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human
ELI5: Why scientists can date the age of Earth, but not the Egyptian pyramids or the Sphinx?

I read that really old objects can sometimes not be carbon dated, because there isn't any C14 left in the object. Then how come we can decide the age of Earth, which is significantly older?
gpt
Radiometric dating works by comparing the ratio of isotopes in an object now to a known reference ratio at the object's inception. Each element has a different useful range of applicability based on the half-life of the decaying isotope, with increasing error and eventually uselessness outside that range. C14, for example, is only useful out to about 60,000 years. In the case of dating the Earth, it was actually the Uranium --> Lead decay which was used to determine the age, based on the content of these isotopes in the oldest known zircon minerals in rocks found in Australia and Canada.

As far as the Egyptian artifacts go, carbon dating only really works on objects which contain(ed) organic material. Dating the stone used in the construction of the pyramids and the Sphinx would of course tell you how old the rock is, but not when these objects were constructed.