| MadSci Network: Earth Sciences |
The dating of rocks depends entirely on the fact that the atoms have been in existence for quite some time. Carbon dating is the most commonly employed method for dating fossils and bones. It involves measuring the amount of carbon-14, an isotope of 'naturally occurring' carbon, carbon-12. 14C has two more neutrons than 12C. Given a certain amount of 14C, it is known that half of the atoms in the sample will decay to 12C in a period of approximately 5730 years. This decay is a physical property inherent to atoms of 14C. All living organisms have a certain amount of 14C in their body. When alive we eat foods that have some 14C, while loosing meolcules that have 14C in them through sweat, shedding of skin, etc. However, when an organism dies, this constant intake/loss of 14C stops. In the case of fossilized organisms, the main way 14C has been lost is by its conversion to 12C. Scientists can calculate how much 14C a living organism should have in it. They then compare the amount found in a sample of fossilized material and can deduce an approximate age of the fossil --> half as much ~ 5730 years old; one fourth as much ~ 28,000 years old, etc.
Very old rocks may be 'dated' by the presence of many kinds of isotopes, some of which have decay half lives far longer than 5730 years. The theory is much the same as for carbon dating -- once the rock was formed, there was no further influx/efflux of the isotope. The amount found in the sample of rock can be compared with amounts in equivalent types of rocks more recently formed, and an aprroximate age determined.