Parent radiometric dating
If, however, the rock is subjected to intense heat or pressure, some of the parent or daughter isotopes may be driven off.
Radiometric dating has shown that very old rocks--3.5 billion years or older--occur on all the continents.
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.
Ideally, the mineral crystals in igneous rocks form a closed system--nothing leaves or enters the crystal once it is formed.
This means that as radioactive parent elements decay, they and their daughters are trapped together inside the crystal.
We know the Earth must be at least as old as any rock on it.