Uranium and Thorium Dating

Uranium and thorium each have radioactive isotopes with half-lives of billions of years, or roughly the same order of magnitude as the age of Earth itself (see tables 13.4 and 13.5). These elements go through a series of radioactive decay that involve several radioactive isotopes with very diiferent half-lives. The net result is the same, though. Both elements end their respective series with stable isotopes of lead. Both uranium and thorium are common enough in rocks that the ratios of uranium to lead or of thorium to lead in a rock can be used to date rocks that are billions of years old. 

Parent Isotope Daughter Isotope Decay Mode Half-life 

Using these decay series, geologists have determined that the best estimate of the age of Earth is 4.6 bilHon years. The oldest rocks that have ever been found, however, are younger than this—3.5 to 4 bilHon years old. Because of plate tectonics—the movement of large masses of Earth s crust over Earth s mantle—rocks subside into Earth s interior, where they melt and become molten material. Eater, the same material may again rise to Earth s surface or pour out as magma during a volcanic eruption. The rocks eventually cool, but when they do, they are different rocks than the ones they were before subsidence. Thus, no one has found a rock that is actually the same age as Earth itself. 

---