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But how is it dated? What does radiometric dating actually mean? And what methods of dating can be used to date which kinds of items? Radiometric dating is a method of establishing how old something is — perhaps a wooden artefact, a rock, or a fossil — based on the presence of a radioactive isotope within it. The basic logic behind radiometric dating Define radiometric dating that if you compare the presence of a radioactive isotope within a sample to its known abundance on Earth, and its known half-life its rate of decayyou can calculate the age of the sample.
Radiometric dating is useful for finding the age of ancient things, because many radioactive materials decay at a slow rate. The of protons or neutrons in the atom changes, leading to a different isotope or element. Radiocarbon dating is possible because all living things take in carbon from their environment, which includes a small amount of Define radiometric dating radioactive isotope 14 C, formed from cosmic rays bombarding nitrogen When an animal or plant dies, it will not take in any more carbon, and the 14 C present will begin to decay.
This can raise complexities in archaeology when, for example, a society uses a piece of wood that was felled hundreds of years prior. There are also issues because the rate of cosmic ray bombardment of the planet over time has not always been stable: but this problem is largely redressed by a calibration factor.
Radiocarbon dating is not suitable for dating anything older than around 50, Define radiometric dating, because 14 C decays quickly its half-life is 5, years and so will not be present in ificant enough amounts in older objects to be measurable. More recently, Australian scientists used radiocarbon dating to figure out the age of wasp nests in rock artand thereby establishing a date range for the art.
Potassium-argon dating is a method that allows us to calculate the age of Define radiometric dating rock, or how long ago it was formed, by measuring the ratio of radioactive argon to radioactive potassium within it.
Radioactive potassium 40 K — a solid decays to radioactive argon 40 Ar — a gasat a known rate. When volcanic rocks are formed and cooled, all argon within the rock is released into the atmosphere, and when the rock hardens, none can re-enter. This means that any argon present in a volcanic rock must have been produced by the decay of radioactive potassium, so measuring the ratio can enable a scientist to date the sample. However, there are potential issues with potassium-argon dating. Argon-argon dating is an updated method, based on the original K-Ar dating technique, that uses neutron irradiation from a nuclear reactor to convert a stable form of potassium into the argon isotope 39 Ar, and then measures the ratio of 40 Ar to 39 Ar.
Argon-argon dating was used to determine that the Australopithecus Lucywho rewrote our understanding of early hominin evolution, lived around 3. This technique involves measuring the ratio of uranium isotopes U or U to stable lead isotopes Pb, Pb and Pb. It can be used to determine ages from 4. This method is thought to be particularly accurate, with an error-margin that can be less than two million years — not bad in a time span of billions.
U-Pb dating is most often done on igneous rocks containing zircon. The uranium content of the sample must be known; this can be determined by placing a plastic film over the polished slice and bombarding it with slow neutrons — neutrons with low kinetic energy.
This bombardment produces new tracks, the quantity of which can be compared with the quantity of original tracks to determine the age. This method can date naturally occurring minerals and man-made glasses. It can thus be used for very old samples, like meteorites, and very young samples, like archaeological artefacts. Fission-track dating identified that the Brahin Pallasitea meteorite found in the 19 th century in Belarus — slabs of which have become a collectors item — underwent its last intensive thermal event 4. This method involves calculating the prevalence of the very rare isotope chlorine 36 Clwhich can be produced in the atmosphere through cosmic rays bombarding argon atoms.
Chlorine was also released in abundance during the detonation of nuclear weapons between and It stays in the atmosphere for about a week, and so can mark young groundwater from the s onwards as well. However, they do use radioactive material. These methods date crystalline materials to the last time they were heated — whether by human-made fires or sunlight. Exposure to sunlight or heat releases these, removing the charges from the sample.
The material is stimulated using light optically stimulated luminescence or heat thermoluninescencewhich causes a al to be released from the object, the intensity of which can provide a measure of how much radiation was absorbed after the burial of the material — if you know the amount of background radiation at the burial site.
This method can date archaeological materials, such as ceramics, and minerals, like lava flows and limestones. It has a normal range of a few decades toyears old, but some studies have used it to identify much older things. There are several other radioactive isotopes whose ratios can be measured to date rocks, including samarium-neodymium, rubidium-strontium, and uranium-thorium.
Each of these have their own advantages and idiosyncrasies, but they rely on the same logic of radioactivity to work. The Royal Institution of Australia has an Education resource based on this article. You can access it here. Cosmos is published by The Royal Institution of Australia, a charity dedicated to connecting people with the world of science. Financial contributions, however big or small, help us provide access to trusted science information at a time when the world needs it most. Please support us by making a donation or purchasing a subscription today.Define radiometric dating
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What is radiometric dating?