# K/ar-40 dating

The potassium-argon K-Ar isotopic dating method is especially useful for determining the age of lavas. Developed in the s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale. Potassium occurs in two stable isotopes 41 K and 39 K and one radioactive isotope 40 K. Potassium decays with a half-life of million years, meaning that half of the 40 K atoms are gone after that span of time.

## K–Ar dating

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Atomic number, atomic mass, and isotopes. Current time: Video transcript We know that an element is defined by the number of protons it has. For example, potassium. We look at the periodic table of elements. And I have a snapshot of it, of not the entire table but part of it here. Potassium has 19 protons. And we could write it like this. And this is a little bit redundant. We know that if it's potassium that atom has 19 protons.

And we know if an atom has 19 protons it is going to be potassium. Now, we also know that not all of the atoms of a given element have the same number of neutrons. And when we talk about a given element, but we have different numbers of neutrons we call them isotopes of that element. So for example, potassium can come in a form that has exactly 20 neutrons. And we call that potassium And 39, this mass number, it's a count of the 19 protons plus 20 neutrons.

And this is actually the most common isotope of potassium. It accounts for, I'm just rounding off, Now, some of the other isotopes of potassium. You also have potassium-- and once again writing the K and the 19 are a little bit redundant-- you also have potassium So this would have 22 neutrons. This accounts for about 6. And then you have a very scarce isotope of potassium called potassium Potassium clearly has 21 neutrons.

And it's very, very, very, very scarce. It accounts for only 0. 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 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 so what's really interesting about potassium here is that it has a half-life of 1. So the good thing about that, as opposed to something like carbon, it can be used to date really, really, really old things. And every 1. So argon is right over here. It has 18 protons. So when you think about it decaying into argon, what you see is that it lost a proton, but it has the same mass number.

So one of the protons must of somehow turned into a neutron. 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 And you see calcium on the periodic table right over here has 20 protons. So this is a situation where one of the neutrons turns into a proton. This is a situation where one of the protons turns into a neutron. And what's really interesting to us is this part right over here.

Because what's cool about argon, and we study this a little bit in the chemistry playlist, it is a noble gas, it is unreactive. And so when it is embedded in something that's in a liquid state it'll kind of just bubble out. It's not bonded to anything, and so it'll just bubble out and just go out into the atmosphere. So what's interesting about this whole situation is you can imagine what happens during a volcanic eruption.

Let me draw a volcano here. So let's say that this is our volcano. And it erupts at some time in the past. So it erupts, and you have all of this lava flowing. That lava will contain some amount of potassium And actually, it'll already contain some amount of argon But what's neat about argon is that while it's lava, while it's in this liquid state-- so let's imagine this lava right over here. It's a bunch of stuff right over here. I'll do the potassium And let me do it in a color that I haven't used yet.

I'll do the potassium in magenta. 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. 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. 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. 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.

## Argon–argon dating is a radiometric dating method invented to supersede potassium-argon 40Ar/39Ar dating relies on neutron irradiation from a nuclear reactor to convert a stable form of 40Ar* refers to the radiogenic 40Ar, i.e. the 40Ar produced from radioactive decay of 40K. 40Ar* does not include atmospheric argon. This dating method is based upon the decay of radioactive potassium to radioactive (K-Ar) dating, for example, because most minerals do not take argon.

Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials. Geologists have used this method to date rocks as much as 4 billion years old. It is based on the fact that some of the radioactive isotope of Potassium, Potassium K ,decays to the gas Argon as Argon Ar

Potassium, an alkali metal, the Earth's eighth most abundant element is common in many rocks and rock-forming minerals. The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present.

If you are having problems understanding concepts such as Average Nuclear binding Energy and nuclide stability; What is it that drives fission; fusion; and other nuclear reactions; Types of radioactive decay, alpha, beta, gamma, positron, and a summary of characteristics; Nuclear reactions; Nuclear equations; The use of nuclide charts to visually chart out nuclear reactions; The U decay series shown on a nuclide chart. See the Nuclear Reactions Page. If you are having problems understanding the basics of radioisotopes techniques, such as.

## Argon–argon dating

The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. The sample is generally crushed and single crystals of a mineral or fragments of rock hand-selected for analysis. These are then irradiated to produce 39 Ar from 39 K. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace. Heating causes the crystal structure of the mineral or minerals to degrade, and, as the sample melts, trapped gases are released. The gas may include atmospheric gases, such as carbon dioxide, water, nitrogen, and argon, and radiogenic gases, like argon and helium, generated from regular radioactive decay over geologic time.

## Potassium-argon dating

If you're seeing this message, it means we're having trouble loading external resources on our website. To log in and use all the features of Khan Academy, please enable JavaScript in your browser. Science Biology History of life on Earth Radiometric dating. Chronometric revolution. Potassium-argon K-Ar dating. K-Ar dating calculation. Atomic number, atomic mass, and isotopes. Current time:

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How Accurate is K-Ar Dating? Messel, "A Modern Introduction to Physics" vol. Sydney, p: The radiogenic argon that builds up in potassium-rich minerals after they have crystallized, therefore, furnishes a good measure of the age of the sample.

## Potassium-argon (K-Ar) dating

Potassium-argon dating , method of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock. This dating method is based upon the decay of radioactive potassium to radioactive argon in minerals and rocks; potassium also decays to calcium Thus, the ratio of argon and potassium and radiogenic calcium to potassium in a mineral or rock is a measure of the age of the sample. 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. The potassium-argon dating method has been used to measure a wide variety of ages. The potassium-argon age of some meteorites is as old as 4,,, years, and volcanic rocks as young as 20, years old have been measured by this method. We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind. Your contribution may be further edited by our staff, and its publication is subject to our final approval. Unfortunately, our editorial approach may not be able to accommodate all contributions. Our editors will review what you've submitted, and if it meets our criteria, we'll add it to the article.

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In this article we shall examine the basis of the K-Ar dating method, how it works, and what can go wrong with it. It is possible to measure the proportion in which 40 K decays, and to say that about Potassium is chemically incorporated into common minerals, notably hornblende , biotite and potassium feldspar , which are component minerals of igneous rocks. Argon, on the other hand, is an inert gas; it cannot combine chemically with anything. As a result under most circumstances we don't expect to find much argon in igneous rocks just after they've formed.

## Historical Geology/K-Ar dating

O Timothy, keep that which is committed to thy trust, avoiding profane and vain babblings, and oppositions of science falsely so called: Which some professing have erred concerning the faith. Lies of Evolution: K-Ar Dating. Christopher J. Johnson Published:

Kevin R. Most quantitative analytical methods, including any water analyses for organic or metal contaminants Skoog and West, , chapters 25 and 26 , require standards to provide accurate results. With water analyses, a calibration curve is established by analyzing several known standards. The concentrations of the unknowns are then determined by where they plot on the calibration curve. Ar-Ar dating also relies on standards to provide quantitative results.

Potassium—argon dating , abbreviated K—Ar dating , is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium K into argon Ar. Potassium is a common element found in many materials, such as micas , clay minerals , tephra , and evaporites. In these materials, the decay product 40 Ar is able to escape the liquid molten rock, but starts to accumulate when the rock solidifies recrystallizes. The amount of argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors.

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