Imagine that throughout your life youve taken many pictures. Certain groups of fossil animals and plants occur in the geologic record in a specific order.
Rather than putting them into a photo album they are stacked up in a box. If a scientist finds one of those fossils, they can then assume the age of the rock based on the age of the fossil.
Among the best-known techniques are radiocarbon dating, potassium-argon dating and uranium-lead dating.
Relative dating of fossils is based on what
In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter.
Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years (e.g., tritium) to over 100 billion years (e.g., Samarium-147).
If you did it like many people, you based your sentences on what you know normally happens when you make a lunch.
This is similar to what geologists do to determine age.
Where are the oldest pictures, by year, going to be in your stack? Often igneous intrusions will cut across layers of rock.