Difference between relative absolute dating
There are two main methods determining a fossils age, relative dating and absolute dating.
Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
These isotopes break down at a constant rate over time through radioactive decay.
By measuring the ratio of the amount of the original (parent) isotope to the amount of the (daughter) isotopes that it breaks down into an age can be determined.
We define the rate of this radioactive decay in half-lives.
If a radioactive isotope is said to have a half-life of 5,000 years that means after 5,000 years exactly half of it will have decayed from the parent isotope into the daughter isotopes.
The same rock formation also contains a type of trilobite that was known to live 415 to 425 million years ago.
Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of 415 to 420 million years.
It can only be used to date fossils younger than about 75,000 years.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.Then after another 5,000 years half of the remaining parent isotope will have decayed.While people are most familiar with carbon dating, carbon dating is rarely applicable to fossils.