The atoms of some chemical elements have different forms, called isotopes.
These break down over time in a process scientists call radioactive decay.
Geologists often need to know the age of material that they find.
They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years.
For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes.
Another example is luminescence dating, which measures the energy from radioactive decay that is trapped inside nearby crystals.
Stable and Unstable Isotopes Elements can be made up of different isotopes. Different Types of Radioactivity How is it measured?
Isotopes are atoms with the same number of protons and electrons, but a different number of neutrons. These are the elements that we see around us and find in nature. Radioactivity is measured using a unit called the "curie". The curie measures how many atoms spontaneously decay each second.
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.
The table below shows characteristics of some common radiometric dating methods.
Geologists choose a dating method that suits the materials available in their rocks. Measuring isotopes is particularly useful for dating igneous and some metamorphic rock, but not sedimentary rock.
Because of the fairly fast decay rate of carbon-14, it can only be used on material up to about 60,000 years old.