So, we have a “clock” which starts ticking the moment something dies.Obviously, this works only for things which were once living.by Tas Walker A geologist works out the relative age of a rock by carefully studying where the rock is found in the field.The field relationships, as they are called, are of primary importance and all radiometric dates are evaluated against them.Radiometric dating is the method for establishing the age of objects by measuring the levels of radioisotopes in the sample. It decays to nitrogen 14 with a half life of 5730 years. Carbon 14 is created by cosmic rays in the upper atmosphere.
Here he can see that some curved sedimentary rocks have been cut vertically by a sheet of volcanic rock called a dyke.
C14 is continually being created and decaying, leading to an equilibrium state in the atmosphere.
When the carbon dioxide, containing C14 as well as stable C12 and C13, is taken in by plants it is no longer exposed to the intense cosmic ray bombardment in the upper atmosphere, so the carbon 14 isotope decays without being replenished.
Biostratigraphy: One of the first and most basic scientific dating methods is also one of the easiest to understand.
Layers of rock build one atop another — find a fossil or artifact in one layer, and you can reasonably assume it’s older than anything above it.