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Uranium-lead and potassium-argon dating

Atoms that can only have got there by decaying from something else

Uranium-lead dating can be used to date rocks that are as old as the Earth, some 4.5 billion years.

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Zircon is a very common crystal found in sand that easily incorporates uranium atoms but not lead atoms.  So any lead atoms found in zircon must have come from the decay of uranium-238 and uranium-235 at the end of their decay chains.

By analysing the ratio of lead to uranium the age of the crystal can be calculated.  The more lead there is, the older the sample.

A similar technique is used to date rocks using the decay of potassium-40 to argon-40.

back to Lesson 15: Half-life part 2