A radiometric dating method based on the fact that potassium-40 in volcanic rock decays into argon-40 and calcium-40 at a known rate. The half-life of potassium-40 is approximately 1. 3 billion years. Chronometric dates are determined by measuring the amount of argon-40 in a sample. Similarly, argon-40 and argon-39 ratios can be used for chronometric dating.