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## Radiometric dating - Wikipedia

Radiocarbon dating is a method that provides objective age estimates for carbon-based materials that originated from living organisms. The impact of the radiocarbon dating technique on modern man has made it one of the most significant discoveries of the 20th century. Archaeology and other human sciences use radiocarbon dating to prove or disprove theories. Over the years, carbon 14 dating has also found applications in geology, hydrology, geophysics, atmospheric science, oceanography, paleoclimatology and even biomedicine. Radiocarbon, or carbon 14, is an isotope of the element carbon that is unstable and weakly radioactive. The stable isotopes are carbon 12 and carbon Carbon 14 is continually being formed in the upper atmosphere by the effect of cosmic ray neutrons on nitrogen 14 atoms.

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 measurements of time discusses how geological time can be measured Selected areas that are being discussed include Radio Carbon Dating. Radiocarbon dating is essentially a method designed to measure residual radioactivity. By knowing how much carbon 14 is left in a sample, the age of the. The technique of comparing the abundance ratio of a radioactive isotope to a reference isotope to determine the age of a material is called radioactive dating. If we were to measure the ratio of 14C to 12C today, we would find a value of.

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.

Radiometric dating is used to estimate the age of rocks and other objects based on the fixed decay rate of radioactive isotopes. Learn about half-life and how it is . Geologists use radiometric dating to estimate how long ago rocks formed, By measuring the quantity of unstable atoms left in a rock and comparing it to the. Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to . Precision is enhanced if measurements are taken on multiple samples from different locations of the rock body. Alternatively, if several different .

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. 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 datingwe 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 datingwe 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. 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 inis 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 datingalso 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. 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. Let's review. 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 decaywhich 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 datingexist 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. Create your account. Already a member?

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Try it risk-free for 30 days. Register to view this lesson Are you a student or a teacher? I am a student I am a teacher. Try Study. Cancel anytime. What teachers are saying about Study. Just checking in. The time that it takes for half of a sample to decay is known as the half life of the isotope.

Some isotopes have half lives longer than the present age of the universebut they are still subject to the same laws of quantum physics and will eventually decay, even if doing so at a time when all remaining atoms in the universe are separated by astronomical distances.

Various elements are used for dating different time periods; ones with relatively short half-lives like carbon or 14 C are useful for dating once-living objects since they include atmospheric carbon from when they were alive from about ten to fifty thousand years old. See Carbon dating. Longer-lived isotopes provide dating information for much older times. The key is to measure an isotope that has had time to decay a measurable amount, but not so much as to only leave a trace remaining.

Given isotopes are useful for dating over a range from a fraction of their half life to about four or five times their half life. Symbolically, the process of radioactive decay can be expressed by the following differential equation, where N is the quantity of decaying nuclei and k is a positive number called the exponential decay constant. The meaning of this equation is that the rate of change of the number of nuclei over time is proportional only to the number of nuclei.

This is consistent with the assumption that each decay event is independent and its chance does not vary over time. We can measure directly, for example by using a radiation detector, and obtain a good estimate of by analyzing the chemical composition of the sample.

The half-lifespecific to each nuclide, can be accurately measured on a pure sample, and is known to be independent of the chemical composition of the sample, temperature and pressure. The half-life of carbon is approximately 5, years. Approximately how old is the bone? Thus the bone is approximately 17, years old. Our input data had two significant figures, so reporting a more accurate result would be meaningless. This is based on the decay of rubidium isotopes to strontium isotopes, and can be used to date rocks or to relate organisms to the rocks on which they formed.

It suffers from the problem that rubidium and strontium are very mobile and may easily enter rocks at a much later date to that of formation. This method for rock dating is based on the decay of potassium into argon: until the rock solidifies, argon can escape, so it can in theory date the formation of rock. One problem is that potassium is also highly mobile and may move into older rocks. This depends on the decay of uranium and uranium to isotopes of lead.

Due to the long half-life of uranium it is not suitable for short time periods, such as most archaeological purposes, but it can date the oldest rocks on earth. A important limitation of radiometric dating often overlooked by layman and not always made clear in scholarly works as well is that any date is actually a range, following the 68—95— This leaves out important information which would tell you how precise is the dating result. Carbon dating has an interesting limitation in that the ratio of regular carbon to carbon in the air is not constant and therefore any date must be calibrated using dendrochronology.

Another limitation is that carbon can only tell you when something was last alive, not when it was used.

## How Does Carbon Dating Work

A limitation with all forms of radiometric dating is that they depend on the presence of certain elements in the substance to be dated. Carbon dating works on organic matter, all of which contains carbon. However it is less useful for dating metal or other inorganic objects. Most rocks contain uranium, allowing uranium-lead and similar methods to date them.

Radiometric dating involves dating rocks or other objects by measuring the extent to which different radioactive isotopes or nuclei have. Debunking the creationist radioactive dating argument. The argon age determination of the mineral can be confirmed by measuring the loss of potassium. Radiometric dating methods. In geology, an absolute age is a quantitative measurement of how old something is, or how long ago it occurred, usually expressed.

Other elements used for dating, such as rubidium, occur in some minerals but not others, restricting usefulness. Note that although carbon dating receives a lot of attention, since it can give information about the relatively recent past, it is rarely used in geology and almost never used to date fossils. Carbon decays almost completely withinyears of the organism dying, and many fossils and rock strata are hundreds of times older than that. Radioactive elements "decay" that is, change into other elements by "half lives.

The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life in other words raised to a power equal to the number of half-lives. If we knew the fraction of a radioactive element still remaining in a mineral, it would be a simple matter to calculate its age by the formula. To determine the fraction still remaining, we must know both the amount now present and also the amount present when the mineral was formed.

Contrary to creationist claims, it is possible to make that determination, as the following will explain:.

# How is radiometric dating measured

By way of background, all atoms of a given element have the same number of protons in the nucleus; however, the number of neutrons in the nucleus can vary. An atom with the same number of protons in the nucleus but a different number of neutrons is called an isotope. For example, uranium is an isotope of uranium, because it has 3 more neutrons in the nucleus. It has the same number of protons, otherwise it wouldn't be uranium.

The number of protons in the nucleus of an atom is called its atomic number. The sum of protons plus neutrons is the mass number. We designate a specific group of atoms by using the term "nuclide. Only K40 is radioactive; the other two are stable. K40 can decay in two different ways: it can break down into either calcium or argon.

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 gasit 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. Carbon dating: See Carbon 14 Dating in this web site. Rubidium-Strontium dating: 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