The process of radiometric dating uses the decay of uranium-238 to lead-206 to determine the age of ancient rocks.
U-238 decays to stable Pb-206 largely by the emission of α particles, with a half-life of about 4.5 × 10⁹ a.
Thus, geologists can determine the age of rocks by measuring the relative amounts of U-238 and Pb-206.
However, the method is not “exact”.
The rock must be more than ten million years old so that enough uranium can decay to make the amount of lead measurable.
The age uncertainty for 100-million-year-old rocks ranges from 100 000 a
to 3 000 000 a.
The error margin for older rocks, say, 2.5 billion years old, can be as low
as 2 000 000 a. That’s a long time, but it’s an uncertainty of less than 0.1 %.
