44: From the Abyss

Last episode, I introduced you to our final location of Season 2: the Isua region of southwest Greenland. This area is just below the Arctic Circle, 100 km north of Greenland’s capital, with no roads in between. The only way to reach these rocks is by helicopter. Despite this intense remoteness, Isua is still a hotbed for geologists- hundreds of papers have been published here over a half-century of research. Perhaps it’s fortunate that such old rocks are so far afield- if not, humans would likely have stolen them all away by now.

Last episode was an appetizer, an introduction to the Isua area and its age. Here’s a brief recap. Isua is best described as a giant U-shape or smiley face spread across the Greenland tundra over 36 km. As we walk from south to north, the rocks get younger, from 3.8 to 3.7 billion years old. On our Earth Calendar, that’s March 3 to 11. Give yourselves a pat on the back: we’re solidly beyond February now, working our way toward springtime.

Over the next few episodes, we will walk forward in time across Isua, from south to north. The sheer variety of rocks here is staggering, unlike anything we’ve seen on the show- literally miles of ancient volcanos and seafloors to explore. This abundance is a double-edged sword. On one hand, Isua gives us a wealth of new data, our widest glimpse onto Earth’s surface. On the other hand, I can’t cover every single stone like earlier locations. Furthermore, many of Isua’s stories are ones we’ve already heard. For example, we’ve already discussed banded iron formations twice this season, and they appear multiple times in Isua.

Here's my gameplan, to avoid turning this exciting gift into a boring slog. I’m going to focus on new rocks and new ideas, or at least adding more detail to previous topics. We’ll give a nod to old friends, but focus on the highlights of this grand buffet. In terms of space and time, we’ll broadly work south to north, old to young, but sometimes we’ll bounce around to cover more ground. In other words, this story is less of a strict year-by-year chronology, and more of an epic narrative. Hopefully you don’t mind. 

Today, we start with a special treat: a rock we have not seen before on the show. This rock has been pulled up from the depths like a strange deep-sea fish. It is an exile from a realm humans can’t even visit today. It’s time to meet the oldest piece of Earth’s mantle.

Part 1: From Theory to Reality

Before we examine these exciting mantle rocks in Greenland, let’s review what we’ve learned about the mantle over two seasons of the show.

As you’re listening to this podcast, you’re either on, in, or above Earth’s crust, our planet’s outermost solid layer. The mantle is the next layer down. The mantle is not that far below us, around 40 km or 25 miles, less than a marathon. But close as the mantle is, no human has ever visited this layer, and we’re not likely to visit anytime soon. The pressure and temperature is far too much for any human or machine to handle. And that’s just the mantle’s shallow end. The mantle keeps going down for 3000 kilometers or 2000 miles, the thickest Earth layer by far.

OK, if no one has visited the mantle, how do we know what it’s made of? There are actually several clues about what’s below our feet.

Imagine the mantle like the deep seafloor. If you want to see what’s hidden deep in the ocean, you can bounce sound waves around and get an image with sonar. If you want to see what’s deep inside the Earth, you can watch seismic waves from earthquakes bounce around and get a similar sonar image. For example, an incredible woman named Inge Lehmann discovered Earth’s inner core using only earthquakes. For more info about Inge and this seismic sonar, check out Episode 6: Working out the Core.

Scientists quickly figured out that Earth’s core was made from iron, giving us a magnetic field and the northern lights. The mantle material in between was trickier to ID, but scientists narrowed it down to a few candidates, a few tough minerals that could survive at such intense pressures and temperatures. The final piece of the puzzle came from volcanos and meteorites. Certain deep volcanos kept pulling up strange green crystals that were uncommon on the surface. The same green crystals were found in ancient meteorites, the building blocks of the early Earth. Together, earthquakes, volcanos, and meteorites tell us the mantle is made from the mineral… olivine!

Longtime listeners will be nodding in recognition, perhaps even smiling to see an old friend once again. Olivine was the first mineral I introduced on this show, way back in Episode 4, and was a star player in Season 1. She’s taken a backseat in Season 2 but takes center stage today. Olivine is typically olive green, forming blocky, translucent crystals. Gem fanciers might know olivine better as the gemstone peridot (pear-ih-dot or pear-ih-dough), a birthstone for August, very fitting for this August 2025.

We’ve talked about the mantle and olivine many times on this show. So what’s new about today’s episode?

Until now, all our mantle discussions have been theoretical. Again, no one has visited the modern mantle. We can make educated guesses, but we can’t go there. The situation is even trickier for the ancient mantle, a hidden realm in a hidden time. But sometimes, a rare sliver of mantle is pulled up from below and shoved onto the surface, where we can touch and glimpse the mantle directly.

Today, I’m proud to introduce a new type of rock to the show, one made almost entirely of olivine, one that comes straight from Earth’s mantle. Meet peridotite.

Part 2: Escaping the Underworld

Peridotite gets its name from the green gemstone peridot. Most rocks are far from gem-quality, but are still quite beautiful. A piece of peridotite is made of small, vibrant green olivine crystals scattered with black flecks. At first glance, it almost looks like moss, but one touch quickly reveals a hard crystal nature. You’ve probably never seen peridotite before, or even heard of it, but it’s arguably the most abundant rock on Earth. Peridotite dominates the mantle, and the mantle is 2/3rds of Earth’s mass.

The reason that Earth’s most common rock is uncommon on the surface is density. When the early Earth assembled in Season 1, the lightest minerals floated to the surface like crystal corks. Meanwhile, heavy metal iron sank down into the core. Peridotite falls in between, which is why the mantle sits between the light crust and heavy core. For more info, check out Episodes 5, 6, and 7.

If peridotite is denser than surface rocks, how does it make it up here? How does it escape the underworld? There are a few pathways to raise a peridotite up to our level. It’s time to bring back an old bit from earlier episodes: the game show comparison: Is This Your Peridotite?

I’m going to introduce three potential candidates for our ancient Greenland peridotites, three ways to bring the mantle to the surface. Each candidate has a very different story to tell.

Let’s start with Candidate #1: Eruption.

Imagine a raging mountain stream. That stream is not just water. You’ll also see rocks and pebbles rolling along the riverbed, getting carried away by the flow. Some of those stones were probably carried miles away from their original locations up the mountainside. Eventually, they’re dumped along the riverbank or into the sea, far from their homeland.

Now imagine that this river is made of boiling lava instead of water. Instead of a mountain stream, we now have a volcanic eruption. Much less pleasant to be around, but the effect is similar. The lava flow plucks rocks from its edges and bottom and carries them downstream. Some of these rocks were stolen from deep, deep underground. Eventually, the lava cools and their passengers are trapped inside, far from their homeland.

Even so, it takes a powerful volcano to bring up pieces of mantle. Notable examples are found across Africa, including Sierra Leone, the Congo Basin, and South Africa. Here massive vertical pipes over a mile deep are filled with shattered green peridotite. These pipes were formed by rapid, violent eruptions, but fortunately the volcanos are long dead. These regions are also famous for another mineral, far more famous and precious: diamonds. Turns out, the same eruptions that bring up peridotite are also the best places to mine diamonds today. So does Greenland have any diamonds to share? Let’s check in on our other contestants first.

Contestant #2: Collision.

The violent eruptions I just mentioned happen under thick continental crust, up to 70 km or 40 miles away from the mantle. Fortunately, there are many places where the crust is much, much thinner: the seafloor. Ocean crust is 10 times thinner than continents, you can almost smell the mantle here.

If you dig just below the brown mud and black volcanic rocks, you would find all the green peridotite you could ever ask for. It’s called “abyssal peridotite” since it’s found below the ocean abyss. This is all fine, but the seafloor is still a hard place to reach. Most abyssal peridotites are found very far from shore, where you need submersibles or deep-sea drills.

Obduction can push mantle slivers onto land, while subduction makes trenches. Source: LibreTexts Geology of California, Ch. 11.2

So how can we bring the seafloor, and the mantle below it, onto dry land, where it’s much easier to study? Why, simply shove the seafloor against a continent! I’m not joking, this has happened multiple times in Earth history. Let’s revisit a place we’ve seen many times before: a deep-sea trench. The best way to visualize a trench is to take your bare hand and slowly shove it below another object. It could be your other arm, the edge of your table, or a comfy bedsheet. Go ahead, give it a shot now. Take a pause if you need to- this is an important demonstration. For the best effect, start with a short-sleeved shirt.

Your sinking arm is ocean crust, getting shoved down below the lighter, higher continent. The crack in between is the deep-sea trench. For a more detailed description, check out Episode 31. Now, grab a long-sleeved shirt, coat, or sweater, and try the exercise again. The fabric of your shirt should bunch up against the trench as your hand plunges down. Imagine this as layers of seafloor getting literally scaped and shoved onto land. If you shove hard enough, some pieces of mantle might also come up. But let’s keep your arm safe and move to…

Contestant #3: The Cheater

Layered peridotite from northern Italy, Borghini et al., 2016, J. of Petrology

Our previous contestants brought us green peridotite directly from the mantle, either by eruption or by continental collision. It takes hard work to get the real deal. Now Contestant #3 also makes peridotite, but it’s not exactly “pure”. It doesn’t come straight from the mantle. Instead, it’s made much closer to the surface. If you cool a lava or a magma in just the right way, a little bit at a time, you can make bright green peridotite in your own backyard, so to speak. If you want to learn more about this partial freezing process, check out Episode 27, I’m not going into great detail here. The important point is this: these “shallow” peridotites don’t tell us as much about the mantle deeper down.

These “imposters” can be fairly easy to spot: they often form green layers on the bottom of ancient magma chambers, much like a bathtub that desperately needs cleaning. I feel bad for giving these guys a hard time: they can tell us a lot about volcanos, but that’s not today’s goal! We want to know if these ancient Greenland peridotites are actually from the mantle.

To answer that question, we must return to the field, to Isua.

Part 3: A Friend Indeed

A peridotite exposure from Isua. Light gray are layered, near-surface deposits. Dark gray are abyssal peridotites. Friend et al., 2002.

If you remember from the intro, the Isua region is a giant U-shape on the Greenland tundra. If this U is a smiley face, these ancient peridotites are well below the bottom lip, getting into chin and beard territory on the southern edge. The peridotite exposures themselves are not that extensive. The largest are the size of a football field, it doesn’t matter what type of football you’re playing. Most are half that size or smaller.

These peridotites form irregular blobs scattered around the tundra, but a few well-preserved outcrops share a rough bulls-eye pattern, a ring around a central core. The outer rim is made of layered stone, almost like rings in a bathtub. As we learned last section, this is contestant 3, the Cheater. While these outer rings are peridotites, they aren’t straight from the mantle. But! Don’t despair. If you keep heading inward toward the center of the bulls-eye, you’ll see a different candidate.

This center has no layers, no bathtub rings, so it’s not the Cheater from the surface. We also don’t see any evidence for violent explosions, and sadly, no diamonds. So it’s not our Eruption candidate either. Instead, this is our Collision candidate: a slice of mantle scraped up from under the sea. In other words, an abyssal peridotite.

These Isua peridotites are the oldest known pieces of Earth’s mantle, 3.8 billion years old, March 3 on the Earth Calendar. They were brought to the spotlight by a friend of Allen Nutman’s research team from last episode. And by friend, I really mean Friend with a capital F. This new researcher’s name is Clark Friend, and he’s been overdue for a brief mention.

Clark Friend in Greenland. Source: Nutman 2014, Precambrian Research.

When we first met Allen Nutman on Akilia Island, I took care to mention his work was part of a larger team. Clark Friend is the most consistent member of Team Nutman, the most frequent co-author on the group’s papers. Friend started working for the Greenland Geological Survey in the 1970s and started working with Nutman a decade later. By my count, they have penned at least 60 papers together, most of them on ancient Greenland rocks. Friend specializes in plate tectonics, volcanic rocks, and metamorphism, especially focused on gneisses, those gnarliest of metamorphic rocks. So while Clark Friend was the first to describe Earth’s oldest mantle slices in detail, it’s not even his most famous achievement, and we’ll see more of his work in future episodes.

Friend worked at Oxford Brookes University in the UK until the early 2000s, not to be confused with the more famous Oxford University. His time was cut short when the school shut down the entire Department of Geology. Such an event is surprising, but not unprecedented. For example, North Dakota State University closed its entire Geology Department in 2023, especially surprising in an oil-rich land. Since then, Friend has worked for the Chinese Academy of Sciences, as a freelance researcher, and most intriguingly, assessing the risk of old unexploded bombs and mines in the UK, left over from World War II. Though he has not worked for a university in over a decade, he is still a publishing machine alongside Team Nutman. It just goes to show that even if life throws you a curveball, if you’re persistent and lucky, you can sometimes find a path to do what you love.

We’ll leave on that sentimental note for today, but we will continue looking into the old peridotites of Isua. Now that we know the identity of these rocks, we can now ask questions about the ancient mantle and the nagging question of early plate tectonics. It’s time to put these rocks to work.

Summary: Greenland contains the oldest slices of Earth’s mantle: 3.8 billion years old or March 3 on the Earth Calendar. These rocks are called peridotites, rich green stones filled with our old crystal friend olivine. Peridotites give us tantalizing glimpses of a world humans have never visited. Sometimes they reach the surface in violent, diamond-filled explosions. Sometimes they reach the surface when tectonic plates collide. The Isua peridotites are the latter category, scraped from below the early seafloor. The fact that they have survived nearly 4 billion years is truly incredible.

Next episode, we will learn how the mantle has changed over time, and see if these peridotites can shed any more light onto a long-standing debate: does Greenland preserve evidence for plate tectonics, or for a more alien type of geology?