Thus the age of the oldest terrestrial rock gives a minimum for the age of Earth, assuming that no rock has been intact for longer than the Earth itself.
Studies of strata, the layering of rocks and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence.
In 1830, geologist Charles Lyell, developing ideas found in James Hutton's works, popularized the concept that the features of Earth were in perpetual change, eroding and reforming continuously, and the rate of this change was roughly constant.
This was a challenge to the traditional view, which saw the history of Earth as static, He assumed that Earth had formed as a completely molten object, and determined the amount of time it would take for the near-surface to cool to its present temperature.
He calculated the amount of time it would have taken for tidal friction to give Earth its current 24-hour day.
His value of 56 million years added additional evidence that Thomson was on the right track.
For more information, see the latest announcement from the NBER's Business Cycle Dating Committee, dated 9/20/10.