Potassium-argon and argon-argon dating techniques are used on

And it erupts at some time in the past. So it erupts, and you have all of this lava flowing. That lava will contain some amount of potassium And actually, it'll already contain some amount of argon But what's neat about argon is that while it's lava, while it's in this liquid state-- so let's imagine this lava right over here. It's a bunch of stuff right over here. I'll do the potassium And let me do it in a color that I haven't used yet.

I'll do the potassium in magenta.

It'll have some potassium in it. I'm maybe over doing it. It's a very scarce isotope. But it'll have some potassium in it.

And it might already have some argon in it just like that. But argon is a noble gas. It's not going to bond anything. And while this lava is in a liquid state it's going to be able to bubble out. It'll just float to the top.

K/Ar Dating

It has no bonds. And it'll just evaporate. I shouldn't say evaporate. It'll just bubble out essentially, because it's not bonded to anything, and it'll sort of just seep out while we are in a liquid state. And what's really interesting about that is that when you have these volcanic eruptions, and because this argon is seeping out, by the time this lava has hardened into volcanic rock-- and I'll do that volcanic rock in a different color. By the time it has hardened into volcanic rock all of the argon will be gone.

It won't be there anymore. 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 there. So then you're only going to be left with potassium here. And that's why the argon is more interesting, because the calcium won't necessarily have seeped out. And there might have already been calcium here. So it won't necessarily seep out. But the argon will seep out. So it kind of resets it. The volcanic event resets the amount of argon So right when the event happened, you shouldn't have any argon right when that lava actually becomes solid.

And so if you fast forward to some future date, and if you look at the sample-- let me copy and paste it. So if you fast forward to some future date, and you see that there is some argon there, in that sample, you know this is a volcanic rock. You know that it was due to some previous volcanic event. You know that this argon is from the decayed potassium And you know that it has decayed since that volcanic event, because if it was there before it would have seeped out. So the only way that this would have been able to get trapped is, while it was liquid it would seep out, but once it's solid it can get trapped inside the rock.

And so you know the only way this argon can exist there is by decay from that potassium So you can look at the ratio. And so for every one of these argon's you know that there must have been 10 original potassium's. And so what you can do is you can look at the ratio of the number of potassium'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. 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. And so let's dig in the ground. So let's say this is the ground right over here.

And you dig enough and you see a volcanic eruption, you see some volcanic rock right over there, and then you dig even more. There's another layer of volcanic rock right over there. So this is another layer of volcanic rock.

So they're all going to have a certain amount of potassium in it. This is going to have some amount of potassium in it.

And then let's say this one over here has more argon This one has a little bit less. 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 that's left, or using the ratio of the potassium left to what you know was there before, you say that this must have solidified million years ago, million years before the present. And you know that this layer right over here solidified.

Let's say, you know it solidified about million years before the present. Potassium K is one of the most abundant elements in the Earth's crust 2. One out of every 10, Potassium atoms is radioactive Potassium K These each have 19 protons and 21 neutrons in their nucleus. If one of these protons is hit by a beta particle, it can be converted into a neutron.

With 18 protons and 22 neutrons, the atom has become Argon Ar , an inert gas. For every K atoms that decay, 11 become Ar How is the Atomic Clock Set? When rocks are heated to the melting point, any Ar contained in them is released into the atmosphere. When the rock recrystallizes it becomes impermeable to gasses again. As the K in the rock decays into Ar, the gas is trapped in the rock. The Decay Profile In this simulation, a unit of molten rock cools and crystallizes. The ratio of K to Ar is plotted. Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C graph.