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- Half-life and carbon dating

The ratio of calcium formed to argon formed is fixed and known. Therefore the amount of argon formed provides a direct measurement of the amount of potassium present in the specimen when it was originally formed. Because argon is an inert gas , it is not possible that it might have been in the mineral when it was first formed from molten magma. Any argon present in a mineral containing potassium must have been formed as the result of radioactive decay. F, the fraction of K40 remaining, is equal to the amount of potassium in the sample, divided by the sum of potassium in the sample plus the calculated amount of potassium required to produce the amount of argon found.

The age can then be calculated from equation 1. In spite of the fact that it is a gas, the argon is trapped in the mineral and can't escape. Creationists claim that argon escape renders age determinations invalid. However, any escaping argon gas would lead to a determined age younger, not older, than actual. The creationist "argon escape" theory does not support their young earth model. The argon age determination of the mineral can be confirmed by measuring the loss of potassium. In old rocks, there will be less potassium present than was required to form the mineral, because some of it has been transmuted to argon.

The decrease in the amount of potassium required to form the original mineral has consistently confirmed the age as determined by the amount of argon formed. See Carbon 14 Dating in this web site. The nuclide rubidium decays, with a half life of Strontium is a stable element; it does not undergo further radioactive decay. Do not confuse with the highly radioactive isotope, strontium Strontium occurs naturally as a mixture of several nuclides, including the stable isotope strontium If three different strontium-containing minerals form at the same time in the same magma, each strontium containing mineral will have the same ratios of the different strontium nuclides, since all strontium nuclides behave the same chemically.

Note that this does not mean that the ratios are the same everywhere on earth. It merely means that the ratios are the same in the particular magma from which the test sample was later taken. As strontium forms, its ratio to strontium will increase. Strontium is a stable element that does not undergo radioactive change. In addition, it is not formed as the result of a radioactive decay process.

The amount of strontium in a given mineral sample will not change. Therefore the relative amounts of rubidium and strontium can be determined by expressing their ratios to strontium It turns out to be a straight line with a slope of The corresponding half lives for each plotted point are marked on the line and identified.

It can be readily seen from the plots that when this procedure is followed with different amounts of Rb87 in different minerals , if the plotted half life points are connected, a straight line going through the origin is produced. These lines are called "isochrons". The steeper the slope of the isochron, the more half lives it represents. When the fraction of rubidium is plotted against the fraction of strontium for a number of different minerals from the same magma an isochron is obtained.

If the points lie on a straight line, this indicates that the data is consistent and probably accurate. An example of this can be found in Strahler, Fig If the strontium isotope was not present in the mineral at the time it was formed from the molten magma, then the geometry of the plotted isochron lines requires that they all intersect the origin, as shown in figure However, if strontium 87 was present in the mineral when it was first formed from molten magma, that amount will be shown by an intercept of the isochron lines on the y-axis, as shown in Fig Thus it is possible to correct for strontium initially present.

The age of the sample can be obtained by choosing the origin at the y intercept. Note that the amounts of rubidium 87 and strontium 87 are given as ratios to an inert isotope, strontium However, in calculating the ratio of Rb87 to Sr87, we can use a simple analytical geometry solution to the plotted data. Again referring to Fig.

## Half-life and carbon dating (video) | Nuclei | Khan Academy

Since the half-life of Rb87 is When properly carried out, radioactive dating test procedures have shown consistent and close agreement among the various methods. If the same result is obtained sample after sample, using different test procedures based on different decay sequences, and carried out by different laboratories, that is a pretty good indication that the age determinations are accurate.

Of course, test procedures, like anything else, can be screwed up. Mistakes can be made at the time a procedure is first being developed. Creationists seize upon any isolated reports of improperly run tests and try to categorize them as representing general shortcomings of the test procedure.

This like saying if my watch isn't running, then all watches are useless for keeping time. Creationists also attack radioactive dating with the argument that half-lives were different in the past than they are at present. Log in or sign up to add this lesson to a Custom Course. Login or Sign up. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

The aging process in human beings is easy to see. As we age, our hair turns gray, our skin wrinkles and our gait slows. However, rocks and other objects in nature do not give off such obvious clues about how long they have been around. So, we rely on radiometric dating to calculate their ages. Radiometric dating, or radioactive dating as it is sometimes called, is a method used to date rocks and other objects based on the known decay rate of radioactive isotopes.

Different methods of radiometric dating can be used to estimate the age of a variety of natural and even man-made materials. The methods work because radioactive elements are unstable, and they are always trying to move to a more stable state. So, they do this by giving off radiation. This process by which an unstable atomic nucleus loses energy by releasing radiation is called radioactive decay. The thing that makes this decay process so valuable for determining the age of an object is that each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life.

So, if you know the radioactive isotope found in a substance and the isotope's half-life, you can calculate the age of the substance. So, what exactly is this thing called a half-life? Well, a simple explanation is that it is the time required for a quantity to fall to half of its starting value. So, you might say that the 'full-life' of a radioactive isotope ends when it has given off all of its radiation and reaches a point of being non-radioactive. When the isotope is halfway to that point, it has reached its half-life.

There are different methods of radiometric dating that will vary due to the type of material that is being dated.

### Radiometric Dating

For example, uranium-lead dating can be used to find the age of a uranium-containing mineral. It works because we know the fixed radioactive decay rates of uranium, which decays to lead, and for uranium, which decays to lead So, we start out with two isotopes of uranium that are unstable and radioactive. They release radiation until they eventually become stable isotopes of lead. These two uranium isotopes decay at different rates. In other words, they have different half-lives. The half-life of the uranium to lead is 4.

The uranium to lead decay series is marked by a half-life of million years.

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These differing rates of decay help make uranium-lead dating one of the most reliable methods of radiometric dating because they provide two different decay clocks. This provides a built-in cross-check to more accurately determine the age of the sample. Uranium is not the only isotope that can be used to date rocks; we do see additional methods of radiometric dating based on the decay of different isotopes. For example, with potassium-argon dating , we can tell the age of materials that contain potassium because we know that potassium decays into argon with a half-life of 1.

With rubidium-strontium dating , we see that rubidium decays into strontium with a half-life of 50 billion years. By anyone's standards, 50 billion years is a long time.

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In fact, this form of dating has been used to date the age of rocks brought back to Earth from the moon. So, we see there are a number of different methods for dating rocks and other non-living things, but what if our sample is organic in nature? For example, how do we know that the Iceman, whose frozen body was chipped out of glacial ice in , is 5, years old? Well, we know this because samples of his bones and hair and even his grass boots and leather belongings were subjected to radiocarbon dating. Radiocarbon dating , also known as carbon dating or simply carbon dating, is a method used to determine the age of organic material by measuring the radioactivity of its carbon content.

So, radiocarbon dating can be used to find the age of things that were once alive, like the Iceman. And this would also include things like trees and plants, which give us paper and cloth. So, radiocarbon dating is also useful for determining the age of relics, such the Dead Sea Scrolls and the Shroud of Turin.

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With radiocarbon dating, the amount of the radioactive isotope carbon is measured. Compared to some of the other radioactive isotopes we have discussed, carbon's half-life of 5, years is considerably shorter, as it decays into nitrogen Carbon is continually being created in the atmosphere due to the action of cosmic rays on nitrogen in the air. Carbon combines with oxygen to create carbon dioxide. Because plants use carbon dioxide for photosynthesis, this isotope ends up inside the plant, and because animals eat plants, they get some as well. When a plant or an animal dies, it stops taking in carbon The existing carbon within the organism starts to decay back into nitrogen, and this starts our clock for radiocarbon dating.

A scientist can take a sample of an organic material when it is discovered and evaluate the proportion of carbon left in the relic to determine its age. Radiometric dating is a method used to date rocks and other objects based on the known decay rate of radioactive isotopes. The decay rate is referring to radioactive decay , which is the process by which an unstable atomic nucleus loses energy by releasing radiation. Each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life or, in other words, the time required for a quantity to fall to half of its starting value.

There are different methods of radiometric dating. Uranium-lead dating can be used to find the age of a uranium-containing mineral. Uranium decays to lead, and uranium decays to lead The two uranium isotopes decay at different rates, and this helps make uranium-lead dating one of the most reliable methods because it provides a built-in cross-check.

Additional methods of radiometric dating, such as potassium-argon dating and rubidium-strontium dating , exist based on the decay of those isotopes. Radiocarbon dating is a method used to determine the age of organic material by measuring the radioactivity of its carbon content. With radiocarbon dating, we see that carbon decays to nitrogen and has a half-life of 5, years. To unlock this lesson you must be a Study. Did you know… We have over college courses that prepare you to earn credit by exam that is accepted by over 1, colleges and universities.

## What is Radioactive Dating? - Definition & Facts

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Explore over 4, video courses. Find a degree that fits your goals. Learn about half-life and how it is used in different dating methods, such as uranium-lead dating and radiocarbon dating, in this video lesson.

## Half-life and carbon dating

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