The Origin of Continents and Oceans/Chapter 2

by Alfred Wegener

CHAPTER II

RELATION TO THE CONTRACTION THEORY, AND TO THE DOCTRINES OF LAND-BRIDGES AND OF THE PERMANENCE OF THE OCEANS

Geology has not yet completely freed itself from the idea of a shrinkage of the earth. This contraction theory was especially advocated by Dana, Albert Heim and Eduard Suess, but still prevails as one of the fundamentals in geological text-books, as for example in those of E. Kayser^[1] and Kober.^[2] Just as a drying apple becomes wrinkled by folds on its surface through the evaporation from its interior, so should the earth through cooling and the consequent shrinkage form folded mountain chains on its surface. Suess coined the short phrase: “Der Zusammenbruch des Erdballes ist es, dem wir beiwohnen” (“The breaking up of the earth, that is what we see”).^[3] The historical service of this theory cannot be denied; it formed for a considerable period a sufficient epitome of our geological knowledge. As a result of this long period, during which the contraction theory has yielded such logical results from a large amount of varied research work, the immense simplicity of its basic principle and the multiplicity of its applications still give it a strong hold. Nevertheless, there can be no doubt that the contraction theory stands in direct contradiction to all the recent conclusions of geophysics, and that the course of geological research tends to deviate more and more from it.

The already difficult explanation of the mountain chains by reason of a shrinkage of the earth has been made to appear even more unsatisfactory by the discovery of the imbricated sheet folding or over-thrusting in the Alps. This new conception of the structure of the Alps and of numerous other mountain ranges, developed by the work of Bertrand, Schardt, Lugeon and others, involves far greater amounts of contraction than do previous suppositions. Whilst Heim, according to the latter, calculated a shortening of one-half for the Alps, he finds to-day, on the basis of the present generally accepted sheet-folding structure, a shortening to one-quarter or one-eighth.^[4]

Since the present breadth amounts to about 150 km., there would thus have been thrust together a portion of the crust of about 600 to 1200 km. (5 to 10 degrees of latitude). Any attempt to deduce such a decrease in diameter from a lowering of the temperature of the earth’s interior is bound to fail. E. Kayser remarks that a contraction of 1200 km. constitutes only 3 per cent. of the circumference of the earth, so that the radius must also be decreased about 3 per cent. These figures are significant only when the temperatures to which they correspond are calculated. On the basis of the average value of the four linear coefficients of expansion of nickel (0.000013), iron (0.000012), calcite (0.000015) and quartz (0.000010) giving 0.0000125, a loss of heat of about 2400° C. is required merely to explain the Tertiary folding. For more ancient periods, when tectonic movements were much more universally in operation, still higher values will be required. This, however, is in direct contradiction to the results of theoretical physics, for Lord Kelvin calculated from the present weak flow of heat from the centre of the earth to the surface that a temperature of the body of the earth so much higher in former periods is quite out of the question. Rudzki^[5] has, of course, drawn attention to the fact that in Lord Kelvin’s calculation no allowance is made for the work done in compression under the force of gravitation, in consequence of which, in spite of the loss of heat, the temperature remains nearly constant, and contraction is almost all that occurs; but nevertheless Rudzki hastens to add the conjecture that the above-quoted coefficients of expansion might probably be considerably decreased on account of the high pressure existing in the earth, so that Lord Kelvin’s calculations would again be correct. To sum up, it may be said that theoretical geophysics on this aspect of the subject does not yet lend itself to definite conclusions. The researches on radium appear to offer more unequivocal results. Great quantities of heat are generated through the automatic disintegration of radium. This element in any case exists in all rocks in such large traces, according to Joly’s determinations, that if the same radium content persisted to the centre of the earth,^[6] the constant conduction of heat from the earth’s interior, which can be verified by measurement of the increase of temperature with depth in mines, would be more than compensated.

Whether we must therefore assume, as Strutt believes we should, that radium only occurs in the uppermost crust of the earth, is still uncertain, but it is evident in any case that a noteworthy contraction of the earth from loss of heat by radiation cannot be seriously maintained. We know, indeed, no way of avoiding the conclusion that the heat-content of the earth is on the increase.

But even if, in spite of all this, such a contraction did take place, we should be forced to the fatal assumption, made, for example, by Heim, that the shrinkage of a complete great circle takes effect only at one single point upon it. This involves the impossible assumption of a transference of pressure within the earth’s crust over an arc of 180 degrees. Numerous authors, as Ampferer,^[7] Reyer,^[8] Rudzki,^[9] Andrée,^[10] among others, have objected to this and have demanded that the contraction must affect the whole surface of the earth in a manner similar to that of the puckering shown by the drying apple. Recently Koszmat, in particular, has repeatedly emphasized the fact that an explanation of mountain building must take into account immense tangential crustal movements, which do not fit in with the notion of a simple contraction theory.^[11] Thus doubt after doubt is experienced, so that the standpoint of geology has for some time been summarized in the statement: “The contraction theory has long ceased to be completely accepted, and meanwhile no theory has been found that will replace it and is capable of explaining all the facts.”^[12]

But in my opinion it is chiefly in regard to another problem that the contraction theory is forced to declare its complete bankruptcy, namely, the problem of the ocean basins and the continental blocks. A. Heim had already thrown a little light on this problem in the statement, “that until accurate observations on the continental oscillations of the past are made, … and until we have more complete measurements of the amounts of the average contraction of the majority of mountain ranges, scarcely any essential and certain progress in our knowledge of the causal connection between mountains and continents and the form of the latter in relation to one another can be expected.”^[13] The problem becomes increasingly more urgent as the soundings of the seas of the earth become more numerous and the contrast between the extensive smooth surfaces of the oceans and the similar level surfaces of the continents, about 5 km. higher, becomes more sharply marked. E. Kayser^[14] wrote in 1918:—

“Contrasted with the volume of these stone colossi (the continental blocks), all the elevations of the mainlands appear to be small and trifling. Even such high ranges as the Himalayas are only ripples, insignificant in altitude, on the surface of those pedestals. This fact already makes the old view, according to which the mountain chains form the constructive framework of the continents, appear nowadays as untenable. … The converse rather must be assumed, that the continents are the older and the determining factors, the mountains being only of subsidiary and more recent formation.” But how can these colossi of stone be explained by the contraction theory? It says that detached portions of the crust are left behind at the time of a general foundering and remain as steps or horsts under the action of the pressure due to arching. But no account is taken of the enormous areas which are affected thereby. This whole conception, already refuted theoretically by Hergesell,^[15] of a stationary and everywhere effective arching pressure in the outermost layers of the crust, is in absolute contradiction to the more recent and increasingly well-proven doctrine of isostasy or the flotation of the crust of the earth on a plastic lower shell.

The contraction theory, with its conception, due to Lyell, of a limitless alternation of the emergence of the deep sea floor above water and the submergence of the continents to the deep sea floor, is also in contradiction to the doctrine of the permanence of the oceans and continents. As will be shown later, we cannot completely admit this doctrine, but its arguments directed against the contraction theory are quite valid. From the standpoint of the universally admitted theory of isostasy, it appears to be physically impossible that a whole continent should sink such a large amount as 5 km. On the other hand, the marine deposits in the present continents show that these—with insignificant exceptions—were never deep sea areas, but were covered by the shallow seas of the continental shelves. In this way the contraction theory is refuted in a striking manner by the greatest features of the face of the earth.

The theory of the displacement of continents avoids all these difficulties. It permits the assumption of the requisite horizontal contraction in the folded mountain chains; in fact, it makes this possible for the first time. For if the earth as a whole does not diminish to the same amount as its crust is compressed, then every compression of the crust corresponds to a rift at another place, and we thus arrive of necessity at the idea that the outermost skin of rocks no longer clothes the whole world. In addition, the continental masses, with their contrast to the extensive oceanic areas, certainly do not lend themselves to any other explanations. The theory of the displacement of continents thus replaces the contraction theory, which must be completely rejected.

Further, a separate explanation is also needed with reference to the doctrine of the sunken bridging-continents and the already mentioned contrary doctrine of the permanence of the oceans. The relation of the displacement theory to both these doctrines is different from that to the contraction theory. To anticipate the results. The arguments which are led out into battle by both these doctrines are correct, and so therefore are their mutual refutations. Each is based on only that portion of the facts necessary for a favourable judgment and receives its refutation directly the other portion is introduced. The displacement theory will fit the entire facts, and therefore prepares the way for a reconciliation of these hostile doctrines in a manner which satisfies all reasonable demands of both parties. In order to do this we must go further into the subject.

The advocates of the bridging-continents rely on the nowadays well-established fact that the close affinities of fauna and flora of widely separated continents require extensive land connections in the past. The ever-increasing stream of separate discoveries allows the picture of these connections to grow under our eyes, and to-day a very far-reaching agreement already prevails among the various specialists upon the most important of the land-bridges, although individuals cannot always “see the wood for the trees.”^[16] We refer in this connection to the summary given in Chapter V of the views of twenty specialists on the several bridges, whether favourable or hostile. It is regarded as certain that there was a land connection, sometimes broken, between North America and Europe, which finally broke in the Glacial Period; a similar one between Africa and South America, which vanished in the Cretaceous Period; a third, the “Lemurian” bridge between Madagascar and India, which broke down at the beginning of the Tertiary; and finally a “Gondwana” bridge from Africa through Madagascar and India to Australia, which split up in the earliest Jurassic. Formerly also a land connection must have prevailed between South America and Australia, but the view that this was formed by a bridging-continent in the Southern Pacific is advocated by only a few workers. Most of them assume that this connection lay via Antarctica, since this lies on the shortest connection between both continents; moreover, the affinities are confined to such elements as can bear the cold.

Naturally a large number of bridges which to-day are represented by shallow seas are also assumed. The adherents to the doctrine of the bridging-continents have up to the present made no differentiation between the bridges over the oceans and those over the continental shelves. It must be especially emphasized that the displacement theory only deals with the question of land connection over the present-day deep sea areas for which new ideas are developed, whilst for the shelf-bridges, as the Bering Straits between North America and Siberia, the earlier view of submergence and re-emergence of dry land remains absolutely undisputed.^[17]

The adherents to the doctrine of the submerged continental bridges have thus a very strong argument: the former existence of broad land connections between continents which are widely separated at the present day can scarcely be doubted on account of the similarity of the fossil faunas and floras and the affinity of the existing ones. That these land-bridges were formed by continents which later sunk to the depths and form the present floor of the ocean, was assumed, on the grounds of the contraction theory, as self-evident and not requiring further proof, since the possibility of another solution by horizontal displacement was certainly not considered. As Ubisch emphasizes, the displacement theory will fit these requirements just as well as the present-day assumption of sunken intervening continents, indeed even better, because the close affinity of their faunas and floras would still appear rather mysterious on account of the present great distance from each other of these existing continents, even if an exchange of forms was at an earlier date possible over an intervening continent.^[18]

The arguments of the opposing doctrine of the permanence of continents and oceans rest not on biological but on geophysical grounds, and are directed essentially not against the former existence of land connections, but only against that of bridging-continents. The first argument consists in the facts already touched on, that on the continents deep-sea deposits do not occur to any considerable extent, so that the continental blocks as such are undoubtedly “permanent.” Strata taken earlier to be abyssal deposits have been found to be shallow-water sediments—as, for example, Cayeux demonstrated for the Chalk. For a very small number, such as the practically non-calcareous ‘radiolarites’ of the Alps and certain red clays which are reminiscent of the red abyssal clays, a great depth of origin is still assumed mainly because the sea-water acts as a solvent on the calcareous material only at great depths.^[19] The interpretation of these discoveries is, to be sure, still disputed, and it is believed that many are deposited at a depth of 1 to 2 km., which is still to be reckoned as belonging to a continental slope. But even if we follow Koszmat and Andrée, who, for example, assume a depth of deposition of the Alpine radiolarite of 4 to 5 km., the space occupied by such oceanic deposits, compared with the extent of the continents, is still so small that the principle of the general permanence of the continental blocks is not at all damaged by it. The present continental blocks, with but trifling exceptions, have never in the history of the earth formed the floor of the ocean, but were, as at present, continental platforms. The conception of Lyell of a repeated submergence and emergence is thus limited, in that it only concerns the alternating shallow floodings of the permanent continental platforms.

But this gives rise to a great difficulty in the construction of bridging-continents where now oceans, exist. If their re-elevation were not compensated by an equivalent re-submergence in another place, the remaining much diminished ocean basins would not have nearly enough room for the total amount of water of the oceans. The re-elevation of those former continental bridges would raise the surface of the water so much that all the continents, old as well as new, with the exception of lofty mountains, would be completely flooded. In other words, the assumption of continental bridges does not lead to the desired end, which consists of a land connection between extended continents. In order to surmount this difficulty, which was emphasized by Willis and Penck, we must make the assumption, not otherwise warranted, and therefore improbable, that the water on the earth had increased proportionately at the same time as the continental bridges sank. But this hypothesis has not up to the present been seriously considered and championed. With the more probable assumption that the amount of water has remained practically unaltered, the fact that in all geological periods extensive portions of the continental blocks have remained dry land, forces us to the conclusion that the total oceanic area has been essentially constant. This would mean that if the position of the continents is unchanged, hitherto considered as obvious, the several ocean basins have also been permanent features on the face of the earth.

Further, the advocates of the permanence doctrine base it on the geophysical facts of isostasy or the equilibrium in the earth’s crust. According to this theory, the relatively light crust of the earth swims in a somewhat heavier magmatic lower shell. Just as a piece of wood is immersed more deeply into water by loading it, so this uppermost rocky crust sinks deeper into the heavy magma, according to the law of Archimedes, at the place where it is burdened. This happens, for example, with an ice-cap, in which case the shore-lines formed during the depression will be elevated after the melting of the ice. Thus the isobase maps of de Geer, founded on raised beaches, show a depression of the central portion of Scandinavia of at least 250 m. for the last glaciation, an amount which becomes gradually less as we recede from the area, but which assumes still higher values during the Great Ice Age.^[20] De Geer has detected the same phenomena in the case of the glaciated area of North America. Rudzki has shown that on the assumption of isostasy plausible values for the thickness of the inland ice-sheet can be calculated, namely, 930 m. for Scandinavia and 1670 m. for North America, where the depression amounted to 500 m.^[21] Since the magma of the underlying shell is not so mobile as water, but is extremely viscous, all such isostatic movements of compensation must lag greatly. The strand-lines were formed chiefly after the melting of the ice, but before the elevation. Indeed, Scandinavia is still rising, as levelling shows, at the rate of about 1 m. in 100 years. Also deposition of sediments, as was first recognized by Osmond Fisher, results in the depression of the block. Each loading leads to a somewhat delayed sinking of the platform, and thus the new surface lies again at practically the old elevation. Sediments many kilometres thick may originate in this manner, which have, however, all been formed in shallow water.

Gravity measurements are the physical basis of the doctrine of isostasy as deduced by Pratt (the word having been coined by Dutton in 1892). In 1855 Pratt had already ascertained that the Himalayas did not exert the expected attraction in the plumb-line experiments^[22] and correlated with it the fact, which was universally confirmed, that the force of gravity in great mountain chains does not deviate by the expected amount from the normal values. Thus the mountain massifs appear to be compensated in some way by subterranean defects of mass, as the work of Airy, Faye and Helmert, among others, showed, and also as worked out recently by Koszmat in a very lucid article.^[23] It has been pointed out that on the oceans gravity possesses almost its normal value in spite of the evident defects of mass formed by the great ocean basins. The earlier measurements on islands, however, gave scope for differing interpretations, but after Hecker had succeeded in determining the gravity on board ship by means of simultaneous readings on a mercury barometer and on a boiling-point thermometer, in a manner suggested by Mohn, in place of the pendulum, which is not applicable in this case, he was able to carry out the measurements during several journeys on the Atlantic, Indian and Pacific Oceans, and so obtained definite results. The evident mass-defects of the ocean basins must thus—conversely to the case of the mountain chains—be compensated by a subterranean excess of mass. The manner in which these subterranean excesses and defects can be explained has given rise to many different conjectures in the course of time. Pratt thought of the earth’s crust as a dough-like mass, originally of the same thickness everywhere, expanded upwards in continents and compressed in the oceanic areas. This idea was further developed by Hayford and Helmert and applied generally to the interpretation of the gravity observations.

Recently, however, another view, which had already been expressed by Airy in 1859, has become prominent mainly through Schweydar’s work.^[24] According to this view, the continents swim as lighter blocks on the heavier deeper material. Heim was certainly the first to assume that this less dense crust is thickened under mountains, and that the heavy magma was forced to greater depths there (compare Fig. 3). Conversely, this crust must be especially thin under the oceans (according to the displacement theory it is quite absent). The more recent evolution of this doctrine of isostasy has concerned itself mainly with the question of the extent of its validity. For the larger blocks, as, for example, a complete continent or the whole floor of an ocean, isostasy must be assumed without question. But on a small scale, in the case of individual mountains, this law loses its validity.
Fig. 3.—Section through the lithosphere according to the doctrine of isostasy. Such smaller portions can be supported by the elasticity of the whole block, like a stone placed on a floating block of ice. Isostasy is effected between the block and stone as a whole and the water. In a similar manner the gravity measurements very rarely show a deviation from isostasy in continents with structures the diameter of which amounts to hundreds of kilometres. If the diameter amounts only to tens of kilometres, then at the most is there only partial compensation; and if it is only of a few kilometres, then compensation is practically non-existent.

This doctrine of isostasy, the flotation of the crust of the earth, has been confirmed to such an extent by experiments, especially those of gravity, that it belongs to-day to the firmest foundations of geophysical knowledge.

According to this doctrine, it is said that an oceanic area cannot be elevated as a whole above the level of the sea or an unloaded continent sink to the level of the deep sea floor. Small alterations of level, possibly as much as several hundred metres, such as should lead to the emergence or submergence of an area of continental shelf, are obviously possible, for example, in the case of the wandering of the poles due to the lag of the earth in its adjustment to the new ellipsoid of rotation. But it is a mistake to believe that only a difference of degree exists between such alterations of level and the submergence of a continent to the deep sea floor. For the last case would involve the changing-over of the upper frequency maximum of the earth’s crust to the lower, and we should be in need of a physical cause for the favouring of the level of the ocean floor and the absence of the intermediate layers; which is not forthcoming (see Chapter III). The partisans of the permanence doctrine have thus at their disposal good arguments against the doctrine of the submerged bridging-continents.

But since they start from the obvious assumption that the continents have always lain where they do to-day, the advocates of the permanence theory arrive at false conclusions from their accurate premises when they explain: “The great ocean basins are permanent features of the earth’s surface, and they have existed, where they are now, with moderate changes of outline, since the waters first gathered.”^[25] When we bring into consideration horizontal drift movements of the continents, we can only uphold this principle so far as to agree that the total areas of the continental blocks and of the ocean floors, except for the compression of the former in the course of time, remain approximately constant. But this is, however, all that the arguments quoted really prove.

Whilst we must thus totally reject the contraction theory, we need only reduce the doctrines of the land-bridges and the permanence of oceans and continents to the conclusions that can be legitimately drawn from the arguments advanced for them, so as to reconcile both these apparently so opposite doctrines by means of the displacement theory. The latter says: Land connections there were, not through bridging continents which sank later, but by direct contact; permanence not of separate oceans and continents as such, but of the oceanic and of the continental areas as a whole.

In the following chapters the main grounds which indicate the correctness of the displacement theory will be exhaustively treated.

1. E. Kayser, Lehrbuch der allgemeinen Geologie, Ed. 5. Stuttgart, 1918.
2. L. Kober, Der Bau der Erde, pp. 1–324. Berlin, 1921.
3. E. Suess, Das Antlitz der Erde, 1, p. 778, 1885. English Edition, 1, p. 604, 1904.
4. A. Heim, “Bau der Schweizer Alpen,” Neujahrsblatt d. Naturf. Ges., Part 110, p. 24. Zürich, 1908.
5. M. P. Rudzki, Physik der Erde, p. 118 f. Leipzig, 1911.
6. Rudzki, op. cit., p. 122. Wolff, Der Vulkanismus, 1, p. 8. Stuttgart, 1913.
7. O. Ampferer, “Über das Bewegungsbild von Faltengebirgen,” Jahrb. d. k.k. Geol. Reichsanstalt, 56, pp. 539–622. Vienna, 1906.
8. Reyer, Geologische Prinzipienfragen, p. 140 ff. Leipzig, 1907.
9. Rudzki, op. cit., p. 122.
10. K. Andrée, Über die Bedingungen der Gebirgsbildung. Berlin, 1914.
11. F. Koszmat, “Erörterungen zu A. Wegener’s Theorie der Kontinentalverschiebungen,” Zeitschr. d. Ges. f. Erdkunde zu Berlin, 1921, p. 103.
12. E. Böse, “Die Erdbeben” (Sammlung, Die Natur, n.d.), p. 16; compare also the criticism of Andrée, loc. cit.
13. A. Heim, Untersuchungen über der Mechanismus der Gebirgsbildung, Part 2, p. 237. Basle, 1878.
14. E. Kayser, Lehrb. d. allgem. Geologie, Ed. 5, p. 132. Stuttgart, 1918.
15. H. Hergesell, “Die Abkühlung der Erde und die gebirgsbildenen Kräfte,” Beitr. z. Geophysik, 2, p. 153, 1895.
16. “Nevertheless there are still to-day some opponents of the land-bridges. Among them G. Pfeffer is especially prominent. He relies on the fact that many forms now confined to the southern hemisphere are found fossil in the northern. For him there is no doubt that they were once more or less universally distributed. This conclusion is not at all convincing; still less the further assumption that in all cases of a present discontinuous distribution in the south we ought to accept a former universal distribution, even though a fossil occurrence in the north has not been recorded. If he thus explains all peculiarities of distribution exclusively by wanderings between the northern continents and their mediterranean bridges, then this assumption rests entirely on unsafe foundations” (Arldt, “Südatlantische Beziehungen,” Peterm. Mitt., 62, pp. 41–46, 1916). That the affinities of the southern continents can be explained more simply and more completely by direct land connections than by parallel migrations from the common northern region, certainly needs no explanation, even if in isolated instances the process could have taken place in such a manner as Pfeffer assumes.
17. Among the numerous misapprehensions on which Diener’s opposition to the new idea is based (“Die Groszformen der Erdoberfläche,” Mitt. d. k.k. geol. Ges. Wien, 58, pp. 320–349, 1915), already for the greater part refuted by Köppen (“Über Isostasie und die Natur der Kontinente,” Geogr. Zeitschr., 25, pp. 39–48, 1919), the following is found:—“Whoever pushes North America towards Europe breaks its connection with the Asiatic continental block at the Bering Straits.” This objection, obvious as it is on a Mercator’s map, vanishes if a globe be taken; the question is essentially one of rotation of North America about Alaska.
18. L. v. Ubisch, Wegener's Kontinentalverschiebungstheorie und die Tiergeographie. Verh. d. Physik.-Med. Ges. z. Würzburg, 1921, pp. 1–13.
19. A detailed discussion of these possible oceanic deposits is to be found in Dacqué, Grundlage und Methoden der Paläogeographie, p. 215. Jena, 1915.
20. G. de Geer, Om Skandinaviens geografiska Utvekling efter Istiden. Stockholm, 1896.
21. Rudzki, Physik der Erde, p. 229. Leipzig, 1911.
22. At Kaliana, in the Gangetic Plain, 50 miles from the foot of the mountain chains, the northern component of the deviation of the plumb-line amounts to only 1″, whilst the attraction of the mountains should effect one of 58″. Similarly, Jalpaiguri shows 1″ instead of 77″ (according to Koszmat).
23. F. Koszmat, “Die Beziehungen zwischen Schwereanomalien und Bau der Erdrinde,” Geol. Rundsch., 12, pp. 165–189, 1921.
24. W. Schweydar, “Bemerkungen zu Wegener’s Hypothese der Verschiebung der Kontinente,” Zeitschr. d. Ges. f. Erdk. zu Berlin, pp. 120–125, 1921.
25. Bailey Willis, “Principles of Palæogeography,” Science, 31, N.S., No. 790, pp. 241–260, 1910. This is certainly a very blunt statement. Other authors, as, for example, Sorgel (“Die Atlantische ‘Spalte,’ Kritische Bemerkungen zu A. Wegener’s Theorie von der Kontinentalverschiebung,” Monatsber. d. deutsch. Geol. Ges., 68, pp. 200–239, 1916), seek a via media, in which they would allow the bridging-continents to shrink up as much as possible to narrow bridges on the margin of the ocean basins. But this compromise is not a very happy one, since on one hand the explanation of the affinities is rendered more difficult, and on the other the requirements of physics are only insufficiently fulfilled.