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.
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.
Only one sample is required for this method as both the argon-39 and argon-40 can be extracted from the same sample.
In special cases, bones can be compared by measuring chemicals within them.
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.
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.
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.