The Origin of Continents and Oceans/Chapter 10

by Alfred Wegener

CHAPTER X

THE SIALSPHERE

In this chapter the sialsphere, which to-day occurs only in fragments—the continental blocks—on the earth’s surface, will be considered, and be dealt with—in the first place as a whole.

A map of the continental blocks is given in Fig. 27. Since the shelves belong to the blocks, the outlines deviate considerably at many points from the coast-lines. It is important, for our studies, to emancipate ourselves from the usual map of the earth and to obtain a certain intimacy with the outlines of the complete continental blocks. As a rule the 200 m. line gives the best idea of the limits of these platforms, yet some portions, which belong, without any doubt, to the continental blocks, reach a depth of 500 km.

Fig. 28 shows, further, a section of the earth in its true proportions on a great circle through South America and Africa. Mountains, continents and oceanic depressions form such trifling irregularities that they are to be found within the circular line which denotes the surface of the earth in the figure. On the other hand, the thickness of the continental blocks, amounting to about 100 km., is well shown. The core of the earth, probably composed chiefly of nickel and iron, has been termed the “nife” by E. Suess. For comparison the chief strata of the atmosphere are also introduced; that is, the atmosphere of nitrogen (and oxygen) up to 60 km. high, and above that the lighter gases.
Fig. 27.—Map of the continental blocks on Mercator’s projection. The zone of meteorological phenomena, which only reaches up to 11 km. (the troposphere), is too thin to be shown.

It has already been remarked that the material of these continental blocks is chiefly gneiss. But, as is well known, the continents at their surfaces consist frequently not of gneiss, but sediments, and we must on that account be clear as to what rôle these play in the building up of the continental blocks. About 10 km. can be considered as the greatest thickness of the sedimentary strata, a value which the American geologists have calculated for the Palæozoic deposits of the Appalachians; the other limit is nothing, since at many places the primitive rock is bare of any covering of sediments.
Fig. 28.—Section along the circumference through South America and Africa in true proportions. Clarke estimated the average thickness on the continental blocks as 2400 m. But since the total thickness of the blocks may be estimated as about 100 km., it is clear that this covering of sediments only means a superficial zone of weathering, the entire removal of which would, moreover, result in the blocks practically ascending to their former altitudes by the restoration of isostasy; so that the relief of the earth’s surface would thus be but little altered.

It is, indeed, not improbable that in the most ancient “pre-geological times” the film of sial covered the whole earth. It could then have been, not 100 km., but only about 30 km. thick, and have been covered with a “Panthalassa,” of an average thickness calculated by A. Penck as 2.64 km., which probably left exposed only small portions, or none at all, of the earth’s surface.

The correctness of this idea is in any case demonstrated on two grounds, namely, the evolution of life on the earth and the tectonic structure of the continental blocks.

“Certainly no one can seriously doubt but that the life of the fresh-water as well as of the dry land and of the air has come from the sea.”^[1] Before the Silurian we do not know of any air-breathing animals; the oldest remains of land plants are obtained from the Upper Silurian of Gothland. According to Gothan,^[2] those from the Lower Devonian are still, in the main, only moss-like plants without true foliage. “Traces of real, spreading leaves are rare in the Lower Devonian. Nearly all the plants were small, herb-like, and of slight rigidity.” On the other hand, the flora in the Upper Devonian is already similar to that of the Carboniferous, and evolves “through the appearance of large, well-developed, veined leaf-blades, by the accomplishment of the division of labour in the plant as a result of the development of the supporting and assimilating organs. … The character of the flora of the Lower Devonian, its lowly organization, its small size, etc., suggest the opinion, already expressed by Potonié, Lignier, Arber, among others, that the land flora was derived from the water. The advance observed in the Upper Devonian is to be understood as an adaptation to the new mode of life in the air.”

It appears, on the other hand, as if the sial crust could actually become sufficiently increased, by the smoothing out of all the folds in the continental blocks, for it to embrace the whole earth. The continental blocks, however, with their shelves nowadays only occupy a third of the earth’s surface, but we obtain for the Carboniferous a considerable increase in area (to about a half of the earth’s surface). But the further back we go into the earth’s history, the more wide-spread are the foldings. E. Kayser writes: “It is of great importance that the most ancient Archæan rocks are strongly disturbed and folded everywhere on the earth. It is only from the Algonkian onwards that we find, besides folded rocks, here and there non-folded, or but slightly folded deposits. If we pass to the post-Algonkian period, we see how the extent and number of the rigid unyielding masses increase more and more, and correspondingly the area of the foldable portions of the crust become increasingly limited. This applies especially to the Carboniferous and Permian thrusting. The folding forces gradually weaken more and more in the post-Palæozoic, in order, however, to awake again in the Upper Jurassic and the Cretaceous, and to attain a new maximum in the Lower Tertiary periods. But it is very significant that the area affected by this most recent great mountain thrusting is considerably less than that of the Carboniferous folding.”^[3]

According to this, our assumption that the sialsphere once surrounded the whole earth is not at all in opposition to previous ideas on the subject. This movable and plastic skin of the earth was then, on the one hand, torn apart, and on the other thrust together by forces the nature of which will be discussed later. Thus the origin and the enlargement of the deep-sea basins is only one aspect of this process, the other of which consists of folding. Biological facts also appear to verify the fact that the deep-sea basins were first formed during the course of the earth’s history. Walther^[4] writes: “The general facts of biology, the stratigraphical position of the present-day deep-sea fauna, as well as tectonic investigations, force us to the conviction that the deep-sea basins are, as biological regions, no primitive property of the earth, dating from the most ancient periods, and that their first establishment occurred at the same time as that in which in all parts of the present continents, tectonic folding movements set in and essentially transformed the relief of the earth’s surface.” The earliest rifts in the sialsphere, in which the simasphere was exposed for the first time, may well have been similar to those which to-day form the East African rift-valleys. They opened more and more as the folding of the sial made greater progress. It was a process which we can compare to some extent with the folding of a round paper lantern—on one side opening, on the other compression. It is extremely probable that the area of the Pacific Ocean, which is universally regarded as very old, was first deprived of its sial mantle in this manner. Small pieces broke off from the margin of the sial covering in the tearing-open, as well as during the widening of the rift, and perhaps also later in the course of the western drift of the entire continental masses, and remained fast in the sima, and now rest on the deep-sea floor as islands or submarine elevations. The rows of the Pacific islands show a remarkable parallelism. Arldt has measured 19 lines which all strike very nearly N. 62° west.^[5] It might possibly be assumed that this strike of the Pacific indicates the old direction of displacement through which the ocean basin opened, or it may be expanded. It is not inconceivable that the ancient folds in the gneiss massives of Brazil, Africa, India, and Australia are the equivalent of this opening of the Pacific; the later, northerly direction of strike in Africa would fit in very well with the direction of the rows of the Pacific islands (that is, it cuts this direction at an angle of 90°).

This compression of the sialsphere must naturally have a thickening as its consequence, and therefore an elevation, whilst at the same time the deep-sea basins increase in size. Hence the flooding of the continental blocks must—without taking into account their change of position—have in general gradually diminished in the course of the earth’s history. This law is generally acknowledged. It also appears very distinctly from the consideration of our three maps of reconstruction (pp. 6 and 7).

It is important to observe that the evolution of the sial crust must be one-sided, even if the forces vary, for tension cannot smooth out the folds of a continental block, but at the most can only tear them to pieces. The alternating action of the forces of compression and tension is thus not able to undo its operations, but produces unilateral progressive results: crumpling and dismemberment. The covering of sial becomes continually smaller in area, and thicker, in the course of the earth’s history, but it also becomes increasingly split up. These are complementary phenomena, and are effects of the same causes. The hypsometric curves, which on this basis may be assumed for the past, present and future, are shown in Fig. 29. The present mean-level of the crust coincides with the original surface of the still unbroken sialsphere.

We know very little about the inner structure of the blocks of sial.
Fig. 29.—Former and future hypsometric curves of the earth’s surface.
····· for the future ——for the present,—·— for the past, - - - - - original surface (coinciding with the mean crustal level).The fact that volcanoes exist at numerous places on the continental blocks, which emit magmas of sima composition, has been explained by Stübel on the widely accepted assumption that in the interior of the blocks, surrounded on all sides by solid or at least rather stiff sial, fluid, or relatively fluid, inclusions of sima (peripheral magma reservoirs), occur, which feed the volcanoes. On the other hand, no reason is to be seen why such slightly different materials as the sial and sima should completely separate, or have separated, in the body of the earth; much more probably there has been from the beginning a gradual transition from one to the other.
Fig. 30.—Section through a sial block. I imagine, therefore, the structure of the sial crust to be of such a form as is shown diagramatically in Fig. 30—uppermost a zone of continuous sial, with isolated inclusions of sima; below this a dovetailed zone in which each of the two portions are continuous; and beneath all a zone of continuous sima, in which lie a few isolated masses of sial. The original structure, at least, of the sial crust will have corresponded more or less to this scheme. By compression the crust can purify itself from the sima; most of the sima will then be pressed downwards, but some rises (in volcanoes), and spreads out as flat sheets. In great continental displacements, a kind of gliding plane, which is characterized by the fact that there the mineralogical composition changes especially rapidly, will be formed on the under-margin of the block of sial.

Such a structure of the continental blocks offers an explanation of many phenomena; it is, for example, in this way comprehensible that in the track of many drifting blocks (as Australia) the deep-sea floor is studded with numerous elevations, which may cause uncertainty as to whether they are to be considered as part of the deep-sea floor or of the continental block. The fact that the upper edge of the block retains its outline, whilst the deeper portions are drawn out, as in the neighbourhood of Iceland, also appears to be easily explicable in this way. Finally, it is perhaps possible to explain the frequently described “lability” of the geosynclinals, by the fact that, in the upper part of the sial crust, very numerous and large inclusions of sima are present. The surface of such a block would lie lower than its surroundings through the greater specific gravity of such masses, and the greater fluidity of these inclusions would also increase its mobility in the vertical direction, so that it easily sinks under the load of sediments. If, then, mountain-building compressive forces occur, such a part of a block, for the same reason, will be predestined for folding. The great eruptions of lava, by which mountain-building is always accompanied, appear to confirm the correctness of this idea. The inclusions will be forced out by the folding.

The surface of the earth also offers numerous other indications that the essence of vulcanicity is to be sought in a passive extrusion of the inclusions of sima from the sial crust. This is best shown by the island festoons. On account of the curvature, compression must occur on the concave inner side; and tension on the convex outer margin. Actually, the geological structure is strikingly uniform: the inner side always bears a chain of volcanoes; the outer shows no vulcanicity, but strong fissures and faults. This universally recurring arrangement of volcanoes is so remarkable that it appears to me to be of the greatest importance for the question as to the nature of vulcanicity. “A volcanic inner zone and two outer zones can be distinguished in the Antilles, of which the outermost is built of the most recent deposits and has a lower altitude (Suess). The contrast of a highly volcanic inner zone and an outer zone with restricted vulcanicity also occurs in the Moluccas (Brouwer) and in Oceania (Arldt). The analogy with the arrangement of volcanic zones on the inner side of thrust zones, as in the Carpathian or Variscan hinterland, is obvious.”^[6] The position of Vesuvius, Etna, and Stromboli fits in with this idea; and of the islands of the arc of the South Antilles between Tierra del Fuego and Graham Land, it is just the strongly curved central ridge of the South Sandwich Islands that is basaltic, and one of its volcanoes is still active. Brouwer^[7] describes a particularly interesting circumstance from the Sunda Islands: of the two most southern chains of islands, only the simply curved northern one bears volcanoes, not the southernmost (including Timor), which has already been curved in a reverse direction by collision with the Australian shelf. But at one place, near Wetter, the northern chain is already slightly bent because the southern one (north-eastern end of Timor) presses against it in that region; exactly at this spot on the northern chain there is vulcanicity, formerly of an active character, now obviously dying out because the curving is diminishing. Brouwer also draws attention to the fact that elevated coral reefs only occur where vulcanicity is absent or is dead, which likewise points to the fact that these areas suffer compression. The at first sight paradoxical result, that vulcanicity ceases where compression begins, finds a natural explanation within the compass of our ideas.

1. G. Steinmann, “Die kambrische Fauna im Rahmen der organischen Gesamtentwicklung,” Geol. Rundschau, 1, p. 69, 1910.
2. Gothan, “Neues von den ältesten Landpflanzen,” Die Naturwiss., 9, p. 553, 1921.
3. E. Kayser, Lehrb. d. allg. Geol., Ed. 5, p. 904. Stuttgart, 1918.
4. J. Walther, “Über Entstehung und Besiedelung der Tiefseebecken,” Naturwiss. Wochenschr., N.F., Bd. 3, Heft 46 (quoted from Eckardt).
5. Arldt, Handb. d. Paläogeographie, 1, pp. 231–232. Leipzig, 1917.
6. W. v. Lozinski, “Vulkanismus und Zusammenschub,” Geol. Rundschau, 9, pp. 65–98, 1918.
7. H. A. Brouwer, “On the Non-existence of Active Volcanoes between Pantar and Dammer (East Indian Archipelago), in Connection with the Tectonic Movements in this Region,” Kon. Akad. van Wetensch. te Amsterdam, Proceed., vol. xxi, Nos. 6 and 7, 1917.—“Über Gebirgsbildung und Vulcanismus in den Molukken,” Geol. Rundsch., 8, Heft 5–8, pp. 197–209, 1917.