In short, just as Wegener was “born into other people’s lives” in 1880, so his hypothesis of continental displacements was “born into other people’s careers,” finding, as it turned out, its natural home in the second and third decades of the twentieth century not in geology, which remained either indifferent or hostile to the idea of the origin of continents and oceans, but rather in this comprehensive and uniquely German form of geography represented by those geographers named above.
Wegener’s new self-definition as a geophysicist embodied his dawning realization that in Germany the cosmic physics he had sought to found already had a (university) home in the union of geophysics and geography, and this is where his audience resided. As a “geophysicist” he was enrolling himself as a member of this particular German approach to geography pioneered by Gerland, Hettner, von Richthofen, and Penck. This rapidly growing community also included paleogeographers such as Andrée, Irmscher, and Dacqué.
Among geologists, what support he might hope to find would come from the geophysical side, from those investigating the formation and deformation of mountain ranges by huge lateral overthrusts—a not insignificant group, and one growing rapidly in the twentieth century. He could hope to find some understanding from those geologists who believed that some sort of flow of molten material underneath deeply buried sediments could drive such mountain folding.87
These hopes for future support notwithstanding, Wegener still had to respond to his current opponents, who had tried to discredit him by pointing to a long list of detailed geological structures and locales that seemed to depart from or contradict his overarching scheme. Some of these critiques were more cogent than others. Carl Diener had quite rightly asked of Wegener how South America could move to the west, throwing up huge mountain ranges and deforming along its western margins, while the delicate structure of the Isthmus of Panama remained undeformed, and he had asked why the leading edge of western North America in the United States had no large coastal mountains like the Andes but was dominated by north-south faulting. Koßmat had questioned the timing of the separation of Madagascar and India and had wanted details concerning the complex geology of East Africa. Even Köppen had inquired how an oceanic trench could appear before the leading edge of the continent, as in the case of South America.
Rather than admitting that any phenomena departed from or contradicted his scheme, Wegener asserted (in broad and general terms) that all but the smallest and most local features of Earth were the result (direct or indirect) of the lateral displacement of the continents. He asserted this to be true on every scale, from the continents themselves down to individual mountain ranges and such structures as the submarine canyons at the mouth of the Congo and Hudson Rivers and the fjords of Norway. Continental displacements caused the folding of the Himalayas, Andes, and Alps; they caused the great fault and rift systems of East Africa and the San Andreas Fault in California. They explained everything on every scale in which isostasy might play a role—the continents and their major surface features, the continental shelves with all their complexity and variety, and the flat, barren, cold floors of the deep seas.88
This first section of the book might well have been titled “on continents, their shelves, and the oceans in general.” The morphology of all three of these areas of Earth was, he wrote, governed by geophysics, and the geophysical principles in question were the flotation of the crust on the interior and the rifting, splitting, displacement, and lateral compression of continental fragments. Repeatedly, wrote Wegener, portions of the continents are pulled apart. The opening of a fissure above first reflects this, with the downfaulting of blocks into the emerging rift, while at the underside of the continental block (many kilometers below) the intensely heated material is stretched and thinned. Eventually, such fissures ruptured and exposed the denser Sima below, creating new ocean floor. As the water poured into these fissures created by departing continents, new oceans appeared.89 The origin of continents is the origin of oceans.
As the continental “skin” was pulled apart in one area, it was shoved together in another, and about 95 percent of such compressive thickening happened below the surface. Wegener found an exact analog here to the formation of pressure ridges in sea ice: when two ice floes collide, a pressure ridge—many meters tall—is created. However impressive these ridges may be, only a small portion of their structure appears at the surface, and most of the ice is thrust down deeply into the water. What appears to be “elevation” is simply the buoyant effect of the huge (invisible) mass below—a consequence of flotation. The way sea ice moves, rifting on one side and piling up by resistance on the other, is also the way that continents are created.90 It has been suggested a number of times that Wegener’s experiences with Greenland ice provided fuel for his ideas about drift; this is made fully explicit here and at many other points in the book: the continents are indeed “continental bergs,” and Wegener extensively develops the analogy of Sal and Sima with sea ice and ocean water.
Wegener’s discussion of the details of the folding of mountains in this first section shows his increased awareness of the so-called nappe theory of the Alps: that the entire Alpine system consisted of huge and complex sedimentary folds. Geologists had, since 1900, increased their estimates of the amount of shortening of the crust by such folding from hundreds to perhaps thousands of kilometers. In 1920 European geologists were extending this analysis of the Alps into an explanation of the great arc of mountains extending through the Himalayas to China. Wegener approved of this move and noted that, as in the case of sea ice, when mountains are squeezed together, most of the crustal shortening is absorbed below the surface, and that the surface mapping of the strata would only yield minimum values of the amount of crust swallowed in these episodes of folding.91
His point was plain: with the contraction hypothesis rejected as the cause of the formation of mountains, “the forces which fold mountains must be the very same which cause also the horizontal displacement of the continents.”92 These forces have most recently and extensively produced the great fold systems of the Tertiary period to which the Alps, the Andes, and the Himalayas belong. These most recent fold systems have a remarkable configuration, namely, a compression toward the equator for the Alps and the Himalayas, and an equally extensive meridional folding system for the Andes. For the Himalayas and Alps, “the principal cause of the continental displacements would seem to be a ‘flight from the poles’ [Polflucht] by the continents.”93 The cause of folding of the Andes is less immediately apparent, though it must be somehow connected with the Atlantic Rift, the Rhine Graben, and the East African Rift System, all of which form a series of meridional parallels. Reserving a full discussion of these forces for a later chapter, he notes with some excitement that “[Damian] Kreichgauer believes he is able to detect Equatorial fold zones also for earlier geological periods, especially the Carboniferous folding, and a placement for the Carboniferous equator that contains the coal deposits of Asia, Europe, and North America.”94
Moving on to “continental shelves in general” (Kontinentalrandes), Wegener had considerable work to do. Even within the bounds of his morphological-empirical method, his first edition had been much too schematic. There were details on the scale of hundreds of kilometers he had not addressed, and most of these were on the edges of the continents. Now that he was going “head-to-head” with the permanence theory, which placed so much stress on continental shelves as zones of sedimentary accumulation and deformation and as rising and sinking land bridges, he had to give a much more detailed account. In his earlier work, the edges of continents were vertical or steep angular surfaces that merely indicated that the continent was over and the ocean had begun. He had now to provide a plausible account of the complex phenomena of a region to which he had given absolutely no attention heretofore.
Wegener provided, as was his wont, a cartoon schematic of his new version of continental flotation. Instead of a vertical margin, the continents are now shown in cross sect
ion to be convex on their outer sides. Wegener conjectured that in the part of the continent submerged in the Sima, the forces on the outer margin would be up from below (buoyancy) and inward toward the continental margin from the ocean side (resistance). In the topmost few kilometers between the floor of the ocean and the top of the continental block, the forces would be downward (gravity) and outward (spreading against the lighter resistance of the water). The result was that continental margins (between their surface elevation and the floor of the deep sea) would be preferentially subject to slumping, spreading, and fracture, as a result of all the force vectors just named.
The weakened and preferentially fractured marginal slabs might then have several fates: on the trailing edge of a continent they might be left behind as islands, or if longitudinally compressed like the Asian island arcs, they might spring away in echelon from the mainland. Segments might be torn off and left behind as in the Drake Passage between South America and Antarctica, with the shattered, confused archipelagoes on the western side of South America and islands left behind like the Falklands and South Georgia. Another outcome might be sections being stripped off parallel to the coast and entirely pulled away as in the case of Malaya and Sumatra. In a less extreme outcome, there might be meridional faulting along a scarp many hundreds of kilometers long without detachment, as in the case of the San Andreas Fault in California. Everything depended on the vector addition of forces, each of these geological phenomena representing a determinate but unpredictable outcome of the contest of pushes and pulls on the continental mass.
As for the floors of the oceans, these were not merely the outer skins of a static fluid medium with a density slightly greater than the continental bergs that floated in it, but the upper, outer surface of an Earth shell pursuing its own thermodynamic evolution. The three great ocean regions, Pacific, Indian, and Atlantic, were different in depth based on their age—with the Pacific being the oldest, the Atlantic being the youngest, and the Indian Ocean having an intermediate age, less deep near Africa and deeper beyond India. The depth differences were a result of long-term cooling, but even as the outer shell shrank and solidified (deepening the oceans), this did not bring an end to the mobility of the ocean floor. If the Sima was indeed so fluid that it could pass underneath moving continents, it must also be fluid enough to have local and independent turbulent streams and eddies.95 These currents would then be capable of moving and rotating various pieces of the torn-off crust—islands and other continental fragments—so that Wegener could declare it “ridiculous” to ask the theory of displacements to account for every single local motion, since local idiosyncratic movements of the subcrust were capable of causing them.
All in all, this is a much more confident and well-structured argument than the first edition. Yet it introduces new dangers while eliminating old ones. Particularly at risk is the mixture of solid geophysical results with highly speculative inferences about how some previously unexplained phenomenon might be achieved by such and such a disposition of forces. There is nothing wrong with this approach, except that this was exactly the sort of thing that his (highly positivistic) critics had railed against in their attacks on the first edition. The heady brew of data, established results, and unfounded conjecture (the qualitative notion that continental margins are particularly susceptible to massive rupture) opened a new line of attack for Wegener’s critics.
When Wegener moves on to the next section (chap. 4), “The Displacement of the Continental Blocks,” there is a decisive shift in the narrative, focusing on a history of Earth with its evidences of former connection, as well as the history of the splitting and drifting. The detail is very dense here, much more so than in the first edition of the book. Moreover, even though Wegener postpones treatment of the “cause” of continental displacements until the end of the book, he constantly repeats the theme of folding, thickening, and splitting apart—over and over again. He refers all geological consequences back to the folding and tearing of the outer shell—a cumulative, one-directional process, constantly raising the mean elevation of the continents relative to the floors of the deep sea.
“For some time,” he began, “the lithospheric skin covered the entire globe [Erdball]. It could then not have been 100 kilometers thick, but only about 30 kilometers and was entirely covered by a ‘Panthalassa,’ the mean depth of which Penck estimates at 2.64 kilometers, and from which only a very small part, or perhaps nothing at all of the Earth’s crust remained free.”96 This accords, he continued, “with everything we know about the earliest development of life, all forms of which came from the ocean. Before the Carboniferous we have no record of quadrupeds or insects, before the Devonian no land plants, and before the Silurian, no air breathing organisms.”97
“Then, in response to whatever forces [durch irgendwelche Kräfte], this earth shell, still plastic, and capable of sliding over the interior, was torn open, and on the side [of Earth] opposite this tear, compressed. Thus were formed the first fold mountains and, at the same time, the ocean began to divide itself into deep and shallow seas, that were even then marked off from one another by a steep slope.”98 The early continents were not, at first, large blocks standing above the surface of the sea, but more or less like the ribs of a paper lantern, like the island festoons of East Asia today.99 From the beginning until the present this process of tearing and compression has been continuous, and in the future the mean elevation of the continents will be higher, though their total area on the surface of Earth smaller. The smaller the continents, and the greater their thickness, the more difficult it will be to fold them; this is to say that the rate of the process has not always been the same (and was greater in the past than in the present).
It is remarkable how well Wegener defends this speculation, in contrast with his conjectures about continental shelves in the previous section. When he proposes a “Panthalassa,” he uses the (published) authority of Albrecht Penck. When describing what we know about the emergence of life and its relation to the deep sea, he uses the authority of the great paleontologist Gustav Steinmann (1856–1929) and the well-known geologist and paleontologist Johannes Walther (1860–1937). The appeal to Walther carries another message: for he is the author of Walther’s law, which is that when we see paleontological remains reflecting different environments stacked above one another, these reflect lateral shifts of the paleoenvironments. Walther probably meant this with regard to the advance and retreat of shallow seas over continental edges, but Wegener was able to adapt and adopt his authority to the idea of continental displacement. These men were all active and living at the time of this writing, and his point clearly is not simply to use their authority but to try to win them over.
Finally, now, at the halfway point of this short book, we arrive at detailed reconstructions of the continents. Here Wegener pressed the necessity of viewing Earth as a globe and of using sophisticated map projections. Wegener drew a map of the relative positions of the continents in the Carboniferous. The map is the oblique case of the Lambert Azimuthal Equidistant Projection that he favored, centered on a meridian passing through Russia, East Africa, and the Carboniferous South Pole (for his “land hemisphere”) and centered on the open ocean of the Pacific, touching only a section of North Asia through Kamchatka (for his “water hemisphere”). The caption underneath the map has the parenthetical note that it has been drawn “without regard to water coverage,” meaning that shallow transgressions of various parts of the continents are not indicated. The stippled areas represent the “shelf seas.”
The choice of the Carboniferous was obvious from the standpoint of the reconstruction of the fossil remains of Gondwanaland and of the remains of the Permo-Carboniferous Glaciation, with the single great paleocontinent reassembled around a South Pole. Wegener refers to it later in the book, passingly, as the “Pangäa” of the Carboniferous.100 The North Pole, in this reconstruction, lies in the Pacific, about 20° of latitude away from the Aleutians and Kamchatka, thus fulfilling his prediction f
or the absence of remains of this early glaciation in the Northern Hemisphere: there was no land area adjacent to the pole.
Wegener tells his readers, “The map, naturally, has a peculiar appearance, and suggest[s] the presumption that the procedure in drawing it was somewhat arbitrary. This is, however, not the case. This is an exact reconstruction—within given limits—and it should be noted that in multiple independent repetitions there were no significant deviations from the map as published here.”101
Then, with a sort of naïve enthusiasm, Wegener tells the story of how he produced the map. Working with a globe with a diameter of 0.5 meters (roughly 20 inches), Wegener used tracing paper to produce outlines of the continents, including their shelves, and cut them out. To make them fit better on the sphere, he cut them into smaller sections, and to account for his estimate of the tremendous compression of Eurasia by the Tertiary mountain folding (subsequent to the time this map represents), he increased their lateral extent by 10°–15° of latitude. He then glued these tracings to the globe, assembling them in such a way that South Africa was immediately adjacent to the South Pole and the equator passed through Germany. He used the parallelism of the current continental coasts to guide the reconstruction. This was most secure along what would be (millions of years later) the Atlantic rift and moderately secure in the area of what he called “Dekan,” the proto-India that would later be folded up into the Himalayas in the “Lemurian Compression.” Least secure, he admitted, was the reconstruction around Antarctica, for which there was as yet little real geological data.102
Alfred Wegener Page 77
