Therefore, their ages indicate when they were formed.
Because all parts of the solar system are thought to have formed at the same time (based on the solar nebula theory), the Earth must be the same age as the moon and meteorites--that is, about 4.6 billion years old.
Radiometric dating works best on igneous rocks, which are formed from the cooling of molten rock, or magma.
As magma cools, radioactive parent isotopes are separated from previously formed daughter isotopes by the crystallization process.
Another line of evidence is based on the present-day abundances of the various isotopes of lead found in the Earth's crust. Three of these isotopes (lead 206, 207, 208) result from radioactive decay of isotopes of thorium and uranium.
The fourth, lead 204, is not the result of radioactive decay.
Note that at time 0, the time of the mineral's formation, the crystal contains only parent atoms.
At time 1, 50% of the parent atoms remain; at time 2, only 25% remain, and so on.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.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.The rates of decay of various radioactive isotopes have been accurately measured in the laboratory and have been shown to be constant, even in extreme temperatures and pressures.