Four Revolutions in the Earth Sciences
From Heresy to Truth
James Lawrence Powell
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The Abyss of Time
Gone Right Through It In September 1846, the faculty of the University of Glasgow convened to examine an applicant for its chair in natural philosophy, the previous holder, appointed in 1803, having passed on after a lengthy illness. At age twenty-two, the candidate, William Thomson (1824–1907), was easily mistaken for a student himself. In spite of his youth, the Cambridge graduate had already accomplished more than enough to justify his candidacy, but there was one additional hurdle. Before an appointment could become official, the applicant had to write and deliver, in Latin, an essay assigned by the faculty. Thomson’s topic was to be De caloris distributione per terrae corpus: “The distribution of heat within the Earth.” The title echoed that of Joseph Fourier’s famous book on heat flow. Either Thomson himself or his father, James, a long-time member of the Glasgow faculty and his son’s strongest booster, had proposed the topic.1 Both knew that no one was better qualified to address the question of the Earth’s heat than William, who at age sixteen had already mastered Fourier’s difficult mathematics. William Thomson had learned of Fourier’s The Analytical Theory of Heat in 1839 from the lectures of Professor John Nichol, who told the teenager that “perhaps” he could understand this work of “transcendent merit.” According to William’s later recollection, “I took Fourier out of the University Library; and in a fortnight I had mastered it— gone right through it.”2 The following summer James Thomson took his children on a trip to Germany. Before leaving, William had picked up a book by Phillip Kelland, professor of mathematics at Edinburgh, titled Theory of Heat. Fourier, Kelland alleged, had made an error. Indignant at this slander of his hero and already a budding academic, William penned an article titled “On Fourier’s Expansions of Functions in
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Trigonometrical Series” and submitted it to the Cambridge Mathematical Journal, which published it.3 The author was listed not as William Thomson but, at the suggestion of his father, as the pseudonymous P. Q. R. James Thomson evidently thought it inappropriate for his sixteen-year-old son to rebuke in print a distinguished colleague.4 As further evidence that young Thomson’s essay topic had not been chosen at random, in 1842, at age eighteen, the precocious youngster had published a memoir titled “On the Linear Motion of Heat.”5 Using Fourier’s approach, he solved the differential equation that describes how to determine the temperature in a solid body at any time in the future. For the rest of his long life, William Thomson would return to this early paper and its implications, which “contain[ed] the germs of many of his subsequent ideas.”6 At the end of the paper Thomson speculated on the effect were he to assign negative values to time. The equations then gave impossible results, convincing him “that there must have been an origin to the natural order of the cosmos. There must have been a beginning.”7 In looking back at his body of work from the vantage point of 1882, Thomson wrote that this youthful essay “gave a very decisive limitation to the possible age of the earth as a habitation for living creatures, and proved the untenability of the enormous claims for TIME which, uncurbed by physical science, geologists and biologists had begun to make and to regard as unchallengeable” (186). The implications of Fourier’s mathematics, first encountered by young William Thomson at age sixteen, would occupy him intermittently but without surcease for sixty-eight years, until his death in 1907. By then he was known as Lord Kelvin, the world’s most acclaimed and accomplished scientist.
High Priest of Uniformitarianism Just before Fourier began his mathematical advances, geology began to emerge as a true science. The pivotal insight came from the Scotsman James Hutton (1726–1797). Trained as a physician and chemical manufacturer, Hutton inherited several farms from his father, allowing
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him the opportunity to roam the land and pursue his interest in geology. Not content merely to observe, Hutton set out to explain. Hutton believed that God had created the Earth for man. Since sections of the Earth are visibly eroding, some process must restore it; else our planet would eventually become uninhabitable, surely not God’s intent. Hutton came to believe that eroded sediments are deposited in the sea and subsequently hardened, heated, uplifted, and returned to the continents, where they erode to start the process again. He viewed earth history as a series of endless cycles of decay and rejuvenation, with, in his most famous phrase, “no vestige of a beginning,—no prospect of an end.”8 This view contrasted mightily with the rival theory of geology, which saw earth history as ruled by catastrophe: earthquakes, volcanic eruptions, and the like. Catastrophism fit well with the short chronology of Archbishop James Ussher and his followers, which allowed only a few thousand years for all of geologic time. Hutton’s cycles required vastly longer periods. As his devoted biographer and interpreter, John Playfair, wrote, they required an “abyss of time.”9 Hutton earned his position as the “Father of Geology” for a statement that would become the guiding principle of geologic thought and practice: Not only are no powers to be employed that are not natural to the globe, no action to be admitted of except those of which we know the principle, and no extraordinary events to be alledged in order to explain a common appearance . . . we are not to make nature act in violation to that order which we actually observe . . . chaos and confusion are not to be introduced into the order of nature, because certain things appear to our partial views as being in some disorder. Nor are we to proceed in feigning causes, when those seem insufficient which occur in our experience.10
Charles Lyell (1797–1875) extended Hutton’s theory in a book titled Principles of Geology. The first edition appeared in 1830 and the last, published posthumously, in 1875. The book made Lyell the most influential geological writer in history. Trained first as a lawyer, Lyell’s
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Principles was a “passionate brief for a single, well-formed argument, hammered home relentlessly.”11 Like Hutton, Lyell believed that God created the Earth for humans. But once he set the Earth going, never again did he intervene in its workings. Natural laws are invariant. Moreover, not only are the processes that we observe today the only ones that have ever operated, but they also have always operated at the same rate. According to Lyell, “If in any part of the globe the energy of a cause appears to have decreased, it is always probable that the diminution of intensity in its action is merely local, and that its force is unimpaired, when the whole globe is considered.”12 Lyell disdained catastrophism, writing in 1881 that he needed no “help from a comet.”13 According to Lyell, while change is constant on Earth, it does not lead anywhere. Our planet has always looked about as it does now, its history revealing no evidence of progress. Even extinction does not represent permanent change: “The huge iguanodon might reappear in the woods, and the ichthyosaur in the sea, while the pterodactyl might flit again through umbrageous groves of tree-ferns.”14 Had the Earth been on trial with Lyell as prosecuting attorney, the defense might have pointed out that his thesis divides into two parts.15 First, natural law and earthly processes do not vary. This we may call the constancy of law and process. Second, neither the rate at which those processes operate nor the overall condition of the Earth vary. This we may call the constancy of rate and state. One could accept the first constancy without having to accept the second. In what Stephen Jay Gould has called “the greatest trick of rhetoric . . . in the entire history of science,” Lyell gave both arguments the same name: “uniformity.”16 William Whewell, who reviewed the second edition of Lyell’s book, lumped the two meanings together under the unwieldy name “uniformitarianism,” which stuck. Whewell posed the essential question of geology: “Have the changes which lead us from one geological state to another been, on a long average, uniform in their intensity, or have they consisted of epochs of paroxysmal and catastrophic action, interposed between periods of comparative tranquillity?” He sagely predicted that the question “will probably for some time divide the geological world into two sects, which may perhaps be designated as the Uniformitarians and the Catastrophists.”17
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The defense would go on to point out that the constancy of law and process, which Gould has called methodological uniformitarianism, describes not how the Earth works but how geologists ought to work. They reject supernatural explanations and employ commonplace, simple processes before appealing to rare, complicated ones. Of course, this is not the only way that science works; it is nothing more than common sense. William of Ockham expressed it well in the fourteenth century: “One should not assume the existence of more things than are logically necessary.” All scientists reason from effects back to causes. Nothing is special about methodological uniformity; it says merely that geology is a science. Lyell believed so strongly in the uniformity of rate and state, which Gould called substantive uniformitarianism, that he wrote, But should we ever establish by unequivocal proofs, that certain agents have, at particular periods of past time, been more potent instruments of change over the entire surface of the Earth than they now are, it will be more consistent with philosophical caution to presume, that after an interval of quiescence they will recover their pristine vigor, than to regard them as worn out.18
But by the late nineteenth century, geologists had already found unequivocal evidence that substantive uniformity is false: earth history is marked by change. Glaciers have advanced over the continents and retreated; seas have drowned the continents and withdrawn; mountain ranges have risen and worn away; parts of the Earth now cold were once warm and vice versa. The coup de grace to an unchanging Earth was the progression shown by the fossil record, leading from the simplest life forms in Precambrian rocks to modern Homo sapiens. Lyell accepted evolution only in the 1866 edition of his Principles and then, Gould believes, only because “it permitted him to preserve all other meanings of uniformity.”19 One type of uniformitarianism amounts to the statement that geology is a science; the second, which requires the adoption and maintenance of an a priori position regardless of the evidence, amounts to the statement that geology is not a science.
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In a 1905 book titled The Founders of Geology, Sir Archibald Geikie, from whom we will hear more, wrote that Lyell became “the great high priest of Uniformitarianism—a creed which grew to be almost universal in England during his life, but which never made much way in the rest of Europe, and which in its extreme form is probably now held by few geologists in any country.”20
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A Great Mistake Has Been Made
You Can’t Win, and Eventually You Lose In 1892, the queen made William Thomson a peer of the realm. He chose the name “Kelvin,” for a small stream that wends its way near the University of Glasgow.1 Some thirty years before, Kelvin, as we will henceforth refer to him, had come to despise Lyell’s theory, but not for any of the reasons already given. Indeed, Kelvin operated from too lofty a perch to become embroiled in geology’s internecine squabbles. His objection came from a higher plane: physics. Lyell’s philosophy envisioned an eternal, unchanging Earth. To supply the energy necessary to keep the planet running, Lyell appealed to chemical reactions in the Earth’s interior. They produce heat, he said, which in turn generates electrical currents, which break up the compounds produced in the reactions and start the process over again. But Kelvin knew that such a scheme was impossible, for it would violate the fundamental laws of nature. The first law of thermodynamics holds that in any process, energy in the form of heat and work is conserved. As science students once liked to joke, the first law states: “you can’t win”—you cannot get out more energy than you put in. The second law, jointly discovered by Kelvin, can be stated in several different ways. The simplest may be to say that heat flows spontaneously from hotter to colder places, never the opposite. “Although mechanical energy is indestructible,” Kelvin said, “there is a universal tendency to its dissipation, which produces gradual augmentation and diffusion of heat, cessation of motion, and exhaustion of potential energy through the material universe.”2 The message of the second law is “eventually, you lose.” In describing geologic time as infinite and the Earth as unchanging, Lyell claimed that the Earth is a perpetual-motion machine, one that
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can not only win the energy battle but go on doing so forever. But the first and second laws prove that such a machine is impossible. Lyell’s theory “violates the principles of natural philosophy in exactly the same manner, and to the same degree,” Kelvin wrote, “as to believe that a clock constructed with a self-winding movement may fulfill the expectations of its ingenious inventor by going for ever.”3 Kelvin carried the battle to the geologists, charging in an 1868 address that “it is quite certain that a great mistake has been made—that British popular geology at the present time is in direct opposition to the principles of natural philosophy. There cannot be uniformity. The Earth is filled with evidences that it has not been going on for ever in the present state, and that there is a progress of events towards a state infinitely different from the present.”4 Kelvin set out to refute Lyell by showing that the Earth was born at some finite time in the past and will not survive beyond some finite time in the future. The Earth is neither eternal nor unchanging. As the second law dictates, like everything else our planet is running down. Kelvin knew that Fourier’s mathematics and the second law showed that when a body starts out with different internal temperatures at different places, heat flows from hot to cold regions and eventually removes the differences. In his 1846 candidate’s lecture, he had shown that if one knew how fast the Earth is losing heat and how well rocks transmit heat, one could work the equations back to the point when the process began and thereby estimate the age of the Earth.5 But before he took up that question, Kelvin began to puzzle over the age of the Sun. It also had to obey natural law and therefore had to have an energy source. Kelvin first attempted to explain the Sun’s heat by appealing to the idea that meteorites continually fall into the Sun and give up their gravitational energy as heat. But calculations showed that the process was inadequate to explain the Sun’s abundant heat and light. That theory having failed, the only apparent alternative was that the Sun’s energy is left over from its birth. The second law then requires that the Sun be running down. If the Sun’s energy is waning, it, too, must have been hotter in the past and will one day be cooler, so cool that it will no longer warm and illuminate the Earth.
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In an 1862 article in the popular Macmillan’s Magazine, Kelvin described the results of his calculations of the age of the Sun: It seems, therefore, on the whole most probable that the sun has not illuminated the earth for 100,000,000 years, and almost certain that he has not done so for 500,000,000 years. As for the future, we may say, with equal certainty, that inhabitants of the earth can not continue to enjoy the light and heat essential to their life for many million years longer unless sources now unknown to us are prepared in the great storehouse of creation.6
To Kelvin, whether the Sun was 100 million years or 500 million years old mattered little. The Earth could be no older. By limiting the age of the Sun, Kelvin had refuted Lyell and uniformitarianism. Now Kelvin turned to the Earth, using Fourier’s mathematics to calculate its maximum age. Fourier had attempted the same calculation and gotten 200 million years, such a seemingly absurd figure that he did not even bother to write it down.7 Kelvin estimated the Earth’s initial temperature at 7,000°F, the measured melting point of igneous rocks. A handful of observations suggested that temperature increased with depth by about 1°F for every fifty feet. Having these facts and Fourier’s mathematics at his command, it was a simple matter for Kelvin, if not for many others, to calculate that the Earth consolidated 98 million years ago. But given the lack of precision in his starting numbers, he broadened his estimate to say that the Earth’s formation “cannot have taken place less than 20,000,000 years ago, nor more than 400,000,000 million.”8 His most probable estimate of the age of the Sun, 100 million years, lay well within this range, seemingly giving two independent calculations of the age of the Earth and the solar system. This encouraged him to declare that he had refuted the “Doctrine of Uniformity.”9 Kelvin was no doubt vexed to find that geologists took no notice of his calculations. The latest edition of Lyell’s Principles continued to claim that the Earth is a perpetual-motion machine. A heavier barrage was needed, and, in an 1868 lecture to the Geological Society of Glasgow titled “On Geological Time,” Kelvin loosed it.10
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“A great reform in geological speculation seems now to have become necessary,” he began. Oddly, or perhaps sagaciously, instead of attacking the geologists of his day, Kelvin directed his fire at the longdead John Playfair, who sixty-six years earlier had written “Illustrations of the Huttonian Theory of the Earth.”11 Hutton was a poor writer, and his treatise contained long passages from the French, so that most geologists had to learn of his views from Playfair. “The statement that the phenomena presented by the earth’s crust contain no evidence of a beginning, and no indication of progress towards an end,” Kelvin wrote, “is founded upon what is very clearly a complete misinterpretation of the physical laws under which all are agreed that these actions take place.” Much of Kelvin’s paper was taken up with a third method of gauging the Earth’s age, using the slowing of the Earth caused by tidal friction with the Moon, which provided another corroboration of Kelvin’s view that the Earth could not have existed indefinitely.
“Odious Spectre” This was too much for geologists and biologists, both of whom needed more time than Kelvin would grant. Indeed, in a letter to Alfred Russell Wallace, the co-discoverer of natural selection, Charles Darwin wrote that in order to explain the missing links that mark the fossil record he would like to take advantage of the ample eons before the Silurian period, but “then comes Sir W. Thomson like an odious spectre.”12 To exorcise the “spectre” came “Darwin’s Bulldog”: Thomas Henry Huxley. In an 1860 debate over Darwin’s theory, so legend has it, Huxley had defeated a famous public speaker, Bishop Samuel Wilberforce, known as “Soapy Sam” for his unctuous delivery. Now evolution was under attack not from a cleric armed with the Bible but from Kelvin, the leading British scientist, armed with impeccable and, to Huxley, impenetrable mathematics. Kelvin’s 1868 assault had been directed at geology, and Huxley happened to be president of the Geological Society of London at the time. In his 1869 presidential address, he used the bully pulpit to rebut Kelvin. Huxley found himself in the same seemingly inferior position in which many geologists over the next hundred years were to find them-
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selves: unable to counter an apparently superior quantitative argument from a physicist. Huxley well knew that he was unable to use mathematics to refute Kelvin. But, he said, this left him no worse off than “attorney[s] general,” who must “nevertheless contrive to gain their causes, mainly by force of mother-wit and common-sense, aided by some training in other intellectual exercises.”13 Huxley pounced on Kelvin’s selection of the long-dead Hutton and Playfair as his targets, pointing out that geologists had long since modified the overly rigid uniformitarianism of the founding fathers. “To my mind there appears to be no sort of necessary theoretical antagonism between Catastrophism and Uniformitarianism. On the contrary, it is very conceivable that catastrophes may be part and parcel of uniformity,” Huxley argued.14 In Huxley’s most telling point, he elegantly summed up what today we often put more crudely. “Mathematics may be compared to a mill of exquisite workmanship, which grinds you stuff of any degree of fineness,” he said. “Nevertheless, what you get out depends upon what you put in” (50). Winding up, Huxley pointed out that Kelvin’s results also depended on assumptions and suppositions. Asked Huxley, “Is the earth nothing but a cooling mass and has its cooling been uniform? An affirmative answer to both these questions seems to be necessary to the validity of the calculations on which Sir W. Thomson lays so much stress” (52–53). But Kelvin would not let Huxley off so lightly. In a response only two weeks later, he began: “The very root of the evil to which I object is that so many geologists are contented to regard the general principles of natural philosophy, and their application to terrestrial physics, as matters quite foreign to their ordinary pursuits.” Kelvin added, “A clever counsel may, by force of mother-wit and common sense, readily carry a jury with him to either side, [but] I do not think that the high court of educated scientific opinion will ever be satisfied by pleadings conducted on such precedents.”15 Huxley’s address was the last time for several decades that any scientist would challenge Kelvin. He had shifted the ground of debate about the Earth. No longer could Lyell’s limitless time be countenanced, nor his unchanging Earth, nor reasoning by “mother-wit and common
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sense.” Whatever the fate of Kelvin’s argument about the age of the Earth, geologists heard his message that their science could not remain descriptive while all other sciences became increasingly quantitative. Kelvin pushed geology toward the twentieth century, though once it got there, it would reject his assumptions and his results.
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