# Radio-dating and the age of the earth

I understand there are a few radio-dating methods to determine the age of the earth, uranium-lead to name one (maybe not the best though). The ratio Pb206 + Pb207 to U allows you to find when the uranium came into existence. What I don't understand is how this relates to the age of the earth. Can't the uranium have been rambling for 500 million years in space before lumping together with other elements to form the earth? Which would make the age of the earth in this case 4 billion years instead of 4.55 billion.

Or is it the same as with C14 dating: the C14 is formed in the higher atmosphere through interaction of cosmic rays with nitrogen atoms. This stops when the carbon enters living material, so we know then the decay starts. Is the radioactive uranium created by cosmic rays until it hits earth? But the early earth didn't have an atmosphere which would stop the cosmic rays, did it?

So I'm a bit confused. How do we know the uranium formed at the same time as the earth?

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I think this article should explain: Zircon Chronology: The Oldest Material on Earth

Zircon is an extremely durable mineral, once zircon crystals have formed, they can last billions of years. It is a well understood chemical process that produces zircons, they crystallize today from crustal magmas. Zircons are common on earth, it is reasonable the that the oldest zircons we find in terrestrial rocks are a lower bound on the age of earth's crust.

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Zircon crystals won't incorporate lead, so at the moment of formation it contains some concentration of uranium and no lead. One half life later there are equal numbers of uranium and lead atoms. Two half lives later the lead:uranium ratio is 3:1, and so on. Hence the lead:uranium ratio gives the time since the zircon crystal formed. The ratios for $^{238}$U:$^{206}$Pb and $^{235}$U:$^{207}$Pb can be measured separately to reduce experimental error. This assumes no lead is lost from the zircon crystal. –  John Rennie Mar 5 '13 at 10:35