27: Rare Earth

For the past two episodes, we’ve visited the oldest rocks on the planet: the Acasta Gneiss. Here’s what we’ve learned so far:

The Acasta Gneiss sits in Canada’s Northwest Territories, in the lands of the Tlicho First Nations. The rocks were forged inside underground magma chambers between 4 and 3.6 billion years ago. On our Earth Calendar, that’s February to March. Boiling red magma cooled into dull gray tonalite, one of granite’s many cousins. As the original Acasta tonalites were buried deeper,the gray stone shifted into black and white stripes: this is the contorted Acasta Gneiss we see today. 

OK, so the Acasta used to be very old magma, but why do we really care? What story is the Acasta telling us?

If you’re listening to this podcast, you’re probably doing so on a continent or an island: a piece of land risen out of the sea. Even though Earth is the blue planet, it just wouldn’t be our home without dry land. The Acasta Gneiss tells us how the first land appeared. It turns out- there’s more than one way to make an island.

Last episode, we learned the modern recipe for making dry land: the recipe’s name is partial melting, and we’ll review it again in just a second. Partial melting made most of today’s land, and also made most of the Acasta Gneiss. But there’s a dirty little secret I’ve been hiding: partial melting did not make the oldest Acasta rocks- the very first ones 4 billion years old. Today, we’ll learn another recipe, a secret one hidden in a few rare spots. We’ll learn how this secret recipe works, how it made the oldest Acasta rocks, and finally, we’ll visit a strange modern island, the best window onto the world 4 billion years ago. 

Part 1: The Crystal Parade

Let start by reviewing the major recipe for continents from last session: partial melting.

 Our first ingredient is dark ocean crust, and our first instruction is to slowly raise the temperature. We only want to melt just a few pale wimpy crystals, not the whole rock. You can try something similar with chocolate chip ice cream: the ice cream will melt long before the chocolate chips, forming pure pale vanilla. 

In a similar way, if you take a dark ocean rock and melt a few pale crystals away, those crystals will cool back down into a pale granite or tonalite, like we saw last episode.

That’s partial melting in a nutshell- it’s a recipe we’ll see time and again as we build up the continents.Our alternate recipe for today starts a little differently. 

Our first ingredient is still the same: dark, dense basalt from the bottom of the ocean. But this time, we’ll turn the heat up all the way to 1500 Celsius, melting the entire rock into magma. 

OK, now what? If we cool this magma back down, we’ll just end up with dark basalt again, right? We haven’t added or removed any ingredients at all. 

That’s true if we cool quickly- all the minerals will freeze at once. But if we take our time, let the magma slowly, gently cool down, something different happens.

When the magma cools to 1300 C, our first crystals begin to form. These are not the pale pink and gray crystals of granite, they’re our old green friends from Season 1: Olivine. Olivine crystals will fall to the bottom of the magma chamber, removing themselves and their ingredients from the mixture. Once olivine leaves, our recipe has changed- it’s a little closer to granite and tonalite, a little closer to a continent. As the temperature keeps falling, other darker minerals leave the party- the crystals that make basalt black. As each dark crystal falls away, the remaining magma keeps inching closer to granite. 

Eventually, another crystal appears, one we haven’t seen since Episode 9. This mineral is called plagioclase (pleh-jo-clays). That’s a mouthful even for geologists, so we just call plagioclase “plag” (plej) for short.

You might not have heard of plag, but it’s probably the most common mineral in Earth’s crust. It’s also the most common mineral on the Moon and Mars. It’s not the fanciest mineral, it’s not usually an expensive gemstone or economic resource, but it is a cornerstone of the planet. Just like I did with olivine last season, I’ll pepper in plag to our discussions as we go forward. Any time we talk about the crust or continents, plag will be part of the conversation.

Most importantly for our recipe, plag is usually white or gray, the colors of a continent, not the seafloor. Finally, after years of slow, gentle cooling, our magma chamber has finally, completely turned to stone. Ninety percent of the rock remaining is still dark, just like before. But at the very top of the chamber, there is a pocket of white, pink, and gray continental crust: granite and tonalite.

That’s our secret recipe for granite: I like to call it “partial freezing”. Remember in the last episode, partial melting slowly heated and melted another rock to make granite. In contrast, partial freezing from today slowly cooled a liquid magma chamber down.

So if these recipes are mirror images, why is partial freezing so rare? 

Consider the starting ingredients: there’s a lot more solid rock to melt at Earth’s surface than liquid magma to freeze. Second, as we saw freezing a magma chamber doesn’t make a lot of granite- most of the rocks remaining are still very dark. There are complex chemical reasons for this, but we’ll talk about those another day. 

For now, let’s return to the Acasta Gneiss. The reason we’re talking about partial freezing today is that this recipe formed the oldest rocks on Earth. So how do we know this?

Part 2: Seventeen Sisters

At the end of last episode, we met Jesse Reimink and his team, investigating the oldest zircon crystals from the Acasta Gneiss. These zircons grew very slowly between 4 and 3.6 billion years ago, between February 12th and March 19th on the Earth Calendar. The zircon crystals were the size of sand grains, but inside, Jesse’s team found growth rings like tiny trees. 

For the outer rings, Jesse and his crew found clear chemical evidence for partial melting, the classic recipe that makes modern continents. For more info, I invite you to revisit last episode. 

But when the team looked at the heart of the Acasta zircons, the earliest stages of growth, they found something very different: evidence for partial freezing. In short, the first magma chambers had slowly cooled down over time, making small islands of pale rock. 

So what was the evidence? How did they know?

Like all evidence from zircon crystals, the answer is found on the periodic table of elements.

The periodic table has 118 known elements. We won’t cover every element on this show, but we’ll knock quite a few off the list. We’ve already met carbon in our bodies, oxygen in water, and hafnium in the crust. 

Today, we’ll meet a large family of elements, a powerful toolkit used by most geologists, from volcanologists to fossil nerds like me. These elements are also a hot commodity in geopolitics, one that’s creeping into the news more every day. It’s time to meet the rare earth elements.

Rare earth elements are seventeen sisters: they’re all soft, white, heavy metals. Rare earths are actually pretty common- in fact, some are more abundant than copper or lithium. However, they are spread very thin: pure rare earths are extremely hard to come by and extremely expensive to process.

 But if you do have rare earth materials, they have many uses, including superconductors, cancer treatments, camera lenses, magnets, batteries, lasers, and MRI contrast fluid, to name a few. As technology grows, so does the hunger for rare earths.

At the moment, China is the largest exporter of rare earth elements. China has at times restricted the flow of rare earths by placing export quotas. China claims these policies are to protect the environment, which can be devastated by mining. Other governments claim these restrictions break free-trade agreements and are used for political clout. In either case, the rest of the world is scrambling to find rare earths in their own backyards. Keep your eyes and ears open in the next few months- you’ll probably hear at least one news article about these elements. For now, we’ll stick to science.

Rare earth elements are found in most zircon crystals, just in very small quantities. If you have the right equipment, like Jesse Reimink, you can measure and compare their abundances. Most geologists line the seventeen rare earths all up in a row. In a perfect world, all these elements would behave exactly the same- there should be the same amount of one element as another. 

 But it’s not a perfect world, and each element has its’ own quirks that tell us stories about the ancient Earth. For example, inside the Acasta Gneiss zircons, all the rare earth sisters behave themselves, they’re lined up in a tidy row. All except one, there’s one missing from this lineup, the only one we’ll meet today: europium.

Europium is a silver-white metal that looks identical to its’ sisters, though it is the softest. If you had enough europium, you could cut it with a butter knife. 

But it’s not the softness we’re interested in- it’s how europium interacts with crystals inside a magma chamber. Let’s return to our recipe from Part 1. 

Europium, like most elements, has a favorite location, a spot that it loves to hang out in. It does just fine inside the liquid magma, but as the mixture cools there’s a crystal that europium just can’t resist. Is it green olivine? No, no, that’s not right. How about those dark black minerals? Not those either. What about dull gray plag? Jackpot! 

Given a choice between magma and a plag crystal, europium takes the plag every single time. As plag crystals leave the magma, they suck up any europium around, leaving very little left for zircons. When Jesse Reimink and his team saw europium missing from the earliest Acasta zircons, they knew exactly how the ancient rocks formed.

Let’s back up and recap: the oldest Acasta rocks formed as magma chambers slowly cooled, producing small islands of pale tonalite surrounded by dark seafloor. This is not how most continents are built today, or even most islands. But there is one very special place built this way, a place where Jesse found very similar rocks to the Acasta tonalites. 

This island is relatively young on the Earth Calendar but gives us a living window onto Earth’s earliest days. This island is Iceland.

Part 3: Fire and Ice

Iceland sits high in the North Atlantic Ocean, just on the edge of the Arctic. It is the 18th largest island in the world, around the size of South Korea or Kentucky, covered with volcanoes, glaciers, and plains of dark, moss-covered rock.

Iceland is a geologist’s paradise for a variety of reasons, and could easily be the subject of an entire series. To finish off this episode, let’s get introduced and learn how Iceland is so similar tothe Acasta Gneiss. 

Iceland sits on top of a mid-ocean ridge, a giant undersea mountain chain that stretches around the globe. Mid-ocean ridges are where new crust is being printed out every day. In the center of a ridge far below the waves, the Earth’s crust slowly pulls apart, forming a crack of lava constantly scabbing over and cooling into dark basalt. This motion is gradually pushing North America and Europe farther apart, around an inch every year. 

The only spot on Earth where a mid-ocean ridge breaks the surface into the sunlight is in Iceland. In fact, Iceland sits directly on the center of the Mid-Atlantic Ridge: one half is on the same tectonic plate as North America, the other half is on the same plate as Europe. If you’re a tourist, you can visit the boundary itself at a place called Midlina, like I did back in 2010. By crossing over a small bridge, I walked from one tectonic plate to another and back in under a minute. 

But why here? Mid-ocean ridges span tens of thousands of miles, yet only break the surface here in Iceland. Clearly something different is happening here. 

That difference is found far beneath the crust, at the border between the Earth’s mantle and core. The mantle is solid, but flows slowly like silly putty. The deep mantle near the core is much hotter than the upper mantle near the crust. This temperature difference causes an imbalance: hot mantle at the bottom wants to rise up, while cold mantle at the top wants to sink down. The end result is like a lava lamp, with hot, goopy material constantly rising and falling.

Sometimes, a pocket of mantle will become extra hot, and will rise in a concentrated spot: geologists creatively call these areas “hotspots”. Hotspots can form intense volcanic activity far from any plate boundary: take Hawaii or Yellowstone for example. But what happens when a hot spot punches up beneath a plate boundary, say, a mid-ocean ridge?

That’s how Iceland formed around 20 million years ago, December 30th on the Earth Calendar. It has a very high proportion of magma volcanoes because it is splitting apart at the seams. And now we can see why Iceland is a decent analogue for the original Acasta islands 4 billion years ago. In Iceland, dark ocean basalt is melted into huge magma chambers, which slowly cool over time. As these chambers cool, pale rocks rise up to the surface, forming dry land. Iceland is nowhere near a continent in size, but it is one of the largest islands on Earth and growing every day. 

So back to the Acasta Gneiss: does this mean earth’s oldest rocks were forged by a hotspot, or a mid-ocean ridge? We’re not totally sure. In his papers, Jesse Reimink says it’s possible, but also the ancient Earth was a lot hotter than today, so maybe Iceland’s weird setup wasn’t necessary. 

In either case, I think this is a good place to leave the Acasta Gneiss. There will certainly be more stories to tell, but we’ve wrung out all we can for now. As we survey the Icelandic landscape, let’s go over what we’ve learned about Earth’s oldest rocks.

Summary

It’s 4.03 billion years ago, February 12th on the Earth Calendar. 

Our ship of the mind has landed on an island in the middle of a steaming blue sea. Fog and geyser mists obscure the island’s size: perhaps it’s the size of a state, perhaps it’s less than a mile wide. 

We walk down the gangplank onto the beach, dressed in spacesuits- there’s no oxygen in the air for us to breathe. Muffled through our helmets, we hear the low rumblings of volcanos in the distance. 

Our boots touch down on dark black sand, and already we can feel the heat pumping up from below. Huge pockets of molten rock are roiling far beneath our feet, melting and cooling in turn, recycling the dark rocks of the Hadean into something new. While most of the rock around us is black, in the hills just before the fog begins, we can see a few pale gray tonalites standing like statues.

In the blink of an eye, we travel forward 4 billion years. The island, the ocean, is now completely replaced by sunlit trees and moss-covered boulders. An arctic fox, startled by our arrival, scampers into the green. In its’ place is a pavement of zebra-striped stone: the Acasta Gneiss, the oldest rocks on the planet. They have been warped and overgrown, but thanks to a few dedicated people, their story is no longer a secret. 

So now it’s time for new secrets and new stories. As we flash back to the ancient Eoarchean world, we notice the clouds have cleared, but something is off. The sun is sitting directly overhead at high noon, but the land still feels dim, like early morning. It’s not our visors, it’s not some trick of the light. The sun 4 billion years ago is weaker than we know it today. What is going on?

Next episode, we’ll set our sights for the sun, learn about one of the greatest paradoxes in Earth history, and meet the man who proposed this paradox: Carl Sagan.