This relatively new technique was developed in order to achieve more accurate dates than those obtained from the potassium-argon method.
The older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted.
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 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.
Another useful chemical analysis technique involves calculating the amount of nitrogen within a bone.The level of nitrogen gradually reduces as the bone decays.Absolute dating is not possible with this method because the rate at which the nitrogen content declines depends on the surrounding temperature, moisture, soil chemicals and bacteria.Knowing when a dinosaur or other animal lived is important because it helps us place them on the evolutionary family tree.Accurate dates also allow us to create sequences of evolutionary change and work out when species appeared or became extinct. These are: Where possible, several different methods are used and each method is repeated to confirm the results obtained and improve accuracy.The technique can, however, provide the relative ages of bones from the same site.Most fossils are found in sedimentary rocks deposited in layers.Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date.A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age.The heat from a volcanic eruption releases all the argon from the molten rock and disperses it into the atmosphere.Argon then starts to re-accumulate at a constant rate in the newly formed rock that is created after the eruption.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.Each volcanic eruption produces a new deposit of ash and rock.Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.