National Geographic : 1961 Oct
A Clock for the Ages: Potassium Argon By GARNISS H. CURTIS, Ph.D. Department of Geology, University of California AS GEOLOGISTS whose business is ex ploring the earth's prehistoric past, my partner Dr. Jack F. Evernden and I are used to surprises. Yet nothing ever star tled us more than our recent discovery that man the tool-maker is 1,750,000 years old. Until this year scientists had believed that man developed as a tool-making creature less than a million years ago. But thanks to the potassium-argon process of dating, we need no longer speculate about the age of the earliest known tool-maker. In the preceding pages, my colleague Dr. Louis S. B. Leakev has told the story of his momentous discoveries at Olduvai Gorge. Coupled with the new potassium-argon dat ing results, they promise to revise man's basic concept of prehistoric time. Atomic Change Reveals the Past What is this new scientific tool that can probe millions of years into the past and un lock the secrets of ages? Perhaps the simplest way to describe it is to call it a type of atomic clock. Like the now-famous carbon-14 process, the potassium-argon dating method utilizes a slow but constant atomic change in cer tain types of matter.* The potassium-argon "clock," however, runs far longer than that of carbon 14. It is ideal for dating events millions of years in the past, whereas carbon 14 operates accu rately only from about 50,000 years ago. To watchmakers, the mechanism of a clock is known as the movement. The potassium argon "movement" is the gradual conversion 590 of an unstable element, potassium 40, into Fossil skull of pioneer tool-maker, Zinjanthro pus, dates from 1,750,000 years ago. New dat ing techniques established its astonishing age. the elements calcium 40 and argon 40. Scien tists call the original element-in this case, potassium 40-the "parent," while the newly formed elements-the calcium 40 and argon 40-are called "daughters." I have said our clock was long-running. The actual change of a potassium 40 atom into a daughter atom takes place in a frac tion of a second, but it occurs only rarely. For example, if you were to imprison 18 po tassium 40 atoms in an airtight test tube for one and three-tenths billion years, you would find at the end that only nine had changed into daughter atoms. The rate of change, then, is enormously slow, but it is constant. We have only to measure the amount of that change, or, as scientists call it, decay, to know how long the process has been going on. I should explain that the calcium 40 daugh ter atoms are useless for our purposes, since they are virtually impossible to distinguish from other calcium 40 atoms not related to the movement of our clock. In determining the ages of rock samples, we depend upon the argon 40 daughter atoms, which can be distinguished from other types of argon atoms. To learn the birth date of a rock, then, we need only find the number of parent atoms and daughter atoms in one of the potassium bearing minerals contained in the rock sam ple. But not every rock will do. In Olduvai Gorge, if we picked up just any rock sample, it would tell us little or nothing about the age of prehistoric man there. What * For an account of carbon-14 dating. see "How Old Is It?" by Lyman J. Briggs and Kenneth F. Weaver, in the August, 195S. NATIONAL GEOGRAPHIC.