Alfred Wegener

by Mott T. Greene

  There was no need any longer, under these circumstances, for a detailed rebuttal of his geological critics, or even to spend time discussing their concerns. He ended his consideration of the geological evidence with the following observation:

  If one surveys the results of this chapter, it is impossible to escape the impression that the displacement theory can today be regarded as well founded geologically even down to its detailed assertions. It is true that there are many opponents of the theory among geologists today, and objections have been raised on different topics by Soergel [35], Diener [108], Jaworski [109], W. Penck [111], A. Penck [110], Ampferer [68], Washington [113], Nölke [114] and many others. However, it can be said that generally speaking these objections—where they are not simply misunderstandings (as in the case of Diener, in particular)—concentrate on peripheral issues of minimal significance for the basic concepts of displacement theory.44

  The next section of the book contains Wegener’s two chapters on paleontological and biological evidence and on paleoclimatic evidence. The distinction here is somewhat artificial from the standpoint of subject matter, since the evidence for paleoclimate was also paleontological, especially where the Glossopteris flora was concerned. Why Wegener did this becomes somewhat clearer when we think of this book in its new format and with its new function as a means to address workers in different fields of earth science. The chapter on paleontology and biology is addressed to paleontologists and biologists, and the chapter on paleoclimate is addressed to climatologists; it is a matter of where the focal interest and background lay for each group.

  These two subject matters (these two chapters) served different functions for the theory of continental displacements. Paleontology mostly served Wegener in showing that the fossil record demonstrates continuity between (now) widely separated continents in the past, continuity later disrupted as these separated continents underwent continued biological evolution and their flora and fauna continued to diverge. Paleontology was therefore mostly about disjunction. Paleoclimatology, on the other hand, did not support continental displacements as much as continental displacements made sense of otherwise irresolvable contradictions when using latitude zone reconstructions. Moreover, as we approach the present, and as the continental configurations of the recent past more and more approximate those of the present, continental displacement has much less to say about climate, as pole movements alone can then handle most of the variation. In the deep past, only displacing the continents from their present positions back to some earlier close conjunction made sense of the coal, desert, and ice distributions.

  Wegener did not try to use the paleoclimate chapter to compress the complete contents of his work with Köppen on climates of the past (1924) into a form where it could be interpolated into the story of continents and oceans. While the chapter on paleoclimate surveyed the problem of climate zones and gave a substantial historical introduction, Wegener concentrated all his energy in showing how only a combination of continental displacement and displacements of the pole, superimposed on one another, makes sense of the differences between the Carboniferous and the Eocene. Thus, though he did include in the paleoclimate chapters his maps for the Carboniferous and Permian (using the Hammer elliptical projection) and a map using the same projection to show the distribution of ice traces in the Carboniferous—now on widely separated continents—he restricted his discussion to the evidence that shows the stark contrast between the Carboniferous Earth and that of the more recent geological past. Using the new radioactive dates from the middle 1920s, his maps contrast Earth of 320 million years ago with Earth 20 million years ago.45

  The next two chapters, one on continental drift and polar wandering and one on the question of the displacing forces, are also to a certain extent artificially divided, as in the case of paleontology and paleoclimate. The first of these two chapters, entitled “Fundamentals [Gründsatzliches] of Continental Displacement and Polar Wandering,” can be considered as a discussion of the “kinematics” of these topics, and the second chapter, “The Displacing Forces,” as a consideration of the dynamics. That is to say that the first is about the reality of such motions without any question as to why or how they happen, and the next addresses the question of why and how these may happen (physically) without any reference to specific historical displacements.

  The “Fundamentals” chapter has the dubious distinction of being the least clear thing Wegener ever wrote and published. As he was a master of scientific explication, one can only surmise that here he was either not sure or very conflicted in what he wanted to say. Wegener had asked Milankovich in 1927 to find him a deep physical explanation for what could move the pole of Earth, not in the simple oscillatory sense that governed a shift of a few degrees back and forth with a regular time interval reliably measurable over hundreds of thousands of years, but something that could actually displace the pole of rotation. Milankovich had been unable to do this, and Wegener could find nothing in his own knowledge of physics or in the work of his contemporary colleagues which could help him overcome his perplexity.

  Wegener’s editing (in summer 1928) of a manuscript by Gutenberg (part of the Lehrbuch), entitled “Movements of the Earth’s Axis and Polar Wandering,” brought things to a head for him and forced him finally to rethink everything he had said and believed about the motions of the pole. Gutenberg, in his contribution to the geophysics book, had approached the problem head-on. He gave a discussion of the precession of the pole and of its nutation: the first a gyroscopic 23,000-year cycle, and the second a perturbation played out over some hundreds of days. He then presented the question of pole wandering and the variation of climate.

  While not criticizing or otherwise commenting on Köppen and Wegener, Gutenberg pointed out that their theory of the timing of the ice ages used a parameter involving very slight oscillations of the pole of a few degrees on a scale of a few tens of thousands of years. He also, however, included a diagram of the motion of the North Pole relative to Europe specified and mapped in Köppen and Wegener’s work, which showed displacements of the pole of more than 20° of longitude in as little as 500,000 years. These two things within the covers of the same book stood in direct contradiction to one another.46 Was it moving 3° or was it moving 20°, and could it be doing both at the same time? Gutenberg suggested that the 3° motion was real and that the 20° motion was an apparent pole wander, caused by the northerly motion of the whole Eurasian continent.47

  Wegener began, therefore, to construct a new argument asserting that the expressions “continental displacement” and “polar wandering” were being used in a variety of “different and contradictory senses in the literature” that had appeared thus far, and that it was necessary to clear up these confusions, to specify the relationships, and to give a precise definition of both terms. One should say at the outset that a good deal of this confusion had to be owned by Wegener, as he had himself at different times used the notion of pole wandering and continental displacement in different senses. It is very likely that he was, in fact, talking chiefly about himself.48

  Writing in the summer of 1928, Wegener argued that the assertions of displacement theory related entirely to relative displacement of the continents with respect to an arbitrarily chosen portion of the crust, namely, Africa. This was true for the book on the climates of the past, and it had been true for the 1922 edition of the book on the origin of continents and oceans, but not true of the 1920 edition, which clearly showed Africa moving north toward the equator from a more southerly position. Wegener wanted to emphasize now that the choice of a reference frame was entirely arbitrary, although in the past he had chosen Africa because it seemed to have moved absolutely the least of all the continents.

  Wegener wanted to make a second argument: that his new definition of continental displacement as relative only has nothing to say about changes of the longitude of the pole, or of any changes of the substratum relative to the crust of Earth. He asserted that from the standpoint of
his arbitrarily chosen frame of reference we might say that America has drifted westward, or that Africa has drifted eastward, or that both may have happened. This was a logical consequence of choosing an immovable Africa as the anchor of an arbitrary reference frame.49

  But immediately one sees that this thoroughgoing relativism is in conflict with the sorts of geological ideas he originally hypothesized (now newly confirmed by du Toit) that the Andes in South America have been upthrust as a consequence of the resistance of the Sima through which the continent of South America moved as it shifted to the west and away from Africa. The disconnect here is startling, and it is not at all apparent how both things can be true, at least when offered as evidence for one and the same hypothesis. It is true that Wegener had once said that the Sima might have moved eastward underneath South America, and this would still create a situation in which the west coast of South America was crumpled. On the other hand, if Africa moved away to the east and South America remained stationary, the geological argument for the folding of South America’s western margin disappeared, as it was no longer an “advancing edge.” The physiographic and structural argument (upthrust mountains) appears to have been uncoupled from the argument for displacements.

  There is more. Wegener then asserted that pole wandering, a “geological idea,” was also only relatively determinable. He had said something like this before, meaning that the evidence for the supposed migrations of Earth’s pole was inferred from fossil and geological (salts, coal, etc.) evidence of former climate. Now he was abandoning the whole idea of a detectable, directional, physical motion of the pole. Instead, Wegener asserted that an abstract system of parallels of latitude should be imagined to rotate relative to the full surface of the globe, or contrarily a rotation of the whole surface relative to the system of parallels; once again, everything was relative. Under these terms, any physical relationship of the crust of Earth to the interior, and any relationship between such a “geologically shifted” pole and the possible motion of the actual axis of spin of Earth, was entirely irrelevant for this (new) definition of “relative pole motion.” There is no mistake here. Wegener says quite plainly, “Geophysics can give no opinion concerning the existence or possibility of such a movement.”50

  Once again, this is a sharp contrast with almost everything he had previously asserted about the movements of the pole. In the past versions of his theory there were real motions of the pole of rotation of Earth, and these actual physical motions were potentiated by the actual motion of continents, in a dynamic interplay that was supposed to have controlled the entire history of the crust, with rearrangements of the crust forcing changes in the position of the pole consequent upon a redistribution of surface masses, with the newly reconfigured oblateness of Earth (given its new pole position) creating still further opportunities for high-latitude continents to “flee from the poles” toward the equator and, in consequence, control the succession of oceanic transgressions and regressions onto the continental surfaces.

  The contradiction to every previous version of his theory here is so stark that had one been given this chapter without Wegener’s name on it, one would have to imagine that it had been written by quite another theorist. But Wegener is the author, and he is quite serious. He produced a map to indicate that all the pole positions he and Köppen had ever calculated were relative to an arbitrary African reference frame, and to demonstrate this, he produced a (parallel) Southern Hemisphere pole-wandering path for every period from the Cretaceous through the present, as it would appear if the reference frame were South America held in its current position, with Africa having drifted to the east. This path is entirely different from any pole-wandering path he had ever drawn. He was now saying, and we must be clear about this, that he doesn’t know where the pole has moved, or where the continents have moved, except relatively speaking in their distance from an Africa arbitrarily held at its current latitude and longitude.51 He says that it is impossible to know in what direction the pole has moved from one period to another because the continents are moving at the same time that the poles are moving. Therefore, he says, his maps of the continental displacements do not show their actual motions, and therefore the actual vector of the migrating pole cannot be known, only known relatively to an arbitrarily shifted continent, in the context of an arbitrarily fixed continental frame centered on Africa.

  This is rather difficult to reconcile with his previous constructions. There is no hint in Klimate der geologischen Vorzeit that the pole positions are arbitrary or even relative; polar, temperate, desert, and equatorial geological markers uniquely determine them. In the maps, from one period to the next, we were always shown an arrow indicating the direction of motion of the pole. Moreover, in the time period between 1922 and 1924 Wegener altered the timing, position, and extent of the migrations of the pole in order to accommodate Milankovich’s calculations of very slight polar motions in the Quaternary, without moving any of the continents differently. In 1924 the motions of the continents were relative to the fixed reference frame of Africa, but there was nothing to have us believe that the motions of the pole and the latitude zones were anything but displacements of the actual physical pole of rotation, via some motion of Earth’s axis of rotation which was determinate and measureable.

  Having proposed this new relative scheme, Wegener introduced two new concepts, this time of real and geophysical (not relative and geological) motions: Krustenwanderung, or “crustal wandering,” and Krustendrehung, or “crustal rotation.” The latter he imagined to be an overall rotation of the whole crust of Earth relative to the interior and axis of rotation, toward the west. This is obviously what he formerly called Westwanderung. The other of these two new concepts, the “crustal wandering,” is a movement of the superficial crust equatorward, away from the axis of rotation. This is what he formally called Polflucht, and moreover these new names become a source of confusion because of the redefinition of Wanderung as a matter of latitude rather than longitude.52

  Wegener’s map of the path of the South Pole since the Cretaceous. On the left is the shift of the pole seen from South America (in its current position); on the right, the shift of the pole seen from a stationary Africa. The point is that in this edition, all motions are relative to arbitrarily chosen reference frames. From Wegener, Die Entstehung der Kontinente und Ozeane (1928).

  He also said that in understanding what these motions mean, we have to separate the idea of an internal axial shift (innere Achsenverlagerung) from that of an astronomical axial shift relative to interstellar space.53 These are very difficult concepts to discriminate, as it is hard to imagine an actual shift of Earth’s axis which would not cause it also to point to a different position in interstellar space; Wegener obviously believed, for reasons not entirely clear, that this was a meaningful distinction.

  From this point on, Wegener’s argument is even less clear. He wishes to reintroduce his entire theory of transgressions and regressions of the oceans caused by the actual physical motion of the pole, but he wishes to have their driver, these “internal axial shifts,” be held separate from any pointing of the axis of Earth very far away from the stars at which it currently points. So this internal axial shift does not appear to be what we would call today “inertial interchange true polar wander,” in which Earth’s axis, consequent on some internal mass shift, may point in any direction relative to the fixed stars. Instead, it is some unknown process whereby the crust does not slide relatively over the interior with respect to a fixed axis of rotation, but in which the axis of rotation shifts without somehow pointing anywhere else in space. Moreover, having made all these careful distinctions between superficial and geophysical polar wandering, he persisted in this same chapter in drawing maps giving vectors for the actual motion of the pole in the Devonian and the Carboniferous periods.54

  With regard to this strange distinction of two kinds of pole movements, one can see that Wegener would be reluctant to move the astronomical pole—the ecliptic angle
(the angular distance between where the axis points in space and the plane of Earth’s orbit)—as any appreciable shift in the ecliptic angle would make it difficult to interpret the entire history of Earth’s climate in terms of latitude zones defined by the kind of seasonality and distribution of precipitation which characterize Earth in the present. He was trapped between his own theory of displacements and motions of the pole of rotation on the one hand and the climate theory of Köppen on the other. These at one time had fit together, but since 1924 they could no longer easily be conjugated. The sticking point was the need to maintain the explanatory power of assuming latitude zones like those in the present—polar, temperate, and equatorial—at every point in the past, in conjunction with continental displacements, in order to make sense of the paleontological record. The depth of these contradictions had clearly not emerged in his mind until he attempted to rewrite the theory in 1928.

  Had Wegener never embarked on a collaboration with his father-in-law and not embraced the idea that shifts of the pole were “givens” that needed to be “supplemented” by continental displacements in order to make sense of Gondwanaland (and the post-Carboniferous dispersion of this giant Southern Hemisphere continent), there is no reason why he could not, in 1928, have accomplished everything in his theory with continental displacements alone. One can see that in terms of another, but plausible, evolution of the theory of continents and oceans, the entire notion of large-scale motions of Earth’s pole of rotation as an explanation for anything could have been discarded in favor of complex motions of the continents alone. The “theoretical cost” would have been high, but Wegener was already in the midst of yet another radical revision of his ideas; here the constraints on Wegener’s theory were personal and biographical, much more than geological or geophysical.55