Cost of k ar dating
In the Ar-Ar technique, the K and Ar are measured on the same sample aliquot.First the sample is irradiated in a nuclear reactor, where fast neutrons convert some of the 39K to 39Ar.This is going to have some amount of potassium-40 in it.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.
We know that if it's potassium that atom has 19 protons.
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.
And you know that this layer right over here solidified.
Let's say, you know it solidified about 150 million years before the present. 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. It allows us to date things that are very, very, very old and go way further back in time than just carbon-14 dating.