Measures the decay of a rock or fossil to measure the age.

Question

anonymous · Answer

So what's the question?

alllie · Answer

I'm pretty sure you are asking what word fits the definition you wrote? If not then sorry lol. 

So based on what you said, the word to fit that definition would be Radiometric Dating, also more commonly known as Radioactive Dating. I did a little bit of research and I saw that Radiometric Dating was a method to used to date rocks and other various objects based off of it's known decay rate of radioactive isotopes. So I believe that's the word you're looking for. 

Sorry if this wasn't what you were asking, as I was a little bit confused what your question was. Hope I could help! c: