Dating igneous volcanic rocks
If a geologist claims to be 45 years old, that is an absolute age.
Superposition: The most basic concept used in relative dating is the law of superposition.
Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.
Here I want to concentrate on another source of error, namely, processes that take place within magma chambers.
When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, 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 long time periods are computed by measuring the ratio of daughter to parent substance in a rock and inferring an age based on this ratio.
This age is computed under the assumption that the parent substance (say, uranium) gradually decays to the daughter substance (say, lead), so the higher the ratio of lead to uranium, the older the rock must be.
Geologists generally know the age of a rock by determining the age of the group of rocks, or formation, that it is found in.