How Do Scientists Know the Age of Rocks?

(Student Reading)

Pick up a rock, any rock, and examine it. Can you see any way to determine its age? Is it ten years or ten million years old? You cannot tell just by looking at it. For this reason, scholars for nearly all of human history had no way of knowing the age of the Earth or its rocks. Up to the 19th century, for example, many people in Europe believed the Earth was only about 6,000 years old. Yet, to those who looked, the Earth offered bizarre clues of a deeper past.

One such person was Leonardo da Vinci, the great artist and engineer who was also an extraordinary scientist and observer of the world around him. He had seen fossil remains of animals where they should not be. In one of his notebooks, he asked,

Why do we find the bones of great fishes and oysters and corals and various other shells and sea-snail on the high summits of mountains by the sea, just as we find them in low seas?

—Leonardo Da Vinci

The Notebooks of Leonardo Da Vinci, Vol II (Jean Paul Richter, ed.), Dover Publications, Inc.: New York, 1975, p. 217

Fossil Life

In the 17th century, progress was made in dating rocks when Nicolaus Steno, a Danish anatomist, discovered what would be called the law of superposition. He realized that the layers of rock closest to the surface generally were younger than those underneath. In other words, the deeper the layer, the older the rocks.

In 1796, William Smith, an English surveyor, noticed that the most distinctive characteristics of many rock layers were their fossils. He concluded that layers of rock containing the same fossils were the same age. Therefore, by indexing their fossils, rocks, regardless of their composition or location, could be correlated with each other by when they were made.

An even more difficult puzzle was the discovery of the bones of animals that no longer lived. In 1770, for example, the huge jaws of a primeval beast were discovered in Holland (later identified as belonging to a Mosasaurus) and elephant bones were discovered in Paris. These and other fossil remains persuaded scholars that the Earth's past must have been quite different from its present.

Listening to the Atoms

Scientists had discovered geological time and were able to determine the relative ages of rocks, but the rocks' actual ages remained a mystery until the twentieth century when scientists were able to peer inside their atoms. Scientists first saw that living organisms contain carbon-12 and carbon-14 isotopes in the same ratios. Isotopes are forms of the same element that have different numbers of particles in their nuclei. Carbon-12 has six neutrons and 6 protons, while carbon-14 has 8 neutrons. When an organism dies, however, the carbon-14 decays into another isotope—nitrogen-14. Most importantly, the carbon-14 decays at a constant rate; half the atoms decay every 5,730 years, which is called the element's half-life.

Therefore, by examining the ratio of carbon-14 atoms to carbon-12 atoms, scientists could determine the age of organisms' remains up to about 70,000 years. In 1960, this feat, called radiocarbon dating, earned its discoverer, American chemist William Frank Libby, the Nobel Prize for Chemistry.

Scientists then applied this dating technique to inorganic materials like rocks. They were able to measure the decay of various isotopes like uranium- 238 and thorium-232 in rocks to determine their absolute ages. The half-life of uranium-238, for example, is 4.5 billion years. Based on this science, we have been able to date rocks tens and hundreds of million years old and have determined that the Earth itself is some 4.6 billion years old.

Like so many other discoveries in science, our ability to date rocks is built on the careful observations and scientific testing of many people, often over the course of centuries. Maybe someday, you will make an observation or discovery that will contribute to our understanding of our Earth and the universe we live in.