The find the approximate age of the granite one must know the half-life of potassium-40. The half-life of an element is the period of time in which half atoms of the same substance changes into the isotope of the element. The isotope of potassium-40 is Argon 40. Potassium-40 has a half-life of approximately 1.3 billion years. The rock is approximately 1.3 billion years old.
