Sir Arthur Holmes (1890-1965), the British geologist who contributed to our understanding of Earth's age. Photo courtesy of University of Edinburgh, Department of Geology and Geophysics.
English geologist Arthur Holmes made not one but two major contributions to our under-standing of how the Earth works. He was the first earth scientist to grasp the mechanical and thermal implications of mantle convection, and he widely applied the newly-developed method of radioactive dating to minerals in the first attempt to quantitatively estimate the age of the Earth.
Holmes was fortunate that the phenomenon of radioactivity was discovered during his years as a graduate student at London’s Imperial College of Science. Holmes had come there to study physics, but switched to geology before graduating in 1910. Meanwhile, in 1905, English physicist Ernest Rutherford had suggested that the energy emitted by radioactive minerals in the form of particles and rays could be used to date the minerals. Called radioactive dating, this technique measures the rate of decay of certain unstable atoms, such as uranium, contained within minerals. Using this new technique, Holmes was able to determine the age of minerals and thus the rocks they are in, and in 1913, he formulated the first quantitative geological time scale. He estimated the age of the Earth to be 1.6 billion years, far older than was believed at the time. Holmes revised this estimate throughout his life, as measuring techniques improved. In 1953, an American geochemist, Clair C. Patterson, finally established the true age of the Earth at 4.55 billion years old.
Holmes also made major contributions to the theory of continental drift. This theory was proposed by German meteorologist and geologist Alfred Wegener in 1912 and states that the position of the continents on the Earth’s surface has changed considerably over time. Wegener’s idea was far from universally accepted, since it was not clear what would cause large continents to move across the surface of the Earth.
It was Holmes, in 1919, who suggested the mechanism: that the continents are carried by flow of the mantle on which they sit, and that the mantle is flowing because it is convecting. Warning that his ideas were “purely speculative,” he suggested that rocks in the interior of the Earth would buoyantly rise toward the surface from deep within the Earth when heated by radioactivity and then sink back down as they cooled and became denser. Holmes theorized that convection currents move through the mantle the same way heated air circulates through a room, and radically reshape the Earth’s surface in the process. He proposed that upward convection might lift or even rupture the crust, that lateral movement could propel the crust sideways like a conveyor belt, and that where convection turned downwards, the buoyant continents would crumple up and form mountains. Holmes also understood the importance of convection as a mechanism for loss of heat from the Earth and of cooling its deep interior. Not until after World War II could scientists produce the hard evidence to support Holmes’s fundamental concept. (To learn how this came about, read the profile of Harry Hess) Holmes’ theories have continued to be reinforced by new data from seismologists, mineral physicists, and geochemists.
Holmes began his major work, Principles of Physical Geology, while standing watch against German firebombs in the laboratories of Durham University, where he was head of the Geology Department. Published in 1944 and in a substantially revised edition in 1965, shortly before Holmes’s death, it is one of the most important and clearly-written books about the earth sciences. The depth and range of his thinking, which incorporated almost all aspects of physical geology, establish Holmes as a brilliant earth scientist.