Radiometric Dating:
The phenomenon of radioactive decay provided scientists who study the Earth and human history with one of their most important methods of determining the age of materials. This remarkable technique, which depends on our knowledge of the half–life of radioactive material is radiometric dating.
The best known radiometric dating scheme involves the isotope carbon–14. Every living organism takes in carbon during its lifetime. At the moment, your body is taking the carbon in your food and converting it to tissue, and the same is true of all other animals. Plants are talking in carbon dioxide from the air and doing the same thing. Most of the carbon, about 99%, is in the form of carbon–12, while perhaps 1% is carbon–13. However a certain small percentage, no more than one carbon atom in every million, is in the form of carbon–14, a radioactive isotope of carbon with a half–life of about 5700 years.
As long as something is alive, the carbon–14 in its tissues is constantly renewed in the same small proportion that is found in the general environment. When a living thing dies, however, it stops talking in carbon of any form. From the time of death, therefore, the carbon–14 in the tissues is no longer replenished. Like a ticking clock, carbon–14 disappears atom by atom to form an ever–smaller percentage of the total carbon. Scientists determine the approximate age of bone, piece of wood, cloth, or other object by carefully measuring the fraction of carbon–14 that remains, and comparing it to the amount of carbon–14 that scientists know must be in the material when it was alive. If the material happens, to be a piece of wood taken out of an Egyptian tomb, for example we have a pretty good estimate of how old the artifacts is and probably, when the tomb was built.
Carbon–14 dating often appears in the news when a reputedly ancient artifact is shown to be from more recent times. In a highly publicized experiment, the Shroud of Turin, a fascinating cloth artifact reputed to be the burial cloth of Jesus Christ, is from the twelfth century.
STOP AND THINK!
Stonehedge was built over a long period of time, starting in 2800 B.C.E. This date comes from carbon–14 dating. However this monument is made of stone. Therefore how do you suppose scientists could date this structure.
Carbon–14 is instrumental in mapping human history over the last several thousand years. When an object is more than 50,000 years old, however the amount of carbon–14 left in it is so small that this dating scheme can not be used. To date rocks and materials that are millions of years old, scientists must rely on similar techniques that use radioactive istopes of much greater half–life. Amoung the most widley used radioactive clocks in geology are those based on the decay of potassium–40 (half–life of 1.25 billion years), uranium–238 (half–life of 4.5 billion years), and ribidum–87 (half–life of 49 billion years). In these cases, scientists measure the total number of atoms in a given element, together with the relative percentage of a given istope, to determine how many radioactive nuclei were present at the beginning.
