26: The Magma Forge

Last episode, we met the oldest rocks on Earth- the Acasta Gneiss in Canada’s Northwest Territories. The Acasta Gneiss is not one single boulder sitting on a cold tundra: it’s an area of banded rock the size of a large city. The oldest spots in this wide expanse are 4 billion years old, February 12th on our imaginary Earth Calendar.

If you walk around the Acasta Gneiss, you’ll see black and white stripes everywhere. Sometimes, these stripes are thin and straight like a bar code at a supermarket. Other times, they’re wider than your hand, contorting into snake-like curves. The rocks did not look like this originally. The stripes were forged long after the Acasta’s birth, under intense heat and pressure. The Acasta Gneiss is an area that stared destruction in the face and survived, but not unchanged.

So what was the Acasta before it was Gneiss? Over the next few episodes, we’ll paint a portrait of Earth’s oldest rocks in their prime. Today, we’ll start with the basics: what did the original rocks look like 4 billion years ago, and how do these rocks form today? Along the way, we’ll review some concepts we learned last season- so don’t worry if you’re a little rusty.

Let’s start at the beginning.

Part 1: Taken for Granite

Let’s start by using our imaginations. I want you to visualize a piece of Acasta Gneiss in your mind: a chunk of rock the size of your fist, covered with zebra stripes of dark and light crystals. What we’re going to do is rewind the clock backwards, like an old video tape. As we head further back in time, the stripes disappear into random patterns around the rock. When we reach 4 billion years ago, the fresh-faced stone has a salt-and-pepper texture, with large speckles of white, black, and gray.

This is the Acasta before it was squeezed into gneiss. Looking at this rock, you’d probably say “That looks a lot like granite”. And you’d be very close, but it’s time to meet a new rock for our collection.

We’ve met granite a few times already, in Episodes 2 and 13. Granite is born in magma chambers: huge pockets of molten rock deep in Earth’s crust. As the magma slowly cools, pink, black, gray, and clear crystals the size of coins start to form- crystals you can see in any granite countertop.

Granite is not the only pale rock born inside magma chambers- it has a lot of close siblings: rocks that have slightly different recipes but look essentially the same. Together, granite and its’ siblings are called granitoids. The granitoid we’ll meet today, the origin of the Acasta Gneiss- is called tonalite.

So what makes tonalite different than plain old granite? If I placed them side by side, the most notable difference is color: many true granites have pink minerals, while tonalites are dull gray with more dark crystals. These colors are telling us a story: pink crystals are some of the last to form inside magma chambers- you need to process magma a lot to get any pink out of it. Therefore, slightly darker gray tonalites are a bit more primitive than their pink granite siblings.

There’s one other difference between tonalite and granite: quantity. There’s a reason granite is so famous and tonalite, well, isn’t. If we zoom out and scan the globe, granite forms the backbone of the continents, around 30% of the planet’s surface. In contrast, tonalites are far more rare, only popping up in certain mountain chains and islands. For example, the word tonalite comes from the Tonale Pass in the Italian Alps- it’s a famous ski resort that used to be a boiling magma chamber deep underground.

But while tonalites are rare today, that wasn’t always the case. Tonalites were far more common in the Archean chapter of Earth’s history, and they will be our close friends for the next four seasons. They will tell us the story of how continents rose up from the sea- how dry land came to planet Earth. You may not have heard of tonalites before this show, but without them, Earth would still be a waterworld.

So how exactly does tonalite form today?

Think of tonalites like shopping malls- they were huge and important in their day, and some are still around, but they’re echoes of a bygone era.

Part 2: The Crust, Revisited

The last time we talked about the crust was in Episodes 12-13. If you’re new, or if it’s been a while, I would really recommend brushing up on those sessions before continuing. For those who just can’t wait, here is a two-minute summary of Episode 12- how Earth’s crust forms today.

Everything starts with the mantle, the layer beneath the crust. The mantle is extremely hot, but most of it is solid thanks to intense pressure deep within the Earth. At the very upper edge, however, the mantle is less pressurized, more relaxed, and importantly- still hot. As the pressure eases up, the solid mantle melts into liquid magma.

Now here’s the important part, where things really get interesting. Rocks do not melt all at once: wimpier crystals melt first, while tougher crystals stick it out until the bitter end. This idea is called partial melting. Think of it like melting chocolate chip ice cream on the ground- the ice cream will melt much earlier than the chocolate chips and start flowing away.

Let’s take the idea of partial melting back to the mantle. Some crystals melt right away, boiling up to Earth’s surface, while others don’t care at all, staying behind in the mantle. The few escapees, the molten bunch, burst up to the surface… and are quickly frozen back into rock again near the cold seafloor.

But this ocean rock is not the same as the mantle it left behind. Remember, we only melted a few mantle minerals, not all of them. It’s like re-freezing our melted ice-cream as it flowed away- there would no chocolate chips left. You can see the difference between mantle rocks and ocean crust with your eyes. Mantle rocks are green, dominated by our old friend olivine. In contrast, ocean crust is black- the best example is basalt.

Now what about dry land, what about continental crust? What about our new friend tonalite?

There are a few ways to make continental crust, but for now let’s stick with the most familiar: partial melting. This time, our starting ingredient is ocean crust. All ocean crust is eventually destroyed, pulled back down into the mantle at ocean trenches like the famous Mariana Trench. As we heat up these dark rocks, the wimpier crystals will melt away and boil upwards to the surface. As this new generation cools back down, they form pale rocks, white, pink, and gray. These light rocks form many volcanic islands, as well as the continents- granite and tonalite.

In short, when you partially melt the mantle, you’ll get  crust like basalt. If you take that ocean crust and partially melt it again, you’ll get pale islands and continents- granite and its’ cousins. Each step is less dense than the last, which is why continents are higher up than the seafloor.

Phew! All right, that’s the basic version of modern crust formation.

Part 3: Crystal Memories

In 2014, a team of Canadian researchers including Jesse Reimink traveled to the Acasta Gneiss. Plenty of folks had visited in the previous 30 years, but this crew was searching for the best samples they could find- the crème de la crème in a sea of altered stone and pine forest.

Eventually they hit a jackpot: small pockets of our new friend tonalite. The tonalites were so well-preserved, they earned their own name- Idiwhaa (eee-dee-wha), or “ancient times” in the local Tlicho language. Technically the Idiwhaa Tonalite is still gneiss, just less altered, but I’ll just call them tonalites for now.

Most importantly, the Idiwhaa Tonalites held a treasure trove of zircon crystals. Last episode, we used zircons as time pieces to date the Acasta Gneiss. Today, we’ll use them as time capsules, pockets of information about the ancient world.

The zircons were as wide as a human hair and could only be observed under a microscope. Despite their tiny size, they started out even smaller. Like crystals of ice, salt, or sugar, zircon crystals grow over time, gathering ingredients from surrounding magma. These growth patterns can be seen on a microscopic scale, like tiny tree rings around a central core. The core formed first, the outer rings later.

Looking inside a time capsule gives us glimpses of the past: toys, books, and pictures that tell us how people lived and felt. If we open the zircon time capsules of the Idiwhaa, what do we find inside? We see elements- atoms from the ancient world that changed as they passed through different environments.

We learned about two elements when we tackled the Jack Hills zircons in Season 1. In Episode 13, we met the element hafnium, which helps trace the origins of magma chambers. To see how, let’s revisit Part 1 and the idea of partial melting. When a rock begins to melt, some elements prefer to be in liquid magma, while others remain behind in the solid crystals. Hafnium prefers the magma, and escapes towards new adventures in new rocks. This means that as the green mantle turns into black ocean crust, and eventually into pale islands, each stage will have more hafnium.

Here's where things get interesting- the Idiwhaa tonalites are pale, but when we peek inside their oldest zircons, we find hafnium levels closer to darker oceanic rocks. These zircons are acting like a memory bank, remembering the dark parent that recycled itself into the pale daughter of tonalite. The parent was destroyed, but the child survived and still remembers what it was- that’s the idea hidden in your back pocket.

Finally, there’s a far more familiar tool in our time capsule, one we met in Episode 15: oxygen. Oxygen does far more than pass through your lungs- it really gets around. Oxygen is a building block of water, living things, and many minerals like zircons. As oxygen travels from one place to the other, it changes and leaves clues of where it’s been before. For example, the Jack Hills Zircons from Season 1 have the fingerprints of cool liquid water. In other words, the rocks that partially melted into the Jack Hills magma chamber had clearly interacted with the ocean.

What about the Idiwhaa zircons? Unsurprisingly, they also tell us that Earth had an ocean- it’s good to know it didn’t disappear on us.

As I mentioned at the start of the episode, this session was dedicated to showing the similarities between the Idiwhaa Tonalites and the modern crust- dark rocks recycling into light ones, with water playing across Earth’s surface. If you want more detail, I would again recommend Episodes 12 through 15, where we tackled similar issues in the Jack Hills Zircons. Now that we’ve reviewed the rules, it’s time to start breaking them.

Summary:

The oldest rocks in the Acasta Gneiss are the Idiwhaa tonalites- dull gray cousins of granite- 4 billion years old, February 12th on the Earth Calendar. These tonalites were recycled from much older, much darker stones- most likely basalts below the Hadean sea from Season 1. As we move forward, tonalites will form the first building blocks of today’s continents. We know these stories by looking at elements trapped in zircon crystals: hafnium tells us about crust recycling, while oxygen tells us about water.

Next episode, we’ll learn a secret, alternate recipe to make continents, a recipe that formed the very oldest rocks in the Acasta Gneiss. This recipe can only be recreated in one of the strangest places today. Stay tuned to find out just where that is.