Scientists usually express this as an age range e. Uranium-lead dating method is usually performed on the mineral zircon. But later in "Geologists working in the mountains of western Australia have discovered grains of rock that are 4. A sample of the kind of data that leads to such a projected age is the rubidium-strontium isochron of lunar sample which yields a time to last melting of 4. Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density. The process of dating finds the two ratios between uranium and lead; and uranium and lead At right, the crystalline structure of a zircon. This data is compared to a curve called the Concordia diagram. The method is usually applied to zircon.
The following radioactive decay processes have proven particularly useful in radioactive dating for geologic processes:. Note that uranium and uranium give rise to two of the natural radioactive series , but rubidium and potassium do not give rise to series. They each stop with a single daughter product which is stable. Some of the decays which are useful for dating, with their half-lives and decay constants are:. The half-life is for the parent isotope and so includes both decays. Some decays with shorter half-lives are also useful. Of these, the 14 C is unique and used in carbon dating. Note that the decay constant scale in the table below was kept the same as the table above for comparison. Parent isotope radioactive Daughter isotope stable Half-life y Decay constant 10 yr -1 10 Be 10 B 1. Of those isotopes, are stable and 70 are radioactive. Eighteen of the radioactive elements have long enough half-lives to have survived since the beginning of the solar system. The table above includes the main isotopes used for age studies. The natural radioactive series which involve lead as a daughter element do offer a mechanism to test the assumptions.
The nitty gritty on radioisotopic dating Radioisotopic dating is a key tool for studying the timing of both Earth's and life's history. Radioactive decay Radioisotopic dating relies on the process of radioactive decay, in which the nuclei of radioactive atoms radioactive dating uranium lead particles.
This releases energy in the form of radiation and often transforms one element into another. For example, over time, uranium atoms lose alpha particles each made up of two protons and two neutrons read article decay, via a chain of unstable daughters, into stable lead.
Although it is impossible to predict when a particular unstable atom will decay, the decay rate is predictable for a very large number of atoms. In other words, the chance that a given atom will decay is constant over time. For example, as shown at left below, uranium has a half-life of million years.
At the same time, the amount of the element that it decays into in this case leadwill increase accordingly, as shown below. How old would you hypothesize the rock is? Study the graph at left above. At what point on the graph would you expect the ratio of uranium to lead to be about 39 to 61? At around million years i. Thus, you would calculate that your rock is about a billion years old. Scientists usually express this as an age range e. With modern techniques, these ranges have gotten narrower and narrower, and consequently, even very ancient rocks can be dated quite precisely.
Some are much shorter. Final, dating website white label exist? allows scientists to date events that are more or less this web page. For example, carbon decays into nitrogen and has a half-life of just 5, years.
Hence, carbon dating can only be used to estimate much younger ages, up to around 60, years. Slightly different dating techniques are used with different radioactive elements, but the same basic logic of estimating backwards based on radioactive decay remains the same. The geology behind radioisotopic dating Though the basic logic behind radioisotopic dating relies on nuclear physics and quantum theory, many geologic processes also factor into our ability to date a particular rock.
How do scientists find the right rocks for dating? How do they know that the rock isn't contaminated with elements that would throw off the dating? How can the formation of a rock be correlated with a particular ancient event? The answers to all of these questions lie in our understanding of the geologic processes that affect the deposition of radioactive elements. To see how it works, we'll start at the beginning, using uranium as an example:.
At left, a zircon crystal in a thin section cut from granite. At right, the crystalline structure of a zircon. In the magma, crystals of zirconium silicate called zirconsas well as other crystals, form. If these crystals were pure, they would contain just zirconium, silica, and oxygen; however, uranium happens to have a similar arrangement of outer electrons to zirconium, and so as zircons form, "mistakes" are radioactive dating uranium lead made, and uranium is substituted for zirconium.
Because lead the stable daughter of uranium has a very different arrangement of electrons, it does not make its way into the crystal as it is forming. The formation of crystals in the magma marks the moment that the radio-isotopic clock starts ticking. When the eruption occurs, zircons are released in the ash and lava, which then become rocks like rhyolite.
Geologists hunt for these particular sorts of rock to date the volcanic more info in which the rock formed. Geologists extract the appropriate minerals from the rock in this case, zircon crystals and use a technique called mass spectrometry to figure out the relative amounts of uranium and lead radioactive dating uranium lead the zircon.
Thus, when a geologist dates a rock using uranium-lead dating, he or she is actually getting an estimate on the age of its zircon crystals, which formed "shortly" before the volcanic eruption. Of course, link this case "shortly" is meant in terms of geologic timescales.
The zircon formation may have occurred tens to hundreds of thousands of years before the eruption and deposition. However, when dealing with rocks that are hundreds of millions of year old, the time between zircon formation and eruption really is short in comparison. The amount of material involved in these estimates is small, but can be used to generate powerful results. Nevertheless, with modern techniques, scientists can measure these amounts very precisely.
Furthermore, to gain confidence in their estimates, geologists date five to ten zircon crystals from the same rock. Normally, these crystals will all point to the sameyear window. Occasionally, an outlying crystal will date to a much earlier time period than the others from the sample, and in these cases, geologists know that the rock sample has been contaminated by zircons from a different eruption. Furthermore, other radio-isotopic systems can be used as independent lines of evidence to validate the results from the uranium-lead method.
Using such techniques, scientists can very accurately date ancient volcanic events and can extrapolate from these dates to learn about the ages of other rock strata. To see how it works, we'll start at the beginning, using uranium as an example: At left, a zircon crystal in a thin section cut from granite. Search Glossary Home. Support this project. Studying mass extinctions.