How do archaeologists date their finds? Nowadays, fossils are usually dated using the radio carbon dating method. All living things contain carbon atoms. While 99 per cent of this carbon is regular carbon-12, carbon-13 makes up about one per cent and carbon-14 makes up about 0.000001 per cent. The rarer carbon-14 is radioactive.
In the late 1940s, an American chemist called Willard Libby realised that the radioactive decay of carbon-14 could be used by archaeologists to date their finds.
This dating method is based on the fact that radioactive elements eventually decay into stable atoms (nitrogen-14 in the case of carbon- 14). Half of the radioactive atoms will change in a given period of time, known as the half-life. This rate of decay is constant and is not affected by any chemical or physical conditions.
By knowing the half-life of carbon-14 and how many carbon-14 atoms a fossil specimen had when it was alive, archaeologists can work out how old it is by determining how many carbon-14 atoms it has left.
Carbon-14 has a half-life of 5,730 years. Therefore, if a living thing had 1,000 carbon-14 atoms before it died, 5,730 years after it died it would have 500 carbon-14 atoms and after 11,460 years it would have only 250.
The one drawback with radio carbon dating is that you have to know how many atoms of carbon- 14 the fossil had when it was a living thing. In the case of animals such as dinosaurs which have been extinct for millions of years, this can only be guessed at.