What is the half life of potassium argon dating
So for example, potassium can come in a form that has exactly 20 neutrons. And 39, this mass number, it's a count of the 19 protons plus 20 neutrons. But this is also the isotope of potassium that's interesting to us from the point of view of dating old, old rock, and especially old volcanic rock.
And this is actually the most common isotope of potassium. This accounts for about 6.7% of the potassium on the planet. And as we'll see, when you can date old volcanic rock it allows you to date other types of rock or other types of fossils that might be sandwiched in between old volcanic rock.
Join Britannica's Publishing Partner Program and our community of experts to gain a global audience for your work! This dating method is based upon the decay of radioactive potassium-40 to radioactive argon-40 in minerals and rocks; potassium-40 also decays to calcium-40.
Thus, the ratio of argon-40 and potassium-40 and radiogenic calcium-40 to potassium-40 in a mineral or rock is a measure of the age of the sample.
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.
To understand argon-argon dating, you need to understand potassium-argon dating.
But since floods jumble materials of different origins and ages together, that meant the scientists had to date dozens of different minerals.
The youngest crystal in the footprint layer would represent the oldest possible age for the prints; the oldest crystal in the layer above it would represent the youngest they could be.
The calcium-potassium age method is seldom used, however, because of the great abundance of nonradiogenic calcium in minerals or rocks, which masks the presence of radiogenic calcium.
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.
By converting potassium-39 to argon-39 then measuring the argon-39:argon-40 ratio, you can calculate the sample's potassium-40:argon-40 ratio, remembering potassium-40:potassium-39 is fixed.