“It is perhaps a little indelicate to ask of our Mother Earth her age,” he wrote in his introduction — then proceeded to reveal that she was roughly 1.6 billion years old.When Holmes presented the findings at a meeting of the Geological Society of London two years later, he was “violently attacked” by critics.

The number of parent atoms originally present is simply the number present now plus the number of daughter atoms formed by the decay, both of which are quantities that can be measured.

Samples for dating are selected carefully to avoid those that are altered, contaminated, or disturbed by later heating or chemical events.

All rocks and minerals contain long-lived radioactive elements that were incorporated into Earth when the Solar System formed.

These radioactive elements constitute independent clocks that allow geologists to determine the age of the rocks in which they occur.

] A century later, William Smith realized that rock layers at distant locations came from the same time period.

He created a catalogue of strata (which all got colorful names such as Lias Blue, and Ditto White) and argued that each one represented a distinct time in Earth's history — a principle known as fossil succession.

The accumulating evidence pointed to an extraordinary new idea: that the history of Earth goes back much, much further than any human memory.

In 1788, Scottish geologist James Hutton published his “Theory of Earth,” which introduced the world to the idea of “deep time.” The implications of the treatise were revolutionary: Not only was the Earth not young, but it was not static, Hutton said.

This spawned several earnest — if not entirely successful — attempts to determine the age of the Earth based on ongoing natural processes.

One calculated how long it would take rivers to deliver enough dissolved minerals to the ocean to give it its current saltiness (answer: 90 million to 100 million years).

Radioactivity was discovered in 1896 by French physicist Henri Becquerel.