Radiometric dating according to creationists
It’s hard to find rocks on the surface of the Earth that have not been altered over time.
Radiometric dating is possible because this decay occurs at a known rate, called the “half-life” of the radioactive element.
Radiometric dating requires that one understand the initial ratio of the two elements in a given sample by some means.
In this case, Argon-40 is a gas that easily bubbles out and escapes when it is produced in molten rock.
Once the rock hardens, however, all the Argon-40 is trapped in the sample, giving us an accurate record of how much Potassium-40 has decayed since that time.
So, if we find a rock with equal parts Potassium-40 and Argon-40, we know that half the Potassium-40 has decayed into Argon-40, and that the rock hardened 1.3 billion years ago.