Geological dating radioactive elements
Only the tiniest fraction of the Earth, the crust, is accessible.
Those rocks available for analysis (ie, the oldest ones) have been heated and squeezed many times in their GEOLOGICAL HISTORY, because for billions of years continents have been drifting over the Earth's surface, colliding and producing mountains and new ocean floors.
They were found, however, in much younger sediment and it is not known where these zircons originated.
The second approach, which is more indirect but gives an answer currently believed correct, involves a comparison of the Earth with meteorites.
The first involves sampling as much of the Earth's crust as possible and dating these rocks.Since 1950, radiometric methods have been developed to a very sophisticated level in several countries, including Canada.It has been demonstrated that when rocks which have led an undisturbed history are analysed, all methods reveal the same age.It often proves possible to date even severely disturbed rocks.To date the time of formation of a planet 12 740 km in diameter and 70.8% covered by water is not easy.