

Radioactive Isotopes
 the "Clocks in Rocks"

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Numerical and Relative Ages for Rocks

A numerical (or "absolute") age is a specific number of years, like 150 million years ago. A relative age simply states whether one rock formation is older or younger than another formation. The Geologic Time Scale was originally laid out using relative dating principles. Numerical dating, the focus of this exercise, takes advantage of the "clocks in rocks"  radioactive isotopes ("parents") that spontaneously decay to form new isotopes ("daughters") while releasing energy. For example, decay of the parent isotope Rb87 (Rubidium) produces a stable daughter isotope, Sr87 (Strontium), while releasing a beta particle (an electron from the nucleus). ("87" is the atomic mass number = protons + neutrons.) 

Many minerals contain radioactive isotopes. In theory, the age of any of these minerals can be determined by 
1)  counting the number of daughter isotopes in the mineral, and 
2)  using the known decay rate to calculate the length of time required to produce that number of daughters. 
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