The Geophysical Argument
When Wegener, leaving precursors and history behind, actually begins his summary of the geophysical arguments against the contraction theory and in favor of his own notion of continental displacements, it is immediately clear how much his powers of exposition had benefited from the two years he had spent writing the undergraduate textbook Thermodynamics of the Atmosphere, a volume already popular with students for its lucidity and comprehensibility. Wegener’s explanatory skill had further benefited from reducing the scope of that 331-page book down to a 70-page digest for the review article he produced in 1911 for Abderhalden’s journal.
Wegener’s writing on continents and oceans for Petermanns Mitteilungen is concise without being telegraphic, diffident without being casual, exact without being pedantic, and instructive without appearing didactic. He gets to the point immediately, without parenthetical commentary or peripheral excursions. “The problem of how one may explain the platform-like elevation of the continents above the deep-sea floor is an old one.”31 The answer cannot be the forces that produce mountains, for (and here he invokes the authority of Albert Heim [1849–1937], the great Swiss Alpine geologist and theorist of mountain formation) “the continents appear as immense broad pedestals from which, making up only about one five hundredth of their bulk, mountain ranges rise like diminutive ribs.… The movements of the crust, which sever the continents and oceans from one another, are thus no doubt different from those which have since wrinkled the crust on the great continental plateaux.”32
Wegener turned immediately to the initial inspirational idea that led him to this entire line of investigation: the realization that the continents at the 200-meter (656-foot) depth contour have exactly the outline of their sea level surface. He says nothing here about the experience of reading Andree’s Atlas; instead, in support of his argument, he reproduces the “hypsometric curve” of Earth’s crust, the distribution of the elevations and depths of the continents and the oceans, which he had taken from the first volume of Krümmel’s Handbook of Oceanography. As one can see in the accompanying diagram, the vertical axis is elevation above and depth below sea level, expressed in meters, and the horizontal axis is the area at each of these elevations expressed in millions of square kilometers. The result is a mean continental elevation of 700 meters (2,297 feet) above sea level and a mean oceanic depth of 4,300 meters (14,108 feet) below that level: Earth’s distribution of elevations is bimodal. The shape of the curve shows that the continental shelves down to about 200 meters are indeed a part of the continents, and the steep drop thereafter leads to a much gentler curve of distribution of different ocean depths, the abyssal ocean floors.
Wegener’s “Hypsometric Curve of the Earth’s Crust (after Krümmel).” The vertical axis is elevation in meters; the horizontal axis is in millions of square kilometers. The chart shows a bimodal elevation: a mean level for the continents, and a mean depth for the ocean—both of these very far from sea level. Many of Wegener’s critics would have great difficulty understanding the significance of this bimodality. From Alfred Wegener, “Die Entstehung der Kontinente,” Petermanns Mitteilungen 58 (1912).
So the continents “exist.” They are not an artifact of sea level but a distinct layer in Earth, one of two preferential modal distributions of altitude. But why are there two distinct layers? “The opinions which exist today,” Wegener continued, “concerning the origin of these remarkable tabular elevations of Earth’s crust present an extraordinary instance in natural science of self-contradictory befuddlement. Although in this sort of work polemics ought, in principle, to be avoided, we cannot avoid casting a brief critical glance at these prevailing views, in order to see what we forfeit when we replace them with our hypothesis.”33 In 1912 many geologists were still using the analogy of a baked apple coming out of the oven, with the corrugation of mountain ranges compared with the shrinking of the cooling apple skin over the reduced interior. This simple analogy, of course, broke down completely, because the contraction theory was held responsible not only for the origin of mountains but for the origin of continents and oceans, with no analogical explanation or picture as to how first a continent might sink, and then the adjacent ocean basin, and then again a continent, and so on. “However,” Wegener wrote, “it is precisely the relentless consistency with which Sueß has worked through these ideas that has already opened the eyes of many to their weaknesses and has thus indirectly opened the way for a more correct interpretation.”34
The self-contradictions of the contraction theory in its various simple and qualitative analogies were a persuasive argument against it, but the idea was now also faced with two formidable obstacles of quite recent origin. The first of these was the discovery of radioactivity in Earth’s crust, which challenged the notion that Earth necessarily had to cool and shrink at all, because radioactivity provided an additional source of internal heat. Not only was Earth not cooling down, but it might possibly be heating up.35 More pressing and less conjectural than this objection was the great mass of information collected concerning Earth’s gravity field: “However, even if all of these arguments against the breakdown of the terrestrial globe did not exist, we would still have to reject the conception, because it contradicts gravity measurements. If the oceanic deeps were nothing more than sunken continents, they would consist of the same material as the latter. Gravity measurements show, however, with inescapable logic, that under the ocean lies rock heavier than that of the continents and not only heavier, but so much every that the difference in elevation is compensated and equilibrium prevails.”36
While European geologists, in the main, were still in the grip of the hypothesis of universal long-term contraction of Earth, North American geologists had embraced the notion of “isostatic compensation.” Extensive gravity measurements carried out by the U.S. Coast and Geodetic Survey indicated that Earth’s interior only some scores of miles below the surface must be in a yielding state, as a consequence of heat and pressure, and therefore the surface had to be seen as floating on the interior, because it was composed of lighter material. There was simply no other plausible explanation for these many thousands of gravity measurements.37 Wegener noted that however correct American geologists might have been in opposing the notion of “foundering continents,” and having instead asserted the notion of “the Permanence of Continental Platforms,” the American geologists had attached to this well-founded and geophysically well-supported conclusion the “dubious doctrine of the Permanence of Oceans.”38 Of course, hundreds and hundreds of paleontological finds of both terrestrial and shallow-water organisms, confirming unhindered communication back and forth across abyssal oceans that such organisms could not possibly have traversed, contradicted this notion.
So here we come to the crux of Wegener’s argument. In Europe we have the notion of foundering continents and newly created oceans, and in North America the notion of permanent continents and permanent oceans: “thus the two conceptions stand in complete opposition to one another. Both parties have good, incontestable arguments, but both to draw inadmissible conclusions from them. I shall try to show that the correct claims of both can be encompassed more simply in the context of the hypothesis of the rifting and horizontal displacement of the continental blocks.”39 Wegener saw his hypothesis of continental displacements as a way to reconcile two different geological communities, each attached to a partial truth and to a demonstrable falsehood. He imagined that both groups would welcome a third way out.
Wegener then pursued an excellent, detailed, nuanced, but extremely technical discussion of the various ways to measure gravity over continents and oceans, and he evaluated the existing survey data on Earth’s gravity field, all of which led him to the nearly inescapable conclusion that the material under the oceans was more dense than the material of which the continents were composed; the ocean floors could not have ever been former continental surfaces. The intellectual issue involved was relatively straightforward. Since water is only
about half as dense as rock, gravity should be less over the oceans, but measurement shows that it is not. Why? The likely answer is a higher specific gravity of rocks under the ocean.40
To this discussion Wegener attached an equally sophisticated and detailed notion of “isostasy.” The flotation of light crust on the denser subcrust would lead to near-equilibrium values of gravity over the oceans and continents in spite of the higher specific gravity of the suboceanic rock. This principle, known as isostatic equilibrium, had a history of investigations going back to the middle of the eighteenth century.41
Wegener then tried to entice geologists to accept this geophysical argumentation by showing the utility of isostatic equilibrium for a variety of geological problems. For if the continents were in a floating equilibrium with the layer of Earth below them, the loading and unloading of the continents must produce a sinking into this magma and a rising out of it. Such loading of a continental surface, on a continental shelf, could explain how to accommodate long-continued sedimentation in a region, a so-called geosyncline, where the sedimentation surface was always just below the surface of the water, but many tens of thousands of feet of sediment might accumulate over time, by pushing down of the subcrust under the weight of this load.
Isostasy also provided a good explanation of Charles Darwin’s celebrated theory of coral atolls that, as they grew and added weight, must cause the underlying island to subside, encouraging the further growth of coral above. Wegener applied isostasy to a number of other well-known geological phenomena, such as the “postglacial rebound” of Scandinavia with the removal of the continental ice sheets: well-documented measurements along the shores of the Baltic had shown the progressive rising of the land over more than 150 years, previously unexplained but now characterized as an “isostatic rebound” some thousands of years after the removal of the ice load. Calculations of the amount of such sinking and rebound, given the relative specific gravities of ice and the Scandinavian rock, even provided a way to estimate the thicknesses of these ice sheets. Wegener was at great pains here to try to show geologists that a little bit of geophysics went a long way to help them pursue their own aims without geophysics trying to conquer them or tell them to solve new and different problems.
In this context, we need not spend much time determining the details of his argumentation on gravity, isostasy, and the impossibility of the ocean floors ever having been continental surfaces. His sources were recent, and he had read them carefully, interpreted them correctly, and presented them fairly. The theoretical situation—or, if you will, the theoretical crisis—to which he thought he was addressing himself in 1912 was actually there, and it was there in precisely the terms he presented it. Subsequent very detailed accounts of this episode by historians of science have confirmed the accuracy of Wegener’s historical sense, his physical intuition, and his mastery of the relevant argumentation.42
More to the point than his physical sophistication was his political naïveté. What could possibly have led him to imagine that a short piece like this, presenting results already in the literature, written by someone who was not a geologist, would convince the many thousands of busy field-workers and the handful of theoretical workers in the global community of geologists to readily abandon the theories so laboriously erected on a foundation of many decades of fieldwork stretching back to the beginnings of the nineteenth century? The answer is that he was a young man from a field—atmospheric physics—that had just been revolutionized by a series of novel theoretical developments of the very sort he thought he was presenting: new results, all less than a decade old, discovered with new instruments, and many times confirmed, leading to an entirely novel picture of the structure of Earth’s atmosphere and the dynamic behavior of the weather.
The situation he saw in geology and geophysics was precisely that which he thought he had just lived through in atmospheric physics. Most of the confirming (or disconfirming) evidence that he was presenting in his paper concerning the inadequacy of the contraction theory, the structure of Earth’s interior, and the probable origin of continents and oceans was less than a decade old. Most of the gravity work had been done since the turn of the century, and the most significant and decisive material, that of the North American Survey, had been published only in 1909. In seismology (which we have yet to discuss) most significant work was also less than a decade old, with crucial parts of Wegener’s argument underwritten by material first published in 1907 and 1910. The same is true for his work on solid mechanics, plasticity, viscosity, fracture and flow in Earth’s interior, and the chemical composition and behavior of the outer shells of Earth.
Wegener assumed that geologists would be persuaded by these new developments in physics, even though they had no particular physical training themselves. The world meteorological community had been persuaded and had actively welcomed the new developments in atmospheric physics as a positive revolution in their science, even though most meteorologists were field observers busily collecting and collating data from a synoptic network of observing stations, extending the “atmospheric map” across the face of Earth with the same relentless avidity of field geologists extending their geological mapping. Wegener assumed that geology would welcome “a new head from physics” because this had happened to him already in meteorology.
To a certain extent, however, Wegener was also encouraged to proceed with this novel hypothesis and spend January and February elaborating it, not because of what he did not know about geology, but because of what he did know. The geologist he knew best was Emanuel Kayser, in the front rank of the science, president of the national association in Germany, and editor of its newest and most forward-looking journal, in which he had invited Wegener to publish. Kayser was a great champion of intercontinental correlation of sedimentary rocks and paleontological data, and he had doubts about the adequacy of the contraction theory. As a geologist predisposed to welcome physics, closely associated with the geophysicist Richarz at his own institution, and close to the group at Göttingen led by Wiechert, he was clearly inclined to accept the notion of a comprehensive earth science much more amply defined than nineteenth-century “geology.” Kayser was perhaps a significantly misleading model of a geologist for Wegener.43
Wegener also assumed, again somewhat naïvely, that his arguments would carry themselves without any particular reputation of his own to endorse them. His own experience in atmospheric physics may have led him to believe that this was true, and to a certain extent it was. But Wegener, in spite of his gratitude for the patronage he received, may not have understood what it meant for him to have been advanced by Aßmann, praised and supported by Hergesell, aided by Süring, and to have the full support, cooperation, and collaboration of a giant like Wladimir Köppen. What Sueß was to world geology, Köppen (along with Julius Hann) was to meteorology and increasingly to atmospheric physics. As we return to a brief consideration of the rest of Wegener’s chief geophysical arguments, we can better understand his fresh and infectious enthusiasm to present this interesting material in a completely ingenuous way, as a consequence of his own previous experiences.
Moving forward from the idea that continents are blocks of crust of specific gravity lighter than that of the ocean floor beneath them, Wegener proposed the question, how thick are these blocks? Once again, gravity measurements provided some guidance. Friedrich Helmert, Wegener’s old professor at Berlin, had made an extensive study of this problem, measuring gravity along the coast of the ocean at fifty-one stations and finding, as one moved measuring equipment closer and closer to the coast, that the value of gravity showed “positive anomalies.” In other words, “the measured value of gravity was higher than should have been expected.” Subsequently, measuring gravity at sea and moving the instruments progressively farther offshore, the values suddenly became “negative,” meaning that gravity was less than might have been expected. Still farther out to sea, the anomalies vanished. Helmert had interpreted these data as the result of a sharp coastal
boundary between dense suboceanic and light continental material, and he calculated, based on the size of the anomalies, a thickness of continental crust of 120 kilometers (75 miles). This was a very encouraging corroboration for Wegener of the North American Gravity Survey, which had estimated the thickness of the “level of isostatic compensation” (and therefore the thickness of the outer layer of Earth) at 114 kilometers (71 miles).44
Friedrich Helmert’s gravity profile of a continental margin. The peak of the curve to the right is a higher-than-expected value for gravity on land, the trough to the left a lower-than-expected value at sea. Helmert’s (and Wegener’s) interpretation was that the anomalies were caused by the adjacency of dense ocean floor and lighter continental material. The dense ocean floor makes the continental value “too high,” and the light continental material makes the oceanic value “too low.” Farther out to sea, or farther inland, the anomalies vanish. From Wegener, “Die Entstehung der Kontinente.”
It is characteristic (and habitual) in physical argumentation to use a variety of instrumental sources to investigate the same phenomenon and compare the results. A result achieved by one method alone is not to be trusted. Thus, Wegener turned from gravity data to the corroboration available from earthquake wave studies pioneered by his senior colleague, Emil Wiechert, at Göttingen. These studies, following the oscillation of fixed waves that could only exist in a thin and elastic crust, arrived at a value for the thickness of the outer shell of Earth of around 100 kilometers (62 miles). Wegener also invoked very recent and very interesting work on earthquake travel times by the Croatian meteorologist Andrija Mohorovicic (1857–1936), suggesting the lower boundary of the solid material of the crust, in places, at depths of no more than 50 kilometers (31 miles). Although Wegener did not know Mohorovicic personally, they had a broad range of shared interests, including tornadoes, the lapse rate in the atmosphere, and the character of clouds; Mohorovicic’s work had been published in a report from a meteorological observatory, so that Wegener, as a meteorologist, came upon it before many geophysicists were aware of it.45 Finally, Kayser had produced a comparative study of the depth of focus (the hypocenter) of eleven major earthquakes around the world since 1873, producing a mean depth (which Wegener took to be accurate only to an order of magnitude) of about 100 kilometers.46
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