Alfred Wegener
Page 79
“Although,” he wrote, “on first inspection the movements of the continents present a quite variegated picture, behind this variety one can see the outlines of a great System: the continental blocks move equatorward and they move westward. It is advisable to treat these two components of the movement separately.”116 He titled this chapter “System, Cause, and Effects of Continental Displacements.” The displacements form a system of movements, these movements have a cause, and this cause has a series of effects—these should be considered as three separate entities.
In this novel scheme, the principal displacements are in the direction of the equator, and Wegener calls these (a coinage he will attribute to Köppen) “Polflucht” (pole-fleeing). Such motions affect large blocks more than small blocks, and their effects are most visible in middle latitudes, as evidenced by the great Eurasian fold systems of the Tertiary period. Evidences of this are, however, available for every continent, including North America, Australia, Lemuria, and South America, and even, in some periods, Antarctica.117
Equally primordial is the secondary component, the “Westward Migration” (Westwanderung) of the continents. This is evident even in the time of Pangäa, where the reconstruction of the single protocontinent shows a crumpled (western) leading edge and a torn, trailing (eastern) edge.118 These westward movements are probably derivative from the Pohlflucht and represent a consequence of Earth’s rotation, where here the movement toward the poles is deflected to the west, by analogy with the trade winds in the global atmospheric circulation. It is also possible, though there aren’t any data to support this sufficiently, that solar and lunar tides in Earth might play a role; the problem is that for the short time periods involved Earth’s behavior is probably purely elastic. In any case, “the question must for the time being remain undecided whether we must choose between these two different causes for an explanation of the westward migration or whether they are both simultaneously effective.”119
The fundamental point he wishes to drive home before turning to the “cause” of the “system” of movements is that even if “these two components, the Polflucht and the Westwanderung,” cannot explain every individual detail of the displacements, and if such explanation must wait for the future, nevertheless the principal movements (Hauptbewegungen) both in the present and in the past are clearly evident. In 1912 Wegener had assigned no causes at all for the displacements, and in 1915 he had mentioned a few suggestions. As Köppen later said of him, this well-considered caution came from his geophysical training, which made him wary of beginning from some plausible geophysical explanation and then proceeding deductively toward the evidence, rather than the reverse, which is to examine the empirical evidence and establish a pattern in the phenomena without specifying a necessary causal mechanism.120 Even here, in 1920, Wegener was extremely reluctant to specify a cause and devoted not quite two pages to the question. Köppen, he said diffidently, has recently proposed an explanation for displacement on a rotational ellipsoid, which he calls Polfluchtkraft, and which he has allowed me to summarize from an article that will appear in Petermanns Mitteilungen.
Here was Köppen’s idea. In a floating body, the center of gravity (what we should now call the center of mass) is determined by the figure and density of that body under the attraction of gravity. However, for a floating body, the relative densities of the body and the medium in which it floats determine the center of buoyancy. A simple calculation shows that for the relative densities of the continents and the Sima, the center of gravity in a continental block lies on average 2.4 kilometers (1.5 miles) above the center of buoyancy. On a rotating (viscous fluid) body like Earth, the disparity between the center of buoyancy and the center of gravity results in a small but detectable resultant gravitational force. Where the center of buoyancy lies higher than the center of gravity, this resultant will be in the direction of the pole; where the center of gravity lies higher than the center of buoyancy, the resultant will be in the direction of the equator. It is obvious that for objects at the pole or at the equator there will be no resultant force, and that the largest resultant would appear in midlatitudes.121
Under the action of such a force, however slight, masses as large as the continents would eventually slide toward the equator from the midlatitudes. Köppen had only described this qualitatively, Wegener said, and had not made any calculations. If we knew the elevation and area of the continents in different periods, he wrote, we could conceivably predict the path of the pole through numerical integration, though this is far beyond our capacity now.
If the causes are conjectural, it is the system—the motions—and not the cause of the motions which produces the effects. The principal effect of continental displacement is the migration of the poles, for reasons already described in an earlier section. The second effect of continental displacement is meridional rifting: if the continents were not moving, the stretching and thinning of the lithosphere which produce rifting would not take place. The third consequence of the motions is oceanic transgression and regression. If displacements of continents force the migration of the pole, the viscous Earth will adjust with a considerable lag time, but the water in the oceans will respond immediately, resulting in a recession of ocean water in front of the moving pole and a transgression of ocean water on the land behind it. Many writers on the displacement of the poles had discussed this, but never in connection with a sound geophysical explanation.122
Wegener has shifted quite radically here from a theory emphasizing changes in longitude to one emphasizing changes in latitude. He has also shifted emphasis from the creation of mountain ranges along meridians of longitude to creation of mountains by compression along parallels of latitude, as the continents shift toward the equator.
This theoretical move creates an interesting situation. In both 1912 and 1915 Wegener could assert that the American Frank B. Taylor, who had an idea somewhat similar to his own with regard to the rifting and drifting of the Atlantic continents, had made an offhand suggestion in a long paper mostly concerned with the argument that the great mountain ranges from the Alps through the Himalayas to the ranges of Asia were the result of a compression, as continents moved away from the poles and toward the equator. Similarly, Wegener had barely mentioned Kreichgauer as an interesting proponent of the idea that the shift of the poles and the movement of the lithosphere were connected phenomena.
Now, however, Wegener found himself accepting the same general motions of the Taylor theory (without the highly suspect geophysical basis) and also found himself accepting the basic idea and the basic mode of presentation of Kreichgauer’s 1902 book Die Aequatorfrage in der Geologie. There were important substitutions here, of course. Kreichgauer had assumed that Earth had been spherical in the Carboniferous and had only assumed its oblate ellipsoid over time, with the centrifugal force driving the sphere toward an ellipsoid offered as the cause of the continents fleeing the poles; this was a geophysical absurdity. Moreover, because he had no lateral displacement of continents, his paleoequators were conjectural and often on the wrong continents. Nevertheless, Wegener now found himself in a position where he had to acknowledge the general tendency of the crust to slide from the poles toward the equator, mentioned by a number of authors, among them notably Kreichgauer and Taylor.123 In this context the reader is referred to the epigraph at the beginning of this chapter, in which Köppen remarks on the great difficulty of tracing the lineage of ideas, which appear over and again in a variety of forms and in a variety of ways, associated with good ideas and bad such that the idea of priority becomes extraordinarily difficult to establish, and the actual path followed in the development of the theory is not linear or intuitive.
This leads to another interesting perplexity for Wegener: if others have proposed both the equatorward and the westward displacement of continents, and if others have connected these ideas with displacements of the pole of rotation because of shifts of mass on Earth’s surface, and if others have documented these sorts of shifts wit
h reference to former geological continuities and the distribution of animals and plants, what exactly is new and different about Wegener’s theory of “the origin of continents and oceans”?
Wegener’s answer to this very good question is to throw everything into the question of measuring displacement: “Among all the theories with similar far-reaching ambitions, the displacement theory has the advantage that only it is capable of being tested by exact astronomical position finding.”124 Here he had great hope but slender means. His handwritten notes in the rebound copy of his 1915 book show that he had been informed of an error in his own transcriptions of supposed changes in longitude between Europe and North America. In 1912 and again in 1915 Wegener had recorded the change in longitude between Cambridge, Massachusetts, and Greenwich, England, as having been 4 hours, 44 minutes, 31.065 seconds of longitude in 1870, and 4 hours, 44 minutes, 31.12 seconds in 1892. However, the second figure was quite wrong, and the published figure was actually 31.032 seconds, a change 10 times smaller than Wegener had recorded in order to declare a continental displacement of 4 meters (13 feet) per year over twenty-two years. Moreover, Wegener’s notes show that the actual measurements carried out in the Azores in 1914 by the Geodetic Institute, before the cable was cut, gave a distance between Cambridge and Greenwich of 31.039 seconds, virtually the same longitude as in 1892. Finally, Wegener had to admit that Galle’s 1916 article in the Deutsche Revue had noted this discrepancy and had concluded that there had been no longitude shift measured since 1892.125
Wegener had clearly made a major error in using these telegraph time signals, and even though he corrected it in this edition, he persisted in another error, of claiming that Koch’s longitude measurements in 1906, published in 1917, lent confirmation to the idea that Greenland was drifting to the west. Examination of Koch’s published measurements, in the context of Wegener’s own published review of the same, shows a much greater uncertainty about the accuracy of these measurements.126 The reader may refer to the earlier chapter in which these measurements were made (or not made) to connect the longitudes of the German expedition of 1870 to the measurements of the Danmark Expedition, on a trip to Sabine Island in November 1906 which nearly cost both Wegener and Koch their lives, and on which Koch neglected to wind his chronometers, arriving there with no way to determine the longitude at all. Even though Koch had returned to try to make these measurements again, discrepancies of hundreds of meters (almost a kilometer) remained in the measurements.
One important change in his attempt to measure displacement was a shift from the exclusive focus on the opening of the North Atlantic, or the shift of Greenland, to the amount of displacement per year required for different separations: across the Arctic, the North Atlantic, and the South Atlantic. To this he added estimates of the drift of Madagascar from Africa, as well as of Southeast Asia from Madagascar, and finally a single estimate of the displacement between Tasmania and Wilkesland (East Antarctica).
He made it quite clear that he was using recent (1910) estimates of the number of years elapsed since the beginning of different geological periods and dividing these by the number of kilometers of separation since the proposed rifting. This produced, for every region of Earth other than Greenland, Iceland, Norway, Scotland, and Labrador (all supposed to have begun their separations between a half-million and one million years ago), comparatively small annual displacements in centimeters per year, not meters. For instance, across the arc from Buenos Aries to Cape Town, the displacement was only 30 centimeters (12 inches) per year. Between Tasmania and Antarctica it was 36 centimeters (14 inches) per year, and between Brazil and West Africa it was 24 centimeters (9 inches) per year. While he was still proposing recent spreading rates as high as 20 meters (66 feet) per year in the high Arctic, he hoped that these smaller rates would do much to assuage his critics and supporters alike, almost all of whom had proclaimed that his spreading rates were unbelievably high in terms of the physics of Earth.127
In this context, given the stakes involved, it is no wonder that Wegener turned to an increasing reliance within his theory on shifts in latitude rather than shifts in longitude. The International Latitude Service had discovered and published shifts of anywhere between 0.17 and 1.51 seconds of latitude in the past half century for a variety of stations around the Northern Hemisphere. Wegener admitted that such shifts could, of course, be either displacements of the continents or displacements of the pole of rotation and were not themselves definitive, but these figures gave a good match to the distance the pole would have had to travel from the Eocene to the time of the ice age, assuming a time gap of about ten million years.128
Here, quite characteristically, Wegener ended his book, with no conclusion, no summary, no retrospective judgment, and simply a final sentence. It was April 1920, and he was exhausted. He had worked on this revision every day (every evening) since October 1919 while holding down a very demanding job and teaching at the university. As he signed his name to the introduction and sent off the manuscript to Vieweg, he awoke again to the life of his family, where Else was giving birth to their third daughter, Lotte, born on 16 April.
The book was gone, but the work was not complete. Even as the book was going to press, he and Köppen were working together on two detailed articles for Petermanns, one on the relationship of pole migration and continental drift to the history of climate, and a second, more technical article on the causes and effects of continental displacement and migration of the poles.129 As if to underline the continuity of this commitment, Wegener began, in April 1920, to teach a course of lectures at the University of Hamburg entitled “Climatology.”130 He was preparing to change his profession once again.
16
From Geophysicist to Climatologist
HAMBURG, 1920–1922
The ways in which WEGENER’S aforementioned hypothesis and DARWIN’S theory developed are widely different. While DARWIN gathered arguments during a period of about twenty years until his ideas had matured, WEGENER put forward a supposition, which afterwards underwent many alterations. Possibly a dissertation may one day be written about all these alterations. Many of them are highly interesting if one knows the background.
C. E. WEGMANN (1948)
Through the fall of 1919 and into the spring of 1920, Wegener was following two parallel careers. By day he was a senior meteorologist in an oceanographic institution, and in the evenings, on holidays, and on weekends he was a geophysicist working on the history of continents and oceans. It was a contrast of extremely short-term and extremely long-term events, as his work at the observatory was principally concerned with forecast meteorology, on a scale of one to three days. The time horizon of prediction for his work on continents and oceans, on the other hand, was from decades to millions of years.
The intermediate ground between these two different imaginative timescales was the study of climate. Climatology is the study of the long-term pattern underlying day-to-day weather. In English-speaking countries, climatology used to be regarded as merely the statistical aspect of meteorology, in contrast to the French and Russian view that meteorology was a subsection of climatology. From the standpoint of Köppen and most other Germans, it was not so easy, as meteorology referred mostly to the dynamics and physics of the atmosphere and was a theoretical science, while climatology was a descriptive science where one did not abstract very far from the full ensemble of factors acting on a region.
Wegener’s attempts, begun in 1911, to convince his father-in-law of the reality of continental displacements had taken hold only in 1918, in their extended correspondence on isostasy. Köppen could see the linkage between geophysics and climate once he had grasped the idea that continental ice sheets were so heavy that they could actually push down large sections of Earth, not by fracturing them, but by forcing the fluid material below to move away laterally. This began to make real sense to him when applied to the postglacial landscapes of Scandinavia, including the long-term uplift of the shoreline of the Baltic, as lat
erally displaced fluid rock far below the surface flowed back beneath the landscape and elevated it, thousands of years after the ice melted.
It was only at that point that Köppen began to believe that continental displacements could be the “Ariadne’s thread through the labyrinth of climatology” and could resolve intractable problems in understanding climate zones in the past. As we have seen, since the 1870s the primary strategy for climatologists and paleontologists to explain tropical plant fossils in high latitudes had been to (conjecturally) displace Earth’s pole of rotation. Köppen knew, as did all other climatologists, and as did Wegener, that conspicuous absurdities remained if one simply moved the pole; given the resulting pole positions, polar flora and temperate flora, or temperate flora and tropical flora, appeared simultaneously in the same latitude zones in the reconstructions. This was intolerable not only because it made it difficult to understand past climate but also because it threatened all climate schemes based on latitude zones. There would, of course, in any latitude zone, be differences based on the configuration of land and water, since oceans moderate climates and continental interiors are both hotter in the summer and colder in the winter than coastal regions. But the anomalies in the geological record were much too great for this to resolve the differences.
Wegener’s and Köppen’s interests had grown together in the fall of 1919. The more Köppen learned about the ability of continental displacements to explain past climate, and the more he interrogated Wegener on the connection between continental shifts and climate shifts, the more Wegener was drawn in to seeing his theory as a solution not so much to a geological problem as to a climate problem. In 1920 his situation was very different from that in 1912 or 1915. Wegener’s targets in earlier years—the contraction theory and the theory of sunken land bridges—he could now dismiss. The remaining contest was between continental permanence and continental movement. Here the question was no longer (simply) how certain kinds of shallow water and terrestrial creatures could have gotten from one side of the deep ocean to another, but how one could explain (latitude-based) climates over the full Earth in every period of the past. For Wegener, the climates of the past became the key to demonstrating the superiority of his displacement hypothesis, just as, for Köppen, the displacement hypothesis had become a potential key to the understanding of past climate. The outfall of this was not just that Wegener increasingly became a climatologist, but that Köppen increasingly became a geophysicist.