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.
K-Ar dating calculation (video) | Khan Academy
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. 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.
K-Ar dating calculation
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.
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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.
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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. And let's say you feel pretty good that this soil hasn't been dug up and mixed or anything like that. It looks like it's been pretty untouched when you look at these soil samples right over here. And let's say you see some fossils in here. Then, even though carbon dating is kind of useless, really, when you get beyond 50, years, you see these fossils in between these two periods. 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 million and million years old.
Then you have these fossils got deposited.
These animals died, or they lived and they died. And then you had this other volcanic event. So it allows you, even though you're only directly dating the volcanic rock, it allows you, when you look at the layers, to relatively date things in between those layer. In practice, each of these values may be expressed as a proportion of the total potassium present, as only relative, not absolute, quantities are required. To obtain the content ratio of isotopes 40 Ar to 40 K in a rock or mineral, the amount of Ar is measured by mass spectrometry of the gases released when a rock sample is volatilized in vacuum.
The potassium is quantified by flame photometry or atomic absorption spectroscopy. The amount of 40 K is rarely measured directly. The amount of 40 Ar is also measured to assess how much of the total argon is atmospheric in origin. Both flame photometry and mass spectrometry are destructive tests, so particular care is needed to ensure that the aliquots used are truly representative of the sample.
Ar—Ar dating is a similar technique which compares isotopic ratios from the same portion of the sample to avoid this problem. Due to the long half-life , the technique is most applicable for dating minerals and rocks more than , years old. For shorter timescales, it is unlikely that enough 40 Ar will have had time to accumulate in order to be accurately measurable. K—Ar dating was instrumental in the development of the geomagnetic polarity time scale.
One archeological application has been in bracketing the age of archeological deposits at Olduvai Gorge by dating lava flows above and below the deposits. In the K—Ar method was used by the Mars Curiosity rover to date a rock on the Martian surface, the first time a rock has been dated from its mineral ingredients while situated on another planet.