Geologists often need to know the age of material that they find.They use methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years. The atoms of some chemical have different forms, called isotopes.
Geologists use radiocarbon to date such materials as wood and trapped in sediment, which indicates the date of the sediment itself.
Measuring isotopes is particularly useful for dating and some rock, but not rock.
Sedimentary rock is made of particles derived from other rocks, so measuring isotopes would date the original rock material, not the they have ended up in.
Because of their unique decay rates, different elements are used for dating different age ranges.
For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.
measures radioactive isotopes in once-living material instead of rock, using the decay of to nitrogen-14.
Because of the fairly fast decay rate of carbon-14, it can only be used on material up to about 60,000 years old.
Each isotope is identified with what is called a ‘’.
When ‘parent’ uranium-238 decays, for example, it produces , energy and ‘daughter’ lead-206.
Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element.
These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.