39: The Shattered Planet

Episode 39: The Shattered Planet

Before we start, some housekeeping. I want to thank everyone who donated to the show these past two weeks. Whether it was $1 or more, I truly appreciate your generosity. If you feel like dropping something in for the first time, or again, there’s a donation button in the description. Don’t worry, I’m not going to do this every episode, but I want to get the word out while it’s fresh.

I also want to thank everyone who took the audience survey- we had over 70 responses! If you haven’t taken it yet, there’s a link in the description. Here’s the summary: I’m going to start a Patreon for small monthly donations, hopefully up within two weeks. There will be extra perks like Geology in the News, Mail Time, and voting for Bonus Topics. Merchandise is coming, but that’s many months away. As expected, ads are divisive: for now there will be none. Finally, your overwhelming favorite way to support the show is hitting the donation button in the descriptions, when you can. If you want more details, I’ll post a short update soon.

Finally, I have a big announcement to make: I have a new research paper, published in the journal Nature. For a geologist, that’s the most prestigious place to publish, the highest rung you can climb. On this show, I’ve mentioned many Nature papers, including: Earth’s oldest surviving crystals, rocks, and evidence for liquid water. It is an honor to be published in the same journal, but it was also many years of hard work and patience. I’m not the main author- I’m one of 11 people on this project. The lead author is Laureline Patry, and the project’s supervisor is Stefan Lalonde, both at my old stomping grounds in Brest, France- where I started this show. Special thanks go to Laureline and Stefan for seeing this paper to publication, as well as everyone else. Their perseverance paid off.

Thanks again for your patience, I promise these preludes will get shorter and fewer as we move forward. Let’s return to the show.

In the last three episodes, we’ve examined rocks in SW Greenland. They’re not the oldest rocks on Earth, but they’re pretty dang close, and are much more extensive and well-preserved than previous locations. Here’s what we’ve learned so far: the area is called the Itsaq Complex, that’s I-T-S-A-Q. The Itsaq rocks sit just outside of Nuuk, Greenland’s capital. In age, the rocks fall between 3.9 and 3.6 billion years old, Feb 23 to Mar 17 on the Earth Calendar. This episode doesn’t have a specific date, so I’ll keep using 3.9 billion, the age of Itsaq’s oldest rocks.

Most of the Itsaq rocks have been squeezed, folded, and baked into each other multiple times over like bread dough, obscuring their original details. But in a few secluded pockets, we get a better window on the ancient world. Today, we return to one of the biggest questions in early earth history: when, and how, were the continents built?

Like the origin of life, the origin of the continents is one of those giant topics that requires a divide and conquer mentality. We need to answer smaller questions to get to the big one. Here’s the question for today: Are there any alternatives to plate tectonics? In other words, if we get in a rocketship and visit other worlds, would they also have mid-ocean ridges and deep-sea trenches, or would they have something stranger?

In this episode, we’re going to examine alternative Earths through the lens of Itsaq’s most common rock: tonalite. More than 70% of the Itsaq Complex is tonalite, a regular guest of Season 2. Tonalite is a cousin of granite, with a different color palate. Tonalite is dull gray, while granite is usually on the pink side. Both cousins form deep underground in boiling magma chambers, but as their colors suggest, these magmas have different ingredients and form in different places. Most folks look at these rocks and just see gray vs. pink, but geologists, and listeners like you, can turn those colors into a story. Color tells us about chemistry, chemistry tells us about magma, magma tells us about tectonics and how continents are built.

Today, we’ll learn two recipes for tonalite: one from the modern Earth, and a potential one from another planet. We will not definitively solve the tectonic mystery today. The point of this episode is to show you there are different recipes to make a continent, recipes that will show up in many future episodes. As a reminder, I’m not a tectonics expert, so if you are one, please forgive me if I oversimplify or correct me if I get something outright wrong. Who knows, maybe we can set up an interview!

After today, we’ll move away from tonalite for a bit, back to the surface world. But first, we need to dive into Earth’s mantle.

Part 1: Back in the Trenches

For this first recipe, we’re going make a tonalite on the modern Earth. Last episode, we left on a high note, a rare moment of concord in the contentious world of early Earth research. Most scientists agree that tonalite is born much deeper than granite: in the basement of Earth’s crust, near the mantle. Now we come to heart of the debate. How did the ancestors of tonalite get so deep in the first place?

*A classic garnet, hoarder of heavy rare earth elements.*

But first, let’s review how we know this in the first place. As we learned last episode, the major clue comes from the mineral garnet. Yes, that garnet: the semi-precious gem, the January birthstone. Garnet is not just a pretty face, it can tell us a lot about how continents form. Last time, we learned two important facts about garnet. 1: garnet forms under intense pressure, 15x the pressure at the bottom of the deepest ocean. The best way to reach such pressures is to go even deeper, where the crust meets the mantle, ~40 km or 20 miles deep.

2: Garnet has a unique chemical property. This quirk doesn’t really matter to gem collectors, but it’s huge for geologists. When a garnet crystallizes, it sucks in elements from the periodic table like iron, aluminum, etc. You can imagine garnet like a vacuum cleaner hoovering up tiny atoms and storing them inside. But garnet is picky, especially when it comes to a special group of elements: the rare-earth elements that we also reviewed last time. Before you have traumatic flashbacks to chemistry classes, here’s all you need to know: garnet likes heavy rare earths. That’s it: garnet hoards the heavy stuff all away for itself like a tiny red dragon. This means there are no heavy elements left for any other crystal or rock forming in the deep, just like tonalite.

When scientists look at tonalite, it has very few heavy rare earths, garnet stole them all away. This means tonalite and garnet were once neighbors, very deep below our feet. If you want more detail, check out last episode.

So what is the first recipe for making tonalite, how and where does it happen today?

For that answer, we travel to the other side of the Arctic: Alaska.

If you look at a map of Alaska, you’ll see a thin peninsula sticking to the southwest like a pinky finger. If you follow that line into the ocean, you’ll see a long string of islands forming a giant curve. These are the Aleutian Islands, or Unangam Tanangin in the native Aleut language. The archipelago stretches nearly 2,000 km, or 1,200 miles across. That’s roughly the distance from Los Angeles to San Antonio in the States, from Paris, France to Belgrade, Serbia, or Adelaide to Brisbane, Australia. The Aleutians are a wide, scattered, remote land.

*The Aleutian Islands, highlighted in pink, with Adak Island circled in white. The dark blue line beneath is a deep-sea trench, which feeds volcanic activity.*

If you threw a dart in the middle of this great curve, you would hit Adak (ayy-dack or ah-dahk) Island, depending on who you talk to. The town of Adak has 171 people, and is notable for a few reasons. It is the southernmost town in Alaska at 51 N, but still farther north than the lower 48 states. It’s also the westernmost municipality in the USA at 176 W. Adak is the same distance from Seattle, Washington as Tokyo, Japan.

OK, that’s enough geography, let’s get back to geology. Why are we here? If you look out over the town, perhaps eating a diner meal from the Adak Soul Restaurant, you would see a tall, snow-capped mountain in the distance. This is not any mountain: it’s a volcano. In fact, the Aleutians have 57 volcanos in total. This is the northern rim of the Ring of Fire, a chain of volcanos running around the Pacific including New Zealand, Indonesia, Japan, Mt St Helens, Mexico, and Chile. OK, a little bit more geography there, but there’s a point.

All these places are known for major, usually deadly volcanos and earthquakes on the surface. But if you look just offshore from any location, you’ll see the hidden culprit: deep-sea trenches. We covered trenches at length in Episode 31, but here’s a refresher. A trench happens when a piece of old ocean crust smashes into another piece of crust in a tectonic sumo match, a younger ocean or a continent. The older ocean crust is usually cooler, thinner, and denser than its opponent, and gets pinned down, deep down below Earth’s surface into the mantle. Imagine shoving your palm beneath a blanket or a carpet. Your hand is the heavy ocean plate going down, the blanket is the lighter crust riding high. The crack in between is the deep ocean trench.

*Cross-section of a trench, with the buried plate fueling magma chambers and volcanos above.*

Here's how trenches usually make volcanos. When an ocean plate sinks down, it’s not just taking rocks along for the ride. It’s also soaked through with seawater, like a giant, rocky sponge. This seawater reacts with the surrounding mantle, literally melting it into magma chambers which form volcanos. Imagine the mantle like peanut butter: it’s solid but flows slowly given time. If I add a bunch of water to that peanut butter and mix it up, it’s going to turn into a soupy, runny, low-budget peanut sauce. Mixing seawater and the mantle is obviously more complicated, but that’s all we need to know for today: water + mantle = magma. That’s the usual recipe for making volcanos near a trench, like Mt Fuji or Mt St Helens. See Episode 31 for more details.

Back in Alaska, Adak Island sits near a trench, like the rest of the Ring of Fire. But when geologists first looked at Adak’s dull gray volcanic rocks, they found a strange chemical signature. Strange for them, but familiar to us. Just like the Greenland tonalites, Adak rocks were missing heavy rare earth elements, something had stolen them away. There was only one possible culprit: garnet! Here’s the difference between Adak and other trenches on the Ring of Fire.

Everywhere else, seawater + mantle = magma. Below Adak, there’s another ingredient: that old ocean crust sinking deeper and deeper down. As that old rocky seafloor sinks, it gets squeezed and pressurized, the perfect place for garnets to form. The tough garnets stay behind in the deep, the wimpier minerals melt away and bubble up into magma chambers and volcanos. In short, the chemical fingerprints of Adak’s volcanos tell us that the seafloor is melting into the mantle deep below.

Similar rocks called adakites are found around the Ring of Fire: but they’re still uncommon. Adakites are not quite tonalites- there are a few other differences and debates, but for this show, they’re close enough. Which brings us back to Greenland 3.9 billion years ago. Some scientists think that Greenland tonalites formed volcanic islands around deep-sea trenches, just like Adak Island. If so, the early Earth could have been very similar to the modern day. OK, no people or trees or fish or breathable air, but islands and early continents were forming just like today. Case closed.

But there’s more than one way to make tonalite, and more importantly, islands and continents. To see an alternative model, we need to visit an alternative planet.

Part 2: The Evil Twin

I’m going to ask you a question, and I want you to answer with the first thing that comes to mind. Here we go: What’s one thing that makes Earth different from any other planet in the Solar System?

I’ll take many answers to this question: an atmosphere with breathable air, vast surface oceans of liquid water, life as we know it. But there’s another answer that most folks don’t think of, at least not right away. No other planet has plate tectonics quite like Earth. As we learned in Episode 12, Earth’s surface is shattered into many separate plates: some large, some small, some with continents, some beneath the sea. Each plate is crashing into each other or pulling away like giant, slow-motion bumper cars.

I can’t stress this enough: No other world in the Solar System behaves like this, at least not today: All our rocky neighbors have a single solid sheet of crust, sometimes cracked, but never broken into moving plates like Earth. There’s a name for worlds like this, with a single, unbroken crust: they’re called “stagnant lids”. I personally think the word stagnant, which means inactive, sluggish, dull, is misleading and slightly mean. Sure, their crust isn’t drifting around like Earth, but these worlds can still be active, dynamic places. At this point, most science shows talk about Mars, with huge volcanos and canyons, but today we’ll visit somewhere even weirder and more alive, tectonically speaking: Venus. Before we learn about its’ rocks, let’s get an overview.

Venus is the second planet from the Sun. In some ways, it’s a better Earth analogue than our other neighbor Mars, which gets a lot more attention. Venus and Earth are almost identical in size and they both have thick atmospheres with major clouds. Venus’ size, clouds, and closeness to the Sun, make it the brightest object in the night sky after the Moon, poetically called the Dawn Star or Evening Star.

The thick clouds on Venus make it hard to see the planet’s surface. Early researchers guessed that with all those clouds, Venus must be a rainy, wet, jungle world, inspiring many science-fiction tales. Most stories are very pulpy- think damsels in distress from slimy aliens being saved by burly American men. But there are some real classics: my favorites are by Ray Bradbury, a man far more famous for his Martian Chronicles. Bradbury’s Venus stories were set in a world with constant showers. Two short stories: “The Long Rain”, and “All Summer in a Day”, are unnerving glimpses into tortured human minds straining under no sun and unrelenting rain. I highly recommend them- they’re good, but heavy.

Astronomers finally pierced the clouds of Venus with radar and even a few physical machines. The surface has no aliens or damsels or Americans but are a few interesting features. First, Venus has a few continent-sized plateaus surrounded by low, wide plains. There are no oceans, but if you flooded Venus, it would look far more like Earth than if you flooded Mars. I’ve put maps on our website, bedrockpodcast.com.

*Elevation map of Venus (false-color). The large, high regions are called “terra”, like Ishtar Terra in the north, the size of Australia*

But there are a few critical features missing. Unlike Earth, Venus has no evidence of plate tectonics, no deep trenches smashing plates together like the Ring of Fire, no seams splitting the crust apart, like Iceland. If you pressed fast forward on the Earth Calendar, Venus’ continents would rise and sink vertically like corks, but would not drift laterally like South America breaking away from Africa.

Finally, Venus’ surface has far fewer craters than other planets or moons - something erased them away. On Earth, craters are erased by liquid water and life, but we don’t see these on Venus. Instead, the surface has been reshaped by volcanos- lots of volcanos. By counting the craters on Venus and estimating their timing, geologists think there were once huge basalt eruptions covering the entire planet in lava, only half a billion years ago. That’s after this show’s final season, not quite dinosaur age, but pretty close! Astronomers found evidence for smaller eruptions on Venus just last year in 2024. They didn’t witness the eruption, but when they scanned the ground, they could see new lava flows that weren’t there before. Pretty cool stuff!

There are many more stories I want to tell about Venus, that the surface is hot enough to melt lead, that it has acid rain and metal snow, that the sun rises in the west and its day is longer than its year. But those are stories for future episodes. Consider that a teaser.

For this episode, here’s what we need to know: Venus is a dynamic, geologically active world with volcanos and a few continents. On Earth, these usually form by plate tectonics. Tectonics is literally the bedrock of modern geology, you can’t describe today’s world without it. Venus uses a different playbook, and a few scientists argue that the early Earth 3.9 billion years ago, was similar. So what are the rules? How could we make tonalite on Venus if there are no deep-sea trenches.

Venus might not have trenches or tectonic plates, but it seems to share one feature with Earth: “mantle plumes”. We learned about plumes in Episode 27: they’re thin fingers and blobs of extra-hot mantle: think of a groovy lava lamp from the 1960s. Mantle plumes start near the core, thousands of miles down, and rise up to the crust above, making isolated volcanos on the surface called “hotspots”.

Because mantle plumes are born so deep down, they act independently from plate tectonics. A great example is Hawaii, in the middle of the Pacific. Hawaii is nowhere near the Ring of Fire, nowhere near any plate boundary. The reason Hawaii exists is thanks to a mantle plume, not a trench. The Hawaiian Islands are nothing to sneeze at, but they’re not big enough to make tonalite. Sometimes a hotspot just keeps erupting and erupting. Eventually, this hyperactive plume builds a huge plateau, somewhere between island and continent size. Such features are scattered around the surface of Venus.

The crust gets thicker and thicker, reaching farther and farther down into the mantle. For the same effect, imagine throwing different planks of wood into water: the bigger planks rise higher in the air, but also stretch deeper underwater. The same balance happens as the crust gets thicker- it sticks farther down. Eventually, the basement reaches the deep, high-pressure realm of garnet. Here’s where we can finally make tonalites, the whole reason we started this sidequest. Just like the trenches from Part 1, the wimpier rocks melt into tonalites, leaving the tough garnets behind. The effect is the same, but we got there using very different recipes, like heating a meal on a skillet vs in a microwave.

There’s a dirty secret: scientists haven’t seen any tonalites on Venus yet. Shock and awe, I’ve betrayed all of your trust, I hope you can forgive me. In fact, this alien tonalite recipe can sometimes happen on the modern Earth, though it is even rarer than at deep-sea trenches, and that wasn’t common to begin with. So why did I go to hellish Venus instead of sunny Aruba or the Philippines? As lovely as those islands are, I wanted to introduce the idea of a “stagnant lid” world, an entire planet where plate tectonics as we know it doesn’t matter. Nearly all rocky planets have stagnant lids, some researchers think the early Earth did as well, and they point to Venus a potential analogue. But are they correct?

Part 3: The Key to the Past

Ok, let’s pause and review these two main recipes for tonalites. Recipe 1: Normal plate tectonics: two different plates smash together, making a deep-sea trench. Think of an island arc like Japan. Recipe 2: The stagnant lid: Mantle plumes form a thick, deep scab of crust, no trench needed. Think about Hawaii, but on a much larger scale.

In other words, there’s the “home team”: Earth as it is, and the “away team”: a radically different, alien planet. There’s a word for this debate, a big, gnarly word with a big, gnarly history. That 9-syllable, 17-letter word is uniformitarianism. One more time: uniformitarianism. Let’s break that down. The most important part is “uniform”. In this case, uniform means steady, consistent, unchanging. A uniformitarian idea is that events in the past happened as they did today. For example: if I let go of a pebble today, gravity would pull it down. If I let go of that pebble every day of my life, gravity would still pull it down. Therefore, I can safely assume that if I went back 3.9 billion years ago and dropped a pebble, it would also drop straight down. It almost certainly wouldn’t shoot upwards or sideways or levitate in the air.

Uniformitarian ideas are simple, but powerful. It means I can look at ripples in an ancient sandstone and assume that they formed in the same way as ripples on a modern sandy beach: flowing water, not some mysterious ether that doesn’t exist today. We tested this idea in Episode 31, just without a name. We saw strange, rounded pillows of dark volcanic basalt in northern Quebec, 3.8 billion years old. The only way these blobs form today is when volcanos erupt underwater. Therefore, this area was likely an underwater volcano. You could say that the basalt magically popped into existence from nowhere, but that’s not very uniform- we don’t see that happening today. More importantly, you can’t test that idea. In short, uniformitarianism is the Occam’s Razor of geology: the simplest solution is usually the best: if an ancient rock looks like a modern rock, they probably formed in the same way. The present is the key to the past.

The idea of uniformitarianism has a long history with many famous scientists, and honestly deserves an entire episode. To make a long story short, the idea held sway for over a century. But as folks began to look deeper into Earth’s past, some cracks began to emerge. I can already hear you shouting: “Of course things were different in the past: we saw a planet hit the Earth, we saw the Sun change in brightness, we saw life form from simple molecules.” And that’s all true. While Earth history isn’t entirely chaotic and random, it also isn’t entirely uniform. Today, many scientists fall in the middle: most Earth history can be explained by modern actions: waves on the beach, lava from volcanos, pebbles dropping from your hand. But a few things have changed over time, some gradually, some violently: the moon, the sun, life. These are non-uniform events.

If you want to propose a non-uniform event in Earth history, you need two things. First, you need a good reason why: a paradox that’s otherwise unexplainable. A moon that’s unlike any other in the Solar System, an early Earth that should have been frozen, life as we know it. Second, you need damn good proof: moon rocks, the light from a thousand suns, the genetic code of DNA and RNA. The present isn’t the key here, but it still holds clues.

Which brings us back to early tectonics: the show-down with the home team and the away team. The “home team” are uniformitarians of varying degrees. They may quibble on the details, but when they look at Greenland rocks, they see similar patterns to the modern world: plate tectonics and trenches. Same stuff, different day. The “away team” are non-uniformitarians. They look at the same rocks and see a different story entirely, one that could not be made by plate tectonics, one that better fits other worlds like Venus. The truth is out there.

Where does that leave us at the end of this episode? Was Greenland’s Itsaq Complex made by plate tectonics or a stagnant lid? What about all these tonalites? Here’s something the home and away teams would agree on, despite all their differences: tonalites alone are not quite enough to clinch the argument for or against tectonics. They are only the opening round of a much longer game.

Here's where we’re going to go from here. In the next few episodes we’ll return to the surface and chip away at other questions: what was going on with volcanos, the oceans, and even life 3.9-3.6 billion years ago? As we answer these questions, we’ll flesh out arguments for the larger tectonic debate: moving plates or stagnant lid. So if you’re a hard-rock nerd, you’ll still get your fix. But if you’re a surface nerd like me, you’re probably itching to see what’s changed up there. It’s time to return topside once again.

Summary:

Plate tectonics is literally the bedrock of modern geology, you can’t describe today’s world without it. However, Earth is the only world we know that has plate tectonics. Planets like Venus show us there’s more than one way to make continents and volcanos. Such worlds have an unbroken surface, a “stagnant lid” controlled by mantle plumes, those giant lava lamps below the crust. On Earth, mantle plumes make volcanos like Hawaii and Yellowstone, but don’t make continents like Africa. The question remaining is: did Earth have plate tectonics from the very beginning, or did we have a single, stagnant lid like Venus? On Greenland, 3.9 billion years ago, tonalites are just one piece of the, so we’ll need to gather more evidence.

Next episode, we’ll take a refreshing swim in the seas of Greenland, 3.8 billion years ago, in Episode 40.

38: Hidden Gems