Bertram Borden Boltwood pioneered the radiometric dating of rocks, giving impetus to the use of nuclear methods in geology and establishing a new chronology of the earth.
Bertram Borden Boltwood, educated at Yale University and in Germany, began his career as an instructor of analytic chemistry at Yale University. He remained at Yale until 1890, when he left to establish a private laboratory (Pratt and Boltwood, Consulting Mining Engineers and Chemists) in New Haven, Connecticut. He was interested in radium and was fascinated by the theory of radioactive disintegrations proposed in 1903 by two McGill University scientists, Ernest Rutherford and Frederick Soddy.
Bertram Borden Boltwood.
The significance of Boltwood’s work is best appreciated in the light of a chronological controversy raging at that time between geologists and physicists. It had been accepted generally that the earth, at some time in its history, had been a liquid ball, and that its solid crust was formed when the temperature was reduced by cooling. The geological age of the earth was defined, thus, as the period of time necessary to allow it to cool down from the melting point to the present temperature. Several estimates of that time were made in the second half of the nineteenth century by the famous physicist Baron Kelvin, Sir William Thomson (known as Lord Kelvin), who claimed that the age of the earth did not exceed 100 million years. In 1897, Lord Kelvin specified that the actual figure was probably close to 20 million and certainly not as large as 40 million. His calculations were mathematically correct, but they did not take into account radioactivity, which had been discovered the year before.
Geologists believed that 40 million years simply was not enough time to create continents, erode mountains, and supply oceans with minerals and salts. Studies of sequences of layers (stratigraphy) and of fossils (paleontology) led them to believe that the earth was older than 100 million years, but they were not able to prove it. Boltwood and Rutherford proved that geologists were right. This came as a by-product of their research on the nature of radioactivity. The gaseous element helium
Uranium 238 decays naturally, over a predictable period of time, to form lead. Boltwood realized that the accumulation of lead in rocks could be used to determine their age.
In line with this model, and knowing how much helium is produced from each gram of uranium per billion years, Rutherford and his collaborators were able to see that naturally radioactive rocks are often older than 100 million years. The accumulated amounts of helium in the rocks they studied were simply too large to allow other interpretations. The ages of some of the samples exceeded 500 million years. Moreover, Rutherford was aware that the ages they assigned would have to be revised upward in order to account for helium that was escaping from the rocks.
Impressed by these results, and trying to eliminate the uncertainties associated with leakage of helium, Boltwood decided to work on another method of dating the rocks. This resulted from his earlier attempts to demonstrate that, as in the case of radium, all uranium minerals contain the same number of atoms of lead per gram of uranium. Chemical data, however, did not confirm this expectation; the measured lead-to-uranium ratios were found to be different in minerals from different locations on the earth. Yet this still needed to be interpreted. According to Rutherford and Soddy’s theory, a spontaneous transformation of uranium into a final product proceeds through a set of steps in which alpha particles and electrons are emitted, one after another. Boltwood realized that lead must be the final product and that its accumulation could be used to date minerals. By focusing on lead rather than on helium, he hoped to reduce the uncertainties associated with the leakage. He reasoned that lead is less likely than helium to escape from rocks because, once trapped, it becomes part of a solid structure.
Motivated by these ideas, Boltwood proceeded with the development of the uranium-lead method of dating.
Boltwood was aware that even the most carefully performed experiments can lead to highly unrealistic conclusions. In his case, two sources of interpretational errors were possible, one having to do with the preexisting (nonradiogenic) lead and another having to do with the fact that chemicals can migrate through rock boundaries. Suppose, Boltwood argued, that 50 percent of the lead found in a sample was already there at the time of solidification, but a researcher was not aware of that fact. In such a case, the derived age of the sample would be exaggerated by a factor of two. In other words, preexisting lead would make rocks appear older than they actually were. On the other hand, the opposite would be true if one-half of the lead generated from uranium were able to escape through the rock boundaries. Recognizing the dangers of interpretational errors, Boltwood was very careful in selecting samples. He analyzed only deep portions of samples to minimize uncertainties associated with the possibility of lead leakage. Boltwood started his investigations in 1905, and before the end of the year he had analyzed twenty-six samples. One of them was identified as 570 million years old. In a formal publication that appeared in 1907, he described forty-six minerals collected in different locations; their reported ages were between 410 and 2,200 million years old.
Similar results had been reported earlier by Rutherford, from his laboratory in Montreal, and by Robert John Strutt (also known as Fourth Baron Rayleigh), from the Imperial College in London. It is true that similarities were mostly qualitative, rather than quantitative; however, all data showed that the absolute time scale was undeniably longer than anticipated, even by some geologists. In other words, the large margins of error that characterized early results did not interfere with the basic overall conclusion that many rocks were at least ten times older than Lord Kelvin had calculated. It was significant that helium and lead methods, based on different chemical procedures and applied to a large number of very different minerals, yielded similar results at three institutions.
The main result of Boltwood’s pioneering work and that of his successors was the realization that geological time must be expressed in hundreds and thousands of millions of years rather than in tens of millions, as advocated by Lord Kelvin. This was particularly significant for the acceptance of the theory of evolution and, in general, for better understanding of many long-term processes on the earth. Geochronology (the study of geological time) has been used, for example, in investigations of reversals of the terrestrial magnetic field. Such reversals occurred many times during the earth’s geological history. Scientists discovered and studied them by dating pieces of lava that were naturally magnetized during solidification. The most recent reversal took place approximately 700,000 years ago.
It is clear, in retrospect, that the discovery of radioactivity affected geochronology in two ways: by providing tools for radiometric dating and by invalidating Lord Kelvin’s thermodynamic calculations. These calculations were based on the assumption that the geothermal energy lost by the earth is not replenished. The existence of radioactive heating, discovered in 1903 in France, contradicted that assumption and prepared scientists for the acceptance of Boltwood’s findings. Lord Kelvin died in the same year in which these findings were published, but he knew about Rutherford’s findings as early as spring, 1904. He was very interested in radioactive heating but never came forward with any public retraction of his earlier pronouncements.
In its modern form, the uranium-lead method of dating is very different from what it was at the beginning of the twentieth century, primarily because radioactive transformations are better understood and because instruments have been developed that were not available in Boltwood’s day. The most important tool of modern geochronologists is the mass spectrometer. Its widespread use after World War II is responsible for the development of new methods of dating. The availability of several methods is important when redundancy can be used to minimize, and sometimes eliminate, the kinds of interpretational errors recognized by Rutherford, Strutt, and Boltwood.
By the beginning of the twenty-first century, a large number of samples from different geological strata, all over the world, plus lunar samples and meteorites, had been dated by modern methods. The data showed that the oldest known terrestrial rocks solidified 3.7 billion years ago; this is 0.9 billion years less than the ages of lunar samples. The ages of most meteorites, between 4.5 and 4.7 billion years, are essentially the same as the ages of lunar samples. This is consistent with the overall picture, also backed by astronomical data, that the earth’s solar system was formed 4.7 billion years ago, and that the earth remained liquid in the first 0.9 billion years of its existence. Boltwood would be proud to be counted among the contributors to that knowledge.
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Asimov, Isaac. Exploring the Earth and the Cosmos. New York: Crown, 1982. Provides excellent description of the ways researchers learn about the world. Twenty-three chapters cover the entire panorama of findings in astronomy, biology, chemistry, geography, and physics. Chapters 13 and 14 are devoted to chronology and to the age of the earth. -
Badash, Lawrence. “Bertram Borden Boltwood.” In Dictionary of Scientific Biography, edited by Charles Coulston Gillispie. New York: Charles Scribner’s Sons, 1970. Biography of Boltwood includes a detailed description of his family background, professional activities, and private life. -
_______. Radioactivity in America. Baltimore: The Johns Hopkins University Press, 1979. A good account of the history of the early (pre-1920) research in radioactivity and of practical applications of that research, primarily in the United States. Boltwood’s life and professional activities, including his work on radiometric dating, are described in chapters 4, 5, and 6. Chapters 9 and 10 are devoted to medical applications of radium and to the marketing of that material. -
_______, ed. Rutherford and Boltwood: Letters on Radioactivity. New Haven, Conn.: Yale University Press, 1969. Good reading for those who wish to experience the scientific excitement of these two friends and collaborators. Of particular interest is a letter, dated November 18, 1905, in which Boltwood recognizes Rutherford as the initiator of the uranium-lead method of dating minerals. -
Burchfield, Joe D. Lord Kelvin and the Age of the Earth. New York: Science History Publications, 1975. An excellent book for all who are interested in the evolution of geochronologies, both before and after Lord Kelvin. The last chapter describes the dramatic effects of the discovery of radioactivity on geoscience. Highly readable; excellent bibliography. -
Eicher, Don L. Geologic Time. 2d ed. Englewood Cliffs, N.J.: Prentice-Hall, 1976. The author of this short but very informative book, an earth scientist from the University of Colorado, places the concept of geological time in historical perspective. Focuses on significant discoveries that contributed to the growth of scientific geology and describes various methods of dating geological formations. Includes photographs, sketches, and maps. -
Faul, Henry. Ages of Rocks, Planets, and Stars. New York: McGraw-Hill, 1966. Written by a geophysicist with extensive experience using modern methods of nuclear geochronology, this book provides the general scientific background that readers need to understand these methods, together with clear explanations of different techniques and descriptions of interesting findings. -
Friedlander, Gerhart, et al. “Nuclear Processes in Geology and Astrophysics.” In Nuclear and Radiochemistry. 3d ed. New York: John Wiley & Sons, 1981. Useful introduction to the quantitative aspects of various methods of geochronology for those with some background in general physics and chemistry. Includes numerous references. -
Gorst, Martin. Measuring Eternity: The Search for the Beginning of Time. New York: Broadway Books, 2001. Discusses how human understanding of time, and specifically the age of the universe, has changed over the centuries as scientific knowledge has increased. Includes index. -
Hellman, Hal. “Lord Kelvin Versus Geologists and Biologists: The Age of the Earth.” In Great Feuds in Science: Ten of the Liveliest Disputes Ever. New York: John Wiley & Sons, 1998. Focuses on the human side of the debate about the age of the earth between proponents of Lord Kelvin’s calculations and scientists such as Boltwood and Rutherford.
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