​There are two main ways to determine the age of a rock, these are Relative dating and Absolute dating.

Relative dating

Relative dating is used to determine the relative order of past events by comparing the age of one object to another. This determines where in a timescale the object fits without finding its specific age; for example you could say you're older than your sister which tells us the order of your birth but we don't know what age either of you are.  

There are a few methods of relative dating, one of these methods is by studying the stratigraphy. Stratigraphy is the study of the order of the layers of rocks and where they fit in the geological timescale.  This method is most effective for studying sedimentary rocks.

Cross dating is a method of using fossils to determine the relative age of a rock. Fossil remains have been found in rocks of all ages with the simplest of organisms being found in the oldest of rocks. The more basic the organism the older the rock is. This practice supports the theory of evolution which states that simple life forms gradually evolve over time to form more complex ones.

If undisturbed, layers of sedimentary rocks help to determine the relative age of rock: the oldest being at the base and the newest on top. Source: Tes Teach with Blendspace

There are some drawbacks to using relative dating, these include

• This method does not give the age of the rock in years.
• External forces from plate tectonics or erosion can change the sequence of the rock.
• Large gaps in geological information can make dating difficult.

Absolute dating

Absolute dating finds the actual age of the object, this would be like you saying you're 15 and your sister is 11.

In geology, absolute dating can tell us the approximate age in years of the rock. The main way absolute dating is carried out is by Radiometric dating.

Radiometric dating

Atoms are made up of Protons, Neutrons and Electrons, different elements have unique numbers of protons e.g. Carbon always has 6 protons and Oxygen always has 8 protons. Isotopes of an element are atoms that have the same number of protons but a different number of neutrons, an example of this is Carbon-12 and Carbon-14, both atoms have 6 protons but Carbon-12 has 6 neutrons and Carbon-14 has 8 neutrons.

Some isotopes can be very unstable and are likely to break down to form a different atom, in the process they emit radioactivity and are therefore called Radioisotopes. 

Radioisotopes can be used to date rocks.

Rocks often contain traces of uranium which is a radioisotope. It is unstable and eventually decays to form lead, which is stable.

Isotopes have a property know as their half-life. The half life of an isotope is the time it takes for half of the atoms to decay, for example the half-life of Uranium is 4.46 billion years!! This means if you had 10g of uranium it would take 4.46 billion years for 5g of it to decay to lead!

Diagram showing the Uranium-Lead decay in a rock sample. Source: The Society of Vertebrate Palaeontology

The age of a rock can be calculated if we look at the ratio of uranium to lead. The older the rock, the lower the ratio of uranium to lead. Young rocks will have high uranium content and low lead content whereas very old rocks will have low uranium content and high lead content.

By studying the chemical composition of a rock and knowing the half life of the radioisotopes present we can determine the age of the rock in years.