The methods work because radioactive elements are unstable, and they are always trying to move to a more stable state. This process by which an unstable atomic nucleus loses energy by releasing radiation is called radioactive decay.
By the late 18th century, some naturalists had begun to look closely at the ancient rocks of the Earth.But for humans whose life span rarely reaches more than 100 years, how can we be so sure of that ancient date? Even the Greeks and Romans realized that layers of sediment in rock signified old age.But it wasn't until the late 1700s -- when Scottish geologist James Hutton, who observed sediments building up on the landscape, set out to show that rocks were time clocks -- that serious scientific interest in geological age began.Using fossils as guides, they began to piece together a crude history of Earth, but it was an imperfect history.After all, the ever-changing Earth rarely left a complete geological record. The method was developed by Willard Libby in the late 1940s and soon became a standard tool for archaeologists.