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How Can Geochemists Calculate the Age Of A Rock When There Are So Many Assumptions They Make?

Hello Ask an ACS Chemist team, 

     I am coming to you with a question that has been on my mind for some time now.  In geochemistry, scientists use radioisotopic dating as a technique to give an "actual age" to the rock they are studying.  My questions, however, are these--

1. They were not there to see the rock form, so how do they know the amount of parent element present?

2. How did they calculate the half-life of the rock?

3. If decay is a randomized event for each atom that decays, how can they get a constant rate of decay based on all the random decay events?  

4. How do they know that the rate of decay is constant if they claim it is? 

Thank you very much for your time, and I hope to hear your response soon.  



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