This uses radioactive minerals that occur in rocks and fossils almost like a geological clock.It’s often much easier to date volcanic rocks than the fossils themselves or the sedimentary rocks they are found in.Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils.If the fossil you are trying to date occurs alongside one of these index fossils, then the fossil you are dating must fall into the age range of the index fossil. In a hypothetical example, a rock formation contains fossils of a type of brachiopod known to occur between 410 and 420 million years.
It was first recognized by William Smith, a British Surveyor, who while working on open cuts of canals, railroads, and roads, noticed that the fossils change systematically from the older towards the younger rocks.
Studying the layers of rock or strata can also be useful. If a layer of rock containing the fossil is higher up in the sequence that another layer, you know that layer must be younger in age. This can often be complicated by the fact that geological forces can cause faulting and tilting of rocks.
Absolute dating is used to determine a precise age of a rock or fossil through radiometric dating methods.
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.
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.