Radioactive dating of rock samples
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 grains of sand in the top half of the hourglass are the radioactive parents, and those falling to the bottom are the stable daughters.
Radiometric dating works best on igneous rocks, which are formed from the cooling of molten rock, or magma.
As magma cools, radioactive parent isotopes are separated from previously formed daughter isotopes by the crystallization process.
Note that at time 0, the time of the mineral's formation, the crystal contains only parent atoms.
At time 1, 50% of the parent atoms remain; at time 2, only 25% remain, and so on.