Potassium-40 (K-40) decays to Calcium-40 (88.8%) or Argon-40 (11.2%); the half-life is 1.25 billion years. The most critical assumptions of the K/Ar method are that the material lost all its argon at an identifiable time, and that no argon has been lost since that time. Then one can measure the K-40 and the Ar-40 (subtracting out the Ar-40 which is assumed to have come from air) and calculate how far back the rock would have had no Ar-40. This is assumed to be the age of the rock. Click here for more details.
2010 Arthur V. Chadwick, Ph.D.