Radioactive Dating

Radioactive dating is a method of determining the approximate age of an old object by measuring the amount of a known radioactive element it contains. Rocks as well as fossil plants and animals can be dated by this process. It has given paleontologists (a person specializing in the study of fossils) as well as geologists (a person specializing in the study of the origin, history, and structure of Earth) a powerful way of dating ancient objects.

Until the discovery of radioactive dating, scientists had no way of approximating how old any part of Earth was. Once the principle behind this method was discovered, however, it became possible to gather reliable information about the age of Earth and its rocks and fossils. Radioactive dating was not possible until 1896, when the radioactive properties of uranium (a radioactive metallic element) were discovered by French physicist (a person specializing in the study of energy and matter), Antoine Henri Becquerel (1852–1908). When a substance is described as radioactive, it means that at the subatomic (relating to parts of an atom) level, some parts of it are unstable. When a substance is described as unstable, it means that it has a tendency to break down or decay. During this decay, one substance actually changes into another and radiation is released.

As long ago as 1907, the American chemist Bertram B. Boltwood (1870–1927) suggested that knowledge of radioactivity might be used to determine the age of Earth's crust. He suggested this because he knew that the end product of the decay of uranium was a form of lead. Since each radioactive element decays at a known rate, it can be thought of as a ticking clock. Boltwood explained that by studying a rock containing uranium, its age could be determined by measuring its amounts of uranium and lead. The more lead the rock contained, the older it was.

Although this was a major breakthrough, Boltwood's dating method made it possible to date only the oldest rocks. This is because uranium decayed or changed into lead at such a slow rate that it was not reliable for measuring the age of rocks that were younger than 10,000,000 years old. Another drawback was that uranium is not found in every rock. A later method that used rubidium (which changes into strontium) proved more useful because it is found in nearly all rocks, although it still was not useful for younger specimens. Perhaps the best method for rock dating is the potassium-argon method. This method proved useful to date rocks as young as 50,000 years old.

In 1947 another dating breakthrough occurred. The American chemist Willard F. Libby (1908–1980) discovered the radiocarbon method for determining the age of organic materials. Called the carbon-14 dating technique, this ingenious method used the simple knowledge that all living plants and animals contain carbon (a nonmetallic element that occurs in all plants and animals). Libby also knew that while most of this carbon is a common, stable form called carbon-12, a very small amount of the total carbon is radioactive carbon-14. All plants absorb carbon during photosynthesis (the process in which plants use light energy to create food), and animals absorb this carbon by eating plants or eating other animals that ate plants. Libby also found that as long as an organism remains alive, its supply of carbon-14 remains the same. However, once the organism dies, the supply stops and the carbon-14 in its body begins to decrease according to its own rate of decay. Libby realized that this could be a practical dating tool. He eventually designed a device that used Geiger counters (which measure radiation) to accurately measure the amount of carbon-14 left in an organic substance. Libby won the 1960 Nobel Prize in chemistry for his discovery. The discovery allowed him to correctly date a piece of wood from an Egyptian tomb that was known to be about 4,600 years old.

In the last 40 years, radiocarbon dating has been used on more than 100,000 samples in 80 different laboratories. Besides dating plant and animal life, this method has been used to verify the age of such different artifacts as the Dead Sea Scrolls (2,100 years), a charcoal sample from an ancient South Dakota campsite (7,000 years), and a pair of sandals from an Oregon cave (9,300 years). Improvements have raised its accuracy to nearly 70,000 years, with an uncertainty of plus-or-minus 10 percent.

[See alsoFossil; Paleontology ]