You might have noticed that many of the oldest age dates come from a mineral called zircon.
The narrower a range of time that an animal lived, the better it is as an index of a specific time.
With absolute age dating, you get a real age in actual years.
It’s based either on fossils which are recognized to represent a particular interval of time, or on radioactive decay of specific isotopes. Based on the Rule of Superposition, certain organisms clearly lived before others, during certain geologic times.
So carbon 14 is used to date materials that aren’t that old geologically, say in the tens of thousands of years, while potassium-argon dating can be used to determine the ages of much older materials, in the millions and billions year range.
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.
Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.