Fossils have been discovered which are hundreds of millions years old. How does one determine their age? The technique involves measuring the abundance of some elements. One method is commonly used called "Carbon dating". There are three forms (Isotopes) of carbon: the stable forms are carbon-12 carbon-13, while an unstable radioactive form which decays with times is carbon-14.
These forms of carbon are incorporated into the plant kingdom by the process of photosynthesis (conversion of atmospheric carbondioxide into more complex plant parts), and then through the food chain, carbon is passed onto animals. The radioactive carbon -14 is being constantly formed in the atmosphere as a result of bombarment of nitrogen by cosmic rays. Live plants and animals have the carbon-14 being constantly introduced from the atmospheric carbon dioxide but ones the living material die, the amount of carbon-14 starts to diminish with time, the clocks starts ticking.
Carbon-14 has a 'half life' of 5730 years i.e its amount to reduce its half after this time. After ten half lives (about 60,000 years), the amount of carbon-14 diminishes to almost zero and it is there fore not possible to find the age of plant or animal fossils older than 60,000 years by radiocarbon dating.
For determining age of fossils older than 60,000 years one uses a potassium-argon dating technique. Potassium dating has a half life of 1.3 billion years, thus allowing the age of rocks several billions years old to be determined. A more accurate "argon-argon" dating technique (determining the ration between argon-39 and argon-40) has also been developed.
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