The standard geologic time scale was devised according to relative time relationships observed in rocks across the world.
Determining the actual ages of these time spans, and thus establishing the beginning and ending dates of geologic eons, eras, periods, and epochs, became possible with the discovery of radioactivity.
The important boundary between the Paleozoic era and the Precambrian era is dated at about 570 million years ago; the Mesozoic era (the “Age of the Dinosaurs”) started about 245 million years ago and ended 66 million years ago.
This radioactive decay begins after the elements are locked into crystalline mineral structures.
Some elements have variations called isotopes, which are atoms that contain different numbers of neutrons in their nuclei.
For example, uranium has the isotopes U‐235 and U‐238; U‐238 has three more neutrons than does U‐235.
Radioactive decay is the breakdown of isotopes that contain unstable nuclei.
As an element decays it creates a series of daughter products.
For example, uranium‐238 loses protons and neutrons during its decay, going through a series of intermediate daughter products to form its end product lead‐206, a stable isotope. By determining the relative amounts of a radioactive isotope and its decay products in a mineral, the of the mineral can be determined.
Other decay reactions that are used to calculate absolute age are carbon‐14 to nitrogen‐14, potassium‐40 to argon‐40, rubidium‐87 to strontium‐87, thorium‐232 to lead‐208, and uranium‐235 to lead‐207.
An isotope's half‐life is the time it takes for half of a known quantity of radioactive material to convert to its daughter product.
For example, the half‐life of U‐238 is 4.5 billion years.
Thus, if you began with one gram of U‐238, 4.5 billion years later only one‐half gram would remain.
After another 4.5 billion years, only one‐quarter of the original amount would remain.