He was convinced that radioactive dating could yield much more precise results: “Every radioactive mineral,” he wrote, immediately after receiving his doctorate, “can be regarded as a chronometer registering its own age with exquisite accuracy.”7
Exquisite accuracy still lay in the future, though. In one of Holmes’s earliest projects, he made use of the decay rates introduced by Rutherford and Soddy to date a Norwegian rock layer at 370 million years. Shortly afterward, he calculated another rock sample at 1.6 billion years. And when, at the age of twenty-three, he wrote his first book, a good part of it was devoted to explaining why radioactive methods produced such a wild range of dates.8
The Age of the Earth, published in 1913, still stands as a benchmark in geology—but not because it answered the implied question. (In fact, Holmes never actually specifies in it what the age of the earth might be; that would come much later in his life.)
Rather, The Age of the Earth shifted the trajectory of the adolescent science slightly, angling it away from Lyell’s strict uniformitarianism.
Uniformity itself Holmes had no problem with. In fact, he thought that both Hutton and Lyell had done geology a favor by forcing it to give up its theories of origins. “Speculative fancies concerning the origin of the world,” he wrote,
form the subject-matter of many of the earliest writings on record, and throughout the intellectual history of mankind the problem has proved to be one of supreme fascination. It was not, however, until quite recent times that the efforts of imagination gave place to reasoned hypotheses. . . . At first, on having attained the status of a science, geology steadfastly refused to consider seriously the cosmogonic fantasies then current. It was Hutton, who by advocating the direct observation of nature in place of the old scholastic arguments, first delivered geology from the inevitable wranglings that would necessarily have arisen from so premature a discussion of the beginning of things.
But the question could not be ignored forever:
While today we are still unable from geological facts alone to trace back with confidence the details of the earth’s beginning, yet the uncertainty which justified Hutton in entirely disregarding the earth’s genesis no longer exists. Astronomy, physics, and chemistry have all . . . done much to remove our modern ideas from the dangerous quicksands of speculation.9
Lord Kelvin, Holmes concluded, had done geology a great favor by reintroducing the problem of the earth’s age: “He invaded the domain of Geology . . . determined to protest against what he considered the immoderate application of the principle of Uniformity.”
But radioactive theory, not the Second Law of Thermodynamics, would unlock the secret of the earth’s age. Radioactive minerals, Holmes concluded, are “clocks wound up at the time of their origin,” and “we are now confident that the means of reading these time-keepers is in our possession.”
The confidence was a tiny bit premature, since radioactive dating was currently yielding ages ranging from 370 million to 1,200 million years. The factors involved were complicated; much was still unknown, and much work remained to be done. Yet Holmes remained confident. “The problem has advanced from the qualitative to the quantitative stage,” he concluded, “and for the first time in historical geology, accurate measurement founded on delicate experimental work has become possible.”10
In 1913, this was more prophecy than statement of fact. But in the next decades, study of radioactive decay yielded more and better data, and estimates of the earth’s age began to tighten to a very clear range. In later publications, Holmes moved from a stance of 3,000 million years, to 1.6 billion years, to over 3 billion years; by 1930, Ernest Rutherford had followed him, calculating that the earth had been formed “about 4 × 109 years ago”: 4 billion years.11
Within thirty years, almost the entire scientific community had shifted its heading onto Holmes’s new track.
“Uniformity proved a great advance,” Holmes had written, “but in detail it is apt to lead us astray if applied too dogmatically.” Uniformity remained the basic assumption of geology; but it was now tempered by the necessity of understanding the earth’s history as encompassing a beginning, a direction, and (in all likelihood) an end.12
To read relevant excerpts from The Age of the Earth, visit http://susanwisebauer.com/story-of-science.
ARTHUR HOLMES
The Age of the Earth
(1913)
The original text of The Age of the Earth can be viewed online (see http://susanwisebauer.com/story-of-science for a live link) and is also available as a PDF download from Forgotten Books (readable, but the conversion to a downloadable format has altered spacing and reproduced headings so mechanically that they often appear in the middle of a page). A print copy of the original Harper’s Library edition, containing Holmes’s illustrations and diagrams, can be found at most university and many large public libraries. Be careful if ordering a physical copy secondhand, since many copies offered are low-quality print-on-demand reproductions of the electronic text. Look instead for the reprints by Ernest Benn and Nelson & Sons.
Arthur Holmes, The Age of the Earth, Harper & Brothers (hardcover and e-book, 1913, no ISBN).
* * *
* The Second Law of Thermodynamics is, perhaps, better known in its related but simpler phrasing: The universe tends toward entropy. (The gradual, continuing loss of energy is the reason that natural engines in the universe will, ultimately, stop functioning.)
† The formulation of atomic theory is covered, briefly, in Chapter 26.
SEVENTEEN
The Return of the Grand Theory
Continental drift
To determine “truth” . . . [is] to find the picture that sets
out all the known facts in the best arrangementand that
therefore has the highest degree of probability.
—Alfred Wegener, The Origin of Continents and Oceans, 1915
The decay of radioactive elements had reintroduced, into Lyell’s strict uniformitarianism, the ticking clock.
It now seemed impossible to deny that there had been a beginning to earth time (even though no one knew exactly when it might be). So Lyell’s long, slow, predictable, mostly uniform series of changes had a start.
Which led to a series of additional questions. What did the earth look like at that start? How had those slow changes altered its original form? And (in line with Lyell’s principles), could those slow changes still be detected, creeping along in the present?
The Austrian geologist Eduard Suess garnered a wide following with his theory of “thermal contraction,” which was a throwback to Newton’s molten earth cooling over time, combined with Lyellian cycles of erosion and buildup. According to contraction theory, as the newborn, extremely hot earth began to cool into a solid globe, its crust contracted and wrinkled up, like a drying apple. Some of the crust collapsed inward, forming ocean basins and pushing the crust between the collapsed basins upward into continents. Then, as the earth went on cooling (over those millions and millions of years that the measurement of radioactive elements now suggested), it contracted further, and the continents collapsed inward and pushed the ocean basins up. This was a repeating cycle, a “continual interchange of land and sea” that pushed fossil remains from the tops of mountains to the bottoms of seas and back up again—which explained why marine fossils now existed on mountains, and why the same kinds of fossil remains could be found on different and disconnected continents.1
Other geologists accepted the basic model of contraction but argued that the continents and oceans had always been in the same place (“permanence theory”). Erosion and silting, or perhaps volcanic activity followed by earthquakes, created and destroyed land bridges between them. Animals, and eventually people, wandered across the land bridges, which then disintegrated.
But there were massive problems with contraction theory.
For one thing, the idea of “land bridges popping up and down” (in Naomi Oreskes’s phrase) seemed more than a little
ad hoc. Radiation theory introduced another problem: it now appeared that certain atoms generated more heat over time, which didn’t fit at all with the idea that a uniformly hot earth was now cooling.2
And calculations made by more than one physicist suggested that the earth was simply too dense for contraction to produce high mountains and cavernous ocean basins. Physicist Osmand Fisher, as far back as 1881, had suggested that perhaps the globe wasn’t solid; perhaps there was a “fluid substrate” to the earth, a soft and yielding layer farther down that allowed the surface above it to shift, break, and move. Fisher’s contemporary, the American geologist C. E. Dutton, had pointed to the movement of glaciers as an example of how a solid layer could “flow” up and down. But there was no way to prove that a fluid underlayer existed, and no proof that the dry and solid surface of the earth had ever acted like a sheet of ice.3
Alfred Wegener, a German astronomer with a particular interest in the weather, had a different solution.
“Anyone who compares, on a globe, the opposite coasts of South America and Africa,” he wrote in 1915, “cannot fail to be struck by the similar configuration of the two coast lines.” The jigsaw match suggested to him that the continents had once been a single mass, a giant supercontinent that he labeled Pangea; long, long ago, Pangea had broken up and drifted apart.4
17.1 PANGEA AND CONTINENTAL DRIFT
This theory required him to provide an explanation for how solid earth could “drift.” So he proposed that the earth was not actually solid. Instead, it consisted of a liquid core, surrounded by a series of shells that increased in density as they got closer to the surface.
It was a simple and elegant explanation and accounted for almost all the factors that puzzled geologists: the odd similarities between far-distant fossils, the apparent interlocking fit of the continental coastlines, the existence of mountains (which sprang up where the drifting pieces collided and overlapped). And it was greeted, in the world of geology, by shrieks of derision.
The reaction was not entirely irrational. Although continental drift made sense of the map, Wegener had formulated it in the absence of pretty much any other physical evidence. It was a grand theory in the Aristotelian tradition; he had come up with the huge overarching explanation first, and defended it entirely on its internal consistency. This was not particularly “scientific,” as the American geologist Harry Fielding Reid sniped in a review: “Science has developed by the painstaking comparison of observations,” Reid wrote, “and, through close induction, by taking one short step backward to their cause; not by first guessing at the cause and then deducing the phenomena.” The paleontologist Charles Schuchert complained, “The whole trouble in Wegener’s hypothesis and in his methods is . . . that he generalizes too easily from other generalizations.” Schuchert’s colleague Edward Berry, teaching at Johns Hopkins, agreed: “My principal objection to the Wegener hypothesis rests on the author’s method,” Berry wrote:
This, in my opinion, is not scientific, but takes the familiar course of an initial idea, a selective search through the literature for corroborative evidence, ignoring most of the facts that are opposed to the idea, and ending in a state of auto-intoxication in which the subjective idea comes to be considered as an objective fact.5
On the other hand, there wasn’t a great deal of painstaking observation or of concrete fact to support contraction theory either, not to mention vanishing land bridges.
The wholesale resistance to Wegener’s intuitive leap may have had something to do with border protection; Wegener was neither a geologist nor a paleontologist. He was a tinkerer in meteorology, an adventurer who had once been forced to eat his own ponies to survive in an icy Greenland camp, a German who had fought on the side of the Central Powers in World War I. But there were undeniable weaknesses in drift theory. Wegener was unable to explain the mechanism behind it; he could offer no reason why Pangea didn’t simply remain one supercontinent. And continental drift was horribly counterintuitive. It was almost impossible to imagine those enormous landmasses taking to the high seas, plowing through the ocean as if they were the Staten Island Ferry. It required a conceptual leap not unlike the one demanded four hundred years earlier, when the apparently stationary earth was sent hurtling through space at top speeds.
Wegener himself was conscious of the problems, but he believed that the explanatory power of his theory trumped his lack of explicit proof. He argued that, after all, the earth “supplies no direct information” about its configuration:
We are like a judge confronted by a defendant who declines to answer, and we must determine the truth from the circumstantial evidence. All the proofs we can muster have the deceptive character of this type of evidence. . . . It is only by combining the information furnished by all the earth sciences that we can hope to determine “truth” here, that is to say, to find the picture that sets out all the known facts in the best arrangement and that therefore has the highest degree of probability.6
Wegener continued to work on his theory, adding new arguments, reworking and republishing The Origin of Continents and Oceans. The book went into a second edition, and then a third. He summarized his arguments for continental drift in lectures and symposiums, in Europe and in North America. “The theory offers solutions for . . . many apparently insoluble problems,” he wrote, in 1922.7
Most geologists still disagreed. But in 1928 the naval astronomers F. B. Littell and J. C. Hammond compared the longitudes of Washington and Paris in 1913 and in 1927. Their readings revealed beyond a doubt that the distance between the two cities had increased by 4.35 meters—a creep of 0.32 meter per year.
Given that Paris is some 6,000 kilometers from Washington, it would have taken over eighteen million years for the two cities to move that far apart. But the drift was measurable, beyond a doubt. When Wegener published the fourth and final edition of The Origin of Continents and Oceans in 1929, he added Littell and Hammond’s measurements to his final appendix: “The direction and amount of this change,” he concluded, “agree very well with the deductions on the basis of drift theory.”8
Wegener did not live to see the reaction. While the fourth edition was coming off the press, he was organizing his fourth expedition to Greenland. He arrived in the spring of 1930 and managed to establish an observation camp at a site called Eismitte, nearly in the center of the island. But everything that could go wrong did: ice was thicker than expected, the weather was unsettled and harsh, his hired team lobbied for more money, supplies failed to arrive, dogsled teams vanished. “The whole business is a big catastrophe,” he wrote back to a colleague in late August.9
In early November, short on food and facing temperatures plunging far below zero, Wegener and a companion abandoned the observation camp and set out for the better-supplied base camp of Scheideck, 250 miles away. They never arrived. In the spring of 1931, Wegener’s body was located in a grave halfway between Eismitte and Scheideck; his colleague had vanished entirely.
•
Without its defender, the theory of continental drift could easily have faded; but its explanatory power was too great.
Arthur Holmes found it particularly appealing, and suggested that the movement of continents might be caused by convection—the slow movement of the mantle, the heated layer of the earth beneath the crust. This layer, he theorized, might be fluid enough to cycle in lazy currents, like a simmering stew, moving the crust above it as it turned over.
This turned out to be the right explanation, but Holmes, like Wegener, had no ability to peer beneath the crust for proof. Geology needed an extension of the senses through artificial means, instruments to do for geologists what microscopes and “furnaces” had done for chemistry.
Those instruments finally evolved out of war.
Sonar technology, first developed to allow ships to search for attacking submarines, was redeployed in the 1950s to map the ocean floor. For the first time, geologists could see the features of the deep: continental shelves, abyssal plains, midoc
ean ridges and trenches, an entire underwater terrain. One of the mappers was Harry Hess, who had taught geology at Princeton before the war, and commanded a transport outfitted with sonar during the years of combat. In 1962 he published a paper proposing that the newly mapped features of the ocean floor were proof of convection; the midocean ridges were places where the currents, cycling slowly through the mantle, were pushing hot material up into the seafloor, where it formed new crust. Trenches were places where the crust was subsiding back down into the mantle, melting and re-joining the currents. Holmes had been right: “The continents do not plow through oceanic crust impelled by unknown forces,” Hess wrote. “Rather, they ride passively on mantle material as it comes to the surface at the crest of the ridge and then moves laterally away from it.”10
17.2 CONVECTION
Hess (along with two later Cambridge researchers, F. J. Vine and Drummond Matthews) had provided the foundation for plate tectonics—the theory (finally formulated in the late 1960s) that the earth’s crust is made up of separate, continually moving pieces, or plates, of crust, that “float” on the mantle of the earth. This, at last, was the mechanism Wegener had been missing. And the grand theory had finally been justified—half a century later.
For a link to Wegener’s own brief précis of his argument, written in 1922, visit http://susanwisebauer.com/story-of-science.
ALFRED WEGENER
The Origin of Continents and Oceans
(1915/1929)
John Biram’s 1966 translation (made from the fourth German edition of 1929) has been reprinted by Dover Publications and is available in both print and digital formats.
The Story of Western Science Page 15
