​There are two main ways to determine the age of a rock, these are Relative dating and Absolute dating. Relative dating is used to determine the relative order of past events by comparing the age of one object to another. Cross dating is a method of using fossils to determine the relative age of a rock.

How do we determine the absolute age of rocks?

Geologists find absolute ages by measuring the amount of certain radioactive elements in the rock. When rocks are formed, small amounts of radioactive elements usually get included. As time passes, the parent radioactive elements change at a regular rate into non-radioactive daughter elements.