38: Hidden Gems

Welcome back! It’s been a while, and I thank you all for your patience and kind wishes. Here’s a little housekeeping before we return to the rocks. If you want a detailed account of what I’ve been up to, please check out the last update, Bedrock Returns!

In short: I still have my job in Michigan, but it’s still not permanent- just one more year. The job hunt continues.

I’m on summer break now. On the plus side, that means time for more regular episodes- at least every other week. On the minus side, that means I’m not getting paid until September. Folks have asked me you can support the show. Here are a few ways, if you want the full rundown, check out the last update.

1) Episodes will now have a DONATE button in their descriptions. Only give what you want- I know times are tough for a lot of folks, not just me. 2) Patreon. On Patreon, you can give a small monthly donation for some extra perks- I’m thinking Mail Time Episodes or Geology in the News. I’ll start a Patreon soon, but I want a solid schedule first. I don’t think it's fair to ask for money if I’m not delivering. 3) Maybe some ads at the start and end of episodes, not interrupting the flow. If you love or hate any of these ideas, please fill out the audience survey in the episode description- I’d love to hear what you think!

If you can’t or don’t want to donate, that’s fine. These main episodes will always be free. Which brings us back to today’s scheduled program. Thanks for listening!

We’re in the final arc of Season 2. Our destination is southwest Greenland, around the capital of Nuuk: a remote land of mountains, fjords, glaciers, and very old rocks. The area around Nuuk is the crown jewel of Season 2 for three reasons. 1: This is Earth’s oldest major slice of rock, the size of a small nation, far larger than anywhere else we have visited. 2: Nuuk’s rocks cover a huge timeframe, from February to March on the Earth Calendar, 3.9 to 3.6 billion years old. We are officially done with any date before 4 billion years- we’re moving forward. 3: The rocks around Nuuk are the most well-preserved of their age, though there is a range of quality. Last episode, we learned about three stages of metamorphism, three altered rocks that tell us different stories. Do you remember their names?

Different styles of metamorphism. It’s hotter to the right, and more pressurized up top. Check out greenschist, amphibolite, and granulite!

If not, here’s a review. On the extreme end of metamorphism are the grand old granulites. They’ve been pressure-cooked beyond imagination, but they still tell large-scale stories of continents, mountains, and tectonics. Next is a group that’s not great, not terrible: you can feel ambivalent about the rock amphibolite. Amphibolites give us more info than their great granulite cousins, but still not a lot. Finally, we have the lightly cooked greenschists, giving us the green light to explore the surface world in greater detail: oceans, volcanos, and even fossils. When I say greenschist, you should get excited.

In SW Greenland, the rocks are mainly amphibolites- middle of the road. For other places on Earth, this is not exciting news, but for 3.9 billion years old, these Greenland amphibolites are looking pretty good.

So, where do we go from here? We have an entire month of the Earth Calendar before us, with many different rocks at our fingertips. This is our biggest buffet yet. As usual, we’re going to divide and conquer. Today, we’ll start with the big picture, with plate tectonics. In other words, what did Greenland look like 3.9 billion years ago?

Were continents smashing together into mountains, were they pulling apart into seas? Were there any continents at all, or was something stranger happening? These tectonic questions might sound esoteric, but they tell us how the hellish lava world from early Season 1 became the Earth we know and love. They will set the backdrop for future Greenland episodes on oceans, volcanos, and maybe even life. It's time to set the scene.

Part 1: A Quilt of Stone

Let’s start by making a simple mental map of these Greenland rocks using our imagination. Remember, the island of Greenland is huge, the size of Mexico or Saudi Arabia. Don’t worry, today we’re zooming in on a small sliver on Greenland’s SW coast.

The Itsaq Complex, modified from Nutman et al., 1993. Red are rocks 3.9 to 3.8 billion years old (Ga), yellow are 3.8 to 3.7 Ga, blue are 3.7 to 3.6 Ga, and pink are much younger (“only” 2.5 Ga).

I want you to imagine a long, thin banner hanging vertically. On this banner is embroidered a map, a map of Greenland’s oldest rocks. The top of the banner is north, the bottom is south. At the bottom of the banner is the cold blue Atlantic Ocean. Here, tucked in the bottom left corner, is the harbor of Nuuk, Greenland’s capital of 20,000 people. It might not be large, but it is the gateway to Earth’s oldest major slice of rock. If you were to wander around the streets of Nuuk, you would see very old rocks: April to June on the Earth Calendar, 3.2 to 2.5 billion for those keeping score. But we won’t see rocks that age until Seasons 4 and 5. We need to look elsewhere for the really ancient stuff.  

Back to our imaginary banner, we see islands and inlets lining the bottom of the tapestry, its ragged southern edge. These islands have some of the oldest rocks in the entire area, and some folks have argued for the traces of life here. Both the dates and the possible fossils on these islands have been subject to heated debates. If you listen on the sea breeze, perhaps you’ll still hear the arguments of passionate scientists. We will focus on these islands and give them a name in two episodes.

As we gaze upward, northward, into Greenland’s interior, we see a broad stretch of dull gray stone, pushed upwards into snow-capped mountains and sliced through by modern blue fjords. These rocks have seen a lot of action, even in the distant past. In the middle of the tapestry sits a large patch of pink and brown, looking for all the world like a wine stain on the tapestry. This stain happened in Season 5, but we’ll have to talk about it in a few episodes, since dramatically altered the surrounding older rocks.

If you look closely in the grey background, beyond that gnarly pink stain, you’ll see smaller flecks of original color peppered throughout the banner: patches of black, green, and even red. That might ring a bell, especially for folks listening recently. These were the colors we saw in Nuvvuagittuq in the Canadian north, Episodes 30-33. Those rocks, colorfully named “greenstone belts”, were windows onto the ancient surface: oceans, volcanos, and maybe life. Most of Greenland greenstone patches are far smaller than their Canadian cousins, but there’s one major exception.

At the banner’s northern boundary, we begin to see greater detail on a wider scale. It’s great curve of dark stone shaped like the letter U or a welcoming smile, inviting us to explore further. This greenstone belt is 200 km2, 10x larger than Nuvvuagittuq, larger than Liechtenstein, larger than Washington, D.C. Here we can see lava flows, here we can see rusty iron seafloors, and wait, is that possibly some fossilized pond scum? When suddenly, we’re cut off by a wall of white glacial ice at the northern border, just like Game of Thrones. The stone banner continues beneath the glaciers, but for now, those stories are hidden. As humans continue to change Earth’s climate, one unintended bonus could be revealing ancient secrets beneath the ice. Up to you if that’s worth it.

For now, let’s appreciate what we have. This banner of stone, stretching from the sea to the glaciers has a name: the Itsaq (it-sock) Complex, that’s I-T-S-A-Q. In the native Kalaallisut language, the word “itsaq” means “ancient”. Pretty on-the-nose there.

The word complex is also there for a reason- this is a complex cluster of rocks. I just described Itsaq like a single gray banner, but it’s more like a giant chaotic quilt. When geologists look closely at Itsaq, they find many different gray patches shoved together, different rocks that formed at different times. A single cobblestone in your hand could have a 3.9 billion year old crystal neighboring one 3.6 billion years old. For context, that’s Feb 23 vs Mar 19 on the Calendar, nearly a full month sitting in the same hand sample. Even for geologists, you usually can’t find a single small rock with two original ages that far apart.

What happened here? We’ll go over the details in later episodes, but here’s a handy visual for now. You can even follow along at home if you have a few different colors of Play-doh or modeling clay. Feel free to pause if you need to grab them or put them in your mind’s eye. Take the pieces of Play-Doh and smash them together, the more colors the better. Now stretch them all out as long as you can. Take this colorful noodle and fold it in half, tip to tip. Repeat this stretching and folding two or three more times. In the end, you should have a demented rainbow of clay, with many different pieces tortured into long, stringy threads. Replaces these colors with black, white, and gray, and you’ve made a rock we met in Episode 25: gneiss, the most metamorphosed rock you can find.

More than 95% percent of the Itsaq Complex are these complex stretched-out gneisses. You can tell some stories with them, but it’s very difficult, even after 60 years of research. But in a few protected pockets, the rocks are less stressed out, keeping more of their original features. They’re still messed up, but they can tell us what their neighbors once looked like. Today, let’s focus on the most abundant rocks in the area. What would we see if we walked across the Itsaq quilt?

Part 2: Stone Soup

More than 70% of the Itsaq Complex is or was just one type of rock, an old friend of ours. It’s a dull gray stone peppered with black crystals, stretching from the fjords to the mountaintops. A non-listener would say “That’s just granite!”. They’re close, but long time listeners would know better. You would say “Actually, that’s tonalite,” but I leave it to you whether pedantry is more important than pleasantry, especially on the Arctic tundra.

For a refresher on tonalite, check out Episode 26. Here’s the abridged version. Tonalites are one of granite’s many cousins. Both form deep underground inside boiling magma chambers. The magma slowly cools into crystals the size of coins, making beautiful mosaics that look great on countertops. Some magmas make tonalites, many more make granites. Both cousins tell us stories of plate tectonics: the birth and growth of continents.

Gray tonalite

Pink granite

So how can you tell a rare tonalite from a common granite? Color. Granites usually have more pink or red or brown crystals, while tonalites have more gray and white crystals. Now for most homeowners, and honestly for most geologists, the difference between granite and tonalite is seriously splitting hairs. You’re not going to get more money for a gray tonalite counter than a pink granite one. But for this show, at the edge of prehistory, this color difference tells us a story. The key players are those pink crystals in granite. If you really want a name, they’re called orthoclase, BUT that is not on today’s test. We’ll do a deeper dive in a later episode. Just remember that granites are usually pinkish.

As a magma chamber cools, the pink crystals in granite are some of the last to form, very late to the party. Furthermore, our pink pals are picky- they can only form in magmas that have been repeatedly recycled upward from the mantle into seafloors into continents. Check out Episode 12 for more detail on that cycle. In short, a pink or red or brown granite takes a lot more work, a lot more refinement than dull gray tonalite. If our old friend tonalite is grape juice, granite is wine.

I’m not ragging on tonalites here. They’re crucial rocks in Earth’s early history, and we’ve seen them in Episodes 26, 27, and 30. For example, Earth’s oldest surviving rock, the Acasta Gneiss, was once a tonalite. Back to today’s episode, most of Greenland’s Itsaq Complex, that great stone banner, was once tonalite. This rock will be a constant companion for the next few seasons. You’ll grow sick of hearing me say it. In short, tonalite was extremely common on the early Earth, but is relatively rare today, replaced by its more popular cousin granite.

So what changed? What does tonalite tell us about Greenland 3.9 billion years ago? For that answer, we need to return to the original magma chambers of their birth.

When most people think of magma, they imagine a thick, bubbling, red-hot liquid, deadly to the touch. And they’re absolutely right! A geologist would see and feel the same thing you do. But there’s another way that geologists think of magma, one that gives us even more information. To us, that pot of magma is a giant stew of minerals and chemicals, one whose recipe changes as rocks melt and recrystallize.

To start, I want you to imagine a rock sitting in a pot. As we turn up the temperature, the rock begins to melt. Softer, wimpier crystals melt first, while tougher crystals stick it out until the bitter end. If you removed some of that very early partial melt with a ladle, it would have a different chemistry, a different “flavor” than the later, completely molten rock.

Now let’s cool the temperature back down. The hot magma begins to freeze into different crystals at different times: tough ones first, wimpy ones last. If you plucked out the very first crystals as they cooled, they would have a different chemistry from the later, completely recrystallized rock.

These ideas have names in geology: “partial melting” and what I call “partial freezing”. For sticklers, the freezing process is actually called “fractional recrystallization”, but I think partial freezing describes it far better for non-science folks. If you want more info, Episode 26 is dedicated to partial melting, and 27 is for partial freezing.

Let’s bring this back to the oldest Greenland tonalites, 3.9 billion years old. We can’t watch these ancient rocks melt and freeze from scratch, but we can look for clues in their chemistry. Just like certain crystals melt more easily or freeze more easily, certain elements on the periodic table jump immediately into magma while others cling desperately to the last remaining crystals until they’re forced into the pool.

In short, the chemical patterns of tonalite can tell us where it melted and where it froze, and if there are any modern spots where we see the same patterns. The smallest elements can tell stories about the birth of continents.

Part 3: The Great Carbuncle

In the 1960s, a Kiwi geologist named Vic McGregor began to untangle the contorted Greenland rocks, the “Play-Doh” knot we made earlier this episode. Vic realized that the oldest rocks might be older than any known at the time, but he needed dates. He sent the samples to Oxford, where they confirmed his suspicions, kicking off a string of publications dating these incredibly old rocks.

The year is now 1974, a decade after Vic’s research began, and we’re back in Oxford, England. The initial rush of dating has cooled down (as it does for many relationships), the crew now turns to deeper questions beyond age alone, questions of chemistry.

In all honesty, the chemical nitty-gritty is less important than the larger story, but I do want to highlight a group of elements we’ve met before and will meet again many times: the rare earth elements. I’ve described the rare earths as 17 sisters, 17 soft metals that behave similarly but have their own unique quirks. Despite their relative rarity, these elements are increasingly important ingredients for technology, from electric cars and smartphones to cancer treatments and lasers.

Narsaq, southern Greenland, the site of the Kvanefjeld rare earth element deposits and political controversy. NOT where our old rocks are this season.

For those of you up on the news, hearing the words “Greenland” and “rare earth elements” might make your ears perk. A huge mine on Greenland’s southern tip has been in the geopolitical crosshairs for decades. It’s a long, tangled, and honestly tense story that weaves together nuclear physicists like Niels Bohr, Chinese mining firms, the 2021 Greenland election, environmental concerns, and this year, heightened interest from the U.S. government. If I start a Geology in the News series, this would probably be the first story. Otherwise, we’ll see these contentious rocks in Season 7: September on the Earth Calendar.

My point is that we’ll talk about rare earths in Greenland today, but they’re NOT the newsworthy ones. Mining them in Itsaq would be a total waste. Scientists must sift the old tonalites with a fine-toothed comb to get anything. Which brings us back to Oxford in the 70s. Keith O’Nions and Robert Pankhurst are looking at a chart of rare earth elements from their samples 3.9 billion years old. They’ve laid out the 17 sisters all in a row, lightest on the left, heaviest on the right. They see a clear, repeatable signal: these ancient tonalites had lots of light elements, but very few heavy ones. Imagine a bar chart with the highest bars on the left, getting smaller and smaller to the right. Something had stolen those heavy elements away.

Now you the listener might be bracing yourself: whenever I bring up a new dataset, there’s usually a heavy debate, one that’s still unresolved. What will they fight about this time? Well, you can unclench yourself, I’ve got some rare good news: When the Oxford bunch saw this rare earth data, they knew exactly what was going on. After decades and dozens of new samples, scientists see the same signals and reach the same broad conclusions. Peace, love, and harmony have finally been achieved, so what do we all agree on?

I’m going to answer that question visually first, then chemically. Let’s return to our imaginary rock in the pot. This time, we don’t have just any old rock, this rock is scattered through with beautiful, dark red crystals. As we crank up the heat, wimpier minerals start melting into magma, but these red gems remain stable, cool as a cucumber. We’ve only melted part of this original reddish rock, but that’s OK, it’s all we need. If we remove that little bit of early magma to another pot and let it cool, voila, it turns into dull gray tonalite. Meanwhile, all those hardy red gems remain in the first pot.

If we zoom in to the chemical scale, we would see these beautiful red crystals have hoarded all the heavy rare earth elements behind for themselves. Very little of these heavy elements are left for our poor dull tonalites, which is exactly what the Oxford crew saw.

If you’re wondering where this is all going, let’s take a step back. The big question we’re trying to answer is: how did the Greenland tonalites form, 3.9 billion years ago? We know they formed underground, inside boiling magma chambers. We’re trying to get more specific: was this magma below the ocean or a continent, was the crust pulling apart or crashing together? The chemistry tells us what minerals were around, and the minerals tell us where this was all happening.

To wrap up this episode, let’s meet this red mineral, and learn where it forms. We first met this gem way back in Episode 7 in the early magma ocean. I mentioned it would be a character actor, appearing infrequently but dramatically. This old friend is garnet.

Garnets from Fujian, China.

Garnet is a mineral most folks are familiar with. Here are a few random facts before getting into the science. Garnet comes in almost every color, but is usually dark red or bright green. As a gem, it’s not as fancy as diamond or ruby, but it can fetch a pretty penny. It’s been used as jewelry ever since ancient times, and has importance or shoutouts in texts from Hinduism, Buddhism, and Judaism. A more poetic name for red garnet is a carbuncle, which you’ll see in literature from Shakespeare to Sherlock Holmes. In more practical terms, garnet is used as an abrasive because it’s very tough. 

Which brings us back to science. Garnet is one of my favorite minerals, and not just because it’s my birthstone. Garnet forms under intense pressure deep, deep in the Earth. Last episode, we took an elevator ride down into Earth’s crust. Garnet begins to form at the bottom of that elevator ride, the real basement of the crust. As a reminder, that’s 40 km or 20 miles deep, hotter than a flame, 10x the pressure of the deepest ocean.

If you see a rock with garnets in it, it likely formed in the deepest parts of the crust. Our Greenland tonalites, 3.9 billion years old don’t have garnets, but their chemistry tells us they were born in the same deep basement. It’s like footprints in concrete: the people are gone, but were clearly once here.

Ok, after all that story, we actually have a location to set our ancient tonalites: magma chambers at the very bottom of the crust, right next door to the mantle below. There are still many questions to answer: what does this deep horizon tell us about ancient plate tectonics? Was it anything like the modern Earth, or was it completely alien? Those are questions for the next episode, which I promise is in two weeks, not another year.

Summary: Greenland contains the largest slice of rocks in Season 2: the Itsaq Complex. This area is a complicated patchwork quilt that formed over a month of the Earth Calendar: February 23 to March 27, 3.9 to 3.6 billion years old. Most of these rocks were tonalites, the dull gray cousins of granite. But don’t let that dullness fool you. These tonalites were born deep in the crust, 40 km down near the mantle boundary. We know this because they have the fingerprints of garnet, a gemstone forged under intense pressures.

Next time, we will learn more about this deep realm, we’ll visit one of our cosmic neighbors for guidance, and we’ll referee one of the biggest debates in early Earth history, in Episode 39: The Shattered Planet.