Radiometric Dating

Precise method of dating Earth materials by measuring the percentages of a long-life radioactive parent element with respect to its daughter products or measuring the presence a short-life radioactive element; those measurements are precise because the rates of decay of many isotopes have been extensively documented and do not vary with physical conditions found in the Earth’s outer layers. Consequently, each radioactive isotope used in the dating process has been decaying at a known rate since it was formed in the rock in which it is contained and the decay products have also been accumulating at a corresponding rate. For example, when a mineral that contains uranium crystallizes from magma, that magma contains no lead that would be the product of a previous decay process. Therefore, the radiometric clock starts ticking at that moment. As the uranium in the new mineral begins decaying, its daughter products are trapped and measurable quantities of lead will eventually accumulate.

Historical Background: In 1896 the discovery of the natural radioactive decay of uranium by the French physicist Henry Becquerel opened the door to a cornucopia of scientific discoveries. In 1902 the physicist Ernest Rutherford and chemist Frederick Soddy, working at Canada’s McGill University determined that radioactive elements, such as uranium and thorium, broke down at a fixed rate over time into other elements in a predictable sequence or series. Their discovery led to the identification of half life and led to their disintegration theory of radioactivity, which proposed that over time atomic nuclei of an unstable atom split to form other elements. Their research into radioactive decay, coupled with the work of their colleague, Kasimir Fajans, resulted in the Radioactive Displacement Law of Fajans and Soddy that described the products of alpha and beta decay.

That discovery intrigued Bertram B. Boltwood (1870-1927), a radiation chemist working at Yale University. Boltwood was spurred on when in 1905 during a lecture at Yale University Rutherford had challenged the scientific community to use radioactive decay to date rocks. Boltwood began studying the radioactive series Rutherford and Soddy had defined earlier in 1905 and found that lead was always present in uranium and thorium ores. He concluded that lead was the final product of the radioactive decay of uranium and thorium. In 1907, he reasoned that once the rate at which uranium decays is known (the half decay period or half-life), the proportion of lead in the uranium ores could be used as a kind of measuring device, or clock, since it would tell geoscientists when that ore, and therefore the Earth’s crust, formed. Boltwood’s pioneering research, although somewhat crude when measured against today’s far more sophisticated techniques (for example, the use of the mass spectrometer to identify atoms by weight), put the Earth’s age at 1.2 billion years, which, for that time, was a dramatic increase in what scientists believed was the right direction.

Shortly after Boltwood’s discovery in 1907 that uranium decayed slowly to stable lead, Arthur Holmes, an undergraduate physics student at University College of London, was so taken by the geological implications of that discovery that he switched majors to geology. By 1911, using only analytical chemistry applied to a few mineral samples, Holmes established a framework for the geologic time scale that proved to be uncannily accurate, considering the relatively primitive nature of his approach (since it predated the discovery of isotopes). Building on Boltwood’s pioneering work, Holmes performed the very first uranium-lead analysis of rock specifically determined for age-dating purposes. That research resulted in a date of 370 million years for a Devonian specimen. Although only 21 years old and still an undergraduate, Holmes had embarked on a lifetime’s quest “to graduate the geological column with an ever-increasingly accurate time scale.”

In 1913 he wrote The Age of the Earth, a book that almost immediately became justly famous. In that book Holmes estimated the Earth’s age at 1.6 billion years. It is quite extraordinary that at that time he was only 23 and had not completed his doctoral studies. After publication Holmes became recognized as the world’s authority on geochronology. But, opposition from established geologists who clung to the belief that the Earth was 100 million years old was formidable. Key advocates of the opposing position included scientists who supported ideas the famous Scottish physicist William Thompson, perhaps better known as Lord Kelvin, had advocated shortly before his death in 1907. Other well-known opponents included German physicist Hermann von Helmholtz, American astronomer Simon Newcomb, and Charles Darwin’s astronomer-mathematician son, George H. Darwin.

However, by the early to mid-1920s Holmes’s work was vindicated when both the British Association for the Advancement of Science and the National Research Council of the U.S. National Academy of Sciences came down on the side of the Earth being between 1.6 and 2.0 billion years old. From the mid-1920s through the early 1940s, a group of physicists, geophysicists, and geochemists succeeded in devising techniques that continued pushing back the age of the Earth. That group included Holmes, Alfred Nier, E. K. Gerling, Friedrich Georg Houtermans, and Clair C. Patterson, who ultimately produced accurate “primeval” lead isotopic measurements from minerals collected from five meteorite fragments at Canyon Diablo, Arizona.[1] By 1956, Patterson’s research had determined the age of the Earth at almost 4.6 billion years.

Since Patterson’s and Houtermans’ pioneering research in the mid-1950s, additional data have been accumulated, instruments have become more precise, and analytical techniques have improved. Moon rocks and many more meteorites have been sampled and dated. Decay constants have been measured with ever increasing accuracy. Remarkably, certain technical adjustments to and corrections of Patterson’s 1956 computation have canceled each other out. Today’s best estimate of the age of meteorites (4.55 ± 0.02 billion years) is identical to Patterson’s except for a smaller error range. That value has been confirmed by dozens of scientists working independently.

Today it is a nearly universally accepted scientific principle that radioactive decay occurs at a constant rate that is specific to each radioactive isotope. Since the 1950s, geologists and geophysicists have used radioactive elements as natural “clocks” for determining ages of certain types of rocks. Radiometric clocks are set when each rock forms. “Forms” means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism cools. That setting and resetting process allows geoscientists to date rocks that formed at different times and under different circumstances. Another commonly used radiometric dating technique is based on the decay of potassium (K-40) to argon (Ar-40). In igneous rocks, the potassium-argon clock starts the moment the rock crystallized from magma. Precise measurements of the amount of the isotope K-40 relative to Ar-40 in an igneous rock determine the time that has passed since crystallization (knowing that the half-life of K-40 is about 1.3 billion years). If an igneous or other rock is metamorphosed, its radiometric clock is reset. Potassium-argon measurements are then used to determine the number of years that have passed since metamorphism. See isotopic dating.

Author’s Note: It is critical for students to realize that no scientific method is free from ambiguity. In addition, most scientific techniques in and of themselves are subject to considerable latitude in terms of the interpretation of results. Consequently, all physical-chemical methods of dating rocks have uncertainties associated with them. Several basic assumptions are made when geoscientists determine the age of rock samples. The most significant assumption is that the sample is from a closed system in which no parent or daughter isotopes were gained or lost over time. Another assumption involves the amount of daughter isotope present at the time the sample rock was formed. For rare isotopes, that amount is generally assumed to be zero. Because of those and other uncertainties, the strongest evidence for the age of a rock is obtained when two different radiochemical dating methods produce similar results.

Since we live in a real world where our convenient, highly intellectualized categories and classifications are seldom found in nature, it is likely that geoscientists unknowingly (or knowingly according to many creationist critics) put one or both of those assumptions into play when rock samples are dated by radiometric decay techniques. But, despite what Creationists like to assert, simply because a specific dating technique fails to determine a reliable or verifiable date for a rock sample is no reason to reject all radiometric dating techniques. After all, when your car fails to start one winter morning, surely you don’t automatically assume that all cars therefore are useless pieces of junk.

Interested students may wish to consult one of the standard works on the topic. I recommend highly to anyone with a high school background in science G. Brent Dalrymple’s classic, The Age of the Earth. Stanford, California: Stanford University Press, 1991. It is well-written, well-reasoned, and powerful in its explanations. Radiometric dating has been widely attacked by Christian fundamentalists, or Creationists, as unreliable, riddled with inaccuracies, and unscientific. A variety of their views may also be found on the internet under the entry, radiometric dating. Curious students owe it to their intellectual development to examine that alternate universe. However, for the point of view of a Christian geophysicist, see: Roger C. Wiens, PhD, Radiometric Dating: A Christian Perspective, material written in 1994 and revised in 2002: http://www.asa3.org/ASA/resources/Wiens.html.

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[1] Where, quite coincidentally, my great uncle, Earl Cundiff, had been murdered in 1926.