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- Thank you to Skillshare for supporting PBS. Hey, smart people. Joe here. This is a video about the oldest rocks on Earth. This is one of them. Hidden inside is a tiny clock. It's too small to see with the naked eye that's been slowly counting down since Earth's earliest days. We're gonna find out where these rare, super-ancient rocks come from and how scientists measure those clocks to help shine a light on one of the most mysterious times in Earth's history, when our planet was first cooling from a molten,
inhospitable blob to become a world with oceans, continents, life. And ultimately, you and me, and also our friends from Howtown, who join me to help figure all of this out. This rock is about to help us travel back into Earth's history farther than we ever have before. (gentle music) We were about to take a journey, billions of years back in time to the earliest days of Earth. But the immense scale of Earth's geologic history is pretty hard to wrap our measly human brains around. So let's translate that to something a little easier to understand by squishing the planet's entire life into one calendar year.
Let's begin late on December 31st, just as the calendar's about to tick over to the new year. The American Revolutionary War is just one second before midnight, a New Year's Eve. In fact, our species comes onto the scene in just the last half-hour of the year. The first horses, dog-sized creatures, gallop into view on December 27th, while the dinosaurs say goodbye on December 26th. The first flowers bloom the afternoon of December 21st, while the earliest ancestors of mammals arrive on the 15th. And Tiktaalik barely crawls on the land before we turn the page.
November sees the first ancestors of sharks, the first land plants, and the Cambrian explosion, when most modern animal groups appear. But we've still got more than 11 months to go. Fungi mushroom onto the scene in September, in a world that's almost entirely single-celled creatures. Mitochondria are swallowed up around July. - [Person] The powerhouse of the cell! (lightning crashes) - And in June, our atmosphere first filled with oxygen as photosynthesis kicks off.
We take a huge jump all the way back to February to find the last common ancestor of all living things in one of Earth's earliest life forms, an unknown microbe called LUCA. We know that sometime in mid-January, the young Earth collided with an early planet called Theia, melting both bodies and forming our Moon in the process. And we finally arrived at the beginning of our planet on January 1st, one second after midnight. When we look at the geologic calendar like this, most of the things that have shaped life as we know it today happened in the last month or so.
Flipping back through the earlier months, the scale of time is so immense, it's honestly hard to imagine. And when we get back to January and February, well, the calendar's pretty empty. We just don't know very much about what our planet was like back then. It's geology's Dark Ages. To shine some light on this mysterious era of early Earth, we need a time machine. Unfortunately, those don't exist. But what we do have are some very special crystals, and they're hiding chemical clues about what the Earth was like in those earliest days. And some of the only stuff that remains from the beginning
of Earth's history are super old crystals, a mineral called zircon. And some people have given zircon a nickname, the Time Lord, because of how much it can teach us about deep geologic time. And for one thing, zircons are super tough. They're extremely resistant to things like erosion or weathering that could change or completely erase other minerals, so they could potentially survive for billions of years after they form. - So when everything else in this rock changes have changed and their chemistry has been altered, the zircon stay the same. So the zircons are sitting in this rock, relatively undisturbed by billions of years of chaos around them.
- [Joe] That's Mike Ackerson. It's his job to find these special crystals and tease out their secrets. - Zircons are generally very beautiful minerals are pretty simple. This is a 3D model of a zircon. So if you took this 3D model and you shrunk that down thousands of times. Honey, I shrunk the zircon down to a hundred microns. So the width of a human hair, basically. - [Joe] So tiny, that one misplaced breath could send it flying, never to be seen again. Yet that crystal's tough enough to have survived billions of years of geologic torture that would make even a diamond weep for mercy.
- They really do not like being destroyed. Zircons want to be around. - [Joe] So not diamonds are forever, zircons are forever. - Yeah, zircons are way more forever than diamonds, yeah. - That foreverness is a big help for geologists because zircons also carry tiny atomic clocks that can tell us how old they are. That's why the current title for the oldest thing on Earth goes to a handful of microscopic zircon crystals. They were found in a spot in Western Australia called the Jack Hills. And they're 4.37 billion years old, meaning that they formed around January 19th on our geologic calendar.
The rocks and minerals that once held the Jack Hills zircons eroded away long, long ago. So the crystals are sort of just sitting there in the sediment, hidden like microscopic needles in a sandy haystack. But recently, Mike's been on the hunt for something else, the oldest rock on Earth. As in, chunk of stuff you can hold in your hand. If zircon crystals are hiding in there, inside the rock they formed in. They'll help us tell a much more complete story of the early Earth. Question is, where do you go looking for rocks like that?
Now, finding rocks from the earliest days of our planet would be easy if it wasn't for plate tectonics. - [Person] My boy- - That's a bit of a Catch-22, though, because- - Plate tectonics is the reason we have a habitable planet. It's the reason that we have atmospheres that are stable for billions of years. It's the reason we're able to have stable oceans and chemistry that makes life able to form and evolve over eons. So we love plate tectonics, but if you wanna study old rocks, you kind of hate plate tectonics, 'cause it also ruins the rocks. - For Mike and his colleagues to find old rocks, that means finding old crust.
Luckily for me, I recently teamed up with my friends from Howtown on an episode for their channel all about plate tectonics. So I asked them. If I wanna find really old crust, where should I look? - Oh, man. I mean, probably just deep in a canyon, like the bottom of a canyon, like the Grand Canyon. - Right, like there's all those layers. You could just sort of keep going down to like the bottom layer. That's a good idea. You know, there's, the first thing we should say is there's all different ages of rock on the Earth. If you go to a place like a mid-ocean ridge,
you've got rock that's, geologically speaking, you know, it's fresh, it's brand-new rock. - And that's like from the goo of the Earth into the ocean. - Goo the Earth oozing up. Yeah. Into the ocean. - Okay. - And then that crust ages, and it moves, and oftentimes it eventually gets to a subduction zone where it slides under another plate and gets recycled back into the mantle. And a lot of rock that is part of risk crust is getting recycled over time. - Okay. - But there are a few pieces of crust that turn out to be incredibly stable, incredibly old.
They go back billions of years as the continents have moved and the tectonic plates have shifted. These chunks of rock have resisted, and they're called cratons. - It sounds like a cereal. - (laughs) It does. Cratons. (traffic droning) Part of this complete breakfast. The term cratons comes from this Greek word kratos, which means strength. They have sort of staying power and their roots go really deep. So if you imagine like new crust, is sort of like a plate or something very thin, easy to crack and have it degrade. But these cratons are more like a bowling ball where they're sort of very solid
and thick and they're hard to break up. And we actually have a video over on our channel Howtown, about how scientists try to figure out where these cratons were billions of years ago. - [Joss] And you and Joe are gonna ride those continents back in time. - That's right. Yeah. So we hope you'll check that out. But geologists have also mapped where these ancient bits of rock are today. And if you go up to the northern part of Canada, sort of in the middle of the North American plate, you find one of the oldest pieces of Earth crust in the Slave Craton.
- Ooh, okay. So that's where you go. - Yeah. It's a good place to look for the oldest rock in the world. - Yes, it is, guys. Here in a remote corner of the Northwest Territories, hundreds of miles, and 1.6 times more kilometers away from the nearest human settlement is a geologic formation called the Acasta Gneiss. Named for the beautifully striped metamorphic rock that Mike's holding. - But this is the oldest rock on the planet. This is the 4.02-billion-year-old Idiwhaa unit of the Acasta Gneiss.
- That's February 15th on our geologic calendar in case you're keeping track. - [Mike] You know, you can see on this the surface of this rock. It does not look visually like much when you're out in the field. - [Joe] I would walk right by that. - [Mike] You'd walk right by it. And so when you're in the field, even the most trained PhD geologists would not be able to say much about this rock. - These rocks are incredibly rare. If you add up all the area where rocks this age are exposed to the surface, it's roughly the size of the red zone on an American football field, or just a few suburban backyards.
- And one thing that nature has done us a favor with, with this rock, is it's actually magnetic. So we're a bunch of geologists walking around with little magnets along cliffs. - You're like little dowsing rocks almost- - Yeah, exactly. (laughs) Yeah, pretty much. We'll put a fork on it, you know, and then we just take a rock hammer to smash 'em. - [Joe] Mike and his colleagues carry thousands of pounds of rock back to camp on foot, then by plane, then train, so they can sift out the fused zircon crystals hiding inside.
- We take like a football-size sample of this rock. We crush it, we grind it, we turn it into a flour. We're looking at maybe an eighth of a teaspoon of zircons coming outta that rock. - If you took all the zircons we've found that are more than 4 billion years old, they would barely cover the bottom of a bucket. When you think of how hidden and rare these rocks are, it's actually kind of incredible that we even have that much. But there's still one big question we have to answer. How do we actually know how old these things are? If you wanna tell time accurately for billions of years, you need a clock that's dependable and invincible. And zircon is both of those things.
You can think of zircon kinda like an hourglass. The super-strong crystal itself is like the glass container, and locked inside, instead of sand falling through at some unpredictable rate, you've got atoms, radioactively decaying. Let's let my friends from Howtown explain how to build an atomic clock out of rocks. - Deep within a magma chamber, zirconium and other elements are cooling and crystallizing into this lattice formation. - Okay, and the elements go into that shape at a certain temperature under like certain conditions. - Exactly. They start to form up and every once in a while, a uranium atom sneaks into that lattice.
It's about the right size and charge to sort of masquerade as zirconium. It's very important that lead is not the right size and charge, so it gets excluded. So at the beginning of this process, there's no lead in the zircon crystals. But uranium is radioactive, so over time, subatomic particles will go whizzing away, and eventually, that uranium will become lead. So if we find any lead in the zircon crystal today, we know it used to be uranium. We just need to know how long it takes for this uranium to degrade into lead. And it's basically impossible to know how long it will take an individual atom. But if you have a whole big pile of atoms, you can say, for example, for uranium-238,
it will take 4.47 billion years for half of that uranium to turn into lead. - Hmm. - [Adam] For a different isotope, uranium-235, half of that pile will decay into a slightly different form of lead in a brisk 710 million years. - So if you get two uranium isotopes and two lead isotopes, you measure them, you're gonna get two different ages. And if those ages agree, then you have a high level of confidence that the age that you're getting for that crystal is actually the age at which you're crystallized. - But you need to still measure that somehow. So how do they actually capture and measure the uranium-to-lead ratio inside those crystals?
- Yeah, like a lot of scientific techniques, this one starts with lasers. A beam, a few thousandth of a millimeter wide, is fired at the zircon crystal, blasting away this cloud of atoms. - Pew, pew! - Those atoms are zap, so they become ions, and atom with an electrical charge, and they're sent flying through a magnetic field. So heavier atoms like uranium can be separated from lighter atoms like lead and be counted. This tell scientists the fraction of uranium and lead in their sample. And then using some math and the half-life number we talked about before, they can calculate how old this crystal is.
Basically, the more lead you have in the ratio, the older that rock is, it's had more time for the uranium to turn into lead. - Wow. Okay. So that's how you find the oldest rock on Earth. - And that's how scientists know that the oldest rocks on Earth are more than 4 billion years old. And beyond just their age, those little zircon crystals hold a bunch of other chemical clues that are completely changing scientists' theories about the early Earth. - So prior to the discovery of those zircons, we had nothing.
It was purely dark. We knew the age of the Earth, not from Earth. We knew the age of the Earth from meteorites, and then that was it. Then we had a huge blank swath of Earth's history to which we had zero access. The first 567 or so million years of verse history is called the Hadean Eon. The Hadean Eon is named after Hades, the Greek god of the underworld. Because for the longest period of time, we thought that part of Earth's history was a nasty, barren hellscape. - I mean, that's always the image I've had in my head when I picture that time. Just magma and barren lava rock everywhere, no water in sight.
A lot more like Mordor than our nice blue planet today. Ancient zircons are completely rewriting that history. - For example, if we go back all the way to 4.37 billion years, the oldest zircon material we have from Western Australia, we know from the chemical signatures of those crystals that those zircons formed in a magma that was interacting with low-temperature surface waters. So what that tells us is that we had large bodies of liquid water on the early Earth, and we know from the titanium concentrations that the zircons crystallized that relatively low temperatures in granite. So we know that we had continental crust made of granites, and we know that we had liquid water oceans at 4.37 billion years,
which again, without those rocks, we would have no idea. - As we sort of shine a light on those geologic dark ages on our calendar, we're writing a totally different story of how our planet came to look like it does today. It's starting to look like, as soon as the Earth cooled off after the giant impact that made the Moon, Earth pretty quickly became this blue planet, unlike anything else we've found since. - We're trying to address fundamental questions about how Earth became the planet it is. I mean, Earth is unique among all known planetary bodies in that it has liquid water, oceans, continents rising above the oceans, and it's teeming with life.
No other planetary body has even one of those features, let alone all three. - Earth is a living planet, and not just in the biological sense. That story is also written in the rocks. And I love that. Stay curious. And now that you've learned about the oldest rocks in the world, follow me over to Howtown, where we collaborated on an episode about how we know what the map of the continents used to look like, way back in deep time. For all you Pangaea fans out there, you don't wanna miss this one. Click right up there or check the link down a description, and follow me over to Howtown. And thank you to Skillshare for supporting PBS.
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The first 500 people to use my link in the description will receive a one-month free trial of Skillshare. Keep learning, keep exploring. Skillshare. And you know what else rocks? Everyone who supports the show on Patreon, we literally could not do episodes like this without you. And if you'd like to become a part of that community, help support our show directly. We would really love to have you. - This whole rock that I'm holding, all the chemicals in this are 4.02 billion years old, oldest Earth material.
- [Joe] I'm gonna go find one that's 4.021. (Mike laughing)
