Buried bones absorb chemicals, such as uranium and fluorine, from the surrounding ground and absorb more of these chemicals the longer they remain buried.The rates of absorption depend on a number of factors which are too variable to provide absolute dates.This form of uranium usually decays into a stable lead isotope but the uranium atoms can also split – a process known as fission.During this process the pieces of the atom move apart at high speed, causing damage to the rock or mineral.Where the rocks are not strongly folded or tilted it is possible to work out the order in which the layers were formed.The oldest rocks and fossils are at the bottom and the youngest are on top.The age of volcanic rocks and ash can be determined by measuring the proportions of argon (in the form of argon-40) and radioactive potassium within them.
Most fossils are found in sedimentary rocks deposited in layers.
The number of tracks increases over time at a rate that depends on the uranium content.
It is possible to calculate the age of a sample by measuring the uranium content and the density of the fission tracks.
This newer method converts a stable form of potassium (potassium-39) into argon-39.
Measuring the proportions of argon-39 and argon-40 within a sample allows the age of the sample to be determined.
Argon is gas that gradually builds up within rocks from the decay of radioactive potassium.