Unfortunately, none of the original rocks still exist, so scientists had to use less direct evidence to determine the age of the Earth. Radiometric dating shows that almost all meteorites are between 4.5 and 4.7 billion years old. Scientists assume that meteorites and moon rocks were not subjected to the extensive alteration that Earth rocks have undergone. Therefore, their ages indicate when they were formed. Another line of evidence is based on the present-day abundances of the various isotopes of lead found in the Earth's crust.
Natural lead is a mixture of four stable isotopes.
The fourth, lead 204, is not the result of radioactive decay. This means that all of the lead 204 on the Earth has been around since the formation of the Earth. Based on extensive sampling of the Earth's crust, scientists determined the present-day abundances of the four isotopes of lead relative to each other and to the parent isotopes that produced three of them. Because the original abundances of lead on the planet cannot be measured, scientists use meteorites to get at the Earth's original lead composition. Some meteorites contain the four lead isotopes but no uranium or thorium parents.