The Origin of Continents and Oceans/Chapter 9

by Alfred Wegener

CHAPTER IX

THE FLOOR OF THE OCEAN

We have seen above that in the most rapid continental displacements there is only a question of an annual increase of about 10 m. in a distance of about 1000 km. If this amount of 10 m. be uniformly distributed over the whole distance, then each metre would show an increment of 0.01 mm. in the year. This is a very small quantity. Since the rock of the oceanic floor will naturally be intersected by all kinds of cracks, a very trifling expansion of these fine fissures would be sufficient to extend the total distance the required amount. In the deeper layers the sima would, without difficulty, be stretched about this amount. It is thus not necessary for the molten sima ever to come to the surface during this whole process. It is, however, on the other hand, probable that these processes take place irregularly. At one place the surface would receive no increment, in another the increase would, in compensation, be so much greater. And thus, at least here and there, highly heated portions of the sima would be uncovered.

But it need not be assumed that the outcropping of such highly heated material on the sea-floor would be attended with anything like catastrophic events. The “critical pressure” of water only amounts to 200 atmospheres, and will therefore be reached at a depth of 2000 m. Thus, however great the heat, no formation of steam can occur at such depths, but the water heated above its critical temperature only attempts to ascend by virtue of its diminution of gravity, whereby it is naturally soon mixed with the practically freezing bulk of water of the deep sea. The submarine effusions of lava are wont to take place in complete quietude in a similar manner. According to Bergeat, such submarine eruptions have taken place, for example, at a depth of 700 to 1000 m. in the years 1888, 1889 and 1892, in the neighbourhood of Vulcano,

Fig. 24.—Map of the oceanic sediments, after Krümmel.
1. Red clay. 2. Radiolarian ooze.

and have led to a rupture of the cable from Lipari to Milaggo, which was the reason that they were first noticed at all. It is a well-known characteristic of such submarine eruptions that they are practically noiseless in their effects.^[1]

The depths of the three great oceans are not nearly the same. Kossinna^[2] from Groll’s charts calculated the average depth of the Pacific as 4028 m., of the Indian as 3897 m., and the Atlantic as 3332 m. A faithful picture of these depth relationships is also given by the distribution of the oceanic sediments (Fig. 24), to which Krümmel personally drew my attention some time ago. The red deep-sea clay and the radiolarian ooze, both real “abyssal” (deep-sea) sediments, are confined essentially to the Pacific and the eastern Indian Oceans, whilst the Atlantic and the western Indian Oceans are covered with “epilophic” deposits, the greater lime content of which is causally connected with the lesser depth of the sea. That these differences in depth are not accidental, but are systematic, and that they are connected with the distinction between Atlantic and Pacific types of coast, is best shown by the Indian Ocean, of which the western half has an Atlantic and the eastern half a Pacific character. For here the eastern portion is also considerably deeper than the western. These facts have a special interest in connection with the displacement theory, for a glance on the map plainly shows that it is the most ancient oceanic floors which have the greatest depths, whilst those which have been uncovered for the first time in relatively recent time show the smallest depths. Thus on Fig. 24 we see in a surprising manner the trace of the displacements.

The reason for these differences of depth might lie naturally in the different specific gravities of the recent and ancient sea-floors. It is conceivable that the composition of the sima has altered during the course of the earth’s history through the crystallizing out of certain components, or from other causes. It recalls, for example, the mineralogical difference of old and recent eruptive rocks, and also the distinction of the present-day Atlantic and Pacific lavas. But, according to this, it might well be expected that the recent, and not the ancient, sea-floors would be the deeper. For that reason, in my opinion, the differences of depth are also to be explained by the temperature relationships. The old ocean-floors became more strongly cooled, and are therefore of greater density than the younger. If the specific gravity, say, of sima amount to 2.9, then it would change to 2.892 by an elevation of temperature of 100° C. on the basis of the cubical coefficient of expansion for granite of 0.000 026 9. Two ocean-floors with a difference of temperature of 100° C., which are in isostatic equilibrium with each other, must then show a difference in depth of 300 m., in which the warmer floor is at a higher level. It is certainly difficult to imagine, for example, that the floor of the Atlantic should have preserved its higher temperature throughout a period of time estimated at millions of years, even if the original difference of temperature may have been very much higher (1000° to 1500° C.). Still, we do not know from what source the internal heat of the earth has been derived. If it is produced by the disintegration of radium, or only partly maintained in this manner, the idea that freshly exposed deep-seated layers could show, even throughout geological ages, an increased temperature on account of the greater content of radium, is not to be entirely rejected off-hand.

If the sima is actually a viscous, fluid body, comparable with sealing-wax, it would be remarkable if its ability to flow only manifested itself in yielding in front of the drifting sial block, and if streams of independent character did not occur also. The maps give in places a direct appearance of more such local flows of sima by the distortion of chains of islands which were, apparently, formerly in straight lines. Two examples of this are shown in Fig. 25, namely, that of the Seychelles and that of the Fiji Islands. The crescent-shaped Seychelles shelf, which bears the individual islands formed of granite, does not fit into either Madagascar or India, the straight outlines of which denote rather an earlier immediate connection.
Fig. 25.—Upper: Madagascar and the Seychelles Bank. Lower: The Fiji Islands. (Depth contours 200 and 2000 m.; outline of ocean deeps dotted.) Therefore the explanation suggests itself that we have a molten mass of sial which has risen from the underside of the block, was then carried off by the flow of the sima and has travelled a large part of the way in the direction of India. This stream of sima, which Madagascar itself is also following, runs exactly in the track of India, and was perhaps produced through its displacement, or possibly, on the contrary, the current caused the displacement of India, as indicated by the breaking-away of Ceylon. Movements in liquids, including viscous ones, are only rarely of so simple a kind that cause and effect can be clearly separated. Our knowledge of these matters is still all too defective. It is, therefore, absurd to demand of the displacement theory that it should link up and explain every relative movement that is observed. We consider these matters only for the explanation of the flow phenomena in the sima, and these show, by the recurved ends of the Seychelles shelf, that the movement of the stream of sima diminishes on both sides of the median line from Madagascar to India. We may also assert that the stream runs most strongly in the freshly exposed sima, whilst the older deep-sea floors north-westerly and south-easterly therefrom move more slowly. The second figure shows, in the Fiji Islands group, a form which is suggestive of a two-armed spiral nebula, and indicates a spiral movement of flow. Its formation seems to me to be connected with the alteration of movement which Australia underwent when it ruptured its last connection with Antarctica and began its still recognizable movement towards the north-west and left the New Zealand festoon behind. Before this rolling together, the Fiji Islands probably formed a parallel chain near the Tonga ridge, and both together comprised an outer festoon of the Australia-New Guinea block, and, as in the case of all festoons of Eastern Asia, adhered on the outside to the old deep-sea floor, and therefore became detached on their inner margin from the continental block, the inner chain being swept together by the separation of the block. The New Hebrides and the Solomon Islands may be two further echeloned island festoons left stationary on the way.^[3] Of the Bismarck Archipelago, New Britain (Neu-Pommern) has, as already mentioned, remained adherent to New Guinea, and been dragged round, whilst on the other side of the great Australian block the spiral curvature of both of the most southerly chains of the Sunda Islands indicates a whirling flow in the sima, similar to that of the Fiji Islands.

No positive picture can yet be obtained, on the basis of the observations made up to the present day, as to the nature of the ocean deeps.^[4] They are, with a few exceptions of probably different formation, always opposite the outer (convex) side of the island festoons, where the latter impinge against the old oceanic floor, whilst on the inner side, where the newly exposed floor crops out, window-wise, a deep is never found. It appears as if only the old ocean-floor is in a position to form deeps, because its cooling and hardening extend down to a greater depth. Perhaps they may be considered as marginal rifts, one side of which is formed of the sial of the festoons, and the other of the sima of the deep-sea floor. The profile of this area, shown in Fig. 26, in reality very flat, need not lead one astray, for naturally it is strongly levelled down by gravity.

Fig. 26.—Section through the Yap Deep (after G. Schott and P. Perlewitz), exaggerated five times.
(The upper interrupted line shows the true proportions.)

The origin of the deep channel, bent at right angles, south and south-east of the island of New Britain (Neu-Pommern) obviously depends on the violent dragging of the island towards the north-west, as a consequence of its adherence to New Guinea; the island block, reaching down to a depth of 100 km., ploughs up the sima, which, flowing behind, has not yet quite filled the furrow. This is perhaps just the case in which we can give the most accurate account of the formation of an ocean deep.

Still another explanation seems to be possible for the Atacama Deep west of Chile. If we bear in mind that all layers beneath the level of the deep-sea floor are pressed down by the elevation of this mountain mass (compare the section on mountain-building which follows), then the neighbouring ocean-floor must be also dragged down with it. There is a still further cause for the sinking of the continental margin, namely, the melting of the downward directed mountain folds and the carrying off, by the westerly drift of the block, of the molten masses eastwards, where they are partly elevated, according to our assumptions, in the Abrolhos Bank. The continental margin must thereby sink and the adjacent sima be dragged down with it.

However, all these ideas on the nature of the oceanic deeps are still in need of thorough confirmation by further more accurate investigation, especially by gravity measurements. On this point there are to be found up to the present, to my knowledge, only Hecker’s observations on the Tonga deep, which gave a strong disturbance of gravity (∆ g = −0.25, in comparison with +0.13 to 0.22 on the Tonga plateau).^[5]

This would agree with our idea that here isostatic compensation has not yet been effected by the inflow of the sima. But it would be of great importance if, as a result of still further observations on other deeps, we could understand more accurately the nature of these interesting disturbances of gravity.

1. E. Kayser, Lehrb. d. Geologie, I, Allgem. Geologie, 5th Ed., p. 784. Stuttgart, 1918.
2. E. Kossinna, “Die Tiefen des Weltmeeres,” Veröff. d. Inst. f. Meereskunde a.d. Univ. Berlin, N.F.A. Geogr. naturw. Reihe, Part 9.
3. Hedley also arrives at the same result on biological grounds, namely, that New Guinea with New Caledonia, the New Hebrides and the Solomon Islands form a unit.
4. The term “ocean troughs” (deep sea graben) is less fortunate, since it implies the assertion that they are similar to the trough faults of the continental blocks.
5. O. Hecker, “Bestimmung der Schwerkraft auf dem Indischen und Groszen Ozean und an den Küsten,” Zentralbureau d. Internat. Erdmess., N.F., No. 16. Berlin, 1908.