping-hot core below, and the cool, brittle crust above. Heating causes lower portions of the mantle to rise up. When they reach the crust, these mantle plumes cool back down and sink. If this sounds like a lava lamp from the 1960s, you’re not far off- this same principle of rising and falling due to heat is called convection. Imagine the mantle as a very slow lava lamp filled with very thick, hot caramel- do not try this at home.
But if the mantle isn’t made of caramel, what is it made of?
In Episode 4, I introduced a little green mineral called olivine, found everywhere from asteroids to beaches. Since then, I’ve peppered olivine throughout the program to keep it fresh in your mind, and here is where the mineral truly shines. Earth’s upper mantle is mostly made of olivine, and we have the rocks to prove it. Sometimes, very rarely, a slice of mantle is shoved up onto Earth’s surface, like a piece of hot birthday cake. This upper mantle material is full of olivine, forming beautiful rocks called serpentines because they resemble green snake-skin.
As we go deeper into the mantle, temperatures eventually reach 4,000 C, or 7,000 F, four times hotter than a magma chamber. And yet, the mantle is still solid, thanks to intense pressure, 1000 times greater than the deepest ocean. The lower mantle is a strange realm full of minerals that are unstable on Earth’s surface, like an anglerfish pulled from the Mariana Trench- they can only survive at intense pressures and temperatures. These minerals are not on the test yet, but I mention them because our friend olivine is only one part of a chemically complex mantle.
Serpentine from Wyoming, USA (Proterozoic), formed from the interaction of water with mantle-derived rocks. The green is derived from original olivine.
Outer Core
When scientists first looked at seismic waves passing through the Earth, they noticed something weird: a dead zone. Just below the mantle, transverse waves completely disappear, like the string of beads we whipped earlier. As we learned in that experiment, this dead zone could only mean one thing- the outer core is a liquid.
Model of the magnetic field within the inner core
The outer core is a mixture of molten iron and nickel. We see a similar mix in the iron meteorites from last episode, the dead cores of other worlds. But if you’re still not convinced, pull out a compass. The reason that compasses work is because Earth has a magnetic field, lining the needle north-south. If you put a kitchen magnet next to your compass, it’s pulled in a different direction. So we know that Earth has a giant magnet below our feet- it’s big enough to line up compasses around the world, but distant enough that your tiny kitchen magnet can override the same compass.
The outer core is more than just a giant kitchen magnet. Like the mantle, the outer core flows up and down due to heat- a lava lamp made from liquid iron. These spiral paths resemble giant slinkies around the core, or more appropriately, a coil of metal wire like an inductor inside a radio. When you turn the radio on, this coiled wire makes a tiny magnetic field shaped like a donut. Earth’s outer core makes a giant magnetic donut around the planet as molten iron spirals up and down like that wire. You can see Earth’s magnetic field with your own eyes if you live close to the poles. An aurora occurs when the solar wind we met in Episode 4 hits the magnetic shield just right, the sky lighting up with beautiful curtains of green and red. So whenever you use a compass or spot an aurora in the night sky, be sure to thank the churning cauldron of liquid iron in Earth’s outer core.
Inner Core
The year is 1925, the place: the University of Copenhagen in Denmark. In one laboratory, we see a geologist, Dr. Niels Erik Norlund, a severe-looking man with a wide moustache, looking over a diagram of the Earth’s interior. Peering over his shoulder, we see the crust and the mantle, but only one circle of liquid iron in the core- something is missing to our modern eyes.
Niels hears a knock on the laboratory door. He opens it and welcomes in a quiet young woman, her wavy hair pulled back. Her name is Inge Lehmann, and she will be the first person to discover Earth’s inner core.
Inge was born in Copenhagen in 1888. As Inge grew up at the turn of the century, she attended a school where girls and boys were given the same classes, activities, and were treated as equals. Inge later would say that her teachers and father were her biggest influences as a child, setting her up to succeed at Cambridge University. She suffered from bouts of poor health and exhaustion during her work, and left academia for seven years to work for an insurance office, where she honed her math skills.
Inge returned to Copenhagen, getting a master’s degree in physical science and math in 1920. She worked as an assistant for several professors including Dr. Norlund, where she got a second master’s degree at the age of forty in geodesy, studying the shape of the Earth. The same year, Inge was made the head of the seismology department, and the national geodesist of Denmark. Her goal was to improve seismology stations around Europe to better understand the Earth’s interior.
Using these updated stations, Inge calculated that certain mysterious seismic waves were caused by interactions with a solid object in the center of the Earth- the Inner Core. Her landmark 1936 study has the shortest paper name I have ever seen- just two characters: the letter P and an apostrophe, which reads as “P-prime”, describing these mysterious waves. To me, that title alone is an incredible flex, daring anyone else to write a title that brief and to the point.
Inge Lehmann’s work was quickly accepted by other major researchers, and she had a long, well-respected career, living to the age of 104. Her story is inspirational on so many levels- growing up in the 1800s with the same opportunities as boys around her, taking time off from academia due to burnout, facing a system dominated by men and becoming a leader in that field. These are still relevant topics in today’s geosciences, one century later. Anyone who is given the right opportunities and has the passion to learn can become a scientist if they wish. There’s a lot we can learn from Inge about the core of a person, and not just a planet.
Speaking of which, the inner core is made from iron and nickel, just like the outer core. The intense pressures in the center of the Earth keep the iron solid, despite temperatures over 5000 C, as hot as the surface of the sun.
We still don’t know exactly when the inner core formed. Estimates range between February and November on the Earth Calendar, 4 billion to half a billion years ago. That’s like not knowing if your furnace was built in the 1990s or the 1890s. Interestingly, more recent studies are leaning towards the younger ages after July- clearly the center of the Earth still has many secrets left to uncover.
Summary
The Earth has four layers: the crust, mantle, inner, and outer core. Humans have only explored the crust, but like Inga Lehmann, we can put our ears to the ground and listen to the echoes of earthquakes. They tell us that the mantle is made of hot crystals that slowly flow like caramel. The outer core contains coiling liquid iron that orients your compass and keeps the Earth safe from the solar wind. Finally, the inner core is a solid ball of iron and nickel, a mysterious realm we will explore much, much later in the Precambrian Era.
Speaking of deep time, this excursion back to the modern world has been fun, but I’m itching to take this knowledge and travel back to the Hadean. Next episode, we’ll see the core and mantle truly take shape 4.5 billion years ago, in the Parting of the Ways.
***
Thank you for listening to Bedrock, a part of Be Giants Media. As the show takes off, I would love to hear your input on style, topics, and people to interview- you can drop me a line at bedrock.mailbox@gmail.com. See you next time.
Images:
Kola Superdeep Borehole: Andre Belozeroff
https://commons.wikimedia.org/wiki/File:Кольская_сверхглубокая_скважина_crop.jpg
Seismic waves: USGS, Vanessa Ezekowitz
https://commons.wikimedia.org/wiki/File:Earthquake_wave_shadow_zone.svg
Tin Can Phone: Chris Potter
https://commons.wikimedia.org/wiki/File:Chris_Potter_-_3D_Tin_Can_Phones.jpg
Mantle Convection: Kakitc from Kronbichler et al., 2012
https://commons.wikimedia.org/wiki/File:Simulation_of_2D_mantle_convection_in_a_quarter_of_annulus_using_ASPECT_mantle_convection_code.gif
Serpentine: James St. John
https://commons.wikimedia.org/wiki/File:Serpentinite_(East_Dover_Ultramafic_Body,_Ordovician;_Adams_Brook,_east_of_East_Dover,_Vermont,_USA)_4.jpg
Inner Core: USGS, Andrew Colvin
https://commons.wikimedia.org/wiki/File:Dynamo_Theory_-_Outer_core_convection_and_magnetic_field_geenration.svg
Compass: Rama
https://commons.wikimedia.org/wiki/File:Compass_IMG_1223.jpg
Aurora: US Air Force, Joshua Strang
Inge Lehmann: National Library of Denmark
https://commons.wikimedia.org/wiki/File:Inge_Lehman.jpg
Music:
Retro Wave by Remember the Future
Night Cruiser by Arenas
TV Mambo by Daniel Belardinelli
Space 80s by Ilegot
Elevator by Gangsterdave
https://commons.wikimedia.org/wiki/File:Elevator.ogg
Ding Dong Bicycle Bell by TeWeBs
https://commons.wikimedia.org/wiki/File:Ding_Dong_Bicycle_Bell_A.ogg
Sweet Georgia Brown by Ben Bernie Orchestra
https://commons.wikimedia.org/wiki/File:1925_(USA)_Archives_1925_03_19_Ben_Bernie_Orchestra_-_Sweet_Georgia_Brown.mp3
Knocking on wood or door by Stephan
https://commons.wikimedia.org/wiki/File:Knocking_on_wood_or_door.ogg