Young Geochronologist

Holmes earned a National Scholarship Award in physics and enrolled at the Royal College of Science in London in 1907. The curriculum required all students to take mathematics, mechanics, chemistry, and physics during their first year, and Holmes took an elective geology course in his second year. The president of the Geological Society, William Watts, taught the course and enticed Holmes to change his course of study during his third year. Fortuitously, Robert J. Strutt (1875-1947) from the Cavendish Laboratory at Cambridge University had joined the college at the same time Holmes enrolled. Strutt was one of the physicists who made public his belief that radioactive elements provided a source of heat sufficient to discredit Lord Kelvin's young estimation for the age of the Earth. Strutt invited Holmes to assist him in examining helium trapped in rocks following radioactive decay. He thought that if they could measure the amount of accumulated helium and establish its rate of production, then they could calculate the age of the rock. The concept seemed simple, but determining the rate of helium production was not a straightforward process. Because helium is a gas, an unknown but significant quantity escapes as it is produced, so only the minimum age could be estimated. (Uranium/helium measurements later were considered unreliable since the helium was not retained consistently.) After graduating from Imperial College (formerly the Royal College of Science) in 1910, Holmes assumed this research project with Strutt as a postgraduate student.

Across the ocean, American chemist Bertram Boltwood (1870-1927) had recently determined that lead was the final product of uranium decay, and he attempted to date several rocks using uranium/lead ratios. From 26 rocks he obtained ages ranging from 92 to 570 million years. Since helium could escape from rocks over time, he thought focusing on the end product would yield more accurate results. unknown to chemists at the time, Boltwood's analysis was flawed because of the existence of several isotopes of both uranium and lead. From his results Boltwood constructed a rough list of geological ages.

Holmes was anxious to use radioactivity to measure the age of a rock, selecting a Devonian rock from Norway that contained 17 different radioactive minerals so he could check each result against the others. After crushing the rock, extracting the minerals, and chemically separating them for analysis, he determined the ratios of uranium and lead and estimated the rock to be 370 million years old. He analyzed several others, dating the oldest at 1,640 million years, then calculated ages of geological periods from measurements published by Boltwood. Holmes wrote up his results showing that as the ratio of lead to uranium increased, so did the age of the rock (since uranium decays into lead), but he wondered if some lead was already present, which would have rendered his analysis flawed.

Strutt presented Holmes's results in April 1911 at a Royal Society meeting, where fellow geologists seemed interested but were wary of the radiometric dating technique. They questioned whether it was acceptable to assume the uranium decay rate was constant, and had trouble accepting the possibility that the Earth was more than 1 billion years old. Though geologists were looking for evidence indicating the Earth was older than 20 million years and knew the old techniques relied on rates of nonuni-

form processes, they were expecting a value closer to 100 million years as suggested by rates of sedimentation and salt accumulation, the so-called hourglass methods.

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