Dating very old rocks
Once scientists have determined the parent-daughter ratio, they can use this measurement along with half-life of the parent to calculate the age of a rock containing the radioactive isotope.
Radiometric dating has shown that very old rocks--3.5 billion years or older--occur on all the continents.
The discovery gave scientists a tool for dating rocks that contain radioactive elements.
As magma cools, radioactive parent isotopes are separated from previously formed daughter isotopes by the crystallization process.Scientists determined the Earth's age using a technique called radiometric dating.