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Absolute dating is method of determining a precise date for archaeological or fossil samples. Some people would like it to be called as chronometric orcalendar dating because the wordabsolute indicates some kind of certainty or accuracy which is very difficult to say in most of the cases of archaeology.
Absolute dating is generally made on the basis of physical or chemical properties of materials used in manmade objects, buildings or any other material modified by human being.
Absolute dating methods are quite different from relative dating methods based which are based on stratigraphy.While absolute dating gives numerical age of a sample, relative dating gives only age sequence.
The three main methods of absolute dating are
Radiocarbon dating is one of the most well known and widely used methods of absolute dating. This method uses C-14 dating to for dating organic matters. This technique uses radioactivity measurements as it is based on radioactive decay. Carbon-14 is an unstable radio isotope of carbon. This is produced by the action of cosmic radiation on the Earth’s atmosphere. Plants absorb this isotope along with normal carbon atoms at the time of photosynthesis of carbon dioxide. This isotope goes up along the food chain when animals eat these plants or predators eat these animals. With the death of plants and animals, its absorption stops, and the isotope starts breaking into Nitrogen-14 atom at a fixed exponential rate. The half life of C-14 isotope is 5,730 years. It takes 10 such half lives for C-14 atom to be completely converted to Nitrogen-14 atom. Therefore, we can do absolute dating of such organic matters only up to about 60,000 years. Carbon dating beyond this period is not possible because of this limitation.
For samples older than 60,000 years, another method of absolute dating is potassium-argon dating. In this case, instead of C-14 radio isotope, Potassium-40 radio isotope is used for dating. Potassium-40 breaks into Argon-40 gas. The advantage of this method is that half life of potassium-40 is much longer than C-14 which is 1.3 billion years. This method gives better results with older samples. Another advantage of this method over carbon dating is that it could be done with inorganic samples also, whereas carbon dating is applicable to organic samples only.
Thermoluminescence testing gives the date when the sample was heated last time. By this method, the sample is heated to 350 degrees Celsius or above releasing the trapped electrons inside the sample which produces luminescence or light on the basis of which the age of the sample is determined.