The half-life of carbon-14 is 5,730 years, so carbon dating is only relevant for dating fossils less than 60,000 years old.
Radioactive elements are common only in rocks with a volcanic origin, so the only fossil-bearing rocks that can be dated radiometrically are volcanic ash layers.
Radioactive dating uses the decay rates of radioactive substances to measure absolute ages of rocks, minerals and carbon-based substances, according to How Stuff Works.
Scientists know how quickly radioactive isotopes decay into other elements over thousands, millions and even billions of years.
For instance, if an object has 50 percent of its decay product, it has been through one half-life.