How do scientists know how old fossils are?
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From the brains behind Brains On, this is the Moment of Um. Moment of Um comes to you from APM Studios. I'm Howie Howerson. My name is Howie, which is funny because my favorite question is also, how? How?

I am always wondering. And I'm always wandering. Like yesterday, I wandered down to the science museum. I checked out the bug room with its 463 different species of beetles. I know, because I wondered how many. So I counted them. Down the hall from the bug room is my favorite part of the museum. The dinosaurs.

I love all the fossils on display with those handy-dandy tags with lots of answers to my questions, like how old the fossils are. But you know what that made me wonder? How do scientists know how old all those dinos are? My good friend Vernon was wondering the same thing. So let's see how an expert answers the question. ♪

Hi, my name is Vernon from Elkins, West Virginia, and my question is, how do scientists know how old fossils are?

My name is Jameel Connor and I am the associate curator of fossil reptiles at the Field Museum in Chicago. I am a dinosaur paleobiologist, which is just a fancy way of saying I am a paleontologist who focuses on dinosaurs. Fossils only come from sedimentary rocks and sedimentary rocks form in layers. So we use something called the law of superposition, which basically means if you have this layer of rock

Any rock on top of it is going to be younger and any rock below it is going to be older. So that's like step one, right? But when we really want to get precise dates, what we're looking for in these sedimentary rocks are usually volcanic layers. So if a volcano goes off and, you know, then you'll get this layer of volcanic residue, right? And it will contain elements that we can use for radioisotopic dating. Some elements overlapped

over time will change into a different element. And the way that this happens is because in the atomic microstructure of these elements, some little parts like the neutrons

are loose, basically. And so if they fall out of that element, it makes that element become a new element. And we know exactly how fast this happens for all the different elements. And so this helps us to then calculate backwards when that rock must have formed. Now, what if you don't have any...

volcanic activity at this time or nearby, then, you know, it becomes a little bit more difficult. And, you know, some rocks will contain these radioactive elements just naturally, like, for example, some fossils in Utah. You'll find uranium present so that you can use that for dating. But otherwise, we use the fossils themselves.

So this is actually the original purpose kind of of paleontology or the main purpose of paleontology amongst geologists was using fossils that we associate with different time periods to say that if you have this fossil present, then you know what age these rocks are. Well, how do you like that?

Scientists can figure out the ages of fossils in a couple of different ways. The way that fossils form in the first place is when living things die and then get quickly covered over by sand or mud. Everything builds up from the bottom. So if scientists find two fossils, one closer to the surface and one deeper down, the one that's deeper in the ground must have gotten there first. So it must be older.

But to actually get the dates for how old a fossil is, scientists use something called radiometric dating.

That means researchers can put little samples of a fossil or a rock that came from a volcanic eruption into a special machine to measure the amounts of super tiny things called isotopes. Isotopes are super cool because they change their shape over time, and we know exactly how long it takes for that change to happen.

So scientists can use that to work backwards and figure out how long ago those special atoms started changing. And that's how long it's been since the fossil was a living thing. How cool is that? If you like this episode, take a second to subscribe to Moment of Um wherever you listen to podcasts.

And if you're curious about how paper is made, check out the Brains On podcast where we have a whole episode all about it. Want to see our shows come to life? Head to YouTube where we've got awesome animated Brains On episodes. Search Brains On Universe on YouTube and subscribe.

If you have a question, we'd love to help you answer it. Drop us a line by going to brainson.org slash contact. See you next time and the next day and every weekday. Until then, um... Maybe one of these days I'll branch out and start asking why.

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