Radiometric dating finds Earth is 2.2 billion years old
In 1902 Ernest Rutherford and Frederick Soddy discovered that radioactive elements, such as uranium and thorium, broke down into other elements in a predictable sequence or series. This amazing fact seemed like alchemy to many, but American chemist Bertram Borden Boltwood (1870-1927) was intrigued.
Boltwood studied this concept of "radioactive series," and found that lead was always present in uranium and thorium ores. He believed that lead must be the final product of the radioactive decay of uranium and thorium. A few years later, in 1907, he reasoned that since he knew the rate at which uranium breaks down (its half-life), he could use the proportion of lead in the uranium ores as a kind of meter or clock. The clock would tell him how long that ore -- and by extension, the earth's crust -- had existed. His observations and calculations put Earth's age at 2.2 billion years. This was a dramatic increase in the estimate of Earth's age for the time.
Boltwood's basic idea and technique have been used ever since 1907, but advances in technology and knowledge of atomic structure have shown the earth to be even older. Uranium decay is so slow it can indicate geologic time. Boltwood's reasoning holds true for other radioactive elements such as carbon-14, which has been used to date artifacts within human history.