Worlds in the making: The evolution of the universe

by Svante Arrhenius

Fig. 9.—The Kilauea Crater on Hawaii

As regards the earth-crust, we know from observations in bore-holes made in different parts of the world that the temperature increases rather rapidly with the depth, on an average by about thirty degrees Cent. per kilometre (about 1.6° F. per 100 feet). It must be remarked, however, that the depth of our deepest bore-holes hardly exceeds 2 km. (Paruchowitz, in Silesia, 2003 m., or 6570 ft.; Schladebach, near Merseburg, Prussian Saxony, 1720 m.). If the temperature should go on increasing at the rate of 30 degrees Cent. for each further kilometre, the temperature at a depth of 40 kilometres should attain degrees at which all the common rocks would melt. But the melting-point certainly rises at the same time as the pressure. The importance of this circumstance was, however, much exaggerated when it was believed that for this reason the interior of the earth might possibly be solid. Tammann has shown by direct experiments that the temperature of fusion only rises up to a certain pressure, and that it begins to decrease again on a further increase of pressure. The depths indicated above are therefore not quite correct. If we assume, however, that other kinds of rock behave like diabase—the melting-point of which, according to the determinations of Barus, rises by 1° Cent. for each 40 atmospheres of pressure corresponding to a depth of 155 m.—we should conclude that the solid crust of the earth could not have a greater thickness than 50 or 60 km. (40 miles). At greater depths we should therefore penetrate into the fused mass. On account of its smaller density the silicic acid will be concentrated in the upper strata of the molten mass, while the basic portions of the magma, which are richer in iron oxide, will collect in the lower strata, owing to their greater density.

This magma we have to picture to ourselves as an extremely viscid liquid resembling asphalt. The experiments of Day and Allen show that rods, supported at their ends, of 30 × 2 × 1 mm. of different minerals, like the feldspars microcline and albite, could retain their shape for three hours without curving noticeably, although their temperature was about a hundred degrees above their melting-point, and although they appeared completely fused, or, more correctly, completely vitrified when taken out of the furnace. These molten silicates behave very differently from other liquids like water and mercury, with which we are more accustomed to deal.

The motion and diffusion in the magma, and especially in the very viscous and sluggish acid portions of the upper strata, will therefore be exceedingly small, and the magma will behave almost like a solid body, like the minerals of the experiments of Day and Allen. The magmas of volcanoes like Etna, Vesuvius, and Pantellaria may, therefore, have quite different compositions, as we should conclude from their lavas without our being forced to believe, with Stübel, that these three hearths of volcanoes are completely separated, though not far removed from one another. In the lava of Vesuvius a temperature of 1000 or 1100 degrees has been found at the lower extremity of the stream. From the occurrence in the lava of certain crystals like leucite and olivin, which we have reason to assume must have been formed before the lava left the crater, it has been concluded that the lava temperature cannot have been higher than 1400 degrees before it left the volcanic pipe.

It would, however, be erroneous to deduce from the temperature of the lava of Vesuvius that the hearth of the volcano must be situated at a depth of approximately 50 kilometres. Most likely its depth is much smaller, perhaps not even 10 kilometres. For there, as everywhere where volcanoes occur, the crust of the earth is strongly furrowed, and the magma will just at the spots where we find volcanoes come much nearer to the surface of the earth than elsewhere.

The importance of water for the formation of volcanoes probably lies in the fact that, in the neighborhood of cracks under the bottom of the sea, the water penetrates down to considerable depths. When the water reaches a stratum of a temperature of 365 degrees—the so-called critical temperature of water—it can no longer remain in the liquid state. That would not prevent, however, its penetrating still farther into the depths, in spite of its gaseous condition. As soon as the vapor comes in contact with magma, it will eagerly be absorbed by the magma. The reason is that water of a temperature of more than 300 degrees is a stronger acid than silicic acid; the latter is therefore expelled by it from its compounds, the silicates, which form the main constituents of the magma. The higher the temperature, the greater the power of the magma to absorb water. Owing to this absorption the magma swells and becomes at the same time more fluid. The magma is therefore pressed out by the action of a pressure which is analogous to the osmotic pressure by virtue of which water penetrates through a membrane into a solution of sugar or salt. This pressure may become equivalent to thousands of atmospheres, and this very pressure would raise the magma up the volcanic pipe even to a height of 6000 m. (20,000 feet) above the sea-level. As the magma is ascending in the volcanic pipe it is slowly cooled, and its capacity for binding water diminishes with falling temperature. The water will hence escape under violent ebullition, tearing drops and larger lumps of lava with it, which fall down again as ashes or pumice-stone. After the lava has flown out of the crater and is slowly cooling, it continues to give off water, breaking up under the formation of block lava (see [Fig. 5]). If, on the other hand, the lava in the crater of the volcano is comparatively at rest, as in Kilauea, the water will escape more slowly; owing to the long-continued contact of the surface layer of lava with the air, little water will remain in it, the water being, so to say, removed by aeration, and the lava streams will therefore, when congealing, form more smooth surfaces.

In some cases volcanoes have been proved (Stübel and Branco) not to be in connection with any fractures in the crust of the earth. That holds, for instance, for several volcanoes of the early Tertiary age in Swabia. We may imagine that the pressure produced by the swelling of the magma became so powerful as to be able to break through the earth-crust at thinner spots, even in the absence of previous fissures.

If, in our consideration, we follow the magma farther into the depths, we shall not find any reason for assuming that the temperature will not rise farther towards the interior of the earth. At depths of 300 or 400 km. (250 miles) the temperature must finally attain degrees such that no substance will be able to exist in any other state than the gaseous. Within this layer the interior of the earth must, therefore, be gaseous. From our knowledge of the behavior of gases at high temperatures and pressures, we may safely conclude that the gases in the central portions of the earth will behave almost like an extremely viscid magma. In certain respects they may probably be compared to solid bodies; their compressibility, in particular, will be very small.

We might think that we could not possibly learn anything concerning the condition of those strata. Earthquakes have, however, supplied us with a little information. Such gaseous masses must fill by far the greatest part of the earth, and they must have a very high specific gravity; for the average density of the earth is 5.52, and the outer strata, the ocean and the masses of the surface which are known to us, have smaller densities. The ordinary rocks possess a density ranging from 2.5 to 3. It must, therefore, be assumed that the materials of the innermost portions of the earth must be metallic, and Wiechert, in particular, has advocated this view. Iron will presumably form the chief constituent of this gas of the central earth. Spectrum analysis teaches us that iron is a very important constituent of the sun. We know, further, that the metallic portions of the meteorites consist essentially of iron; and finally terrestrial magnetism indicates that there must be large masses of iron in the interior of the earth. We have also reason to believe that the native iron occurring in nature—e.g., the well-known iron of Ovifak, in Greenland—is of volcanic origin. The materials in the gaseous interior of the earth will, owing to their high density, behave in chemical and physical respects like liquids. As substances like iron will, also at very high temperatures, have a far higher specific gravity than their oxides, and these again have a higher gravity than their silicates, we have to assume that the gases in the core of the earth will almost exclusively be metallic, that the outer portions of the core will contain essentially oxides, and those farther out again mostly silicates.

The fused magma will, on penetrating in the shape of batholithes into the upper layers, probably be divided into two portions, of which one, the lighter and gaseous, will contain water and substances soluble in it; while the other, heavier portion, will essentially consist of silicates with a lower percentage of water. The more fluid portion, richer in water, will be secreted in the higher layers, will penetrate into the surrounding sedimentary strata, especially into their fissures, and will fill them with large crystals, often of metallurgical value—e.g., of the ores of tin, copper, and other metals, while the water will slowly evaporate through the superposed strata. The more viscid and sluggish mass of silicates, on the other hand, will congeal, thanks to its great viscosity, to glass, or, when the cooling is very slow, to small crystals.