Alfred Wegener

by Mott T. Greene

  These two lines of investigation, the seasonal migration of the reflecting layer in the stratosphere and his continued concern with layering in the upper atmosphere above 80 kilometers, came together in late 1926 with his previous work on meteors, and in a most interesting way. He came across a paper in the Proceedings of the Royal Society of London showing that the visible path of “shooting stars” (Sternschnuppen) had different altitudes in summer and winter; in winter the frequency maximum for the disappearance of the trail was around 75 kilometers (47 miles), whereas in summer it was much higher, closer to 85 kilometers (53 miles). These phenomena of “law-like variability” had long fascinated him, and he wanted to see whether this variation in frequency maxima (yet another bimodal distribution) held for large meteors as well. He went to a new large-meteor catalog compiled by Nießl and Hoffmeister (in Vienna) in 1925 and divided up the data by season. He was able to determine quickly that such “remarkable regularity [in the endpoint of the visible path] was not limited to shooting stars, but also applies to large meteors,” albeit with frequency maxima at different altitudes from the smaller meteors.107

  There was now time for these investigations, the work on Lehrbuch der Physik and lectures notwithstanding. Wegener still had a voluminous correspondence, but he wrote no reports and filled out no forms; he had no long rail commutes each day, and instead of four work sites (as in Hamburg) he now had one—the university. This allowed him to expand his consideration of what otherwise might have seemed “side issues.” His work on the length of meteor paths led to another investigation on the estimation of the speed of large meteors. The issue, too complex to go into here, had to do with the discrepancy between the estimated velocity of a meteor relative to Earth and its velocity relative to the Sun, which involved the matter of Earth’s velocity around the Sun, the angle of incidence of the meteor path, and whether the meteor’s orbit was elliptical, parabolic, or hyperbolic—the so-called Keplerian orbits. This work took Wegener back to the tools he had learned in graduate school estimating asteroid orbits, once again including a result with a bimodal distribution requiring an explanation.108

  His editorial work on Lehrbuch der Physik also forged new intellectual alliances. Especially notable was Wegener’s cordial and warming relationship with seismologist Beno Gutenberg (1889–1960). Both men had been in the Army Weather Service on the western front during the war. After the war, Gutenberg (a student of Wiechert) was one of those who lost his position at the University of Straßburg, and he had great difficulty finding another appointment—as did many other young academics, especially if they were Jews. He ended up taking over his father’s factory in Darmstadt, borrowing seismographic records from the University of Frankfurt and doing theoretical seismology on evenings and weekends. Like Wegener, he was attracted to problems of discontinuities and layering in both the atmosphere and the solid Earth; they had much in common. Gutenberg had even worked on the problem of “the outer zone of audibility,” and in the early 1920s he had followed up the work of Tams and Angenheister on the difference of seismic wave propagation beneath the continents and beneath the oceans; Gutenberg’s work seemed to confirm that they were made of different materials.109 Gutenberg and Wegener found themselves in the amusing position in 1926 that they were both editing major textbooks of geophysics for different publishers; Gutenberg was an author on seismology in Wegener’s volume, and Wegener an author on atmospheric optics in Gutenberg’s volume.

  The winter term wound down quietly before Christmas in Graz in late 1926. Over Christmas the Wegener family left for their (now) annual ski vacation in Ramsau, and when they returned, the New Year’s burden of correspondence was not as large as in some past years. Skiing with his children and with Else had made him think of all the time he had spent in Greenland and all the snow he had traversed. The cross-country trails in Ramsau were just right for the girls, but they didn’t go anywhere; one just skied for a period of time and then went back to a hotel.

  When he returned to Graz in January, he was moved to write a letter to Peter Freuchen, his meteorological assistant on the Danmark Expedition in 1906–1908. Freuchen wrote back warmly and extensively in February; he told Wegener of his life, that he had lived for many years in Greenland as a hunter and trapper and had married a Greenland woman, and that on an exploring expedition in Canada in 1923 he had been so badly frostbitten that one of his legs had to be amputated at midcalf. He was now making a living writing about his “exploits in the frozen North.” Ironical and droll as always, he turned serious in talking about their time with Mylius-Erichsen and commented on how little attention had gone into understanding or preparing for the technical side of exploring in those days. He told Wegener that he had heard that new longitude measurements were to be made in Greenland from the southwestern city of Godthab (now Nuuk) and hoped that they supported his theory of Kontinentaltrift.110

  Wegener continued his lectures on geophysics; the editorial work was dovetailing nicely with these, and he was learning a great deal about seismology. In addition, the work of Benndorf and Hess on the electrical phenomena, especially in the upper atmosphere, was helping him a good deal and encouraging him in the sort of work he had recently pursued on meteor trails and optical phenomena; it was good to feel himself part of a working group.

  Things changed quite a bit toward the end of the semester at the Easter break. He sent off his chapter on optics to Gutenberg, and in April he received a note of thanks. Gutenberg thanked him not only for his work but also for the rapid turnaround time and the excellence of his figures; would that all authors were like him! He thought that Wegener’s contribution probably would be one of the highlights of this section of the text, which was being published in segments and sold as separates. The cost of the whole book was very high, and it was an experiment by Gutenberg’s publisher to let readers pick and choose the sections they needed and wanted. It had been a complaint of Wegener’s that many valuable pieces of research were not read because they were buried in hugely expensive multitopical publications.

  Turning from the editorial correspondence, Gutenberg said, “Now, about the displacement theory.…”111 In 1926–1927 Gutenberg had become convinced through further analysis of seismic records of the ocean floors that Wegener’s simple Sial/Sima model was not correct; instead, a sialic crust covered the whole Earth but thinned to about 5 kilometers (3 miles) or less under the Pacific, yet it was, by his estimate, much thicker—more than 30 kilometers (19 miles)—under the Atlantic. It occurred to Gutenberg that while Wegener was reading his work on seismology, he might not have been aware of the extent to which his most recent analyses seem to contradict some of the basic premises of Wegener’s theory. He therefore made a point to direct Wegener’s attention to the differences between the floors of the Atlantic and Pacific Oceans. Wegener had long argued that the Pacific floor was more isostatically compensated than the Atlantic, and this would explain some of the gravity measurements over the Atlantic, which appeared less dense than it should, if the floor of the ocean were really Sima. Gutenberg told Wegener that isostasy was probably not the cause of the difference between the Atlantic and Pacific floors, and that it was quite likely that most of Earth, except for the Pacific, was a shell of Sial. If the crust floated, and it probably did, Europe, North America, and the Atlantic floor composed a single sheet of thickness varying between 60 kilometers (37 miles) and 30 kilometers. Gutenberg’s letter contained the data to be published in May in Gerlands.112

  Wegener trusted Gutenberg’s scientific acumen; he had read his works on seismology as they came out piecemeal, and he had actually reviewed them for Geographische Zeitschrift.113 In this way he became aware of Gutenberg’s sympathy for the idea of continental displacements and for motion of the pole. Yet this other, newer aspect of Gutenberg’s work offered a fundamental challenge to Wegener’s basic conception of the flotation of continents and the possibility of displacement: if Gutenberg was correct, then the continental surfaces and the ocean floor were the
same Earth shell, floating on a denser medium below, and most of what he had written was now facing the “verdict of the exact sciences,” which he had just declared, in Scientia, to be “final.”

  Wegener, deeply concerned by this development, began to fill his notebook with reading notes, rapidly surveying work on isostasy and the seismic study of the ocean floors going back to the time of Gutenberg’s 1924 publication and even before. The theory of isostasy, he found, was in much better shape than he had left it in 1922. In 1924, a Finnish geodesist, Veikko Heiskanen (1895–1971), had published the results of an extensive gravity survey that demonstrated the extent and exactitude of isostatic compensation across northern Europe and Scandinavia and also seemed to resolve the character of that compensation strongly in favor of the “roots of mountains” hypothesis (Airy isostasy). This was the version that Wegener had depended on: that when mountains were thrust together, much in the way of pressure ridges in ice, most of the compressed material went down rather than up and then was lifted by flotation. Americans had preferred another hypothesis (Pratt isostasy), of a smoothly distributed increase in density down to a spherically symmetrical “surface of no strength.”

  Wegener copied out Heiskanen’s judgment—“Today isostasy is no longer a hypothesis, but a confirmed theory”—and made another note: “s.95 [p. 95] from the discussion of the anomalies in the Caucasus, in the Alps, and in the USA show that the Airy [roots of mountains] view is at least as good or slightly better than the Pratt view [the uniform surface]. Gravity observations consistently show that the thickness of the crust of Earth measured from sea level in different parts of the earth corresponds to the Airy principle with a thickness between 30 and 80 km.”114

  Gutenberg, who, like Heiskanen, was using both gravity data and seismic data to gauge the thickness of the crust, seemed to be extending Heiskanen’s estimate for the minimum thickness of continental crust (30 kilometers) from the continental platforms across the entire Atlantic basin. Wegener was looking at all the isostatic research he could locate and trying to determine the range of the most recent estimates of the thickness of the outer (Sial) crust. Along the way, he discovered much more.

  Throughout April 1927 Wegener was busy finding out just how far behind the debate about his hypothesis he had fallen. His notes indicate that he first came into contact with Argand’s publication supporting him (which had appeared in 1924) only in 1927.115 He read for the first time Ampferer’s paper from the 1925 Vienna symposium on his work, with its suggestion of global convection.116 He read a major publication by the French geologist and proponent of the nappe theory Pierre Termier (1859–1930), through which he discovered the recent work of John Joly, who had developed a theory of continental displacement somewhat similar to his own and based on parts of it.117

  At this time he also first read any of the work of R. A. Daly, the Canadian geologist who in 1926 had already published a book on the subject: Our Mobile Earth.118 Wegener learned from a journal article of 1923 that Daly had proposed “the comparatively recent sliding of North America over the sunken crust of the old, Greater-Pacific basin,” to which Wegener added the notation “!!.” He also learned that there was an expanding literature hypothesizing the trapping of radioactive heat beneath the continents, melting the subcrust underneath continental surfaces, and allowing continents to “slide” laterally more easily.119 One notes in passing how these late discoveries underscore Wegener’s mental distance from the supposed “symposium” in New York in November 1926. Van der Gracht’s summary of the displacement hypothesis in his address to the American Association of Petroleum Geologists in November 1926 contained extensive references to the works of both Daly and Joly, and Wegener was oblivious.120

  In all, Wegener filled about thirty pages of his notebook in April of 1927, bringing himself up to date with most major publications on his theory, pro and con. The majority of these were geophysical publications with data and hypotheses concerning the possibility of lateral drift. Among them he found intriguing suggestions, such as that of the Prague geophysicist Adalbert Prey (1875–1949), who had published an article in 1926 in Gerlands hypothesizing a long-term increase in Earth’s coefficient of viscosity which could have enabled, in earlier ages of Earth, much greater shifts of the continents than were now possible, because of Earth’s lower viscosity.121 Wegener also learned, in copying out his criticisms, that Carl Diener was still, after almost ten years, tirelessly devoting himself to refuting Wegener’s hypothesis.122

  Wegener began to draft a paper in response to Gutenberg’s forthcoming work, finishing it at the very end of the winter semester in April 1927. He had read enough of the literature to be a good deal less alarmed than he had been initially, but his tone is sober and cautious.123 While not criticizing the seismic work, he noted that the gravity profile of the Atlantic which Gutenberg employed (as a check on the seismology) had followed a course between the United States and Gibraltar which missed most of the deepest part of the Atlantic (below 5,000 meters [~16,000 feet]), which Wegener had claimed—based on correlation of dredge samples and echo profiles—to be the exposed Sima. Further, the ship track for the gravity measurements passed over a very wide portion of the Mid-Atlantic Ridge and across Azores, which, as the greatest fragments of Sial in the Atlantic, would have to distort the overall estimate of mean depth. This dependence on an anomalous transect had caused Gutenberg to give a much too large value for the mean thickness of Sial, and his work, far from a fundamental challenge, was such that “the displacement theory as presented to date requires only a minor correction.”124

  It may have been Wegener’s wish that this was a “minor correction,” but it was not. Even the map he provided in his article for Gerlands showed that more than half the Atlantic floor probably consisted of something lighter than Sima; he had been giving ground on this issue for some time, as more and more Sial was dredged from the Atlantic floor. In 1922 he had used the analogy that when an iceberg calves and drifts away, some surficial pack ice was left behind. Yet the amounts in play here were not fragments and not superficial—these were very large areas of the ocean floor, and the thicknesses postulated by Gutenberg would be very difficult to pare away.

  Wegener might well have begun his work on a fourth edition immediately, but there was a problem: he had already promised Koch that he would complete all the documentation of their 1912–1913 expedition to Greenland. During Easter break between semesters in late April 1927, Wegener traveled to Copenhagen.125 Wegener and Koch agreed on the nature of their collaboration. Wegener had already done the meteorology and would now take Koch’s rough notes on glaciology and especially the calculations of the distribution of temperatures in the glacier ice boreholes and in the “firn layer,” the porous recrystallized layer between the recent snow and névé (multiyear recrystallized snow) above and the glacial ice below.126

  Wegener arrived in Copenhagen to find Koch very ill and depressed. As they went over the notes, Koch revealed to Wegener that he had run into insuperable difficulties in trying to reduce the temperature data, and that the equations he had developed and the methods that he had followed had failed utterly to produce a common standard of measurement that would allow him to finish the data and summarize it. Wegener said that this was “a severe blow to him [Koch],” so much so that he didn’t even want to give Wegener the calculations he had made thus far for fear that Wegener would be carried down the same line of attack which he, after such a great expenditure of energy, had discovered to be worthless.127

  Wegener began work on the glaciology almost immediately and could see that it was going to be a very large undertaking. His summer course was “General Meteorology,” nothing that could distract him very much; this was fortunate. The physical descriptions of the glacier ice were not difficult to organize or supplement: qualitative descriptions of what the eastern margin of the ice had been like, a description of the character of the glaciers at their Winter Station, an account of the calving of the iceberg that had almost k
illed them in September 1912 as they were camped on their approach to the Inland Ice. The measurements of the movement of the glacier on which “Borg” (their prefabricated hut) had rested in 1912–1913 were interesting but not difficult.

  The hard part was the science, and this is where Koch had done almost nothing. There was the question of the nature and geometry of the crevasses and the “blue-bands” that formed in the ice. There were other phenomena associated with melting, as well as miscellaneous observations of glacier ice out of which Koch and Wegener had both hoped to make some pattern. The data posing the greatest difficulty were the temperature measurements. It took weeks for Wegener to work out a scheme, but once he found the way, he was just left with the labor of calculation; this was unlike the geodetic work at which Koch was so skilled, and Wegener had always liked calculation, since the days of his dissertation on the Alfonsine Tables.128

  So though the work might seem to have been drudgery, it was not for Wegener. He recalled much of the Greenland trip while working up these calculations. It was, as he had already remarked, one of the happiest periods of his life, and he could feel again what it was like to be out there doing science, and he also felt very powerfully the contrast between their serious scientific work in the winter of 1912–1913 and their trans-Greenland stunt in the summer. Even though they had taken extensive measurements of the altitude of the ice cap, it had mostly been a matter of living through it.