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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.
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.
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.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.