In terms of influence and importance in framing Wegener’s physical arguments, it would be difficult to overestimate the influence of Rudzki’s Physics of the Earth. In chapter after chapter Rudzki gave a dispassionate, calculation-based, comparative treatment of all the chief physical hypotheses still in play in the first decade of the twentieth century to explain the major features of Earth’s surface and interior. In Wegener’s first two published papers on continental displacements, he made free use of Rudzki’s critique of the contraction theory, as well as his critique of Sir George Darwin’s hypothesis that the Pacific Ocean had been created in the separation of the Moon from Earth (at a time when Earth had supposedly become an unstable ellipsoid of rotation). Rudzki’s division of geophysics into topical sections, his histories of how their results had been achieved, and his conception of their proportional weighting in assessing our conception of the actual state of Earth’s interior were all adopted wholesale by Wegener.
It was from Rudzki that Wegener became aware of recent advances in the study of Earth’s gravity field and of seismology, as well as how these together provided the first real quantitative data on the state and structure of Earth’s interior. From Rudzki he also learned of the implications of the distribution of radioactive materials in the crust for the contraction theory and for geology more generally, especially for its likely decisive resolution of questions about the age of Earth. In the matter of displacements of Earth’s pole of rotation, Wegener found in Rudzki data on possible permanent movements of Earth’s pole representing translocations, not merely axial wobble or precession. It was here in this volume as well that Wegener found a sophisticated treatment of the possible physical states of matter at different depths in Earth’s interior. With these came a thorough discussion of the difference between strength and rigidity in solid materials under high confining pressures and high temperatures, supplementing what Wegener had studied on this topic while writing about the various phases of ice for his book on the thermodynamics of the atmosphere. Not only did Wegener follow Rudzki’s arguments and critiques in each of these areas, but he often repeated them verbatim in his published work.
The fundamental task for Wegener was not just to assimilate this new work and recommunicate it but to create an intellectual framework in which he could unite the very solid results of intercontinental correlations of rocks and fossil animals produced by geologists and paleontologists with a sound geophysical theory of Earth’s constitution and therefore its dynamic behavior. This geophysical theory would replace a variety of unsound, unphysical, and impossible theories that he found not just embedded in but used as framing concepts for the large geological, oceanographic, and paleontological syntheses he had just encountered and studied. Such a unification of sound geology and sound geophysics would create a milieu in which his hypothesis of continental displacements could receive a fair hearing.
Whose Earth?
There was nothing inherently difficult, recondite, or hard to access in the geophysical model that Wegener was proposing. The question he faced was not so much intellectual as jurisdictional: “who had the right to speak.” The latter was a matter of academic specialties and academic communities. We may orient ourselves to the problem that Wegener was beginning to discover by looking at the difference between a geologist’s Earth and a physicist’s Earth at the time of his writing. Geology was, in the main, a qualitative and historical science, an advanced natural history of rocks, rather than a preliminary approach to material which planned eventually to reduce itself to an aspect of physics and chemistry. There was chemistry, mineralogical chemistry, of course. And there was a mathematical element to the surveying of the globe, measuring the thicknesses of various strata and mapping their lateral extent, considering rates of erosion and sedimentation in a river delta, and such matters. But with the exception of those geologists who received their training in schools of mines and thus had a pressing professional concern with the behavior of solids under load as required for mine engineering, working geologists typically had no formal knowledge of physics whatever and no mathematics past trigonometry.
With respect to conditions in Earth’s interior, for geologists, the most useful sort of interior and the most convenient Earth would be a thin strong layer over a weaker layer, that is, an underlayer that should be more pliable and capable of ductile deformation than the surface—but how much more? How weak it should be was not quantified, and geologists did not require that it should be, only that they be allowed to assume it.
As opposed to the hot but qualitative and indistinct interior of Earth preferred by the geologists, physicists offered a mathematical Earth, a homogeneous solid cooling from the center outward by conduction. Cooling by conduction alone was probably not a realistic assumption, nor was the notion that Earth was homogeneous, but these assumptions were adopted because the consideration of convective cooling and of internal inhomogeneity made the calculations too difficult. This physicist’s Earth is the one we see in Joseph Fourier’s (1768–1830) Analytical Theory of Heat (1822), in every way a founding document of modern mathematical physics, and therefore the one we see in William Thompson’s (Lord Kelvin’s) extensive theoretical development in the last quarter of the nineteenth century of the “solid Earth” so beloved by British physicists.
These preferences for very different Earths could be maintained because into the very late nineteenth century there were no synoptic data about Earth’s thermal condition with increasing depth, there was no experimental study of the effect of pressure on the melting curves of rocks and minerals, there was no seismology to reflect waves off boundary surfaces within Earth, and there were limited and theoretically troubled data about Earth’s gravity field and the reasons for the departures from “normal” gravity at one place or another on Earth’s surface.
Therefore, almost all thermal and dynamic models of Earth’s interior before about 1900 were based on neither observation nor experimentation but rather extrapolations from astronomical data. These were still being debated during Wegener’s graduate career. Gravity surveys could not provide a criterion by themselves for the condition of Earth’s interior. Solid mechanics could as yet provide no real guidance. Until the very end of the nineteenth century there was no consensus on whether rocks generally contract or expand on solidification.
The situation described immediately above, with the inability of these various parts of physics to provide definitive answers to long-asked questions about the interior of Earth, was a nineteenth-century predicament. While a changed situation in “a new century” is often a historian’s fiction, it fits the sequence of events in geology and geophysics quite well. In the first decade of the twentieth century, and especially toward the very end of this decade and within a year or two of Wegener’s first efforts in this direction, all of this was radically changed.
Wegener’s article in Petermanns Mitteilungen was the first self-conscious attempt to reconcile the disparate results of the old geology with the new geophysics in the twentieth century, and irrespective of its other (and quite considerable) merits, it would be worthy of our attention for this reason alone. Wegener was always a little more daring than most of his contemporaries, and certainly more daring than most of his senior colleagues. He was always ready to bring about a reorganization and reconsolidation of new data and provide a new picture. But in terms of fundamental novelty, his work in continental displacements was an order of magnitude more significant and more penetrating than anything he had done heretofore.
The Formulation of the Hypothesis of Continental Displacements
Wegener was always under time pressure, but perhaps at no time in his life did he endure it more than in January and February of 1912, when he was assembling the data to support his idea of continental displacements. On the day after his initial address to the Geological Association in Frankfurt (his remarks on continental displacements there on 6 January had evoked, as he wrote to Else, “a storm of indignation”), he wrote to Prof.
Warming of the Danmark Expedition Committee to explain his situation. He told Warming of his new hypothesis of continental displacements, on which he had just the day before given his first public address, an idea that had the “strongest possible claims on his attention.”21 He continued, “My publications in the year 1911 amounted to 600–700 pages, and about half of this total belongs to the Danmark Expedition, and at the moment I have six different scientific papers in press.” He went on to tell Warming,
This sweeping hypothesis (on the horizontal displacement of the continents) which I presented publicly, in its development will, I believe, be very interesting to you. This comprehensive work equally concerns geology, geophysics, geodesy, and geography and is not without practical application in the interpretation of the longitude measurements of Danmark Expedition. It will naturally, in the future make very strong claims on my time and I just do not have the heart to leave it alone until I have completely finished my work on the results of the Danmark Expedition … it will be a delicate matter to find the right balance.22
Warming was predisposed to like Wegener, and he appreciated his candor and honesty in this matter: Wegener knew that he would get a dispensation and be allowed to proceed with his work on continental displacements without being accused of abrogating his expedition contract.
No sooner had Wegener cleared the way with the Danmark Expedition Committee to work on his hypothesis of continental displacements than a massive new time constraint descended upon him. In the first week of February Koch wrote to him to inform him that he had managed finally to raise the 50,000 marks in Copenhagen needed to launch the expedition to cross Greenland in the summer of 1912, and that they were “on.” Not only would Wegener have to go immediately to work to raise his share of the money, but he would have to have it in hand by April at the latest.23 However much Wegener had wanted to go on this trip, it was now also an obligation from which he could not escape. It would interfere with his development of the hypothesis quite severely. Thus, in the published version of “Die Entstehung der Kontinente” in Petermanns Mitteilungen (the first of three sections appeared in April) there is a footnote to the title that reads, “Due to my participation in a Danish expedition to Greenland, I am obliged to postpone the detailed treatment I had planned and to publish for the moment only this preliminary communication.”24
Let us turn our attention now to the “preliminary communication” that Wegener published in Petermanns Mitteilungen, beginning in April 1912. On the day he mailed it out to the journal, he had written to Köppen, “The Urkontinent (69 typed pages!) is in one copy off to Prof. Langhans (Petermanns Mitt) from whence it will probably soon return with the modest comment: too long!”25 But Langhans did not cut it, nor did he want it cut, and he considered Wegener’s entire argument essential if it was worth publishing at all. Perhaps some of Langhans’s enthusiasm stemmed from the extremely careful framing of the argument and the extraordinary efforts that Wegener made at the very beginning to put his ideas into context:
In what follows, a first rough attempt is made to interpret genetically the large-scale features of our earth’s surface—the continental platforms and ocean basins—in terms of a single comprehensive principle: the horizontal mobility of continental blocks. Wherever we have heretofore accepted that ancient land-connections sank to the depths of the oceans, we will now suppose a splitting and drifting apart of continental blocks. The picture we obtain in this way of the nature of our earth’s crust is new and, in some respects, paradoxical; yet it does not lack, as will be shown, a physical foundation. On the other hand, in the provisional investigation attempted here, based simply on the major findings of geology and geophysics, so many surprising correlations and simplifications are already revealed that it seems to me, on these grounds alone, justified, indeed even necessary, to replace the old hypothesis of sunken continents with this new, more productive working hypothesis. The demonstrable inadequacy of the former has already been made obvious by the opposing theory of the permanence of oceans. I refer to this new principle, despite its broad foundation, as a working hypothesis, and would like to see it treated as such at least until the persistence of these horizontal displacements in the present shall have been successfully demonstrated by means of astronomical position-fixing, with a precision which illuminates all doubt. It is also not superfluous to point out that this is a first draft. Elaboration of the details will most likely show the hypothesis must be modified in several respects.26
On the one hand, we have “a first rough attempt,” “provisional investigation,” “a working hypothesis,” “a first draft”; on the other hand, Wegener asserts “so many surprising correlations and simplifications … necessary, to replace the old hypothesis,” a “new principle,” and a “broad foundation.” This is an interesting combination of tentativeness and certainty, of caution and bravado. But, in the end, he was quite clear: it is a working hypothesis, not even a theory, though, like theory, a guide to investigation. It was not a demonstration about the world, not an axiom, law, or truth; it was a heuristic device for combining scientific findings. Wegener claims that the hypothesis is “based simply on the major findings of geology and geophysics.” He claims to have discovered no new facts, nor to have invented new justifications. It is his conviction here, as it was with his arguments about the vertical structure of the atmosphere, that the evidentiary foundation for the generalization did not lie in future discoveries but was already amply present in the professional and refereed literature and available for inspection. All that had been necessary was for someone to come forward and say “the obvious.”
Wegener began with a brief preview of the argument. He would begin by asking whether, on the basis of general geophysical and geological knowledge, horizontal movements of segments of rigid crust were possible, and if so, how these movements might take place. He would then make a modest attempt to follow the splitting and displacement of continental blocks through geological history and to show the connection of these displacements with the origin of mountain ranges and the displacement of the poles. Finally, there would be a brief attempt to establish the plausibility of contemporary, continuous, measurable displacement of the continents and to offer an explanation of the displacement of the pole of rotation.
Wegener offered one more preliminary: “Before we begin the presentation, a few brief historical remarks are in order.”27 At issue here were his precursors, those who at some point had proposed the splitting and displacement of continents, especially the Atlantic continents, before Wegener had done so. Precursors come in two different varieties: discredited ones from whom one must distance oneself, and credible ones who must be explained away in order to protect one’s own claim of originality. Wegener had discovered one of each. (He would later find many others.) The discredited precursor was the American astronomer W. H. Pickering (1858–1938). Pickering had also noticed the parallelism of the Atlantic coasts and had proposed that the Atlantic continents had split and slid apart as a part of the cosmic event when the Moon had been created in “fissipartition” from Earth, leaving the Pacific Ocean as the scar of its removal. Osmond Fisher (1817–1914) had popularized this hypothesis in the English-speaking world, in his Physics of Earth’s Crust (1881). Wegener had learned from Rudzki that the original calculations for this theory, made by Sir George Darwin (1845–1912) and later advanced by Henri Poincaré (1854–1912), were based on mathematical errors already documented by the Göttingen astronomer and physicist Karl Schwarzschild (1873–1916). Wegener commented concerning Pickering, “Given the removal of the event into an unverifiable grey past, and its admixture with an obviously incorrect hypothesis, the work is at best only of historical interest.”28
Wegener, however, also had a real precursor to deal with:
On the other hand, a recent paper by F. B. Taylor [the American geologist Frank Bursley Taylor (1860–1938)] may be regarded as a precursor of the present work, even though both arose in complete independence from one another. Taylor l
ocates, as we do, the horizontal displacements in geologically well-known periods, especially in the Tertiary, and connects them with the great Tertiary fold systems. He particularly emphasizes the breakaway of Greenland from North America based on the parallelism of the coasts. Thus our view concerning a second locale on Earth’s surface is already expressed here.… Taylor has, I daresay, not realized the enormous range of consequences associated with the assertion of such horizontal displacements of the continents. Since he fails to subject their likelihood to any investigation: despite their contradiction of established conceptions—for the most part his opinions have been greeted with a shake of the head. They could not, as I have stated, have provided any stimulation for the present work as I became acquainted with them too late.29
This is a reasonable and fair characterization of Taylor’s work. Taylor was a follower of Edward Sueß and was attempting to extend Sueß’s theory of the formation of arcs of fold mountains by imagining compressive stresses acting from polar regions toward the equator, pushing continents into plicated arcs. Taylor appears to have believed that Earth was perfectly spherical until some point in the Tertiary, at which time it began to shed fragments of north polar and south polar continents toward the equator, as part of its attempt to become oblate. Taylor’s descriptions of these movements, especially of the rifting of Canada from Greenland in the latter part of the Tertiary and of the drifting of South America and Africa away from the Mid-Atlantic Ridge in the Carboniferous, are easy to follow. His physical explanations, on the other hand, are qualitative, confused, and in fairly obvious contradiction of all the celestial mechanics from the mid-eighteenth century down to the time of Lord Kelvin, Sir George Darwin, and Henri Poincaré.30 One is inclined to agree with Wegener’s assessment that these arguments could only be greeted “with a shake of the head.”
Alfred Wegener Page 47
