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People always want to know how old everything is. And more specifically, they want to know how we know how old everything is. Well, here at Astronomy Cast, it’s our job to tell you now only what we know, but how we know what we know. And today we’ll begin a series on how we know how old everything is.
This is part one of a double episode.
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This episode is sponsored by: 8th Light
Show Notes
How do we age Earth rocks?
How Old Is Earth?How Does Carbon Dating Work?
Radiocarbon Dating
Radioactive decay
Uranium to Thorium
Paramagnetism
How do Geologists know how old Rocks are?
Transcript
Fraser Cain: Astronomy Cast episode 522. Ages and origins, part one. Fear. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos where we help you understand now only what we know but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today. With me, as always, is Dr. Pamela Gay, a senior scientist for the Planetary Science Institute and the director of Cosmo Quest. Hey, Pamela, how you doing?
Dr. Pamela Gay: I am doing okay enough.
Fraser Cain: Yes, yes. You have a scratched cornea. You are in a deeply uncomfortable time right now. Here’s hoping, if we have this conversation next week, you’ll be in less pain.
Dr. Pamela Gay: The show will go on, however.
Fraser Cain: You are the consummate professional. And I’ve got nothing shameless – well, I always have things that are shameless to self-promote, but I just wanted to give a big shout-out to all of the fans who nagged me to continue reading The Three-Body Problem. The first book was The Three-Body Problem. The second book is called The Dark Forest. The third book – I haven’t gotten there yet. I forget what it’s called. Man, The Dark Forest is so good. The Three-Body Problem was fascinating. It’s written in Chinese, and it’s been translated, and so there’s all this stuff you don’t understand just culture wise.
It was definitely a slog for me. But with the second book, I can’t stop reading it. It is just absolutely gripping and compelling and just has some really wonderful ideas. For those of you who are like me and you read the first one, you’re like, “I don’t understand why everyone’s so excited about this book. It’s a mediocre argument for the Fermi Paradox,” get into the second book. It’s so good. I can’t wait to read that and the third one. That’s all. I appreciate everybody telling me to keep going, and I’m so glad I did. That’s it.
Dr. Pamela Gay: Yay.
Fraser Cain: Yay. All right, well –
Dr. Pamela Gay: I’m gonna shamelessly promote he just came back from his astro tour to Costa Rica. We are both going, in June, to Joshua Tree, and you are invited to come with us.
Fraser Cain: You’ve got until March 26th to make a reservation. Prices have been dropped. Less than $2,000.00 a ticket. You should totally check it out. It is a even better deal, and now’s your chance. If you’re worried about the price, it’s far more reasonable.
Dr. Pamela Gay: So, join us in the desert.
Fraser Cain: Yeah. And then I’m sure you’re gonna talk about your trip.
Dr. Pamela Gay: Yes. If you can’t make it in June and you’d rather see the desert in late August when things are, potentially, a little bit cooler, join me on a trip that departs out of Tucson, Arizona and travels up through the canyon lands and eventually makes it to Las Vegas, baby.
Fraser Cain: Right. Go to astrotours.co to see them all. Astro.tours, actually. That’s the new URL, which is even cooler. All right, so people always wanna know how old everything is, and, more specifically, they wanna know how we know how old everything is. Well, here at Astronomy Cast, it’s our job to tell you not only what we know but how we know what we know. Today, we will begin a series on how we know how old everything is. Go.
Dr. Pamela Gay: Okay, so –
Fraser Cain: You picked, what, a meteorite? What was it? You were talking about how old earth rocks are, right?
Dr. Pamela Gay: Strombolites. So, one of the great quandaries and fascinations, to me, that is completely outside of my normal field of study is, how did life on our planet end up going from being little, tiny methanogen bacteria microbes that created methane that, maybe, like life that might exist on Mars, might exist on Titan, to, instead, being oxygen-generating critters, and when did that happen?
And there was recently some new research studying strombolites, which are, essentially fossilized mats from bacteria. It was pushing back the date at which, it looks like, oxygen started to be generated at depth in the seas. And I was like, “How do they know all of these things, and where are the old rocks?” And so, I went down a rabbit hole. And, when I go down a rabbit hole, it’s usually a good topic to talk about on Astronomy Cast.
Fraser Cain: Your rabbit holes get turned into Astronomy Cast topics.
Dr. Pamela Gay: It’s true. It’s true.
Fraser Cain: No, it’s the – it happens to me as well. I think it’s real – people don’t realize that – I would say a good half to three-quarters of the topics are just whatever we are currently incredibly curious about, and so many of the episodes have been from me, and this one is clearly from you.
Dr. Pamela Gay: So, standard disclaimer, I am an astronomer, not a geophysicist, and I learned pretty much everything I know about geophysics from Emily Lakdawalla, [inaudible] [00:05:24] McKinnon, and the internet. So, kudos to all of them and follow them on Twitter. Now, in this case, we are trying to figure out, what are the various ages of different things on earth? And there’s two different ways to go about this.
The first one is this thing is buried under this thing, which is buried under this thing which is buried under this thing. And you assume that the thing that is closest to the surface is the newest, the thing that is under the most layers is the oldest, and sometimes things get tricky because the earth likes to lift things up and sometimes tilt them sideways.
Fraser Cain: Flip them over.
Dr. Pamela Gay: Yeah, yeah. But more or less, when you’re dealing with what’s called sedimentary rock, you assume the relative ages are based on how things are layered on top of each other. And then, as you go digging through them, if you’re dealing with stuff that’s not too old – so, you’re dealing with things that have occurred within the past few thousand years to 70,000 years or so – what you can do is you can go looking for plant material, biologic material – anything that might’ve ingested carbon as part of its growing process – and measure the ratio of carbon-14 to nitrogen-14 because carbon-14 is perfectly happy to, over time, slowly decay into nitrogen.
And you can count atoms and say, “Okay, this is young because it’s all the carbon-14 in the ratio to carbon – the other isotopes of carbon that we normally see in the air. This all makes sense.” Over time, the ratios change. The amount of nitrogen goes up, and so this let’s you figure out the age of things that aren’t that old.
Fraser Cain: Right. Like wood.
Dr. Pamela Gay: But they have to be old enough. When I was a little kid – I’m gonna fess up to this – I was terrified my teachers would figure out I didn’t do my homework on time by carbon dating my pencil lead. I was a weird child. Also, don’t teach eight-year-olds about carbon dating.
Fraser Cain: No, do still. Whatever is the method of discipline that works best – if it’s giving your child a solid understanding in science – then I think that’s perfectly acceptable.
Dr. Pamela Gay: But don’t give them a partial understanding that makes them terrorized that CSI is going to test their homework and make sure that they turned it in on time.
Fraser Cain: Would that work? I don’t think it’s –
Dr. Pamela Gay: For me.
Fraser Cain: I don’t think you could carbon date a pencil accurately enough to know, within a couple of days, if the homework had been done on time.
Dr. Pamela Gay: Well, you can’t because the half life of carbon is 5,730 years, and so you need –
Fraser Cain: And it would only tell you when the pencil was made, not when the lead was put onto the paper.
Dr. Pamela Gay: Exactly, exactly. And so, yeah, yeah, it’s a bit of a problem. So, you can only use things for certain time ranges. So, with carbon, because of its several-thousand-year half-life, you need to be looking at something that’s old so the decay will have had a chance to have been occurring in a reliable way, but it’s not so old that pretty much all of the carbon would’ve gone away completely. So, we’re looking at –
Fraser Cain: The roof beams of a Roman house.
Dr. Pamela Gay: Totally valid.
Fraser Cain: Perfect.
Dr. Pamela Gay: Go for it.
Fraser Cain: Yeah, they’re like 2,000 years old. That’s a little over halfway through the half-life. You’re laughing. Well, no, half-life is 5,700 years.
Dr. Pamela Gay: Yes.
Fraser Cain: So, yeah, a third of the way through the half-life. You could definitely know that that Roman timber was old.
Dr. Pamela Gay: And there’s other methods that we use as well. So, there’s –
Fraser Cain: And, sorry, I apologize, and I don’t wanna stop you on this new thing, but I think exactly how this works is pretty fascinating, and can we just take a second to – how do we know that, when you start the stopwatch with a piece of wood, for example, that gets used in a Roman house – how do we know that that wood started at that time?
Dr. Pamela Gay: When plants grow, they’re actually sucking gasses out of the air to make themselves. If you’ve ever tried to figure out, where does a tree come up with all the materials to make its leaves, its bark, its everything else? Why doesn’t it hollow out the ground beneath it as it sucks up nutrients? Well, it’s because the carbon that makes up the tree is coming out of the air, and the carbon in our atmosphere has specific ratios of the different kinds of carbon atoms that it has, the different isotopes. The carbon-12, the carbon-13, the carbon-14.
And, of course, it has nitrogen and all these other things, and we know a brand-new piece of wood that you’ve just gone out into your backyard and chopped off of your tree is going to have, when you shove it through the mass spectrometer, certain ratios of these atoms. So, new no longer respirating, no longer ingesting from the atmosphere wood is going to have a representative ratio of isotopes and a ratio of the master parent’s atom and daughter element. Now, over time, some of the carbon-14 is gonna be like, “See ya. I’m becoming nitrogen. I’d rather be nitrogen.”
And, through these decay processes, that carbon-14 decreases over time, and the nitrogen increases, and the carbon-12 and 13 are just sitting there going, “But, but, but.” And so, because you see the ratio of one of the forms of carbon changing in relationship to the other types of carbon that became that chunk of wood, that bacteria, that rat – whatever it is that is biological that respirated – that change in the carbons ratios is one side, and the other side is you just count the child elements. Now, the place that the counting of the parent and daughter atoms is most relevant is, actually, when we deal with a different radioisotope dating mechanism, and this is where we look at the decay of uranium into lead. Earth has radioactive materials in it. It’s kind of everywhere. And so, when you grab a piece of rock that isn’t generally being processed, that piece of rock can tell you how old its surroundings are. This is what we refer to as an Ignatius rock. So, we have three kinds of rock. Ignatius rocks, which are formed of stuff, and they sit there going, “Hi, I am a mineral.”
Fraser Cain: I think it’s igneous.
Dr. Pamela Gay: Yes, it is. I said the name of a saint, didn’t I?
Fraser Cain: Probably. So, I –
Dr. Pamela Gay: Let me start that over. Thank you. Good catch. So, we have three basic kinds of rock. The igneous rock is sitting there going, “Hi. I am a rock. I formed. This is who I am. I am happy like this.” We have metamorphic rock that has probably changed over time. And, when you study the ratio of stuff in metamorphic rock, it tells you when that particular rock metamorphosized, came into being versus how old that surrounding is. So, the Ignatius rock – I said it again. The igneous rock will tell you how old the area is. And then we also have sedimentary rock, which we talked about at the beginning of the show.
With that igneous rock, this is where we start figuring out that the Canadian shield parts of Australia, western Australia and bits and pieces of India and Africa are the oldest places in the planet because we look at this uranium-234 that is decaying to thorium-230. We look at other forms of uranium that are decaying into lead via a variety of different decay chains, and we literally just count up, what is the ratio of uranium to lead? What is the ratio of uranium to thorium? And this tells us how old that chunk of rock is, which then reflects how old that chunk of a continent is, which is kind of awesome.
Fraser Cain: But I mean, aren’t those elements – when you think about, say, the thorium, wasn’t that formed in the supernova or colliding neutron – sorry. I’m not sure how far thorium is up there. It’s gotta be beyond iron, though, right?
Dr. Pamela Gay: Yeah.
Fraser Cain: So, weren’t they formed in the supernova? So, how do you know – how can you get any kind of local measurements when it all just came from whenever that supernova was formed, and why can’t you use it to tell you when the supernova happened?
Dr. Pamela Gay: So, one of the great lies that we inadvertently tell is we will say that all of the atoms of gold in that ring you’re wearing were made in a neutron star collision is the new solution. But what we failed to say is that lead, in that pipe that, hopefully, you aren’t using for drinking water, may have come through the decay process of uranium and was not created in the supernova.
So, sometimes the elements that we’re looking at, they weren’t created in a supernova. Their parent atom was created in a supernova. So, here, we know what the background distribution of atoms is on our plant in general. We know what we should expect. And, when we see a concentration of uranium atoms and lead atoms interspersed inside of a rock, that is a reflection of the parent/child atomic decay process, not some supernova.
Fraser Cain: Right, right. But the point being that, for example, with the carbon-14 turning into nitrogen-14, you know the process that came from the air to make the tree, and you know what the ratio was the moment the tree was formed, and then that allows you to then start the clock. Same thing with magma. If you look at a chunk of lava rock, you know what lava of that variety – what mixture of elements should be in that lava of that variety. Knowing what went into it then allows you to measure the constituents, and you can start the clock and figure out what happened.
And so, for every one of these timekeepers, you need to know that starting condition. You need to know what the ratios should have been. But if you just have a blob of uranium, you don’t know what the starting conditions should have been, so you have no way to know when it was created from a – if you just have one atom of uranium or one atom of lead, you don’t know when that atom of lead was formed because it’s all just random probability.
Dr. Pamela Gay: So, if you have a pure block of uranium, you know it’s a baby block because decays haven’t taken place. If you have a block that’s pure lead, you know nothing. If you have a block that is a mixture of uranium and lead, you can generally say, “This used to be just uranium, but it’s been hanging out for this long.” And the thing is, we also use this in concert with other methods, and uranium, in particular, because of all the energy involved in its decay process, does a bunch of really cool things. One of my favorites is what’s called vision tracks, and this is where you measure the damage in volcanic glasses and other naturally occurring glasses and minerals from when uranium-238 decays. So, you will get damage, faults, things you can see in glasses and minerals that contain uranium as a result of the decays, and you count up all of these things, and that tells you how much decay has happened, and that will tell you how old the glass is. There are other minerals that you have the uranium-234 to thorium. You have the uranium-238 going to lead. With all of these different processes, you can count different atoms because, naturally occurring, when you have uranium, it’s not gonna be just a single isotope. It’s gonna start out just like the carbon, with a variety of isotopes together.
So, with these multiple methods all grouped together around one species of atom in all of its isotope varieties, it gives us multiple ways to say, “This is within this age range.” Now, with the uranium series, you are looking, again, at things that are very old. So, here, you’re looking at the hundreds of thousands of years to billions of years. But there’s other ways. There’s, yet, more that uranium is responsible for.
One of the other things I didn’t even know, before prepping for this show that you can do with uranium is, when things get buried either by human beings digging a hole or, in the case of my house, dogs digging a hole and dropping something into the hole and then covering it back up or just wind-blown dust, sand, mudslides – whatever – when something gets buried, it will have experienced a certain amount of radio-induced magnetism. Paramagnetism is the fancy word.
And, if there are mineral lattices, they’re going to be sensitive to this radiation-induced process from uranium. And this what’s called electron spin resonance can be measured and get us a date of something between 1,000 years and 3 million years. So, uranium, it creates paramagnetism that affects lattices. It creates fission processes that create fractures in glass. It just plain decays into things that you can count, and so this one atom is just responsible for so many different ways that we measure the ages of things.
Fraser Cain: And I’m sure we’re going to be taking a look at uranium again as we look at some of the later shows as well.
Dr. Pamela Gay: Yes.
Fraser Cain: And I’m just sort of imagining this sort of rabbit hole. So, did you reach the bottom of your rabbit hole where you were like, “Okay, how did they know?” So, can you sort of go back to that original story that you were talking about and sort of get a sense of how they knew what they knew?
Dr. Pamela Gay: So, the original story was on these fossilized bacteria mats in Australia. And, here, they were looking at a variety of different radio dating mechanism to get at, how long had these things been buried? And they also looked at the sedimentary layers. And one of the things they didn’t use that I learned about in the process of doing this that works with under the ocean in particular is – we’ve talked about the magnetic pulls of the earth flipping before and how this can actually be seen and how lava from the Mariana Trench, from where the sea floor is splitting and oozing out lava and adding more land under the sea – that land under the sea – and, now, I have a Disney song stuck in my head.
Fraser Cain: Yeah, me too. Get it out, get it out.
Dr. Pamela Gay: As it comes out, locks into the sedimentary record, the alignment of the magnetic field of the earth at the moment that it was created, and you can look at the magnetic field in these different rocks as it flips over time and say, “Ah-ha, this belongs to this magnetic field era.” So, you can also do magnetostratigraphy –
Fraser Cain: What?
Dr. Pamela Gay: – which is a fabulous word that I – it’s a new word. I learned a new word.
Fraser Cain: That’s amazing. Right. So, let me see if I understand this correctly. You’ve got the earth’s magnetic field flipping, and, when those flips happen, it’s fairly well known. I know they look at this with looking at ancient lava flow. They can see when the flips happen because the iron crystals are magnetized to align with the magnetic field as the lava is pouring out.
Apologize to everybody who thinks I say lava wrong. I’m Canadian. Different from Ignatius. And so, then all you have to do is look at the alignment in these rocks, and then you just kind of compare to known times that had a similar shape of the various alignments and durations, and you can go, “Oh, this happened during this time period.” That’s mental. I can’t believe that’s possible. That is amazing.
Dr. Pamela Gay: It’s magnetic.
Fraser Cain: It’s magnetic. That’s a stunning accomplishment. And so, it lets you just – if you can get at the magnetic crystals of any chunk of rock and there’s some kind of, I guess, time or you know its orientation, then you can start to puzzle out when this stuff formed. Wow.
Dr. Pamela Gay: It’s called the polarity of the earth’s magnetic field. And, yeah, it’s kind of awesome. And there’s so much cool stuff that scientists have somehow figured out, and, a lot of this stuff, I’m like, “How did you initially think to look at that?” The magnetostratigraphy, actually, that one makes sense, but there are inclusions in quartz crystals – zircons in Australia, for instance, are some of the oldest rocks around, and the Jack Hills zircons date back between three billion and four billion years ago where our planet had survived the thing that created the moon about 4.54 billion years ago. So, these things are like 500 years after the earth came into its current mass amount.
These crystals contain tiny inclusions, gas bubbles, and each little bubble reflect the earth’s conditions at a different point in time. This is the same technique that we use to figure out where in the solar system meteorites come from. Well, it’s also used to figure out when in the earth’s history zircons come from, and that’s just awesome that these things that we use to make jewelry are little time machines that lock in on the history of our own planet.
Fraser Cain: That’s just amazing. Were there any other methods, or do you wanna save this for next week?
Dr. Pamela Gay: There’s one more method to mention, and this one – I have no idea if I’m saying this right, but I’m gonna embrace the mispronunciation. Tephrochronology. This is looking at how volcanic ejecta layer on top of one another and the chemistry of the lava, and this gets used to figure out how different events may or may not be related to one another.
Fraser Cain: That’s really cool. It’s kind of like it’s that same idea of the moon, and you’re counting up craters on the moon and seeing the way the craters are on top of each other, and we actually – we had a project with that on Cosmo Quest.
Dr. Pamela Gay: And we will again. We’re just rewriting our software right now, so stayed tuned, folks.
Fraser Cain: So, is there a volcano mappers coming?
Dr. Pamela Gay: Not that we currently have planned, although, I would love to do that for series, which is apparently covered in cryovolcanoes, but that’s a different quandary.
Fraser Cain: Yeah. So, you could do that, though, right? You could have –
Dr. Pamela Gay: Yeah.
Fraser Cain: – cryovolcanoes. The ejecta would be out there from one volcano, and then another would be over top of it, and then another would be over top of that, and you could sort of detect by counting up the layers of cryovolcanic ejecta.
Dr. Pamela Gay: The other cool thing about this particular technique is there are also the chemistry and the geographic changes that come with volcanoes. When you look at the shape of a volcano, you can say, “Oh, that one’s super tall and still super pointy. It must be still active.”
As they round, as they slump, you can tell they’re older and older. And it’s from that slumping that takes place that you can also tell the ages of the volcanos, especially in series, where the results of the cryovolcanism taking place aren’t necessarily still evidence for the oldest volcanoes or even the medium old volcanoes. So, it’s from the shape of the volcanoes as they slump back into the surface of series that we can start to say old versus new.
Fraser Cain: All right, where do we go next week?
Dr. Pamela Gay: So, next week, we’re gonna talk about things like what I just brought up with on series and work on figuring out, how do we know the ages of things on other worlds? And I don’t know if we’ll get to it next week or if there’ll be a third episode, but how do we measure the ages of stars and other materials in our universe?
Fraser Cain: All the way out to the cosmic microwave background radiation.
Dr. Pamela Gay: Yep.
Fraser Cain: Amazing. I love it. All right, now, we should thank some people.
Dr. Pamela Gay: Yes. Yes, we should. So, we thank a bunch of you for being awesome, and we can’t thank all of you every episode, so I’m working our way down the list of our Patreon followers. If you want to help us pay Suzy, please click on over to patreon.com/astronomycast. This also pays for our server and, generally, just keeps us going because we can’t do it without you. So, special thanks to Dean, Ryan James, Dwyane Isaac, Glen McDavid, Benjamin Davies, Paul Weller, Russel Peto, Dan Littman, Martin Dawson, Kenneth Ryan, Brian Kilby, Steven Ludking, and Thomas Shepshrup. Thank you. Thank you. We are so grateful for everything you have done to make this show possible.
Fraser Cain: Thank you, everybody. And, Pamela, we will see you next week.
Dr. Pamela Gay: Buh-bye. This episode of Astronomy Cast is brought to you by Eighth Light Inc. Eighth Light is an agile software development company. They craft beautiful applications that are durable and reliable. Eighth Light provides discipline software leadership on demand and shares its expertise to make your project better. For more information, visit them online at www.8thlight.com. Just remember, that’s www. the digit 8 T-H-L-I-G-H-T .com. Drop them a note. Eighth Light, software is their craft.
Female Speaker: Thank you for listening to Astronomy Cast, a nonprofit resource provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can e-mail us at info@astronomycast.com. Tweet us @astronomycast. Like us on Facebook and watch us on YouTube. We record our show live on YouTube every Friday at 3:00 p.m. Eastern, 12:00 p.m. Pacific or 1900 UTC. Our intro music was provided by David Joseph Wesley. The outro music is by Travis Sural, and the show was edited by Suzy Murph.
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Duration: 32 minutes
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