How the Mineral Zircon Became Earth's Best Timekeeper : Short Wave The mineral zircon is the oldest known piece of Earth existing on the surface today. The oldest bits date back as far as 4.37 billion years — not too far from the age of Earth itself at about 4.5 billion years old. And, unlike other minerals, zircon is hard to get rid of. This resilience enables scientists to use zircon to determine when major geological events on Earth happened. As part of our series on time, host Aaron Scott talks to science correspondent Nell Greenfieldboyce about why this mineral is often considered a geologic clock and has earned the nickname "Time Lord."

This episode is part of our series, "Finding Time — a journey through the fourth dimension to learn what makes us tick." Read more of Nell's reporting on zircon here.

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Zircon: The Keeper Of Earth's Time

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EMILY KWONG, BYLINE: You're listening to SHORT WAVE...


KWONG: ...From NPR.


Hey, folks - Aaron Scott here. And today, we're going to be talking about some really, really, really old stuff.

NELL GREENFIELDBOYCE, BYLINE: That's right. It's the oldest Earth stuff there is, literally.

SCOTT: NPR science correspondent Nell Greenfieldboyce, everybody. Hey there, Nell.

GREENFIELDBOYCE: Hey. So, you know, I went to the Smithsonian's National Museum of Natural History recently, and I walked past the blue Hope Diamond...

SCOTT: Cool.

GREENFIELDBOYCE: ...Into the mineral collection - I don't know if you've ever been there...

SCOTT: Love rocks - yes.

GREENFIELDBOYCE: ...And I saw some pretty, golden gemstones that were, like, cut and polished. And these are zircons.

SCOTT: Zircons, which sounds like cubic zirconia, as in the fake diamonds that we see advertised on late-night TV - is that what we're going to talk about, today, Nell?

GREENFIELDBOYCE: No, no. So we're talking about a natural mineral - zircon. And some people have called it the time lord.

SCOTT: The time lord - as in "Doctor Who" now?

GREENFIELDBOYCE: There is a species on "Doctor Who" called Zircon, but these are tiny zircon crystals that are like little geologic clocks. I was talking with Michael Ackerson. He's a geologist at the Smithsonian who studies them.

MICHAEL ACKERSON: They are really the best markers of Earth's time or the history of the Earth.

GREENFIELDBOYCE: He says much of what we know about the timing of major geologic events on Earth comes from zircons.

SCOTT: How old are we talking here, Nell?

GREENFIELDBOYCE: The oldest zircons found in Earth rocks are 4.37 billion years old - billion.


GREENFIELDBOYCE: So that goes all the way back to just after, you know, the proto-Earth was hit by a Mars-sized object - you know, that epic collision that created what's now our moon.

SCOTT: Uh-huh - 4.3 billion years - that's old.

So today on the show, we're going to get to know the time lord and learn what this powerful timekeeper can tell us about Earth's distant past. I'm Aaron Scott, and this is SHORT WAVE, the daily science podcast from NPR.


SCOTT: We're talking about zircons today, a mineral that's a geologic timekeeper. And, Nell, you mentioned seeing big, pretty zircon crystals, like cut gemstones. Is this what gets studied in a lab?

GREENFIELDBOYCE: No, no. So to the naked eye, the ones in the lab look like sand or maybe crud. They are teeny tiny. So scientists will take a rock, grind it up into, like, a fine sand. They look through it all, and they pick out these little specks that are zircon crystals. I was in Ackerson's office at the Smithsonian, and he showed me some of these under a microscope.

Jeez, I hate looking through microscopes. OK. Oh, my goodness. How pretty is that?


GREENFIELDBOYCE: They look like little diamonds or something. You said they weren't going to be pretty.

ACKERSON: (Laughter).

GREENFIELDBOYCE: He told me that one of the ones I was looking at - like, on the left, in the third row - was 4.32 billion years old.

SCOTT: Holy moly. Now, where did they find this?

GREENFIELDBOYCE: Inside some rock from Western Australia.


GREENFIELDBOYCE: So there's this place called Jack Hills. Ackerson showed me a hunk of the rock from that area. It looks kind of like a pinkish cement with little flat pebbles in it.

ACKERSON: This rock formed - we know it formed 3 billion years ago. The reason - we know it formed 3 billion years ago because that is the age of the youngest zircon in the rock.

SCOTT: Wait, wait, wait. So is that saying that, while the rock is 3 billion years old, it actually contains zircon that is older? How did it get there?

GREENFIELDBOYCE: So Ackerson told me zircons initially form in liquid rock - in, like, magma - along with other minerals. And once zircon exists, it's really hard to get rid of it. Say the zircon forms with other stuff to make up, like, a granite mountain.


GREENFIELDBOYCE: Over geologic time, there's weathering, erosion...

ACKERSON: Most of the minerals don't survive. So things like quartz, things like feldspar - they're chemically or physically weathered and eroded to a point where they're no longer quartz and feldspar. One of the main reasons that zircon is so useful is that zircon is very resilient.

GREENFIELDBOYCE: So little zircons survive. They'll fall out, maybe go down a mountain stream. They get deposited on some beach somewhere.

SCOTT: And then I'm guessing that beach turns into another rock, and that's the one we find in Australia.

GREENFIELDBOYCE: Exactly, OK? So, you know, geologists do not have whole rocks that are 4.37 billion years old, but they do have these little, itty-bitty zircons that are.

ACKERSON: So all of the zircons that we're talking about on the early Earth primarily formed from rocks that no longer exist. The zircons were weathered, eroded and eventually deposited in a different formation, forming something like a sedimentary rock, like a sandstone or a conglomerate.

GREENFIELDBOYCE: Now, it is unusual to find zircons that are older than 4 billion years. A 4.3-billion-year-old zircon is extremely rare, but they are the only windows we have into the earliest Earth because the surface of the Earth is constantly changing and being recycled and deformed.

SCOTT: So you're saying these things are really old, which begs the question - how do we actually know how old this zircon is?

GREENFIELDBOYCE: OK, so here's the thing. Zircon loves, loves, loves uranium. As it grows, it takes uranium in. But there is something else - another element - that it hates.

SCOTT: And what's that?

GREENFIELDBOYCE: That is lead. It hates lead.

SCOTT: And if I remember correctly, doesn't uranium decay into lead?

GREENFIELDBOYCE: Exactly. So if you see any lead in a zircon, it most likely comes from the decay of uranium and no other source.


GREENFIELDBOYCE: And, you know, radioactive elements like uranium decay at a steady rate. That means you can look at uranium and lead inside a zircon and learn exactly how much time passed since the zircon formed.

SCOTT: OK, so now what you're talking about is, like, the ratio between the two, right? So if you see a lot of lead in relation to uranium, you know that it's an older rock because that means there's been more time for the uranium to decay, right?

GREENFIELDBOYCE: Yeah, that's the general idea.

SCOTT: Now, it seems weirdly handy to have this timekeeping trick inside a mineral that also just happens to be extremely hardy and long-lived.

GREENFIELDBOYCE: It does kind of boggle the mind how lucky geochronology researchers are. I was talking about this with Jesse Reimink. He's a geologist at Penn State University.

JESSE REIMINK: If you believed in a higher power, you'd say, oh, the higher power created this mineral with this specific system because it is so perfect for Earth.

GREENFIELDBOYCE: He pointed out that everything we know about uranium and how it decays mostly came from the work in nuclear physics.

REIMINK: So there's a really interesting interaction between fields here that, in the '50s, once the Manhattan Project results were made public, all the geologists was like, oh, wait, we can use that. That's really cool. And they're running around looking at rocks, and that's where we get the age of the Earth. And it's a long history of technique development.

SCOTT: And so if scientists have been working with them for, you know, a half a century, dating all these rocks, I'm guessing the techniques they're using have also developed and gotten pretty fancy?

GREENFIELDBOYCE: These days, they use lasers. They can date a huge number of zircons very quickly.

JOSHUA GARBER: So welcome to our lab. This corner is all of the instruments that we use to date zircon.

GREENFIELDBOYCE: That's Joshua Garber at Penn State University. He showed me how he sticks tiny zircon crystals into this machine that blasts out little bits.

GARBER: And then I, you know, torture them in an argon plasma to break them down to their smallest constituents. And then I filter out all the things I'm not interested in, and then I very precisely measure the number of atoms that are hitting a detector at the end.

GREENFIELDBOYCE: They can look at elements like uranium, lead - others as well.

GARBER: And we can get meaningful ages and, more importantly, meaningful data about a range of Earth processes that zircon actually records.

GREENFIELDBOYCE: Because, you know, keeping track of time is cool, but zircon can record information about the magma - the liquid rock - that it grew in. I mean, it's cool to look at these things because the growth of this mineral creates, like, a pattern of little rings. They look kind of like tree rings or, like, the layers in an onion.

SCOTT: Wow. And, I mean, tree rings can tell you about the environment the tree was growing in. Are you saying that these also will tell you about their environment?

GREENFIELDBOYCE: Yeah. They can get clues about temperature and whether there was liquid water around.


GREENFIELDBOYCE: Jesse Reimink told me some of the oldest zircons have oxygen isotopic signatures that suggest water was there.

REIMINK: And those surface waters have to be cool enough - probably below 100 degrees centigrade. So, you know, it's definitely liquid water.

GREENFIELDBOYCE: That means there was liquid water on the Earth over 4.3 billion years ago. And that was a real surprise to come out of these zircons - the oldest ones.

SCOTT: When did people think that the oceans formed before this?

GREENFIELDBOYCE: Much later than that. The first 500 million years of Earth's history has long been called the Hadean Eon.

SCOTT: Hadean as in Hades, as in hell?

GREENFIELDBOYCE: Yeah. So Michael Ackerson told me people thought the Earth was basically just a glowing hot ball of molten lava back then - you know, a hellscape, where nothing could survive. He says the oldest zircons have shown that's just not so.

ACKERSON: We're starting to understand how and when the continents arose, how and when the oceans arose and how that might have helped us set the groundwork for the propagation of life on our planet. I mean, these are big, philosophical human experience questions that we can start thinking about when we look at zircons and when we look at the deep, deep, deep history of our planet.

SCOTT: Love this. So where is research on zircons going now?

GREENFIELDBOYCE: Well, you know, scientists keep looking for older and older ones. I mean, the techniques they have are letting them look at more faster. And they're also looking to see, you know, what other kinds of information might be captured in there beyond what they've already found.

SCOTT: Well, you're going to keep us posted on this, I hope. Thanks for the introduction to the time lord. Speaking of, where does that nickname come from, anyway?

GREENFIELDBOYCE: Honestly, I've only seen it on Twitter. There was this thing called Mineral Cup - I don't know if you're aware of that...


GREENFIELDBOYCE: ...Where people, you know, voted for their favorite minerals and tried to convince others to vote for theirs. And the folks there would refer to zircon as the time lord. And none of the zircon researchers I talked to had ever heard of that.

SCOTT: (Laughter).

GREENFIELDBOYCE: But I think it's appropriate, and I'm going to use it until it sticks.


SCOTT: And if you want more time, check out our series about it with NPR's Science Desk. Last week, we talked about time cells in the brain. And next week, we're going on a quest to understand time itself. I mean, what is time? You can find those episodes wherever you listen to this show or at


SCOTT: This episode was produced by Thomas Lu and edited by Gisele Grayson. Abe Levine checked the facts. Our audio engineer was Gilly Moon. Brendan Crump is our podcast coordinator. Beth Donovan is our senior director of programming, and Anya Grundmann is our senior vice president of programming. I'm Aaron Scott. Thanks for listening to SHORT WAVE from NPR.


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