The Origin of Continents and Oceans/Chapter 13

by Alfred Wegener

CHAPTER XIII

THE DISPLACING FORCES

Although at the first glance the displacement of the continents presents a very variegated picture of different kinds of movements, yet nevertheless there is one great underlying principle: the continental blocks move equatorwards and to the west. It is advisable to consider both the components of this movement separately.

A movement directed towards the equator, the flight from the poles of the continental masses, has already been assumed by several authors, especially Kreichgauer^[1] and Taylor.^[2] It is very generally recognizable, more with large blocks, less with smaller, and strongest in mean latitudes. It is especially manifest in Eurasia in the arrangement of the great girdle of Tertiary folding of the Himalayas and the Alps, which were formed on the equator of that time, as well as in the bulging compressed outline of the east coast of Asia. The drift from the poles is also very clear in the case of Australia, for this continent is moving to the north-west, as is consistently shown by the deformation of the series of islets forming the Sunda Archipelago, by the high and youthful mountains of New Guinea, and by the south-easterly lag of the former festoon, New Zealand. In North America the drift from the poles shows itself in the south-westerly displacement of Grinnell Land relatively to Greenland (or of Labrador relatively to South Greenland), also further in the incipient longitudinal compression of the separating coastal range of California, and in the earthquake fault of San Francisco, connected with it. Even the small block of Madagascar tends towards the equator, for it has moved north-eastwards from its position of rupture from the African continent. It is true it is possible that it has been passively carried along by a stream of sima. Africa and South America lie to-day on the equator, and undergo but slight meridional displacement. The great displacements which South America suffered in the Tertiary period, and which led to the uplifting of the South American Andes, were directed towards the north-west relatively to the position of the poles of that period, and likewise exhibits the drift from the poles. The same thing probably happened to Antarctica.

The compression of Lemuria from the Tertiary to the present day can be conceived in its earlier stages as a drift from the pole. It is true that it lies to-day 10° to 20° north of the equator, so that a drift from the pole could only diminish the folding. It is difficult to say how this movement is to be understood, for we can only determine the relative displacement. Perhaps India was pressed against the interior of Asia by a stream of sima flowing northwards; possibly also it is more correct to trace back a great part of the folding to the drift of Asia from the pole.

The other component, the westward drift of the continents, becomes still clearer by a study of the map of the world. The large blocks move westwards in the sima. Thus the Pangæa of the Carboniferous era had already an anterior margin (America), which became folded (Precordilleras), on account of the resistance of the viscous sima; and a posterior margin (Asia), from which littoral ranges and fragments became detached, and remained fast in the sima of the Pacific as groups of islands. This contrast between the east and west shores of our largest ocean is now strikingly evident, especially in Eastern Asia, where the great process of the detachment and abandoning of numerous marginal ranges is taking place, favoured by the meridional compression. The continental lobes of Further India and of the Sunda Islands, which are stretched towards the south, show a lag towards the east, and testify to the westward drift just as much as the similarly directed detachment of Ceylon from the southern point of India. South of this, in the Australian region, the same processes are taking place, as is shown by the lagging behind of the New Zealand festoon, and the north-westerly advance of the Australian block. The same phenomena as on the coast of Eastern Asia are again also met with on the east coast of America. The Antilles form a beautiful example in Central America of festoons lagging behind to the east; from which it is important to notice that the small islands lag more than the greater. The continental shelf of Florida remains behind in the east just as the southern point of Greenland does. In South America the masses of the Abrolhos Bank emerge from under the continent through the easterly lag; the area neighbouring the Drake Straits, with its trailing points of the mainland and the connecting chains remaining far behind, has already been used as illustrating a standard example of the westward displacement. In Africa the drift to the west manifests itself in the lagging behind in the east of the small block of Madagascar (which combines with its drift from the poles, to give it a north-easterly movement). Perhaps the late East African fault system, of which the separation of Madagascar certainly only forms a part, might be brought into connection with the westward drift, although in this case we are no longer dealing with island festoons, but with large blocks. On the west coast of Africa the Canary and Cape Verde Islands seem, it is true, to have been first detached in very recent times, and in this manner to have departed towards the west; but this small westward advance of the sima may well be accounted for as a result of the general flow of the sima at the opening-up of the Atlantic Ocean. This could only mean that the sima surface of the Atlantic was drawn out like rubber in the progress of its opening, or that there was a predominant flow of sima into the rift.

Whether all the details of the displacements can be explained by these two components of the drift from the poles and of the westward drift, must, indeed, remain undecided. The chief movements in the earth’s crust, however, are apparently fairly well accounted for by them.

It is also to be expected that the arrangement of the rifts in the sial crust should be systematic, for indeed rifts and displacements are inter-related. The drift towards the west would correspond to meridional rifts. The drift from the poles could also occur, in the presence of meridional rifts, especially if these are continued up to the pole. It has already been said that, as a matter of fact, we can recognize a tendency to a meridional direction of the trough-faults and rifts; and the East African fault system, the Rhine Valley, and especially the great Atlantic separation, were given as examples. The prolongation to the Pole may be supported, at least for the former South Pole, by the cases of the tapering southern ends of South America, Africa and India. But in this case also there are only indications of a systematic arrangement; in detail there are many deviations.

The question as to what forces have caused these displacements, folds and rifts cannot yet be answered conclusively. Here information can only be given as to the present position of the investigations concerning it.

Eötvös was the first to claim that a force exists which endeavours to displace the continental blocks towards the equator.^[3] He especially drew attention to the fact “that the direction of the vertical is curved in the plane of the meridian, the concave side turned to the pole, and that the centre of gravity of the floating body lies higher than that of the displaced mass of liquid. As a result, the floating body is subjected to the action of two forces working in different directions, the resultant of which is directed from the pole towards the equator. A tendency would thus prevail in the case of the continents to move towards the equator, a movement which would produce a secular variation of the latitude, as has been suspected for the observatory at Pulkova.”

Without any knowledge of this small inconspicuous reference, W. Köppen recognized the nature of the force producing the drift from the poles and its importance to the question of the displacement of continents, and gave a description of it, although without any calculations: “… The flattening of the various levels thus diminishes with the depth; they are not parallel, but slightly inclined to one another, except at the equator and at the poles, where they are all at right angles to be radius of the earth.”^[4] Fig. 44 shows this on a meridional section between the pole (P) and the equator (A). The interrupted line concave to the pole is the line of force of gravity or plumb-line of the place O. C is the centre point of the earth.

Fig. 44.—Level-surfaces and curved plumb-line. Now the centre of buoyancy of a floating body is situated in the centre of gravity of the medium displaced; that of its weight, on the contrary, in its own centre of gravity; and the direction of each force is at right angles to the horizontal plane at the point of application. Their directions are thus not in opposition, but give a small resultant, which, if the buoyancy point lies under the centre of gravity, is directed towards the equator. Both forces are not perpendicular to the horizontal plane at the surface of the block, since its centre of gravity lies far below, but are somewhat bent in the same direction,^[5] the buoyancy, however, more than the weight of the block. These principles must apply to all floating bodies, the centre of gravity of which lies above the buoyancy point, and in the same way the forces must have a resultant directed towards the pole, if the centre of gravity lies under the buoyancy point; the principle of Archimedes is then only strictly correct on a revolving earth if both points coincide.”

The first calculation of the force of the drift from the poles was carried out by P. S. Epstein.^[6] He found the expression for the force K in the geographic latitude 𝜑 to be

K𝜑 = − 3/2md𝜔²sin 2𝜑,

where m is the mass of the continental block, d half the difference in altitude between the floor of the ocean and the surface of the continental block (or, equal to the difference in level of the centres of gravity of the block and of the displaced sima), and 𝜔 is the angular velocity of the earth.

He used this equation in order to calculate the coefficient of viscosity 𝜇 of the simasphere from the velocity of displacement of the continental blocks (according to the general formula K = 𝜇 v/M, where M is the thickness of the viscous layer), and obtained

𝜇= 𝜚sdM𝜔²/v,

where 𝜚 is the specific gravity of the block and s its thickness. Using now the following numerical values, which are certainly very extreme,

𝜚	=	2.9
s	=	50 km.
d	=	2.5 km.
M	=	1600 km.
𝜔	=	2𝜋/86164
v	=	33 m. per annum,

he found the coefficient of viscosity of the sima to be

𝜇 = 2.9 × 10¹⁶ g.cm.⁻¹ sec.⁻¹,

thus being three times as great as that of steel at room-temperature. He concludes from this that “we can summarize the results thereof in the statement that the centrifugal forces of the rotation of the earth can produce a drift from the poles of the magnitude indicated by Wegener, and that it must produce it.” On the other hand, Epstein thinks the question as to whether the folded mountain chains of the equator could be traced back to this force must be answered in the negative, since this force only corresponds to a fall of surface of 10 to 20 m. between the pole and equator, whilst the raising up of mountains to altitudes of several kilometres and the corresponding submergence of sialic masses to great depths constitute a considerable amount of work against gravity, for which the force of the drift from the poles is not sufficient. Only hills of 10 to 20 m. in height could be produced by it.

W. D. Lambert,^[7] at practically the same time as Epstein, mathematically deduced the force of the drift from the poles with essentially the same results. He finds the force in the latitude of 45° to be one three-millionth part of the gravity. Since the force reaches its maximum in this latitude, it must thus have a rotatory effect on an elongated obliquely-lying continent, and will in fact tend, between the equator and latitude 45°, to bring its long axis into an east and west direction, and, between 45° and the pole, on the other hand, into a meridional direction. “All this is quite speculative of course; it is based on the hypothesis of floating continental masses and on the assumption of a sustaining magma that would, naturally, be a viscous liquid, but viscous in the sense of the classical theory of viscosity. According to the well-known theory, a liquid, no matter how viscous, will give way before a force, no matter how small, provided sufficient time be allowed for the latter to act in. The peculiarities of the field of force of gravity will give us minute forces, as we have seen, and the geologists will doubtless allow us æons of time for the action of the forces, but the viscosity of the liquid may be of a different nature from that postulated by the classical theory, so that the force acting might have to exceed a certain limiting amount before the liquid would give way before it, no matter how long the small force in question might act. The question of viscosity is a troublesome one, for the classical theory does not adequately explain observed facts, and our present knowledge does not allow us to be very dogmatic. The equatorward force is present, but whether it has had in geological history an appreciable influence on the position and configuration of our continents is a question for geologists to determine. At any rate, it may be considered as one of the mechanical curiosities with which this paper deals.”

Finally Schweydar^[8] has calculated the force of the drift from the poles. He obtains for the latitude of 45° the value of about 1/2000 cm./sec., that is, the force amounts to about a two-millionth part of the weight of the block. “Whether this force suffices for displacement is not easy to decide. In any case, it would not explain a westward drift, since the velocity is too small to produce an appreciable westerly deflection by the rotation of the earth.”

Schweydar declares that in Epstein’s calculation the assumed velocity of displacement of 33 m. per annum is too great, and that the viscosity of the sima hence obtained is considerably too small. If a smaller velocity is taken, the required greater viscosity is obtained. “If one assumes for he coefficient of viscosity the order of 10¹⁹ (instead of Epstein’s 10¹⁶) and makes the supposition that the formula used by Epstein is applicable in this case, then one obtains about 20 cm. per annum for the velocity of a block in the latitude of 45°. In any case, it is shown to be possible that the continents undergo a displacement directed towards the equator under the influence of this force.”^[9]

We can combine the foregoing statements in the sentence that there is no longer any doubt as to the existence and magnitude of the force of the drift from the poles. It amounts at the maximum (in latitude 45°) to about a two- or three-millionth part of gravity, and is thus, in any case, still four times greater than the horizontal tidal forces. But since it does not vary as these do, but for millions upon millions of years works on unchanged, it is enabled to overcome the steel-like viscosity of the earth’s body in the course of geological periods, provided only that it is not less than the minimum force (which indeed we do not know) required to produce movement. Now we have already seen that the continents behave like wax and the sima like sealing-wax. The minimum force required is, in any case, very much smaller in the sima than in the sial. On that account it appears to me to be highly probable that considerable displacements of the continental blocks have actually taken place in the sima in the course of geological time as a consequence of the force of the drift from the poles. On the other hand, it seems more doubtful whether this force is sufficient to explain the equatorial mountain chains, although Epstein’s results are probably not the last word in this matter.

We can be more brief in the discussion of the forces which have to be considered in connection with the westerly drift of the continents. Various authors, as E. H. L. Schwarz and Wettstein, among others, have claimed the friction of the tidal waves, which will be produced in the solid earth through the attraction of the sun and moon, for a rotation to the west of the whole crust of the earth over the interior. It is also frequently supposed that the moon had formerly a more rapid rotation, but it was retarded by the tidal friction caused by the earth. It is also easy to see that this retardation of a planetary body by tidal friction must especially effect its upper surface, and lead to a slow, sliding movement of the whole crust, or of the individual continental blocks. There remains only the question as to whether such tides exist at all. The deformation of tides in the solid body of the earth, detected by the horizontal pendulum, is, according to Schweydar, of another kind, namely, elastic, and can thus not be directly drawn upon for the explanation. But nevertheless I believe it possible that even these elastic tides give the impulse to a slight progressive displacement of the crust, due to the viscosity of the sima, which accumulates from day to day by an amount which is certainly very slight, and so not at all apparent in the daily measurements, but which can nevertheless lead to considerable displacements in the course of millions of years. For it is beyond question that we cannot regard the earth as being completely elastic in relation to the tidal forces. In my opinion this question cannot yet be considered as settled, simply because the elastic nature of the measurable daily tides in the solid earth has been established.

In another way (which, however, is again referable to the attraction of the sun and moon), namely, on the basis of the precession theory of the earth’s axis, Schweydar obtained a force which can effect a westward drift of the continents.^[10] “The theory of the precession of the axis of rotation of the earth under the influence of the attraction of the sun and moon is based on the supposition that the individual portions of the earth can undertake no very great displacement relatively to one another. The calculation of the movement of the earth’s axis in space becomes more difficult if the displacement of continents is admitted. In this case a distinction must be made between the axis of rotation of the continent and that of the whole earth. I have calculated that the precession of the axis of rotation of a continent lying between latitudes − 30° and +40° and the longitudes 0° and 40° west, is about 220 times greater than that of the axis of the entire earth. The continent has the tendency to rotate about an axis which deviates from the ordinary axis of rotation. By this means forces exist which work not only in a meridional but also in a westerly direction, and attempt to displace the continents; the meridional force varies in the course of the day and does not enter into our problem. These forces are considerably greater than that of the drift from the poles. The force is strongest at the equator and zero at the latitudes 36°. I hope later to be able to give a more exact description of the problem. In this way also a westerly displacement of the continents might also be rendered possible by it.” As this is only a preliminary communication, one must still await the published detailed account in order to give a conclusive opinion, nevertheless it appears to be so far certain that the most clearly recognizable movement of the earth, the westerly drift of continents, can be explained by the agency of the attraction of the sun and of the moon on the viscous earth.

But Schweydar holds the view that the deviations of the earth’s figure from the ellipsoid of rotation, as shown by the gravity measurements, give rise to flow-movements within the sima, and can thereby also cause displacement of the continents. “One may also, however, suspect a flow of the sima, at least in the earlier epochs. In his latest work Helmert infers, from the distribution of the force of gravity on the earth’s surface, that the earth is a triaxial ellipsoid; the equator forms an ellipse. The difference of the axes of this ellipse amounts to only 230 m.; the long axis cuts the surface of the earth at longitude 17° west (Atlantic Ocean), the short at 73° east (Indian Ocean). According to the theories of Laplace and Clairaut, from which in geodesy we have not departed, the earth is to be considered as constructed like a liquid, that is, the pressure in the solid earth (apart from the crust) is assumed to be of the nature of hydrostatic pressure. Helmert’s results are unintelligible from this point of view. The hydrostatically constructed earth with its oblateness and its velocity of rotation cannot be a triaxial ellipsoid. It might now be assumed that the deviation from an ellipsoid of rotation will be brought about by the continents. But this is not the case. I have made the calculation on the supposition that the continents are floating and have the thickness adduced above (200 km.; difference in density between sial and sima 0.034, water = 1), and found that the distribution of continents and seas produces a deviation of the mathematical shape of the earth from an ellipsoid of rotation which is considerably smaller than that found by Helmert. The axes of the equatorial ellipse are absolutely different from Helmert’s, the long axis falling in the Indian Ocean. Thus great portions of the earth must have variations from the hydrostatic structure.

According to my calculations, Helmert’s result can be explained if the layer of sima 200 km. thick under the Atlantic Ocean has a density greater by 0.01 than that under the Indian Ocean. Such a condition cannot hold in the long run, and the sima will endeavour to flow in so as to restore the condition of equilibrium of the ellipsoid of rotation. A flow is, to be sure, scarcely possible with such a slight difference in density, but the ellipticity of the equator and the difference of density in the sima, and therefore, the flow, could have been of more importance in the earlier epochs.”

Without any more details, it is clear that the forces derived from Helmert’s work can render intelligible the opening of the Atlantic Ocean, for in this region the earth seems elevated and the masses will have striven to flow away to both sides.

But here yet another consideration may be adduced which might possibly be considered as an extension of Schweydar’s ideas. Such elevations of the surface of the earth above its equilibrium level naturally need not be confined to the region of the equator, but can occur everywhere on the earth. It has been shown earlier in the discussion of the transgressions and their connection with the displacements of the poles (in Chapter VIII), that we must expect in front of the moving pole a too high, and behind it a too low, position of the surface of the earth, and that the geological facts appear to confirm the existence of these variations. Amounts are involved, similar to those which Helmert found for the excess of the greater equatorial axis over the lesser, or perhaps double the latter amounts. In the case of the more rapid pole movements, the surface of the earth appears in front of the pole to be some hundreds of metres above, and behind the pole some hundreds of metres below the position of equilibrium. The greatest gradient (order of magnitude 1 km. for a quadrant of the earth) would prevail in the meridian of the displacement of the pole at its point of intersection with the equator, and another nearly as large at both the poles. Forces are thereby set free which draw the masses from the too high to the too low areas, and these forces are many times the normal force of the drift from the pole, which corresponds, in the case of the continental blocks, to a gradient of only 10 to 20 m. for a quadrant of the earth. These forces do not act only on the continental blocks as the force of the drift from the poles does, but also on the sima lying beneath, which is more liquid, and perhaps brings about equilibrium under the more rigid crust. But so long as this gradient exists—the transgressions and regressions bear witness to its existence—the force must also operate on the continental blocks, and must therefore cause displacements and folds of the latter, even if these movements are possibly smaller than the corresponding movements of the more liquid material beneath them. In the case of the apprehension being confirmed that the normal force of the drift from the poles is only sufficient for the displacement of continents in the sima, but not for their foldings, I think that we have a source of force in this deformation of the earth’s figure caused by the wanderings of the poles which, in any circumstances, is sufficient to do the work of folding.

This explanation becomes particularly probable through the circumstance that both the greatest systems of folds to be considered in this connection, namely, the equatorial folds of the Carboniferous and of the Tertiary, were formed in exactly those times in which we must assume, on other grounds, especially rapid and extended polar wanderings (movement of the South Pole from the Lower Carboniferous to Permian from Central Africa to Australia in its then position, and movement of the North Pole from the Lower Tertiary to the Quaternary from the Aleutian Islands to Greenland).

As already mentioned, the question as to the forces which have caused and now cause the continental displacements is still too much in a state of flux to permit of a complete answer satisfactory in every detail. One thing may be taken as certain: continental displacements, folding and rifting, vulcanicity, the alternation of transgressions, and the wanderings of the poles, stand in one great causal connection with one another. That is shown by their common increase in certain periods of the earth’s history. Only in one case, that of the continental displacements, can we point out, in addition to internal, also external cosmical causes. On that account it is probable that we have to consider the latter as the primum movens, the ultimate cause of all these alterations. But then the relations seem to become complicated. I can, it is true, believe the polar wanderings to be direct consequences of the continental displacements, in spite of Schweydar’s objection that such a displacement only means an exchange of position of equal masses. For the continental block, on account of the higher position of its centres of gravity, has a greater axial distance, and therefore also greater moment of rotation than the sima, whose place it takes; so that in my opinion the axis of inertia of the earth must be influenced by the displacements of the continents. But we have seen that the polar wanderings can now in turn produce continental displacements of another sort. These also will react again on the position of the poles. Thus complicated alternating relations result, the total effect of which can to-day no longer be neglected.

1. Kreichgauer, Die Äquatorfrage in der Geologie. Steyl, 1902.
2. Taylor, “Bearing of the Tertiary Mountain Belt on the Origin of the Earth’s Plan,” Bull. Geol. Soc. Amer., 21, pp. 179–226, 1910.
3. Verh. d. 17. Allg. Konf. d. Internat. Erdmessung, 1, 1913, p. III.
4. W. Köppen, “Ursachen und Wirkungen der Kontinentalverschiebungen und Polwanderungen,” Peterm. Mitt., pp. 145–149 and 191–194, 1921; see especially page 149.—“Über Änderungen der geographischen Breiten und des Klimas in geologischer Zeit,” Geografiska Annaler, pp. 285–299, 1920.—“Zur Paläoklimatology,” Meteorologische Zeitschr., pp. 97–101, 1921 (with another figure).
5. That is towards the equator.
6. P. S. Epstein, “Über die Polflucht der Kontinente,” Die Naturwissenschaften, 9, Part 25, pp. 499–502, 1921.
7. W. D. Lambert, “Some Mechanical Curiosities connected with the Earth’s Field of Force,” Amer. J. Sci., vol. ii, pp. 129–158, 1921.
8. W. Schweydar, “Bemerkungen zu Wegeners Hypothese der Verschiebung der Kontinente,” Zeitschr. d. Ges. f. Erdk. zu Berlin, pp. 120–125, 1921.
9. Spaced out in the original.
10. W. Schweydar, “Bemerkungen zu Wegeners Hypothese der Verschiebung der Kontinente,” Zeitschr. d. Ges. f. Erdk. zu Berlin, pp. 120–125, 1921.