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.
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.
Some measure the decay of isotopes more indirectly.This is different to relative dating, which only puts geological events in time Most absolute dates for rocks are obtained with radiometric methods.These use radioactive minerals in rocks as geological clocks.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.