Measuring Deep Time: Nine Scientists Who Taught the Earth to Tell Its Age

From dissecting a shark head and puzzling over stone tongues that matched its teeth, Nicolas Steno concluded that rock layers record time laid down in order. His stratigraphic principles of 1669 are still the first tools a field geologist learns, which makes him the right place to begin.

Abraham Gottlob Werner set out an early theory of the stratification of the Earth's crust, known as Neptunism. Most of its tenets were later set aside, but he is remembered for demonstrating chronological succession in rocks and for the zeal that earned him the title father of German geology.

The Scottish naturalist James Hutton founded uniformitarianism, the principle that the same slow processes visible today built the Earth over an immensity of time far beyond what his contemporaries assumed. He is widely called the father of modern geology.

Charles Lyell carried Hutton's uniformitarianism to a mass readership through his Principles of Geology. He replaced catastrophist explanations with the patient accumulation of ordinary, observable processes over vast time.

Bertram Boltwood showed that lead is the final decay product of uranium and that the lead to uranium ratio is greater in older rocks. In 1907, on a suggestion from Ernest Rutherford, he became the first to measure rock ages from that decay, obtaining results of 400 to 2200 million years.

Willard Libby was the American physical chemist noted for the 1949 development of radiocarbon dating. It lets organic material be dated from its residual radioactivity, and it revolutionized archaeology and palaeontology; he won the 1960 Nobel Prize in Chemistry for the work.

Clair Patterson developed the lead-lead dating method with George Tilton. Using lead isotope data from the Canyon Diablo meteorite, he calculated an age for the Earth of 4.55 billion years, a figure that has remained largely unchallenged since 1956.

Eugene Shoemaker, with Edward Chao, provided the first conclusive evidence that Barringer Meteor Crater in Arizona was an impact crater. He also co-discovered Comet Shoemaker-Levy 9, which struck Jupiter in July 1994, and led the Geological Survey's Astrogeology Research Program.

With his father, the physicist Luis Alvarez, Walter Alvarez led research on the hypothesis that an asteroid impact caused the extinction of the dinosaurs, built on an iridium-rich layer at the Cretaceous-Paleogene boundary. It closes the path by reading a single thin layer, as Steno began it.