Convergence: The Idea at the Heart of Science by Peter Watson

Author:Peter Watson [Watson, Peter]
Language: eng
Format: epub
Publisher: Simon & Schuster
Published: 2017-02-20T23:00:00+00:00

How Continental Drift and the Age of the Earth Fit Together

The theory of continental drift coincided with the other major advance made in geology in the early years of the century. This related to the age of the earth. In 1650, as is well known, James Ussher, archbishop of Armagh in Ireland, using the genealogies given in the Bible, had calculated that the earth was created at nightfall on October 22, 4004 BC.I In the late nineteenth century William Thomson, Lord Kelvin, using ideas about the earth’s cooling, proposed that the crust formed between 20 million and 98 million years ago (chapter 1). All such calculations were overtaken by the discovery of radioactivity and radioactive decay. In 1907 Bertram Boltwood, at Yale, acting on a suggestion of Ernest Rutherford, established that lead was the final decay product of radioactive uranium, and realized that he could calculate the age of the rocks by measuring the relative constituents of uranium and lead, and relating it to the half-life of uranium. The oldest substances on earth, to date, are some zircon crystals from Australia dated in 1983 to 4.2 billion years old. The current best estimate for the age of the earth—linking physics, geology, and time—is 4.5 billion years.10

Wegener took the theory for granted, based on the evidence he had collected, but many geologists, especially in the United States, were not convinced. They were “fixists,” who believed that the continents were rigid and immobile. In fact, geology was divided for years, at least until World War II. But with the advent of nuclear submarines the US Navy in particular needed far more information about the Pacific Ocean, the area of water that lay between it and its main enemy, Russia. The basic result to come out of this study was that the magnetic anomalies under the Pacific were shaped like enormous “planks” in roughly parallel lines, running predominantly north-south, each one 15 to 25 kilometers wide and hundreds of kilometers long.

Walter Pitman didn’t know too much about all this. At the time of his Pacific voyage, he was unfamiliar with the classical arguments for continental drift and not over-familiar with the notion of seafloor spreading.11 But the Eltanin findings produced a tantalizing piece of arithmetic: divide 25 kilometers by 1 million (the number of years after which, on average, the earth’s polarity changes), and you get 2.5 centimeters. Did that mean the Pacific was expanding at that rate each year? The broad answer was yes, though later research suggested that the Pacific has been expanding constantly by 1 centimeter per year for the past 10 million years. Even 1 centimeter multiplied by 10 million equals 100,000 meters, 100 kilometers, or 61 miles. (Today the width of the Pacific, at its widest, is 18,800 kilometers or 12,300 miles.)12

There was other evidence to support the mobilists. In 1953 the French seismologist Jean-Pierre Rothé produced a map at a meeting of the Royal Society in London that recorded earthquake epicenters for the Atlantic and Indian oceans. This was remarkably consistent, showing many earthquakes associated with the mid-ocean ridges.

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