31: Trench Warfare

Episode 31: Trench Warfare

Last episode, I introduced the Nuvvuagittuq area of northern Quebec, a land of hilly tundra paved with extremely old rocks. Just how old is debated: some folks say these are the oldest rocks on Earth and it’s not even close: ~4.4 billion years old, January 14th on the Earth Calendar. However, most folks say the area is tied around second place, 3.8 billion years old, March 3rd on the Calendar. On this show, we’re going with the 3.8 date. 

Now that we know when and where these rocks are, it’s time to explore what these rocks are, and what new stories they tell us. Nuvvuagittuq gives us our first direct view of the Earth’s surface, the realm of oceans, fresh air, and life. Until now, we’ve been stuck underground inside boilingmagma chambers, with only faint clues about the surface world. There’s nothing wrong with magma chambers- they gave us the oldest crystals and the oldest rocks. But today, we’ll meet rocks that were once at the bottom of an ancient sea. We’ll also visit a corner of the modern ocean where the same rocks are forming today. But our story begins back on the frigid tundra of northern Quebec. 

1: How Green Was My Valley

Nuvvuagittuq is not a large location- you could walk around it’s perimeter in a few hours. Just outside these borders is a vast wilderness of dull gray granite and its’ cousin tonalite – the cooled remains of countless magma chambers. But inside Nuvvuagittuq is a rich diversity of colors and stones- faded greens, dark blacks, thin bands of red and silver, pale beige peppered with brown freckles. Consider these rocks a drop of paint on a great gray canvas.

We will see this pattern repeated many times in the next few seasons: thin, colorful slivers of stone surrounded by gray granites. These colors were not lost on early geologists. They called these small pockets “greenstone belts”, named after their long, thin shapes and their most abundant color: green. The rocks weren’t green at first- green crystals formed after the rocks were buried under intense heat and pressure. But despite their transformation, greenstone belts are still the best windows on Earth’s earliest habitats. 

If you want to see the first volcanic lava, go to a greenstone belt. 

If you want to vacation on the first sandy beaches, go to a greenstone belt.

If you’re hunting for the oldest fossils on Earth, go to a greenstone belt. 

In short, greenstone belts are one-stop shops for a variety of geologists, including myself, and Nuvvuagittuq is one of the oldest.

Just how greenstone belts form is another question entirely, one we will ask many times on the show. While the habitats inside these belts are familiar- volcanos, beaches, seafloors, etc., the belts themselves are squeezed and surrounded by granite in ways that do not happen today.They’re tantalizing glimpses of a world in transition, both deeply familiar and deeply alien. 

Today, we’ll focus on the familiar, painting a landscape of Earth’s surface 3.8 billion years ago. Or more accurately, a seascape.

As we return to Nuvvuagittuq, how do we know this greenstone belt formed under the sea? The answer is found in its’ most common rocks.

If you walked around Nuvvuagittuq, most stones would be a pale beige, speckled with brown and red. Imagine a vanilla ice cream with flecks of caramel or strawberry. Now I could tell you the name of this rock and the names of all these colored minerals inside, but honestly, it’s beside the point. The rock has been altered, metamorphosed into completely different colors and minerals. But not all is lost- just walk a few minutes down the hillside. You’ll see a gradual palette change from pale beige and brown to dark green and black. These rocks are still altered, but they’re closer to their original state. 

Even more impressive than their color is their texture. These dark rocks are shaped into large, squished ovals, up to a yard or meter wide, piled high on top of each other. It almost looks like a ruined cobblestone wall from a dark eldritch city.

But as alien as these rocks appear, this frozen scene 3.8 billion years old is one that’s stillhappening as we speak at the bottom of today’s oceans.

2: A Pillow of Lava

We’ve met this dark rock before. I introduced it in Episode 2 and we’ve talked about it a lot inSeason 1, but this is the first time we’re meeting it in the wild. This rock is basalt. Basalt is an igneous rock, forming as molten lava cools on Earth’s surface. In many ways, basalt is the opposite of granite, the other major igneous rock we’ve discussed.

Basalt is dark black, while granite is pale gray or pink. Basalt forms on Earth’s surface, while granite forms deep underground. Basalt is most common on the seafloor, while granite sits high and dry on the continents. The list goes on and on. 

Earlier this season, granite and its’ cousins ruled the Acasta Gneiss, the oldest rocks on Earth. But today, basalt is king in Nuvvuagittuq. Technically it’s not basalt anymore, after billions ofyears, but we’ll just call it basalt for now. 

Basalt today usually forms on the bottom of the ocean, though it does erupt on dry land from time to time. Some examples you can visit include the Palisades outside New York City, the Giant’s Causeway in Northern Ireland, and the Deccan Traps of western India. All these places used to be volcanic lava fields, now transformed into dark cliffsides.

Let’s return to Nuvvuagittuq in Quebec, some of the oldest basalts on Earth. Did these basalts form underwater, or under an open sky? I already gave away the punchline in the episode’sbeginning: these rocks formed deep beneath the ocean waves. But how do we know that?

We’ve been asking such questions for thirty episodes, and until now the answer is usually chemistry- some isotope or element that tells stories about the early Earth. 

But today is different. Today we get to do real field geology, looking at the physical textures of ancient rocks and comparing them with the modern world. I’ve got nothing against chemistry- it’s a powerful tool, but there’s something about seeing and holding rocks in person that can’t be beat. If you want images to follow along with, check out our website: bedrockpodcast.com.

To recap: the basalts at Nuvvuagittuq resemble a large cobblestone wall, with dark rounded rocks up to a meter or yard wide. If we look closer, we notice a few other details. First, while there are a few gaps, many of these rounded basalts sit directly on top of each other. In some cases, it looks like they were molded around their neighbors, like a soft putty or taffy. Looking even more closely at individual basalts, we see tiny holes lining their dark round edges, like a chocolate sponge cake, or just a sponge.

These clues all point to a larger conclusion: even though the Nuvvuagittuq basalts are tough as nails today, they used to be squishy, flowing material. They used to be lava flows. The tiny holes around the edges used to be gas bubbles, boiling away at the top of the lava- you can see and hear these bubbles popping away in modern volcanos on land. 

What you don’t see forming on land are big piles of rounded basalt- that happens at the bottom of the sea. The reason why is temperature. Lava is more than a thousand degrees hot, but the bottom of the ocean is nearly freezing. When hot lava erupts onto the cold seafloor, the outer edge is instantly frozen, but the inside of the flow is still molten and breaks through this shell. Like a tube of toothpaste, the eruption keeps forcing lava up, while the ocean keeps re-freezing the outer edges. Eventually the ocean wins and cools the entire flow, which has ballooned into a large, rounded blob of basalt. The process repeats as lava finds new openings to break through, creating massive piles of dark, lumpy rock on the seafloor. 

Geologists call these structures “pillow lavas”, since they’re irregular but usually rounded, and have a thin outer coating. They don’t make the best pillows, especially as hot lava, but there is something cozy and inviting about their appearance- smooth, round, a little bit misshapen, but doing their best. It might just be me, but there’s an extra layer of comfort in the Nuvvuagittuq pillows. They come from a world 3.8 billion years old, where so much was different- no animals, plants, or continents. But even on that alien world, some things behaved just like today. Check out “pillow lava” on Youtube, and you’ll find beautiful, hypnotic videos of red lava flash-freezing into dark basalt. That video could have been made 3.8 billion years ago. 

Pillow lavas are erupting all around the world as we speak, but scientists have pinned one specific spot in the ocean as a good analogue for Nuvvuagittuq. To round out the episode, let’s dive down to the Mariana Trench.  

3: Letters from Iwo Jima

The Mariana Trench might be the most famous corner of the ocean. This is because it is the deepest corner of the ocean- 7 miles, 11 kilometers deep. If you shoved Mount Everest into this crack, it would not break the ocean’s surface. It takes 5 hours to reach the bottom by submersible, and only 22 people have done so. Humans just can’t resist peering into these depths, out of curiosity, fear, or both.

I’ve taught several sections of intro oceanography, and I ask my students what questions they want to know about the sea. The most common is: what’s going on at the Mariana Trench?

The trench is so deep because here, Earth’s crust is being dragged down into the mantle below, a realm where no human has visited. Now that might sound like a planetary catastrophe, the world eating itself, but it’s a perfectly natural part of geology. In Episode 12, I gave an intro to plate tectonics, how continents and oceans shift across Earth’s surface. Let’s recap what happens at the Mariana Trench. 

Earth’s ocean crust is built from basalt. Earth is constantly making ocean crust in some spots, and constantly recycling it in others. The Mariana Trench falls in the second category- it’s basically a giant basalt shredder. The ocean crust plummets down into the mantle like a giant sheet of paper, and is eventually melted. Those are the fundamentals we learned in Episode 12. Now, we get to the interesting stuff. 

It turns out, when ocean crust is dragged down beneath a trench, it takes an important passenger along for the ride: seawater. Don’t worry- the oceans are not disappearing down these drains like bathwater. Instead, imagine the seafloor like a giant sponge- solid but soaked through inside every pore. As the water gets buried deeper into the hot mantle, you might imagine it instantly turning to steam. But at these depths, something stranger happens- the immense pressure keeps water liquid, even at hundreds of degrees. 

This superhot liquid water mixes and mingles with the mantle, weakening it, and melting it into magma. In short, a hot wet rock is much easier to break down than a hot dry rock. As more water is sucked into the mantle, more magma will form, and will eventually break free back to the seafloor as underwater volcanos. Basalt lives, basalt dies, basalt lives again.

If the eruptions last long enough, small islands will break the surface and you can see the newborn rocks for yourself: islands such as Guam, the Northern Mariana Islands, and Iwo Jima, islands made famous during World War II. These new island basalts are a different than the old ones that were destroyed- they’ve been recooked, recycled, and thoroughly washed with seawater. They’re still basalts, but the recipe is different enough that they deserve a new name: boninites, after the Bonin or Ogasawara Islands off Japan, including the famous Iwo Jima. 

Boninites are a relatively rare rock today- you need a trench in the middle of the ocean, such as the west Pacific. Boninites are also found 3.8 billion years ago in Nuvvuagittuq, Quebec- the wall of black pillow lava we’ve been investigating. As usual on this podcast, the Canadian rocks are still being studied and debated, but it’s a less heated debate than others. Most folks think that an ocean trench was involved at Nuvvuagittuq, just like the Mariana Trench today. At the very least, these volcanic rocks were buried beneath the ocean and recycled before erupting back to the surface. They’re the first visual example we’ve seen of the ancient sea, and they won’t be the last.

Summary:

In our ship of the mind, we’re cruising the bottom of the Archean ocean, 3.8 billion years ago, early March on the Earth Calendar. It’s dark and cold, but getting warmer as we travel on. Eventually, we see huge gobs of lava oozing out of the seafloor, brief flashes of red light and heat before cooling into black basalt. Off in the distance, we see a huge yawning canyon, a trench stretching down to depths unknown. If we rose back to the surface, we might find a few scatteredvolcanic islands in the vast global ocean, like modern Guam and Iwo Jima. 

It’s a scene that’s strikingly modern despite its’ age, but as we keep patrolling the seabed, we start to see a more alien landscape. Instead of black basalt, our headlights scan over a vast flat plain stained blood red. This is a sight you will never find on the modern Earth, but we will see it next episode. Tune in to find out just what this bloody field is, and how it might give us clues about early life.