When was potassium argon dating first used

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.

And he hopes the rock has remained sealed until the time he collected his sample.

the geologist only needs to measure the relative amounts of potassium-40 and argon-40 in the rock at the present time to be able to calculate an age for the rock.

The attraction of the method lies in the fact that one of the daughter elements is argon which is an inert gas.

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.

He thinks this solves his problem of not knowing the initial quantity of the daughter element in the past and not being able to go back in time and make measurements. He assumes that any argon-40 that he measures in his rock sample must have been produced by the radioactive decay of potassium-40 since the time the rock solidified.

He imagines that his radioactive hour glass sealed when the rock solidified, and his radioactive clock started running.

It accounts for, I'm just rounding off, 93.3% of the potassium that you would find on Earth. You also have potassium-- and once again writing the K and the 19 are a little bit redundant-- you also have potassium-41. And then you have a very scarce isotope of potassium called potassium-40. And so what's really interesting about potassium-40 here is that it has a half-life of 1.25 billion years. So when you think about it decaying into argon-40, what you see is that it lost a proton, but it has the same mass number.

So the good thing about that, as opposed to something like carbon-14, it can be used to date really, really, really old things. So one of the protons must of somehow turned into a neutron. 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. So right when the event happened, you shouldn't have any argon-40 right when that lava actually becomes solid.

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