While Holmes's major passion was finding absolutes for geological time, he was also knowledgeable about other subjects. In 1915 German meteorologist Alfred Wegener (1880-1930) proposed the theory of continental drift, suggesting all the continents once were part of an enormous supercontinent that broke into pieces leading to the present distribution of continental masses. This model was exciting because it explained many unusual geological (and biological and climatological) phenomena, but most geologists hesitated to accept it without a plausible mechanism. The English translation of Wegener's book The Origin of the Continents and Oceans in 1924 aroused a heated debate, and Holmes was among the few geologists progressive enough to entertain the idea.
Aware of the enormous energy provided by radioactivity, Holmes suggested that the intense heat generated by the radioactive decay of unstable elements within the Earth's interior was a sufficiently powerful engine for moving continents. The substratum, or mantle, was solid, but he thought that over millions of years it behaved like a thick liquid. Holmes proposed thermal convection as a means to dissipate the heat, causing the cooler material close to the surface to sink, leaving space for hotter, less dense material to rise into and fill. In December 1929 Holmes proposed to the Geological Society of Glasgow that convection currents were responsible for continental drift. He explained that as convection currents in the mantle cooled and descended, they could drag continents horizontally across the Earth's surface. "Radioactivity and Earth Movements," his seminal paper, was published in the Transactions of the Geological Society of Glasgow in 1931. Though Holmes had a respectable scientific reputation, his ideas were mostly ignored until the 1960s, when American geophysicists Harry Hammond Hess and Robert sinclair Dietz independently proposed the concept of seafloor spreading, which in combination with continental drift has evolved into the well-supported theory of plate tectonics.
Holmes was invited to lecture around the world, including in the United States in 1932. He took advantage of this opportunity to solicit help in constructing a geological timescale from American scientists. His demanding schedule forced Durham to hire another lecturer in the geology department in 1933; they selected Doris L. Reynolds, a notable petrologist with whom Holmes had been having an affair since
1931. In 1938 Maggie died from stomach cancer, and Holmes married Reynolds in 1939.
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