The half-life of a sample of potassium-40 (40K) is approximately 1.3 billion years. Potassium-40 decays into argon-40 (40Ar). Some rocks containing potassium-40 have been here since the earth formed and have been accumulating argon-40 to the present day. When rocks are super-heated by volcanic action, all the 40Ar escapes the rock. How would geologists use potassium-argon dating to determine how long ago a volcanic eruption occurred?