Debate On The Age Of The Earth

Arthur Holmes was born on January 14, 1890, in Gateshead, England, to a cabinetmaker David Holmes and a former schoolteacher Emily Dickinson. Gateshead High School provided Arthur with a strong background in the sciences and an oppor tunity to develop his musical abilities in the Operatic Society. His teacher introduced him to the age of the Earth debate that had been recently refueled by the discovery of radioactivity. In 1897 Lord Kelvin (1824-1907), a professor of natural history at Glasgow University and an eminent expert of thermodynamics, announced his newest estimation for the age of the Earth. Believing that the Earth had been gradually cooling from its molten genesis, Lord Kelvin calculated that the Earth's crust consolidated 20 million years ago, based on experimentally determined temperatures at which rocks melt and their rate of cooling. Now his long accepted estimation was being challenged, and not by geologists, who seemed to be intimidated by his stature, but by physicists.

Near the end of the 19th century the age of the Earth was a popular topic for research and discussion among geologists, who thought the Earth was an order of magnitude older than Lord Kelvin claimed. Professor John Joly of Trinity College, Dublin, supported the salinity method for estimating the age of the Earth. As the newly formed globe cooled, water condensed and formed the oceans. The water would initially be pure, but as rocks decomposed and washed over the land into the seas, the water would become saltier. using this assumption, if one measured the salinity of the oceans at two time points separated by a few hundred years, then one could extrapolate back to estimate how much time has passed since the water was pure, that is, when the Earth's crust solidified. From the rate calculated for salt accumulation from erosion, Joly estimated the oceans to be more than 90 million years old. one criticism of this method was that it required the rocks to lose more salt than they ever contained to supply the calculated amounts to the oceans each year. Alternatively, Irish geologist samuel Haughton employed the simple concept that thicker strata took longer to form to estimate the Earth's age. After figuring that sediments accumulated on the ocean floor at a rate of one foot (30.5 cm) in 8,616 years, he estimated that it would require at least 200 million years, or possibly 10 times longer, to lay down the total thickness of rock covering the planet. Problems with this method included inaccurate estimations of the total thickness of rock on the Earth's surface and sedimentation rates that differed significantly according to time and place. Even without a satisfactory means of measurement, Lord Kelvin's revised approximation of 20-40 million years appeared to be a major underestimate.

Then in 1896 French physicist Henri Becquerel discovered natural radioactivity when he observed that uranium emitted invisible rays of energy. Polish physicist Marie Sk odowska Curie studied the ema nations for her doctoral dissertation and found that thorium also emitted such rays, and furthermore, the emanations were a property of atoms and not due to a chemical reaction. Curie named the revolutionary phenomenon radioactivity, and she and her husband, Pierre Curie, proceeded to discover two new radioactive elements, radium and polonium. Ernest Rutherford (1871-1937) and Frederick Soddy (1877-1956) explained radioactivity as the result of the instability of an element that spontaneously emitted particles from its nucleus. For example, uranium released helium atoms as it decayed. In the process an element could transform into another element. Some radioactive elements had very long half-lives; uranium took 4.5 billion years to decay to half of its original amount. In 1905 Rutherford suggested that radioactive decay could be used as a geological timekeeper. Using uranium/helium ratios, he determined the age of a sample of pitchblende to be 90 million years, but he incorrectly assumed that helium did not escape over time.

Pierre Curie and colleague Albert Laborde announced in 1903 that radium emitted enough heat to melt its own weight in ice in less than one hour. This finding that radioactive elements generate heat refueled the debate over the age of the Earth. Lord Kelvin's calculations depended on the Earth's slow cooling in an absence of any external heat source, and these physicists claimed that radioactive elements within the Earth provided enough heat to make his calculations worthless. Holmes was intrigued by these scientists who challenged the authoritative Lord Kelvin and by the potential utility of this phenomenon called radioactivity. As a teenager, witnessing the debate between one of the world's most established scholars and a few lesser known but equally accomplished physicists made a great impression upon Holmes. Now, he was also interested in radioactivity, and these two curiosities would merge to become his lifelong passion, using radioactive decay to determine the age of the Earth.

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