Distinguish between relative dating and radioactive dating
Scientists can use certain types of fossils referred to as index fossils to assist in relative dating via correlation.Index fossils are fossils that are known to only occur within a very specific age range.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.Sedimentary rock is made of particles derived from other rocks, so measuring isotopes would date the original rock material, not the sediments they have ended up in.
Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.The Wheeler Formation has been previously dated to approximately 507 million year old, so we know the trilobite is also about 507 million years old.But, how can we determine how old a rock formation is, if it hasn’t previously been dated?For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes.Another example is luminescence dating, which measures the energy from radioactive decay that is trapped inside nearby crystals.
The table below shows characteristics of some common radiometric dating methods.