37: Metamorphosis

Last episode, I introduced you to Greenland, this season’s final destination. Specifically, we’re looking at a strip of rocks in southwestern Greenland. The rocks start around the port of Nuuk, Greenland’s capital, and stretch inland for 150 km, or 100 miles. The rocks are diverse in form and age: from ~3.9 billion to ~3.6 billion years old, late February to mid-March on our imaginary Earth Calendar, including the youngest rocks we’ve yet seen.

On this show, we’re no strangers to long distances and deep time. But there’s a third variable we need to address today, one I’ve been putting off for a while: alteration. Every rock on Earth has been altered in some way since it was formed. Most stones have been buried and squeezed beneath Earth’s surface by pressure. Many are also cooked by temperature, either deep underground, or next to volcanos. Finally, water can also modify rocks- not just wearing them down, but changing their very ingredients.

The older the rock, the more likely it’s been altered by heat, pressure, water, or all three. For example, the oldest rocks on Earth, the Acasta Gneiss, have been distorted almost to oblivion. As we move forward in time, rocks will become more pristine, which brings us to Greenland. Many rocks in Greenland are extremely altered, but a few pockets have survived in decent condition. They’re still old, still messed up, but in better shape than their neighbors.

Today, we’ll compare these neighbors: why are some areas of the ancient Earth relatively pristine, and what’s happened to the others? We’ll start by learning how rocks literally transform beneath our feet, then we’ll meet a few key steps in those transformations, and finally we’ll return to Greenland armed with new knowledge.

Part 1: The Stone Butterfly

The process of one rock transforming directly into another is called metamorphism, just like a caterpillar metamorphoses into a butterfly. We briefly learned about metamorphism way back in Episode 2, and we’ve touched on it every now and then. Now, it’s time to level up our knowledge.

First, let’s set two ground rules. There are many ways to change a rock but they’re not all metamorphism.

Rule #1: Metamorphism (usually) does not melt a rock. We’ll crank the temperature up hundreds of degrees, but we will not make lava, we will not make magma. If we did, we’d be making igneous rocks around volcanos, like our old friends basalt and granite. The most extreme metamorphism will push rocks to the very edge of melting, but will not push them all the way.

Rule #2: Metamorphism does not break rocks into loose sediment. On Earth’s surface, water grinds rocks down into pebbles, then sand, then mud, the source of sedimentary rocks like sandstone or mudstone. There is also water deep below our feet, but it does not form lakes or rivers. Instead, intense pressure squeezes water into microscopic cracks between crystals. This squeezed water can still flow, but instead of grinding crystals down, it changes their chemistry, as we’ll see in a second.

In short, metamorphism does not melt rocks, and it does not break rocks. It literally transforms them from one solid into another. This can be hard for us surface-dwellers to imagine. Even my usual kitchen analogies start to fail us: metamorphism is not like baking a cake, since we’re cooking liquid batter into a solid. Making toast is closer, since you start with solid bread and heat it into solid toast, but metamorphism is even more extreme. Imagine you put bread into a toaster and got back a cookie, a slice of ham, or a piece of cardboard. That’s metamorphism.

There are many different types of metamorphism. It can happen when a meteor strikes the Earth, instantly applying pressure. It can happen near volcanos, with heat baking but not melting rocks. But most rocks are metamorphosed deep, deep underground, with a combination of heat, pressure, and water.

Let’s start with sandstone and see what happens as it changes. If you look at plain old sandstone under a microscope, you’ll see lots of round quartz grains with empty pockets of air in between. As the sandstone is buried, pressure squeezes those grains and empty pockets together, and something strange happens at their borders.

The edges of the round quartz grains begin to dissolve. They’re not shattering apart, and they’re not melting, though temperatures are rising. The crystals are literally leaking atoms from their outer edges, exiling them beyond their borders. Here’s where water comes in- remember, water is also squeezed into these tight cracks with these exiled atoms. Water carries the atoms away into empty pockets underground, making news quartz crystals as old ones shrink down.

Bigger crystals gobble up their smaller neighbors, and eventually the small, round sand grains have turned into larger geometric shapes. So ironically, as pressure squeezes a rock tighter, the crystals inside become larger- they’re literally reshuffling their ingredients.

Ok, that’s what happens when a rock is made of just one mineral, like quartz? The crystals grow bigger, but their basic recipe hasn’t changed. Quartz in, quartz out. But most rocks have many different minerals inside- just check out a granite countertop. You’ll see pink, white, gray, and black minerals inside. Each crystal has a different recipe, a different set of elements like iron, silicon, etc.

What happens when we squeeze this diverse crystal jambalaya together?

Just like our sandstone, crystals usually grow larger. But something even weirder happens. Unlike last time, we have different minerals squeezed against each other: dark minerals filled with iron, and pale minerals filled with silicon. Slowly, very slowly, these different crystals react with each other. Sometimes they swap atoms across their borders. Sometimes they kick atoms out entirely, where water carries them away.

In either case, this mixing and matching forms new recipes, new crystals that can only form under intense heat and pressure. Sometimes these new minerals are squeezed flat like pancakes, other times they’re large, blocky, and dense. We will meet many of these crystal characters in future episodes.

Metamorphism not only changes chemistry, it changes the orientation of a rock, its’ physical patterns. Pressure squeezes a rock like an accordion, making a series of crinkled layers. Some minerals handle the pressure better than others, and eventually light and dark crystals are separated into zebra stripes, just like the Acasta Gneiss we met in Episode 25. Greater pressures usually mean more distinct layers.

Let’s recap: metamorphism changes rocks in three ways: small crystals grow larger, completely new minerals are formed, and the entire rock’s structure is usually squeezed into layers. There is a lot more nuance to this story: metamorphic geology is a huge field of study. But now that we have the basics down, the microscopic details, let’s zoom back out.

What are some metamorphic rocks we’ll meet going forward? And what do they tell us about Earth’s past? To meet these players, we’ll need to take an elevator ride going down.

Part 2: Three Recipes for Metamorphic Rocks

There are many different metamorphic rocks, and it’s easy to see why.

Think of all the rocks you can name: basalt, granite, sandstone, mudstone, limestone, etc. You can probably list a dozen or so rocks. If you bake each of them, you’ll make a dozen new types of rock, a dozen more names. If you squeeze each of them, you’ll make a dozen more. If you bake AND squeeze them… you get the idea. With metamorphism, the handful of familiar rocks we know and love explodes into more than a hundred different types. If that seems like too much to handle, don’t worry- most geologists think so as well. So today, we’ll look at three broad categories, three new names instead of 100. We will use these names as a helpful shorthand going forward.

All the rocks we’ll meet today have been baked and squeezed deep in the Earth. We’ll start with the least altered and work our way deeper down. The deeper you go, the hotter it gets, and the more pressure you feel.

So how deep do we start changing rocks?

In most places, the temperature rises ~20 C every kilometer down. For my US listeners, that’s ~100 F for every mile down. That’s just an average. For example, the world’s deepest mine is in South Africa, the Mponeng Gold mine. The mine is over 4 km, 2.5 miles deep. For many places on Earth, the mine would be literally boiling, uninhabitable. But South Africa is cooler: the rocks are only 150 F, or 70 C. Ice water is pumped down to cool things off, but the air still feels like a hot summer’s day.

And this mine has only scratched the surface. To start truly changing rocks, to start metamorphism, we need to go much deeper, 10 km, 6 miles down. We are reaching the edge of humanity’s fingertips: the only hole this deep is the Kola Superdeep borehole dug by the Soviets in 1979- check out Episode 6 for more info. No human has ever been 10 km deep and no human is likely to go for a long time.

Temperatures here are three times the boiling point of water. The pressure is twice that of the deepest ocean, 2000 times the pressure you’re experiencing right now. Welcome to the shallow end of metamorphism, the kiddie pool if you will, the first stop on our elevator tour.

Stop 1: Greenschist.

The rocks here are about the least-metamorphosed you can find. Many still show their original features- ripples in sand, bubbles in old lava, and if you’re lucky- fossils.

But they have been altered: new minerals have grown, changing their textures and color. These rocks are called greenschist. Not all greenschists are green, but it’s the most common color change, especially for volcanic rocks. By this point in the podcast, I’ve trained you to associate green with one mineral, our old friend olivine from Season 1.

However! Olivine is not the culprit today. There are many other green minerals in the world. We’re not going to discuss them now (three names is enough for one episode), but there is one thing I want you to remember about greenschist.

From now on, whenever I say greenschist, I want you to think of a green stoplight. Green means go, green means good. Remember, these are the least-altered rocks we’ll see, the best snapshots of the ancient Earth. When I say greenschist, you’ll hear stories about the Earth’s surface: oceans, volcanos, and fossils. As a scientist, when I hear the word greenschist, my heart starts racing- I know that these rocks are the best spots to find Earth’s oldest fossils.

We have not met greenschist on our travels yet, but keep your eyes or ears peeled.

Stop 2: Amphibolite

We’re now twice as deep as greenschist: 20 km, or 12 miles down, well below the reach of humans. Here, the temperature is hotter than a tandoori oven, 600 C, 1000 F. The pressure is 6 times the deepest ocean, more pressure than the chamber of a heavy machine gun.

Don’t worry, I’m not going to quiz you on the numbers- we’re more interested in the rocks. This is the realm where rocks start to act like silly putty: they’re still solid, but they begin to bend and warp, forming new layers. At the same time, new crystals are forming, usually darker gray, black or green. These rocks are called: amphibolite. Amphibolite is a mouthful. Try saying it a few times just to get it down. Hopefully you’re not doing this in public, but who am I to judge?

OK, but what does amphibolite mean?

Whenever I say amphibolite, I want you to think “eh, could be better, could be worse.” There’s a word for this iffy feeling in the English language- that word is ambivalent. When you’re ambivalent about something, you could go either way, like trying to pick lunch. If the word ambivalent sounds like the rock amphibolite, that’s not a mistake. Both words are related: in ancient Greek, amphibolite means unsure, doubtful, ambiguous, ambivalent.

There’s a reason for this name: amphibolites are rocks that can go either way. Sometimes they preserve decent details of the ancient Earth. Other times, they’re pretty messed up. I have seen ancient fossils in amphibolite rocks, but they’ve been through the ringer.

On the podcast, we’ve seen amphibolites before: in Nuvvuagittuq, Quebec (Episodes 30-33). Here, they preserve lava flows, seafloors, and maybe microfossils. They will also play a large role in Greenland.

Stop 3: Granulite

We’ve reached the deepest spot on our tour: 40 km or 20 miles down. At this point, temperature and pressure are just giant numbers, but for due diligence: it’s 800 C (1500 F), about the hottest a bonfire can get. The pressure is ten times that of the seafloor.

We’re reaching the outer edge of metamorphism. Raising the temperature a few hundred degrees will melt the rock into magma, and we don’t want that. Nearly all original textures of these rocks are obliterated. Old crystals have grown larger, completely new crystals have popped in, and all these minerals have been squeezed and twisted into layers like modeling clay. So what the heck can we learn from these guys?

These rocks are called granulite. When I say granulite, I want you to think grand-scale geology, big-picture stuff like plate tectonics and the growth of continents. Granulites can only form under intense heat and pressure, like in deep continental crust, or when mountains form as continents crash into each other. We’ve seen granulites before in the Acasta Gneiss, the oldest rocks on Earth, which told us stories of the planet’s early crust.

Personally, I don’t work on granulites. My specialty is on Earth’s surface, the realm of life and oceans. Granulites take the fossils I love and obliterate them. To be honest, I’ll be learning as much about granulites as you do moving forward.

All right, that’s three new metamorphic faces, let’s review. Greenschists are the greenest pastures, the best-preserved rocks for surface stories like fossils, volcanos, and oceans. When I say greenschist, you should be getting excited. Amphibolites are more ambivalent, more mid. Consider amphibolites the baseline of ancient rocks- could be better, could be worse. Finally, granulites tell us the grand tales of the deep earth: plate tectonics, mountain building. They’re not great for fossil-hunters like me, but they have stories to tell.

Speaking of stories, let’s put our new friends on the map and return to Greenland.

Part 3: The Babyface

The older a rock is, the more likely it will be metamorphosed. Over billions of years, stones get buried deeper and deeper in the crust, through the three zones we just met. Mountain building can speed up the process, as continents crash into each other like bumper cars. We’ve already seen this trend on the show: the oldest rocks on Earth are granulites, telling grand tales of early tectonics. Our second destination of Nuvvugittuq had amphibolites, altered stories of Earth’s surface. No good greenschists yet.

In early geology, this trend was a useful shorthand to guess rocks’ relative ages, older or younger. If you saw a granulite next to a greenschist, you’d probably guess that the fresh-faced greenschist was younger. It’s like looking at people: you can usually tell who’s older. Today, we know the story is more nuanced, it’s not that simple. Some rocks sit near the surface for billions of years- they’re very old, but they haven’t been buried too deep. Other rocks are buried very quickly under mountains- they’re younger, but they haven’t aged well, to put it nicely.

One of the first people to recognize that nuance was Vic McGregor, our friend from last episode. If you remember, Vic was a New Zealand geologist who moved to Greenland, and his first job was to map the rocks around the capital, Nuuk. McGregor wasn’t the first person to see these rocks, a few other scientists had visited and made notes. Here’s what Vic knew about the metamorphic rocks- let’s put those three new words to use.

Most of the rocks around Nuuk were grand old granulites, the most messed up a rock can get. But there was also a wide path of other rocks, stretching like a huge highway from the coast to the interior. This strip was made of ambivalent amphibolite, not the best-kept rocks on Earth, but better than their neighbors.

At first glance, the case seems open and shut. The amphibolite strip was less messed-up, so it’s likely younger. But as Vic walked around the landscape, taking copious notes, he became less sure. He found that the gnarlier granulites were cutting into the more pristine amphibolites, like an axe into a tree. That could only be true if the amphibolites, the better-preserved rocks were older. The amphibolites also had a series of large scars that were absent in the surrounding granulites.

For context, this would be like seeing two men: one with gray hair and the other with black hair, and finding out that the gray-haired geezer is actually younger. But that’s what Vic concluded, that the gnarly granulites were younger than the fresh-faced amphibolite. This is why Vic really wanted his samples dated, he wanted to know their real ages.

The person who dated these samples in 1971 was an Oxford researcher, Lance Black. His supervisor, Stephen Moorbath, was heading out on holiday when the Greenland samples came in. Moorbath was convinced nothing exciting would turn up- he was so sure that he promised Black a bottle of wine for every sample older than 3 billion years. When Moorbath returned, he owed Black eight bottles of wine.

If you had a bottle for every old Greenland date since 1971, you’d have a very full wine cellar. McGregor was right: the baby-faced amphibolites were paradoxically older, February to March on the Earth Calendar. The surrounding, messed-up granulites were much younger, May on the Calendar, Season 4.

This news was incredibly exciting in 1971: not only did Greenland have the oldest rocks known (at the time), they were decently well preserved. Not the best, but good enough to study. And the news got even better. In 1971, a mining company invited Vic McGregor and Stephen Moorbath (the wine-giver) to the edge of the Greenland Ice Sheet. There, they would find something that should make your heart race with excitement. They found the oldest greenschist on Earth, but that’s a story for another day.

Summary:

Metamorphic rocks are often overlooked by most geologists, but for those working on Earth’s earliest days, they’re the only things to work with. Beggars can’t be choosers. Some rocks like greenschists still tell many stories of the surface world, about changing oceans, volcanic eruptions, and even life’s evolution. Some, like granulites have been twisted beyond recognition, but those changes tell deep tales of continental growth and mountain building. And many, like amphibolites, fall somewhere in between.

These three rocks will be close companions for the next few seasons, including this final arc of Season 2. It’s impossible to talk about Greenland without them, and now we have the tools for deeper conversation. Next time, we’ll start at the beginning of Greenland’s history, 3.9 billion years ago.

36: The Ghosts of Greenland