Arthur Holmes transformed two important geologic concepts with theories that have withstood the test of time. First, he proposed an estimate of the earth’s age using the rate of radioactive decay to establish the first estimate based on chronological (as opposed to stratigraphic) dating. Throughout his career, he revised his estimate, arriving at a more accurate figure near the end of his career. Second, he proposed a mechanism to explain the contentious theory of continental drift, explaining that radioactive convection could shift the plates in the earth’s mantle. This theory was vindicated in the early 1960s, when geologic evidence supported Holmes’s hypothesis.
Holmes was born on January 14, 1890, in Hebburn-on-Tyne, in Newcastle, England. His mother, Emily Dickinson, was a teacher, and his father, David Holmes, was a cabinetmaker; both parents came from farming stock. At Gates head High School, Holmes’s physics teacher encouraged him to read William Thomson Kelvin’s Addresses, introducing him to the notion of the geologic age of the earth. In 1907, Holmes matriculated at Imperial College in London to study physics on a scholarship. He earned his bachelor of science degree in mathematics and physics in 1909.
Holmes, who had commenced geologic study under Robert Strutt (later the fourth Lord of Raleigh) at Imperial College, continued this line of study at the Royal College of Science, graduating as an associate in 1910. He then conducted postgraduate research with Strutt, applying the recently discovered phenomenon of radioactivity to geology as a means of dating the age of rocks. Physicist Ernest Rutherford had suggested that radioactive decay, which occurs at a set rate, can accurately record the passing of time; Strutt used this application to determine the age of rocks by comparing the amount of uranium in rock samples to the amount of the byproduct of its radioactive decay—namely, helium. In his very first paper, published by the Royal Society in 1911, Holmes devised a helium/uranium ratio for calculating geologic time, and used this ratio to estimate the earth’s age—1,600 million years, 40 times more than Kelvin’s estimate of 40 million years.
Also in 1911, Holmes embarked on a geologic expedition to Mozambique, where he contracted a case of malaria that prevented him from fighting in World War I. The next year, he returned to Imperial College to teach geology. He published his first book, The Age of the Earth, in 1913, and the next year, he married Margaret Howe; together, the couple had one son. In 1920, he published his second book, The Nomenclature of Petrology, and the next year, he published Petrographic Methods and Calculations; these two books had a profound influence on his field of specialization, petrology, or the description and classification of rocks.
In 1920, Holmes left academia for an industry job leading an oil exploration expedition in Burma as chief geologist for the Yomah Oil Company. Four years later, after failing to locate oil deposits, he returned to England to head Durham University’s geology department, newly established by Irvine Masson.
In 1915, the German geophysicist Alfred L. Wegener had proposed the theory of continental drift, expounding that continents actually migrate. Geologists resisted this hypothesis for lack of an explanation of the mechanism whereby such huge landmasses might move. In a 1929 presentation to the Geological Society of Glasgow, Holmes supplied an explanation: radioactive convection. He theorized that radioactive heat in the earth’s mantle created convection currents capable of shifting the continents. Humbly, he stressed that his ideas were "purely speculative" and would "have no scientific value until they acquire support from independent evidence." Harry Hess provided such evidence in 1962 (following up on his 1960 proposition attributing continental drift to seafloor spreading), when he provided the geologic mechanism responsible for such radical continental shifting—the eruption of molten magma, which could push the continents apart and then cool to create new oceanic flooring. This theory validated and elaborated Holmes’s theory.
In 1938, Holmes’s first wife died, and the next year, he married Dr. Doris Livesey Reynolds, a fellow geologist on the Durham faculty. In 1942, Edinburgh University named him to the Regius Chair of Geology, and the Royal Society simultaneously inducted him into its fellowship. In 1944, he published the book for which he is best remembered, Principles of Physical Geology, which crystallized in simplicity and clarity the field that had previously been plagued by confusing jargon.
Throughout his career, Holmes continued to refine his estimate of the earth’s age, addressing the issue in more than 50 papers; in 1959, the Transactions of the Edinburgh Geological Society published his final estimate—four billion years. In testament to the accuracy of Holmes’s calculations, the current estimate for the commencement of the Cambrian period, 590 million years ago, jives with Holmes’s estimate of 600 million years ago.
The year that Holmes retired (1956), he received both the Wollaston Medal from the Geological Society of London and the Penrose Medal from the Geological Society of America.
In retirement, he remained at Edinburgh as a professor emeritus until 1962. In 1964, he received the Vetlesen Prize. In 1965, he published a completely revised edition of his classic text, Principles of Physical Geology. Holmes died in London on September 9 (or 20), 1965.
