How is radioactive decay used to date fossils?

Answer 1

Dating of geological materials using radioactive decay is known as radiometric dating. There are several different methods, but the priciple is the same in each case.

Basically you measure the amount of a radioactive isotope/element (such as carbon-14, uranium, thorium, potassium....the list goes on) in the item that you are interested in. Provided that you know the decay rate of the particular radioactive isotope you measure, and you know how much of that isotpe was there in your item to start with, then you can work out how much of the radioactive isotope has decayed, and calculate an age.

However....knowing how much of the radioactive isotope was in your item to begin with is the tricky bit!!!

With things such as uranium and thorium, which decay into lead, you can measure the lead to work out how much of the uranium and thorium has decayed.

In practice however, most fossils can't easily be directly dated, either because they are too old for methods such a C-14 dating, or they contain too little urinium, thorium etc. to easily measure. In this case you would try to date some of the layers of rock in the sequence that contained the fossils.

Answer 2

Well all living things are carbon based. Carbon 14 in a naturally occurring isotope of carbon so it occurs in living things. When we find a fossil we can look at how much carbon 14 is left versus how much Nitrogen 14 is left (which is what C14 decays too). The half life (or time it takes for 50% of the parent isotope to decay to the daughter) of C14 is 5730±40 years. So if you have 50% C14 and 50% N14 you can say that the fossil is about 5700 years old. Its a little more complicated that that because you have to account for naturally occurring N14 that didn't come from C14 but that's the basic gist of it.