Different dating methods fossils
The table below shows characteristics of some common radiometric dating methods.
Geologists choose a dating method that suits the materials available in their rocks. Measuring isotopes is particularly useful for dating igneous and some metamorphic rock, but not sedimentary rock.
It took a canal surveyor circa 1800, William Smith in England, who noticed that he could map out great tracts of rocks on the basis of their contained fossils.
The sequences he saw in one part of the country could be matched precisely with the sequences in another.
These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.
Because of their unique decay rates, different elements are used for dating different age ranges.
Early geologists, at the end of the 18th and early 19th century noticed how fossils appeared in certain sequences: some fossil assemblages were always found below other assemblages, not above.
This meant that the ones below were older than the ones on top.
Geologists use radiocarbon to date such materials as wood and pollen trapped in sediment, which indicates the date of the sediment itself.
Any attempt to make a claim about evolution always comes back at some point to the geologic time scale.
But if you are going to be looking at time scales that are that old how do you get the dates?
Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.
Each isotope is identified with what is called a ‘mass number’.