In other words, the half-life of an isotope is the amount of time it takes for half of a group of unstable isotopes to decay to a stable isotope. The half-life is constant and measurable for a given radioactive isotope, so it can be used to calculate the age of a rock.

What is half-life relative or absolute dating?

Absolute dating is often based on the amount of carbon-14 or other radioactive element that remains in a fossil. Carbon-14 has a half-life of 5,370 years .Relative Age.IsotopePotassium-40Decay ProductArgonHalf-life1.3 billion yearsAging of Rocks or FossilsEarths oldest rocks3 more columns•Dec 11, 2015