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.
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.
One line of evidence involves rocks from outside the Earth--meteorites and moon rocks.
Radiometric dating shows that almost all meteorites are between 4.5 and 4.7 billion years old.
Radiometric dating has shown that very old rocks--3.5 billion years or older--occur on all the continents.
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.
Scientists determined the Earth's age using a technique called radiometric dating.
Radiometric dating is based upon the fact that some forms of chemical elements are radioactive, which was discovered in 1896 by Henri Becquerel and his assistants, Marie and Pierre Curie.