But what's neat about argon-40 is that while it's lava, while it's in this liquid state-- so let's imagine this lava right over here. And so what you can do is you can look at the ratio of the number of potassium-40's there are today to the number that there must have been, based on this evidence right over here, to actually date it.And in the next video I'll actually go through the mathematical calculation to show you that you can actually date it.It's not bonded to anything, and so it'll just bubble out and just go out into the atmosphere. That lava will contain some amount of potassium-40. And so you know the only way this argon-40 can exist there is by decay from that potassium-40. So you know for every one of these argon-40's, because only 11% of the decay products are argon-40's, for every one of those you must have on the order of about nine calcium-40's that also decayed.So what's interesting about this whole situation is you can imagine what happens during a volcanic eruption. And actually, it'll already contain some amount of argon-40. And let me do it in a color that I haven't used yet. And so for every one of these argon-40's you know that there must have been 10 original potassium-40's.It's a pretty good indicator, if you can assume that this soil hasn't been dug around and mixed, that this fossil is between 100 million and 150 million years old. by Tas Walker One of the most widely used dating methods is the potassium-argon method, which has been applied to ‘dating’ rocks for decades, especially igneous rocks that have solidified from molten magma.
And then let's say this one over here has more argon-40. And using the math that we're going to do in the next video, let's say you're able to say that this is, using the half-life, and using the ratio of argon-40 that's left, or using the ratio of the potassium-40 left to what you know was there before, you say that this must have solidified 100 million years ago, 100 million years before the present.And the reason this is really useful is, you can look at those ratios.And volcanic eruptions aren't happening every day, but if you start looking over millions and millions of years, on that time scale, they're actually happening reasonably frequent. So let's say this is the ground right over here.And every 1.25 billion years-- let me write it like this, that's its half-life-- so 50% of any given sample will have decayed. And it actually captures one of the inner electrons, and then it emits other things, and I won't go into all the quantum physics of it, but it turns into argon-40. And you see calcium on the periodic table right over here has 20 protons. And what's really interesting about that is that when you have these volcanic eruptions, and because this argon-40 is seeping out, by the time this lava has hardened into volcanic rock-- and I'll do that volcanic rock in a different color. And so if you fast forward to some future date, and if you look at the sample-- let me copy and paste it.So this is a situation where one of the neutrons turns into a proton. By the time it has hardened into volcanic rock all of the argon-40 will be gone. And so what's neat is, this volcanic event, the fact that this rock has become liquid, it kind of resets the amount of argon-40 there. So if you fast forward to some future date, and you see that there is some argon-40 there, in that sample, you know this is a volcanic rock.