Radiometric dating example problems
For example, carbon has a half-life of 5, years and is used to measure the age of organic material. The ratio of carbon to carbon in living things remains constant while the organism is alive because fresh carbon is entering the organism whenever it consumes nutrients. When the organism dies, this consumption stops, and no new carbon is added to the organism. As time goes by, the ratio of carbon to carbon in the organism gradually declines, because carbon radioactively decays while carbon is stable.
Analysis of this ratio allows archaeologists to estimate the age of organisms that were alive many thousands of years ago. Along with stable carbon, radioactive carbon is taken in by plants and animals, and remains at a constant level within them while they are alive. After death, the C decays and the C C ratio in the remains decreases. Comparing this ratio to the C C ratio in living organisms allows us to determine how long ago the organism lived and died.
C dating does have limitations. For example, a sample can be C dating if it is approximately to 50, years old. Before or after this range, there is too little of the isotope to be detected. Substances must have obtained C from the atmosphere. For this reason, aquatic samples cannot be effectively C dated.
Lastly, accuracy of C dating has been affected by atmosphere nuclear weapons testing. Fission bombs ignite to produce more C artificially. Samples tested during and after this period must be checked against another method of dating isotopic or tree rings. To calculate the age of a substance using isotopic dating, use the equation below:. How long will it take for Ra has a half-life of years.
Radioactive dating can also use other radioactive nuclides with longer half-lives to date older events.
For example, uranium which decays in a series of steps into lead can be used for establishing the age of rocks and the approximate age of the oldest rocks on earth. Since U has a half-life of 4. In a sample of rock that does not contain appreciable amounts of Pb, the most abundant isotope of lead, we can assume that lead was not present when the rock was formed. Therefore, by measuring and analyzing the ratio of U Pb, we can determine the age of the rock.
This assumes that all of the lead present came from the decay of uranium If there is additional lead present, which is indicated by the presence of other lead isotopes in the sample, it is necessary to make an adjustment. Potassium-argon dating uses a similar method. K decays by positron emission and electron capture to form Ar with a half-life of 1. If a rock sample is crushed and the amount of Ar gas that escapes is measured, determination of the Ar K ratio yields the age of the rock. Other methods, such as rubidium-strontium dating Rb decays into Sr with a half-life of As of , the oldest known rocks on earth are the Jack Hills zircons from Australia, found by uranium-lead dating to be almost 4.
An ingenious application of half-life studies established a new science of determining ages of materials by half-life calculations. After one half-life, a 1. Present day estimates for the age of the Earth's crust from this method is at 4 billion years. Isotopes with shorter half-lives are used to date more recent samples.
Carbon, Radiometric Dating - CSI
There are a few categories of artifacts that can be dated using carbon; however, they cannot be more 50, years old. Carbon cannot be used to date biological artifacts of organisms that did not get their carbon dioxide from the air. This rules out carbon dating for most aquatic organisms, because they often obtain at least some of their carbon from dissolved carbonate rock. The age of the carbon in the rock is different from that of the carbon in the air and makes carbon dating data for those organisms inaccurate under the assumptions normally used for carbon dating.
This restriction extends to animals that consume seafood in their diet. As stated previously, carbon dating cannot be used on artifacts over about 50, years old.
These artifacts have gone through many carbon half-lives, and the amount of carbon remaining in them is miniscule and very difficult to detect. Carbon dating cannot be used on most fossils, not only because they are almost always allegedly too old, but also because they rarely contain the original carbon of the organism that has been fossilized. Also, many fossils are contaminated with carbon from the environment during collection or preservation procedures.
Scientists attempt to check the accuracy of carbon dating by comparing carbon dating data to data from other dating methods.
Other methods scientists use include counting rock layers and tree rings. When scientists first began to compare carbon dating data to data from tree rings, they found carbon dating provided "too-young" estimates of artifact age. Scientists now realize that production of carbon has not been constant over the years, but has changed as the radiation from the sun has fluctuated. Nuclear tests, nuclear reactors and the use of nuclear weapons have also changed the composition of radioisotopes in the air over the last few decades.
This human nuclear activity will make precise dating of fossils from our lifetime very difficult due to contamination of the normal radioisotope composition of the earth with addition artificially produced radioactive atoms. The various confounding factors that can adversely affect the accuracy of carbon dating methods are evident in many of the other radioisotope dating methods.
Although the half-life of some of them are more consistent with the evolutionary worldview of millions to billions of years, the assumptions used in radiometric dating put the results of all radiometric dating methods in doubt. The following is an article on this subject.
Although the half-life of carbon makes it unreliable for dating fossils over about 50, years old, there are other isotopes scientists use to date older artifacts. These isotopes have longer half-lives and so are found in greater abundance in older fossils. All of these methods are accurate only back to the last global catastrophe i. The assumptions are similar to the assumptions used in carbon dating.
The mathematical premise undergirding the use of these elements in radiometric dating contains the similar confounding factors that we find in carbon dating method. Most scientists today believe that life has existed on the earth for billions of years.
This belief in long ages for the earth and the evolution of all life is based entirely on the hypothetical and non-empirical Theory of Evolution. All dating methods that support this theory are embraced, while any evidence to the contrary, e. Prior to radiometric dating, evolution scientists used index fossils a.
A paleontologist would take the discovered fossil to a geologist who would ask the paleontologist what other fossils searching for an index fossil were found near their discovery. If it sounds like circular reasoning, it is because this process in reality is based upon circular reasoning. The process of using index fossils is describes by the late Creationist author and Ph. Henry Morris as follows:. All radiometric dating methods use this basic principle to extrapolate the age of artifacts being tested.
17.6: Radiocarbon Dating: Using Radioactivity to Measure the Age of Fossils and Other Artifacts
These long time periods are computed by measuring the ratio of daughter to parent substance in a rock, and inferring an age based on this ratio. This age is computed under the assumption that the parent substance say, uranium gradually decays to the daughter substance say, lead , so the higher the ratio of lead to uranium, the older the rock must be. While there are many problems with such dating methods, such as parent or daughter substances entering or leaving the rock, e.
Geologists assert that generally speaking, older dates are found deeper down in the geologic column, which they take as evidence that radiometric dating is giving true ages, since it is apparent that rocks that are deeper must be older. But even if it is true that older radiometric dates are found lower down in the geologic column which is open to question , this can potentially be explained by processes occurring in magma chambers which cause the lava erupting earlier to appear older than the lava erupting later.
Lava erupting earlier would come from the top of the magma chamber, and lava erupting later would come from lower down. A number of processes could cause the parent substance to be depleted at the top of the magma chamber, or the daughter product to be enriched, both of which would cause the lava erupting earlier to appear very old according to radiometric dating, and lava erupting later to appear younger. Other possible confounding variables are the mechanisms that can alter daughter-to-parent ratios.
We can see that many varieties of minerals are produced from the same magma by the different processes of crystallization, and these different minerals may have very different compositions. It is possible that the ratio of daughter to parent substances for radiometric dating could differ in the different minerals. Clearly, it is important to have a good understanding of these processes in order to evaluate the reliability of radiometric dating.