On the other hand, the half-life of the isotope potassium 40 as it decays to argon is 1.26 billion years.
In a way this field, called geochronology, is some of the purest detective work earth scientists do.
There are two basic approaches: relative age dating, and absolute age dating.
Here is an easy-to understand analogy for your students: relative age dating is like saying that your grandfather is older than you.
Absolute age dating is like saying you are 15 years old and your grandfather is 77 years old.
Relative age dating also means paying attention to crosscutting relationships.