The age of a rock can be calculated if its ratio of uranium to lead is known.
This is unstable and eventually decays to lead, which is stable.
Many of us believed that string theory was a very dramatic break with our previous notions of quantum theory.
A decade later, American chemist Bertram Boltwood suggested that lead was a disintegration product of uranium and could be used as an internal clock for dating rocks.
From the mapped field relationships, it is a simple matter to work out a geological cross-section and the relative timing of the geologic events.
His geological cross-section may look something like Figure 2.
Over time, radioactive isotopes change into stable isotopes by a process known as radioactive decay.
Some radioactive parent isotopes decay almost instantaneously into their stable daughter isotopes; others take billions of years.