Potassium argon kar dating of volcanic sediments is

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.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.

The data obtained cover the period of (291.3 ± 6.4) Ma, which corresponds to Carboniferous-Permian transition.

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.

And you know that this layer right over here solidified.

So what's interesting about this whole situation is you can imagine what happens during a volcanic eruption. And actually, it'll already contain some amount of argon-40. And let me do it in a color that I haven't used yet. And so for every one of these argon-40's you know that there must have been 10 original potassium-40's.

But what's neat about argon-40 is that while it's lava, while it's in this liquid state-- so let's imagine this lava right over here. And so what you can do is you can look at the ratio of the number of potassium-40's there are today to the number that there must have been, based on this evidence right over here, to actually date it.

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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.

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