However, relative methods are , which is that if there are layers of deposits, those laid down first will be on the bottom and those laid down last will be on the top. However, geological strata are not always found to be in a neat chronological order.

index fossils and relative dating-24

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.

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.

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.

Using relative dating the fossil is compared to something for which an age is already known.

For example if you have a fossil trilobite and it was found in the Wheeler Formation.

Carbon-14, the radioactive isotope of carbon used in carbon dating has a half-life of 5730 years, so it decays too fast.

It can only be used to date fossils younger than about 75,000 years.

Then after another 5,000 years half of the remaining parent isotope will have decayed.