Dating Techniques

When volcanic rocks are heated to extremely high temperatures, they release any argon gas trapped in them. As the rocks cool, argon-40 (⁴⁰Ar) begins to accumulate. Argon-40 is formed in the rocks by the radioactive decay of potassium-40 (⁴⁰K). The amount of ⁴⁰Ar formed is proportional to the decay rate (half-life) of ⁴⁰K, which is 1.3 billion years. In other words, it takes 1.3 billions years for half of the ⁴⁰K originally present to be converted into ⁴⁰Ar. This method is generally only applicable to rocks greater than three million years old, although with sensitive instruments, rocks several hundred thousand years old may be dated. The reason such old material is required is that it takes a very long time to accumulate enough ⁴⁰Ar to be measured accurately. Potassium-argon dating has been used to date volcanic layers above and below fossils and artifacts in east Africa.