Radioactive isotope dating rocks
These use radioactive minerals in rocks as geological clocks.
The atoms of some chemical elements have different forms, called isotopes.
The amount of C-14 in any sample of carbon containing material can be found by measuring the level of radioactive decay, and comparing that with the decay rate observed in a carbon sample exposed to the continual mixing at the surface of the earth of C-12 and C-14 produced in the upper atmosphere.
Using the ratio of C-14 to total carbon, one can determine the age of the sample.
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.
Other particles, including neutrons, are produced by subsequent collisions.
The collision of a neutron with the nucleus of a N-14 isotope produces C-14, as follows: This form of carbon is radioactive.
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.
Sedimentary rock is made of particles derived from other rocks, so measuring isotopes would date the original rock material, not the sediments they have ended up in.
However, there are radiometric dating methods that can be used on sedimentary rock, including luminescence dating.
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.
Oliver Seely Radiocarbon method The age of ancient artifacts which contain carbon can be determined by a method known as radiocarbon dating.