Carbon dating lava
It's assumed that the clock was set to zero when the study material was formed.
This requires that only the parent isotope be initially present or that the amount of daughter isotope present at the beginning is known so that it can be subtracted.
While there is no proof that the rates were different in the past than they are today, there is also no proof that they were the same.
What do rock layers on the Earth's crust tell us about our origins and the age of the earth?
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