Alfred Wegener

by Mott T. Greene

  Wegener had tried to establish the thickness of the continental blocks using first gravity data and isostasy and then seismology. He then turned to a third line of evidence: the material of which the continents were made, to establish the reality of the differences of specific gravity between the continental blocks and the ocean floors. Wegener knew that geologists were overwhelmingly concerned with sedimentary rocks and their fossil contents; this was the only sort of geology he himself had ever done. Wegener thought he needed to establish, in the minds of geologists, the importance of considering not the detailed, differentiated mineralogy of sedimentary rocks, as well as the fine discriminations of their specific gravities, but the bulk undifferentiated density of the continental massifs as a whole.

  It is simple to calculate that with the removal of the water in the oceans, the continental platforms would plunge downward an additional 1500 m in the dense magma and the current 5 km difference in elevation [produced by the weight of sediment] would thus be reduced to 3 1/2 km. But while the thickness of the sediment is also of the same order of magnitude as this difference in elevation it entirely disappears when compared to the total continental thickness of 100 km; and one first sees this clearly, when one takes isostasy here also into consideration. If we were, of course, to have removed sedimentary cover from the whole earth, the continents would rise to the level of the old surface almost everywhere, and the relief of Earth would be only slightly altered. From this it is obvious that the continental platforms are forms of a higher order, compared to which the processes of erosion and sedimentation play only the role of secondary, superficial phenomena. The continental material is made of an archaen rock, the ubiquity of which, in spite of many objections, is not to be denied. If we, in order to establish these ideas, stick to the principal representative, then we can say: the continental blocks are made of Gneiss.47

  It is clear what Wegener means: from a geophysical standpoint one is concerned not with the details of rock type (a geological matter) but merely with the mean specific gravity of an entire continental mass. A thermodynamic system is only concerned with the bulk properties of matter, not its manifold complexities: just pressure, density, volume, and temperature. Wegener is still in the midst of his geophysical argument and has not yet moved on to make the argument for continental displacements from the standpoint of geology, but it was impolitic, to say the least, to suggest that “the processes of erosion and sedimentation play only the role of secondary, superficial phenomena,” since the results of these processes absorbed the working life and intellectual energy of most geologists.

  Pursuing this line of argument, Wegener was ready to undertake a calculation, the results of which can be seen from inspection of his simple diagram. Wegener imagined the continental block to be 100 kilometers thick, with a mean density of 2.8, and in isostatic equilibrium with (that is, floating on) the material making up the ocean floor. He took 4.3 kilometers (2.7 miles) as the mean depth of the ocean and 1.03 as the specific gravity of seawater. This left 95 kilometers (59 miles) of subcrust of unknown density, from the level of the sea floor down to the bottom of the floating continental block. It is a matter of simple multiplication to produce x = 2.9 as the density of the subcrust within the first hundred kilometers or so, “which harmonizes quite satisfactorily with the assumption that the material is essentially identical to the Sima.”48

  Wegener’s sketch of a floating continental block 100 kilometers thick, density 2.8, adjacent to an ocean with mean depth of 4.3 kilometers and density of 1.3. This yielded a density of 2.9 for the 95 kilometers of rock beneath the ocean, close to the estimate for Earth’s shell of Sima, thus supporting Wegener’s supposition of continental flotation in the denser material. From Wegener, “Die Entstehung der Kontinente.”

  It is near this point in the argument that Wegener refers his readers to a plate at the end of the volume, which contains the accompanying figure. Wegener has this to say about his cross section:

  As a further illustration of the dimensions [of the continental blocks], in Figure 2 of Plates 36 a cross-section (along a great circle) of Earth between South America and Africa is presented in true vertical scale. The irregularities of the crust and even the great hollow of the Atlantic Ocean are so slight that they occur within the thickness of the line representing Earth’s surface. In contrast, the continental blocks are readily identifiable. For comparison, the figure also contains the Wiechert Iron-Core, and the principal atmospheric layers: the Nitrogen Sphere, the Hydrogen Sphere, and the hypothetical Geocoronium Sphere. The zone of clouds (the Troposphere) is not thick enough to be represented.49

  Wegener was at this juncture so anxious to move along to a discussion of the material out of which Earth is made and its physical behavior (we shall return to this in a moment) that he had nothing more to say about this diagram, yet it is certainly worth our attention. Notable here is that the ocean of the oceanographer, Earth known to the geologist, and the troposphere of the dynamic meteorologist have all vanished. What we have instead is the chemically differentiated Earth and its chemically differentiated atmosphere, beginning at the outermost Earth shell with Wegener’s hypothetical geocoronium, and proceeding through layers of increasing density down to the core of Earth. Here is the essence of Wegener’s picture of Earth, a series of spherical shells with sharp surfaces of discontinuity and sudden marked change in physical properties at these boundaries. It is an Earth stripped of almost everything that most “earth scientists” would find interesting, but it accentuates the point that Wegener most needs to make: that the continents are higher-order features of Earth’s constitution and form a distinct Earth shell.

  While Wegener needed no encouragement to think in terms of spherical shells separated by sharp surfaces of discontinuity, it is extraordinary how much support he obtained from every quarter in emphasizing this particular structural model. Seismology, in the work of the Göttingen group and Wiechert, was founded on the existence of surfaces of discontinuity, as well as differences in wave propagation at such surfaces. Geologists appeared universally to be entirely comfortable with a chemically differentiated Earth in three large segments: crust, mantle, and core (Sal, Sima, Nife). All discussions of Earth’s gravity field contained inferences of the existence of “surfaces of compensation,” which were either perfectly spherical or penetrated by bulges, in the case of the idea that mountains had deficits of gravity because they had developed lighter “roots.” Within oceanography, the existence of a “mixed layer,” at the surface of the ocean, with a sharp discontinuity surface separating it from the deeper ocean water, carried this theme into a lighter and faster-moving medium, and of course we need not elaborate further on Wegener’s own conviction about the chemical differentiation of the atmosphere and the importance of discontinuities within it.

  Wegener’s cross section of Earth, showing the floating continents (Sal), the subcrust in which they float (Sima), and the nickel-iron core (Nife), as well as the major layers of the atmosphere, including his “geocoronium layer.” The relief of the continents (mountains) and the oceans are too small to be seen at this scale, underlining Wegener’s contention that they are features of “secondary importance.” From Wegener, “Die Entstehung der Kontinente.”

  In this context, once again Emanuel Kayser emerges as Wegener’s great geological guide. In the introduction to all of his textbooks, Kayser emphasized this viewpoint. He spoke of the surface of Earth in terms of an atmosphere, a hydrosphere, and a biosphere (the organic world) and noted that the subject matter of geology, the solid crust of Earth, had recently come to be spoken of as the “Lithosphere, in the context of these other spheres.”50

  Wegener now moved along to what were unquestionably the most difficult and taxing parts of his explanation, as well as the area of greatest contention and misunderstanding concerning the hypothesis of continental displacements. How is it possible for a continental block made of solid rock to displace horizontally through equally solid material that responds
to the tidal pull of the Moon as if it were as rigid as steel? The intuition that such motions are absurd would pursue Wegener and his hypothesis throughout his entire career. Nowhere was the geologists’ lack of training in physics more of an obstacle than in this matter.

  The essential distinction here is between two characteristic aspects of “solid” matter, never entirely grasped by most geologists during Wegener’s lifetime. These are “strength” and “rigidity.” As late as 1940, the distinguished Canadian geologist Reginald A. Daly (1871–1957) was moved to write a textbook with the title Strength and Structure of the Earth, with the specific aim of teaching geologists to discriminate these two physical quantities.51 The many editions of Harold Jeffreys’s (1891–1989) influential geophysical textbook The Earth, from 1929 onward, devoted much attention in each edition to the establishment of fundamental physical quantities: strength, rigidity, viscosity, elasticity, and so on. Jeffreys remarked in every edition of his work that “rigidity and strength are quite distinct properties, but are habitually confused in the geological literature.”52 Wegener’s task was made no easier by the circumstance that a discussion of Earth’s interior was the most absorbing and interesting area for the establishment of the meaning of these physical terms throughout the first half of the twentieth century. Thus, any debate about the actual behavior of Earth always involved a more fundamental debate about the meaning of the terms of the argument.53

  If we leave the technical discussion aside, we can accomplish much in a small space by considering the difference between strength and rigidity using an example given by Wegener himself. “Pitch offers an extreme example: if one lets a piece of it lie for a long time, it begins to flow under its own weight, and tiny lead balls sink into it in the course of time; yet under the blow of a hammer it shatters like glass.”54 This is to say that strength is the property of matter which represents its response to a long-continued stress, while rigidity is that property of matter which represents its response to an instantaneous stress.

  The critical quantity here is the time of application of the stress. The behavior of Earth under a short stress anywhere from three seconds to about four hours (an earthquake wave) is that of an elastic solid with a strength limit. For intermediate stresses anywhere from three years to about 15,000 years (Chandler wobble, Earth tides, faulting), it is an elastic body recovering its shape slowly after long-continued stress, but still rigid for shorter-term stresses. For longer intervals, from 15,000 to 100,000,000 years and longer, Earth has no rigidity and no strength at all, but a high viscosity, and can be plastically deformed indefinitely. These long-continued stresses would include such processes as mountain building, postglacial rebound, polar wandering, and continental drift.55

  Wegener termed this section of his argument “Plasticity.” Thus, it was clear that he had in mind the long-interval stresses. Crucial to his argument about the ability of continental blocks to displace horizontally within the Sima was the notion that the melting point of the Sal, the continental rock, should be 200–300°C (392–572°F) higher than that of the Sima. Thus, at an identical depth, say, 100 kilometers, the Sima, already virtually at its melting point, under long-continued stress would behave more like a fluid (lacking both strength and rigidity), while the continental block, several hundred degrees below its melting point, would behave more like a solid, though still capable of plastic deformation. Using a variety of experimental data, Wegener inferred (a highly conjectural inference) that from the standpoint of long-continued stresses, the entire 1,500-kilometer-thick (932-mile-thick) layer of the Sima ought to be treated as a viscous fluid, in which floated continental blocks that were plastic solids.

  Wegener’s confidence concerning the behavior of the Sima came from his reading of Rudzki, especially the latter’s remarks on the physical characteristics of ice. Rudzki had argued that what we learned about the behavior of flowing glacial ice, assumed to be at its melting point at the undersurface of the glacier, was generalizable to the behavior of all materials close to their melting point, with glacier ice providing an example of viscous flow of a crystalline solid on a timescale that could easily be observed. “On the basis of our experience with the plasticity of ice we can form a view about the plasticity of solid rock in Earth’s interior.”56

  After a considerably technical discussion of the rigidity of Earth at various densities and various depths and the interplay of pressure and temperature at depth, Wegener was able to conclude to his own satisfaction that “all of these phenomena thus in consequence point to the idea that the Sima represents a plastic but in no sense completely mobile material, and that the Salic crust has a considerably greater strength, yet is not because of it completely devoid of plasticity. We will thus, on this account, have no reason to dispute the possibility of extraordinarily slow but nevertheless great horizontal displacements of the continents, acting in the same way, unaltered, throughout geologic time.”57

  Wegener’s final sentence in this argument points to an attitude toward physical hypotheses: what is one permitted to imagine and to assert? In physical argumentation, then and now, there seem to be two clearly defined schools. On one side we find physicists like Wegener, for whom it is only necessary to establish consonance with the laws of nature and plausibility with regard to known observed and experimental values in order to imagine and sketch out a state of affairs, in this case the character of the interior of Earth. In this school, one may imagine and propose almost anything for which there is not direct contradictory physical evidence to prevent it. On the other side, perhaps more common in the Anglo-American world than in Germany, is the sense that some sort of demonstration of existence is necessary, in addition to the characterization of plausibility. This divide was perhaps more pronounced in the early twentieth century because there was nothing in the Anglo-American world like the theoretical physics appearing in Germany. Additionally, the mathematician David Hilbert (1862–1943) at Göttingen had only recently legitimized the notion of “existence” proofs as opposed to “constructive” proofs in mathematics.

  One has the sense that in the Anglo-American world, then and now, a “theory” is something about which truth claims may be made. There is nothing like that in what Wegener is saying here. While there can be little doubt that he believed he was describing the actual state of Earth, science was, for him, not a matter of belief but a matter of evidence. Wegener was arguing that the evidence from geology and geophysics taken together was more consistent with the idea of lateral continental motions than with the idea of a shrinking Earth.

  This large-scale framework of theory, or of a working hypothesis, had still to make contact with phenomena appearing at the surface of Earth. Even if erosion, sedimentation, and mountain building were second-order phenomena compared to the great motions of the continents plowing through the subcrust, it remained to demonstrate the relationship of these geological objects to the geological theory under consideration.

  Wegener attempted to connect his large-scale hypothesis to important phenomena of geological dynamics, especially mountain building and volcanic activity. The theory of mountain building, or “orogeny,” was one of the most important and theoretically developed aspects of geology, and the theory of mobile continents should have something to say about it. This was especially true because geologists at all interested in the dynamical behavior of Earth’s crust were likely to have a special interest in the origin of mountain ranges and to want to know how the motions of the subcrust would affect the great fault and fold structures at Earth’s surface. Similarly, if the continents were rifting and pulling apart, as Wegener indicated they were, one might plausibly expect volcanic eruptions to be preferentially located on the trailing edge of such continents, where newly exposed high-temperature subcrust finds itself on the ocean floor. That this was not the case, at least with regard to the Atlantic, the heart of Wegener’s hypothesis, clearly required explanation.

  Wegener’s rapidly sketched and qualitative discussion of these m
atters looks something like this. The great fold structures seen in the Alps and elsewhere, assumed to be the largest motions on the surface of Earth, represent, relatively speaking, only the most superficial aspect of mountain building. Underneath the great fold structures or “nappes” of the Alps and similar ranges (including the Himalayas) are flow structures, which absorb compression by the tight folding and then downward flowing of large masses of older rock beneath the sediments. Wegener here also adopted the recently expressed views of Albert Heim, among others, that the amount of compression in the Alps amounted to 4–8 times the current width of the range. Thus, with the Alps being currently 150 kilometers (93 miles) wide, a region between 600 and 1,200 kilometers (373–746 miles) would have been thrust together. “A consequence, that before the lateral compression the continental platform must have had a significantly different outline, has, in my opinion not yet been sufficiently appreciated. If, for example the chains of the Himalayas consist of thrust together landmasses of a corresponding width, where would the southern tip of the Indian subcontinent have had to lie? In particular does any room remain at all for a sunken Lemuria?”58