They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years.

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Each isotope is identified with what is called a ‘mass number’.

When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.

For example, carbon dating is used to determine the age of organic materials.

Once something dies, it ceases taking in new carbon-14, and the existing carbon-14 within the organism decays into nitrogen at a fixed rate.

However, there are radiometric dating methods that can be used on sedimentary rock, including luminescence dating.

All radiometric dating methods measure isotopes in some way.

Geologists use radiocarbon to date such materials as wood and pollen trapped in sediment, which indicates the date of the sediment itself.

The table below shows characteristics of some common radiometric dating methods.

Relative dating and radiometric dating are used to determine age of fossils and geologic features, but with different methods.

Relative dating uses observation of location within rock layers, while radiometric dating uses data from the decay of radioactive substances within an object.

Scientists measure the proportion of carbon-14 left in the organism to determine its age.