7: An Ocean of Magma
Last episode, we took a detour back to the modern day to explore Earth’s interior. We learned that the mantle is solid but flows very slowly, and that the core is divided into a liquid outer core and the solid inner core, both made of iron and nickel. We can estimate the composition of these layers by comparing seismic waves traveling through the inner earth.
Today, we return to the Hadean: 4.5 billion years ago. In our imaginary Earth calendar, we’re approaching January 6, 100 million years after Earth’s formation. Let’s quickly set the scene. We have the sun? Check. The Earth? Check. The Moon? Not yet. All the other planets? Check, check, check, plus a few stragglers from the beginning of the solar system.
Now that we know the inner structure of the modern Earth, let’s compare it with the Hadean world. If we line them up side by side, we notice that Earth’s ancient mantle is liquid. Remember, the modern mantle flows like warm caramel, but it is still technically solid, made of many crystals including our old friend olivine. 4.5 billion years ago, the mantle is still too hot to be solid, so it is technically magma. We’ve talked about modern magma chambers the size of cities, but this is something else entirely: scientists call it the magma ocean.
Today, we will explore this ancient magma ocean, more than 200x deeper than the Mariana Trench. First, we will see Earth’s insides heat up by thousands of degrees. Then, we will watch this ocean cool and crystallize into a solid mantle. Last week, we took an elevator to the core, but for the magma ocean, a submarine seems more appropriate. Climb aboard, and let’s dive into the sea of molten rock.
Part 1: Inferno
The center of the Earth is 7000 C, hotter than the surface of the sun. Fortunately, you the listener can’t feel any of it: less than 0.1% of that heat makes it to the surface- we’re well insulated up here. So why do we care? Why are we talking about Earth’s inner furnace at all?
Earth’s heat stirs up the mantle like a giant lava lamp, and that does affect us surface dwellers. A moving mantle is one reason that the crust is shattered into plates which push and pull on each other. This continental grinding results in earthquakes and volcanic activity, both very relevant on human lifespans.
The Earth’s heat also stirs the outer core, producing a giant magnetic field around the Earth- for more info, check out last episode. What I didn’t tell you is that without the magnetic field, the upper atmosphere would be stripped away, exposing us to dangerous amounts of UV radiation. This happened… in the 2005 disaster film The Core. The reality is less cinematic, but equally devastating: just ask Mars. Mars has a much weaker magnetic field than ours. Billions of years ago, Mars had a thick atmosphere like Earth’s, enough to support rain and rivers and lakes. But without a strong magnetic field, the solar winds peeled the Martian atmosphere away like an onion, producing our cold, dry neighbor today.
The magnetic field and plate tectonics are essential parts of the modern Earth, and both need heat from Earth’s interior. So now that we know why planetary heat is important, what causes it? Why is earth’s interior so hot?
The Hadean Earth had three heaters. Their afterglow still heats the planet today. Let’s look at them one by one.
1: Meteor impacts.
This one is pretty straightforward. If you keep hitting something with a hammer, both objects will heat up. Physical force might not seem like enough energy to heat a planet, but I cannot stress enough just how much crap there was floating around in the early solar system. On this show, we will be bombarded by meteors for the next billion years- the first three months of the Earth Calendar. In these early Hadean days, the objects are much larger and more abundant. Being punched by thousands of asteroids the size of countries will heat you up more than a fist or two, a heat that lingers for billions of years.
2: Radioactive decay.
In Episode 5, we saw the first separation between dense iron and lighter olivine, the first steps in separating the mantle and core. This separation, like oil from vinegar, was sparked by heat from radioactive aluminum. As the Earth grew, aluminum was joined by the superstars of the radioactive world: uranium, thorium, and potassium. Aluminum decays and cools down after a few million years, but uranium burns for billions of years, and today, 50% of Earth’s internal today is from radioactive decay.
That sounds worrying, knowing we’re sitting on top of a giant radioactive ball, but like the heat below us, humans don’t experience most of that radiation. A particle from Earth’s interior only hits you once every 10 days. We know this thanks to very special laboratories called neutrino detectors. We won’t talk about how these work, but do yourself a favor and look them up in Google Images. If you can’t: imagine yourself in a boat, floating in a giant cylindrical room underground. Every inch of the walls, ceiling, and flooded floor are covered with shiny metal domes the size of dinner plates. It feels like you’re a James Bond movie, but these are real chambers designed to filter out the outside world expect for special atomic particles. In this room, particles from the Earth only visit every few weeks, escapees that tell us about a nuclear furnace far, far below.
The Daya Bay Antineutrino Detector, Shenzhen, China.
3 Iron Rain
With giant asteroids crashing from the outside, and radioactive decay within, Earth’s interior is really cooking now: thousands of degrees, Celsius or Fahrenheit. And the temperature’s rising.
On our submarine ride through the magma sea, splotches of metallic liquid fall on the portholes and a drumming noise starts on the roof. We are thousands of kilometers below the surface, and there is no water to be seen, yet it looks like it’s raining. In front of us, we see a giant blob of metal the size of a mountain plummet out of sight.
This rain is dense liquid iron, sinking through the magma. Remember when I mentioned iron separating from olivine, the oil and vinegar? This is what that separation looks like up close- the denser iron is falling down to the center of the Earth to help build the core. As this iron rain sinks, it heats up, just like meteors burning as they fall through the atmosphere.
In other words, the separation of the core from the mantle is providing yet another source of heat inside our sweltering planet. As these last iron raindrops fall, the Earth’s interior is as hot as it will ever get, with heat flow five times more powerful in the Hadean than today. We will return to the effects of this heat for the next several seasons of Bedrock.
Slowly, very slowly, as we wind our way though the underground magma sea, things are cooling down. The asteroids will soften their punches, the radioactive peak will pass, and the last iron blobs will fall into the core. So what’s next?
On our windshield, we hear a tink. Tink tink tink, a sound like hail on your window. Looking out, we see small dark mineral crystals floating in the red-hot ooze like marine snow. It is still thousands of degrees hot down here, but the magma ocean is slowly freezing before our very eyes.
Part 2: Sink or Swim
So how did the magma ocean crystallize into the mantle?
When water freezes into ice, the ice floats because it’s less dense than water. On the other hand, when you let salt or rock candy crystallize in the same glass of water, it settles on the bottom. In our submarine 4.5 billion years ago, we see the same principle with crystals in the cooling magma ocean. Lighter minerals like olivine float up below the crust, while heavier minerals sink closer to the core. As we travel along, we also see minerals just hovering in place like fish, neither rising nor sinking. They’re the same density as the surrounding magma.
Unlike our friend olivine, these lower mantle minerals will never see the Earth’s surface. They can only form at temperatures and pressures far below Earth’s crust, but like distant relatives, we know they’re down there anyways. How?
As we described in detail last episode, scientists like Inge Lehmann can use seismic waves to narrow down what the properties of these mystery minerals are- how dense and how malleable. It’s like reaching into a mystery box and trying to guess an object by feel alone.
Next, scientists and engineers made machines that could replicate conditions inside the Earth, if only briefly. One of these is the diamond anvil- which works just like it sounds- you take two diamonds and squeeze something between them as hard as possible, more than 7,000 times the pressure at the bottom of the ocean. In fact for geologists, this is overkill, more than twice the pressure at the core. What we need is heat, and that can be provided by electricity or lasers. Again, this sounds like James Bond villain stuff, but with a heated diamond anvil, scientists can replicate any temperature and pressure below our feet.
So what do we throw into a diamond anvil to see what happens? What mineral makes the most sense?
Say it with me now: olivine.
When you press and heat olivine between diamonds, it eventually changes into strange blue and green minerals with names like bridgmanite, ringwoodite, wadsleyite and davemaoite. If these sound like people names, you’re right: Drs. Percy Bridgman, Ted Ringwood, David Wadsley, and Dave Mao all made great strides in high-pressure geology and physics. Dr. Bridgman won the Nobel Prize in Physics in 1946 for his early high-pressure research, and would sign manifestos with Albert Einstein against the use of nuclear weapons in the Cold War.
To bring it all back home, these high-pressure minerals have the same physical properties as Earth’s lower mantle, calculated using seismic waves.! And if you’re still not convinced that these minerals exist outside a lab, you can look inside meteorites, though they are quite rare.
The story of bridgmanite and the others is a perfect example of how scientists have to tackle the Hadean. Again, no surviving rocks on Earth are that old, and no one has visited the modern mantle. Yet, by combining observations from earthquakes, meteorites, and laboratory experiments, we are beginning to understand these hidden worlds. Which brings us back to our submarine in the magma ocean.
The hovering crystals of bridgmanite are especially important- as they grow, they form a spherical shell in the middle of the magma sea, creating two separate oceans- one shallow and one deep. In your water, it would be like having ice freeze across the middle of the glass, parallel to the table. You could now only drink the top half.
Both oceans will continue to freeze from the bottom and the top. Like an icebreaker, we crash though multiple sludgy layers of crystal as we escape to the surface. The liquid magma ocean will survive for a while, but it’s good to leave before it completely freezes into a gooey caramel mess. As we break through the last bridgmanite, we’re in the upper mantle now. We don’t see olivine yet, but surprisingly, there’s a mineral here most people are familiar with. In fact, if you were born in January, like me, this is your birthstone. It’s my pleasure to welcome garnet to the podcast.
Garnet crystal, Madagascar
For those not familiar, garnets come in many shapes, sizes, and chemistries but are most commonly red or brown, shaped like a 12-sided die, and contain aluminum, silicon, and a rotating cast of other elements. Garnet will not appear as often as olivine in our story, but it will be a character actor of sorts. Whenever I see a garnet in the field, I think “Hmm, that rock has been pretty pressure cooked.” Garnets you find in a gemstore or a hiking trail were made in the crust, not the mantle, but they need heat and pressure to grow. Therefore, garnets are most common in metamorphic rocks. They won’t tell us stories about life or the changing surface world, but they will tell us how altered a rock is, which is very important when looking at ancient Precambrian samples.
There are other minerals surrounding our Hadean submarine that hardcore geologists are probably dying to hear me talk about, but we’re running a bit short on time, and we will see them very soon. Our submarine rises up into a sea of green, surrounded by olivine as we finally break through the thin crust to Earth’s surface. We have survived the magma ocean. Pour yourself a drink from the ship’s bar and catch your breath while we digest what we’ve learned.
Summary
Welcome to the Hadean Earth, 4.5 billion years ago, January 6th on the Earth Calendar. Our world is almost fully grown, and it’s hard to believe that it was a just a few specks of iron and olivine, but it has been 100 million years.
When we began this episode, the inside of the Earth was completely liquid, melted down by friction and radioactivity into a molten core below a magma ocean. That same heat still fuels the Earth’s interior to this day- you can’t feel it, but it powers every volcano, earthquake, compass, and aurora.
But even this early in Earth history, things are starting to chill out. The magma ocean, hundreds of times deeper than the Pacific, is starting to crystallize. Some of these minerals are strange and very rare, others like garnet are common birthstones. Eventually, the magma ocean will become the mantle- solid, but still hot enough to flow and move continents to this day.
As we wind down, take a look around you.
It’s nighttime. 4.5 billion years ago. The ground is lit with open pools of lava on the first dark steaming portions of a solid crust. The night sky is a constant meteor shower, more vivid, beautiful and terrifying than the Perseids or the Leonids. Left and right, these meteors punch through Earth’s thin skin, sending showers of molten material into the air. There is no water or life that we can see.
Now this world truly deserves the name Hadean- it’s the closest we’ll get to Hell on Earth, but it’s still our home.
And it’s not all hellish. Look, just coming up over the horizon. There’s a moon out tonight.
… But wait, at the beginning, I said the Moon hadn’t arrived just yet. And come to think of it, that thing is getting larger and larger. So what is that object?
That’s no moon, that’s a planet, one of the last leftovers of the early solar system. It’s name is Theia. It’s the size of Mars. And it’s headed straight for us.
I’ve got a bad feeling about this.
***
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:
Mars: https://commons.wikimedia.org/wiki/File:OSIRIS_Mars_true_color.jpg
Daya Antineutrino Detector: https://commons.wikimedia.org/wiki/File:The_Daya_Bay_Antineutrino_Detector_(8056998030).jpg
Core-Mantle Diagram: https://commons.wikimedia.org/wiki/File:Core-mantle_differentiation_processes.png
Diamond Anvil: https://commons.wikimedia.org/wiki/File:Diamond_anvil_cell.jpg
Garnet: https://commons.wikimedia.org/wiki/File:Granat,_Madagaskar.JPG
Music:
Hebrides Overture, Fingal’s Cave: https://commons.wikimedia.org/wiki/File:Mendelssohn_-_Hebrides_Overture_Fingal%27s_Cave.ogg
Sonar Pings: https://commons.wikimedia.org/wiki/File:Sonar_pings.ogg
Their Arrival by Emmett Cooke
Murder Case Investigation by High Street Music