40K19 isotope of potassium has a half-life of 1.4 * 10^I9 yr and decays to form stable argon, 40Ar18. A sample of rock has been taken which contains both potassium and argon in the ratio 40Ar18. A sample of rock has been taken which contains both potassium and argon in the ratio 40K19 isotope of [...]
