Chart of a few different isotope half lifes: In reality, geologists tend to mix and match relative and absolute age dates to piece together a geologic history.

The atoms of some chemical elements have different forms, called isotopes.

These break down over time in a process scientists call radioactive decay.

Relative age dating also means paying attention to crosscutting relationships.

Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.

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.

Pretty obvious that the dike came after the rocks it cuts through, right?

With absolute age dating, you get a real age in actual years.

Geologists often need to know the age of material that they find.