On the other hand, the abundance of argon in the Earth is relatively small because of its escape to the atmosphere during processes associated with volcanism. The potassium-argon dating method has been used to measure a wide variety of ages. The potassium-argon age of some meteorites is as old as 4,,, years, and volcanic rocks as young as 20, years old have been measured by this method.
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Please note that our editors may make some formatting changes or correct spelling or grammatical errors, and may also contact you if any clarifications are needed. Potassium-argon dating Written By: The Editors of Encyclopaedia Britannica. Learn More in these related Britannica articles: This is possible in potassium-argon K-Ar dating, for example, because most minerals do not take argon into their structures initially.
In rubidium-strontium dating, micas exclude strontium when they form but accept much rubidium. In uranium-lead U-Pb dating of zircon, the zircon is found to exclude initial lead…. The radioactive decay scheme involving the breakdown of potassium of mass 40 40 K to argon gas of mass 40 40 Ar formed the basis of the first widely used isotopic dating method. Since radiogenic argon was first detected in by the American geophysicist…. Potassium—argon dating has made it possible to establish that the earliest remains of man and his artifacts in East Africa go back at least 2,, years, and probably further.
Potassium-argon dating , for instance, can provide the age of a specimen by clocking the rate at which radioactive isotopes of these elements have decayed. When radiometric methods cannot be applied, investigators may still ascribe a relative age to a fossil by relating it to the…. More About Potassium-argon dating 5 references found in Britannica articles Assorted References major reference In dating: Analysis of separated minerals In dating: So what's interesting about this whole situation is you can imagine what happens during a volcanic eruption.
Let me draw a volcano here.
So let's say that this is our volcano. 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.
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It'll just float to the top. 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.
K–Ar dating - Wikipedia
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.
- Potassium-argon dating.
- Potassium-argon (K-Ar) dating.