By measuring the amount of the product, it is possible to estimate the time elapsed since the process began. Layers in glacial ice provide similar annual records. Unfortunately, direct time-records like tree rings and glacial layering are useful only for the relatively recent past. Like the growth of trees or the accumulation of snow on glaciers, radioactive decay is a natural process that occurs at a known rate, creates measurable products, and can be used to date geologic materials and events. To understand natural radioactivity , we need a quick review of chemistry.
All matter consists of elements or combinations of elements.
The smallest subdivision of an element that still has properties of that element is an atom. Atoms consist of a central nucleus, containing protons and neutrons, and an outer cloud of orbiting electrons. In contrast, the number of neutrons in atoms of some elements can vary, and the various forms of such elements are called isotopes . Some isotopes are stable and remain unchanged through time. Others are unstable and tend to break down, or decay over time, by giving off subatomic particles and energy.