How are scientists able to tell how old a rock is?
In: Chemistry
You have probably heard of radiocarbon dating which is used to date organic elements based on how much carbon-14 have decayed. This is not usable on rocks because it does not contain carbon. However there are a number of similar radioisotope dating techniques for other elements besides carbon-14. For example potasium-argon dating. This works on any mineral that contains potasium. Some of this potasium is the isotope potasium-40 which with a half life of over a billion year turns into argon-40. You would not expect there to be any argon in molten or dissolved rock because argon is a gas that will just bubble up to the surface and escape into the atmpsohere. So all of the argon-40 must come from potasium-40. By measuring the ratio between these two isotopes you know how much have decayed and therefore how long it have taken. There are plenty of these types of dating techniques. Which ones work depend on the age and composition of the rock. You would usually try a few different techniques and then compare these results to each other for an accurate result.
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