Argon is gas that gradually builds up within rocks from the decay of radioactive potassium.

It is initially formed in the molten rock that lies beneath the Earth’s crust.

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This problem is now reduced by the careful collection of samples, rigorous crosschecking and the use of newer techniques that can date minute samples.

Volcanic rocks – such as tuff and basalt – can be used in dating because they are formed at a particular moment in time, during an eruption.

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.

Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.

Uranium is present in many different rocks and minerals, usually in the form of uranium-238.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.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.Knowing when a dinosaur or other animal lived is important because it helps us place them on the evolutionary family tree.