This uses radioactive minerals that occur in rocks and fossils almost like a geological clock. So, often layers of volcanic rocks above and below the layers containing fossils can be dated to provide a date range for the fossil containing rocks. The atoms in some chemical elements have different forms, called isotopes.

Why are fossils useful in dating rocks?

Geologists obtain a wide range of information from fossils. They help us to understand evolution, and life in general; they provide critical information for understanding depositional environments and changes in Earths climate; and they can be used to date rocks.