If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Fossil dating

Let's explore the relative depth and radioactive dating of fossils. Created by Mahesh Shenoy.

Want to join the conversation?

Video transcript

- [Instructor] If you go to a dinosaur museum, then you'll see guides telling you things like this dinosaur lived 50 million years ago. That one lived 70 million years ago. My question is, how do we know these things? Well, in this video we'll look at some methods we used to date these fossils. There are a couple of ways to figure out how old these fossils are. Okay, let me just write them down. One way, is to figure out something called their relative depth. Relative depth. And the second way is a completely different method. It's called radioactive dating. It's called radioactive dating. Okay fancy names. Let's look at each one of them. Let's start with the relative depth. This method will make a lot of sense if we remember how fossils are formed in the first place. We've talked about how fossils are formed before but to quickly recall, one of the most common ways this can happen is if an animal or an organism dies very close to a lake, then it gets decomposed and the bones, which take a long time to decompose, they will eventually settle down to the bottom and then as time passes by, more and more stuff gets accumulated on top of it. These are called sediments and eventually, these things harden to form rocks. Finally, the water can start flowing in through the tiny gaps which are present and the minerals of that water will start replacing the original content of this bone, turning it into a stone. And that's how, that's one of the ways in which fossils can be formed. And of course if you're not familiar with this process or you need to learn this process in detail, then we've talked a lot about that in a previous video called "Fossils and their Formation." Feel free to go back and check that out. And now think about what this means. This means, as more and more time passes by, more and more stuff gets accumulated on top of it. Does that make sense? So with just this information, we can compare the age of two fossils and tell which one is older. Okay so, let me give an example. Let's see if you can, you can do this on your own. So let's say there are two fossils. One is the fossil of some dinosaur, maybe another fossil is some ancient fish and have given the depths over here. One is over here. Another one is found over here. Just by looking at their depth, can you guess which fossil is older and which one is more recent? Go ahead, give it a try. Okay, hopefully you have tried. So if you look at this fossil, there's not much stuff on top of it. And so we can say hey, this might have this fossil might have formed pretty recently. But if you look at this one, a lot of stuff is present on top of it compared to this one. And so we can say this might have been older. So just by comparing the depth, the fact that this is deeper than this, we can say that this is older than this one. That's one of the ways in which we can date the fossils. Okay, so in this method we basically say, I'm just gonna write that down over here, so deeper you go, deeper you go, older the fossil. And let's make sure you can see it better, fine. So the deeper you go, the older is the fossil. That's the first method. And why is it called relative? It's called relative because we are comparing the depth of one fossil with respect to another, right? This is deeper compared to this. So since we are comparing the depth, we call that as a relative. So relative basically means comparing. All right? So this is one way in which we can date the fossils. But this method is only useful if you already know the age of one of the fossils. For example if you knew how old this fossil was, then you can probably, you know estimate how much older this one is. But what will you do if you didn't know the age of any fossils? Then how do you figure out the date? Well, that's where we go to the second method, radioactive dating. So what is that? For that, let me make some space to the right. There we go. Now you don't have to worry about this in much detail but to get some idea of what this method is, we have to know what radioactive materials are. Radioactive elements are, okay? Without going into too much detail, these elements are the ones which automatically convert into some other elements. So for example you might know of this famous radioactive element called radium. That one is radioactive because automatically, if you don't do anything, automatically it will convert to some other element. It turns out that that converts to lead. Okay, again and don't worry too much about the chemical formula and everything. Another example of a radioactive element is there something called a radioactive carbon. That can automatically turn into nitrogen. And just to give you another example, there's something called radioactive potassium. That can automatically turn into, what does it turn into? Aha, yeah it turns into argon. And just to tell you a little bit more, we today know a lot about radioactivity. We know about why that happens and we know all the mathematical details behind it. We also know how quickly it takes place. For example it turns out that if I give you a bunch of radium, hats off to you I mean, nobody should give you radium, okay. You should never hold radium in your hand but if we have a bunch of radium, then it turns out that in a few thousand years, maybe about 10 to 20,000 years, all of that radium will be converted to lead. The same story for carbon. It's a little slower. Turns out that if you get a bunch of radioactive carbon, not the normal carbon that you might be familiar with, but the radioactive carbon, then again if you wait for something like 50 to 60,000 years, it will convert to nitrogen. So these are pretty quick. Actually you'll say very radioactive. So these are quick and you may be like what, thousands of years is quick? Isn't that really slow? No, it's not slow because if you take potassium for example, turns out it takes billions of years. If I give you a bunch of radioactive potassium, it'll take billions of years. Billion, that's right, billions of years for it to get converted completely to argon. So this is pretty, pretty slow. And so, if you want to date these very, very ancient fossils fossils that might have lived millions of years ago let's say, it will make a lot of sense to use these slower ones. Okay, it'll say why. But how does it work? Okay, just to give you an idea of how this dating process works, the whole idea would be you take a rock, which you might find somewhere close to this fossil and you try and figure out how many potassium and argon elements are there in there. So let's say you find there is a lot of potassium in it, just an example just to give you some idea of how this works. And let's say you find a little bit of some very few traces of argon in it. Not much. Few traces of argon in it. What can we say from this? From this we can say that look not much of the potassium has been converted into argon. We can pretty much say this argon must have come from this potassium due to this process. There is no other way argon must have come over here. So since only few traces are there, not much time has passed since this rock was formed. Okay? And so from this we could guess, not guess we can estimate if you know the mathematics, we can say "Hah, this rock is 50 million years old, okay." that's basically how it works. You just look at the, how much amount of potassium is there, how much amount of argon is there, and you can estimate it. On the other hand, let's say there was another rock which you found out, which you dug out from much deeper. Okay, it was a much deeper rock. And again, let's say you find out how much potassium is there and you find let's say little bit of potassium is left in this and a lot of, a lot of argon is present in this. So let's say there is a lot of argon in this. Lot of argon. Now what does this mean? Ooh, now you see almost all the potassium has been converted to argon. That means a long time must have passed, right? Ooh, this means this might be so mine are now two billion years old rock. This is how radioactive dating basically works. And if you're wondering why we are not using the quicker ones, think about it. If for such rocks which are billions of years old, if you had used carbon dating, then you would have found almost all the carbon would have been converted to nitrogen. Almost all the radium would have been converted to lead. There is no point. We can't use them. So these quicker ones are useless if you want to date dinosaur fossils which we know today are about 650 to 60 millions of years old. So this is basically how radioactive dating works, okay? So, long story short, by figuring out how deep these fossils are and by looking at the radioactive elements present inside those stones, we can get a pretty good estimate of how long ago these ancients creatures might have lived and from that, we can guess how the ancient world might have looked. It's pretty awesome if you ask me.