| VIII. The Evolution of the Stars and the Origin of the Solar System. | Title page | X. The Changing Aspect of North America, or the Geosynclines, Borderlands, and Geanticlines |
[ p. 128 ]
In the previous chapter was described the evolution of the stars that leads on to the origin of the solar system and the earth. As we have seen, most hypotheses derive the sun and planets from an antecedent solar nebula, and other theories develop the earth out of a meteoritic swarm born of the sun. All agree, however, that the sun, like the stars, condensed out of nebulous matter. In regard to the origin of the earth, the theory that is most acceptable is the planetesimal hypothesis, which states that the sun while in its early gaseous condition approached another star and was partially disrupted through tidal action set up between the two bodies. Out of the ejected material, become cold, evolved the planets and their satellites. The early stages of the growing earth, as postulated by this theory, will now be set forth in more detail.
In the previous chapter we saw that the planetesimal hypothesis of Chamberlin and Moulton postulates the growth of the earth and moon through the very slow accretion of cold planetesimals, in the main the size of dust, upon two original nuclei. Since, however, we shall follow here Barrell’s conception of earth growth through planetoids that originally were clustered and averaged large in size, quickly accumulating about the nuclei and giving rise to a molten earth, it is now in order to develop this theory further. Nevertheless we must not forget Joly’s conclusion that the original molten cobdition of the earth may have been due “ to accumulated radioactive [ p. 128 ] condensation.” Of course, it can not be known whether during earth growth the center, or material of the original knot, tended toward a liquid or a solid state. The outer part, however, with a thickness of perhaps the outer quarter of the radius, comprising about one half of the volume of the sphere, seems to have passed into a truly molten condition.
After a long time, the rapid generation of heat by impact of the planetoids lessened, and the fluid sphere, seething with slow convection currents, began to cool. The heavy basic crystals were the first to form, and because of their high specific gravity they sank downward in the convective movement. The remaining higher magma was more siliceous, of lighter gravity, and in crystallization gave to the crust a greater proportion of feldspar and quartz. The original crust of the earth was in consequence a granite.
Spheres of the Inner Earth. — The vast central mass of the earth, the centrosphere or barysphere, is some 6200 miles in diameter. It appears certaia that pressure is the dominant factor within this earth nucleus. If the composition of the earth as a whole is similar to that of the meteorites, then the inference is that the material of the centrosphere is of metallic nickel-iron (NiFe, called nife by Suess). The blast furnace makes familiar the fact that slag is insoluble in iron and, being lighter, gathers in the upper part of the crucible, like cream upon milk; and slag is similar in composition to basaltic igneous rocks. The density of the deep interior suggests that it is layered like the material in the crucible of the blast furnace, and that the silicate rocks form an envelope some hundreds of miles thick, grading down into a great metallic core.
The silicate envelope, something like 900 miles thick radially, ultimately differentiated further, resulting in a rise of the more siliceous and lighter fraction into an outer layer, the very strong lithosphere or rock sphere (SiAl or silica and alumina, called sial by Suess, sal by others), perhaps 50 to 75 miles in thickness. This in turn crystallized into a primordial universal granitic crust, above a thicker basaltic shell below (SiMa, written sima by Suess). (For other details, see Pt. I, pages 263-264.)
Between the lithosphere and the centrosphere lies a hot, basic (mainly of basalts and gabbros), rigid, yet weak shell which Barrell has called the asthenosphere, meaning the sphere of weakness. It is marked by a capacity to yield readily to long-enduring strains of limited magnitude, — a zone of earth weakness composed of yielding matter.
[ p. 129 ]
The earth as a whole is very rigid, as dense and rigid as is armor steel, or glass.
“ The lithosphere was once thought to be the restricted province of geologLsts, but they now lay claim to the entire earth, from the center of the centrosphere to the limits of the atmosphere, and they threaten to invade the region of the astronomers on their way towards the outlying domain of cosmogony. Geology illustrates better than any other science, probably, the wide ramifications and the close interrelations of physical phenomena. There is scarcely a process, a product or a principle in the whole range of physical science, from physics and chemistry up to astronomy and astrophysics, which is not fully illustrated in its uniqueness or in its diversity by actual operations still in progress on the earth, or by actual records preserved in her crust. The earth is thus at once the grandest of laboratories and the grandest of museums available to man ” (Woodward).
¶ Azoic Era
The present diameter of the earth is 7918 miles, but at the close of the growing period or the beginning of the hypothetic Azoic era, it must have been 200 and possibly even 600 miles greater radially, for it is well known to geologists that throughout geologic time it has been losing volume due in part to the loss of heat into space, but probably in greater degree to internal molecular change. Even though the earth has thus been dissipating its inherited energy for at least many himdreds of millions of years, our mundane sphere is still far from having attained the internal stability that will, when achieved, probably result in a featureless earth with a universal ocean, and an atmosphere devoid of carbon dioxide, the basis of life. Then the earth will be in its old age.
Dana, following the Laplacian conception of a molten earth origin, called the first era in the earth’s history the Astral eon, when the earth was thought to be in the condition of a star. This gaseous state evolved into a fluid earth surrounded by a heavy vaporous atmosphere.
The Astral eon was followed, according to Dana, by the Azoic eon, when the hot earth became encrusted, but was still lifeless. This eon he subdivided into an earlier Lithic era, when the earth had cooled enough to have a soKd rocky crust, the original crust, composed wholly of crystalline rocks, dominantly granite; and a later Oceanic era, when the previous heavy atmosphere had condensed into a universal ocean.
Tidal action now set in, oceanic waves and currents and rivers commenced their work about the emerged and emerging lands, and sediments accumulated for the first time. The large excess of carbonic acid in the air and water became a source of rock destruction. [ p. 130 ] Before the close of the era, there arose the formation of limestones and iron carbonates by chemical methods, removing carbonic acid from the air and so commencing its purification.
The Primordial Atmosphere. — Geology now holds that the atmosphere and hydrosphere are essentially of volcanic origin, being the accumulated exhalations of active volcanoes and thermal springs. The gases come from deep within the earth, from heated and altering molten magmas. They are conceived of as dissolved in highly compressed magmas, and when the pressure is relieved, the gases heat the magmas and finally escape into the atmosphere.
Granting the initial fluid state of the earth, Barrell goes on to say, there must have been a hot gaseous atmosphere consisting chiefly of water-vapor, and in lesser amount, carbon dioxide and carbon monoxide, chlorine and hydrochloric acid, with some nitrogen but no free oxygen.
The primitive atmosphere penetrated by solution deeply into the universal molten rock. This penetration of water-vapor made it possible for the fluid rock to remain fluid at 800° C., whereas without water, dry silicate magmas melt only between 1300° and 1500° C. As there would be but little dissociation of water into its component gases, therefore the primordial atmosphere would be one of water gas, with an abundance of carbon dioxide and carbon monoxide.
The sunlight of Azoic time finally illumined the earth and was reflected from the mantle of cloud. The planet shone brilliantly by this reflected light, similar to that which Jupiter and Saturn still possess. Above the zone of cloud the carbon dioxide and other gases, with very minor amounts of water-vapor, extended with diminishing density as an upper transparent envelope.
Henderson in his interesting book, The Fitness of the Environment, says that the nature of the chemical combinations into which the elements hydrogen and carbon at first enter is perhaps open to question. But as the temperature falls in the cooling of a sun or planet, the affinities of carbon and hydrogen for oxygen increase, so that carbonic acid and water must normally result. For oxygen is almost certainly present in the sun; it is found in meteorites, and the vast store of it in the earth’s atmosphere and crust (roughly one half of their total mass) justifies the belief that it is everywhere one of the commonest of elements. Hence an atmosphere containing water and carbonic acid appears to be a normal envelope of a new crust upon a cooling body. Even were these substances not present at first in such an atmosphere, volcanoes must soon belch them forth in enormous quantities to relieve the pressure which inevitable chemical processes set up.
In the earth’s atmosphere, carbonic acid has been very largely converted into oxygen and vegetable matter, which later have been turned into enormous quantities of coal. It is, in fact, possible, in accordance with the suggestion of [ p. 131 ] Eoene, that all the oxygen of the atmosphere has been thus formed from carbon dioxide, and that therefore coal, peat, and other similar substances within the earth are chemically equivalent to the oxygen now free.
The most constant accessory constituent of air is carbon dioxide, which is of vital importance to life. In the present atmosphere there are about 3 volumes of this gas to 10,000 of air, and there is as much more in living things as there is in the atmosphere. On the other hand, there is in the oceans of to-day, according to F. W. Clarke, from eighteen to twentj-seven times more carbon dioxide than in the air (Johnston and Williamson say that at 15° C. there is about seventy times more) , while the still vaster volumes locked up in the sedimentary rocks and in the fuels and carbonaceous deposits of the earth are computed to be 30,000 times greater than the volume in the present atmosphere. These facts are brought forward at this time to show that the constituents of the atmosphere have always varied because of the constant loss of carbon dioxide and oxygen to the sedimentary rocks, but that at the same time there has always been a resupply of carbon dioxide through the periodically active volcanoes and the mineral springs, and of oxygen through the life activities of plants.
Gathering of the Ocean Waters. — When the crust began to cool and changed from a fluid to hard rock, Barrell says crystallization went forward in various areas, convection was slowed, and finally the molten rock froze. Then rain, ever descending from the shield of perpetual cloud, but never heretofore attaining the lithosphere, at last began to splash on the hot surface of the earth. A steaming earth’s surface was of short duration, perhaps only a few thousand years. Then the surface began to assemble an ocean of acid water, probably universal over the lithosphere. Carbon dioxide became the dominant gas in the rare atmosphere, and water-vapor was present in subordinate amounts. Solar heat began to play the principal part in warming the earth through the now thin and broken cloud canopy. For the first time sunlight attained the surface of the lithosphere.
Volcanic activity was still very great and great volumes of gases were liberated, adding juvenile materials to the old or vadose atmosphere and hydrosphere. Ever since, new quantities of juvenile water and carbon dioxide have been added to the surface of the earth by the volcanoes. At first, the volume of water added was great, but since early in the history of the earth it has lessened, we may say that the body of the earth has given forth its oceans. The greatest amount was added during the Azoic and Archeozoic eras, when from 50 to 75 per cent of the present volume is believed to have come into existence. The rest has been added during subsequent geologic time.
Origin of Continents and Oceanic Basins. — It is well known to geodesists and geologists that the continents are built of lighter [ p. 132 ] materials, essentially of granites, while the greater oceanic areas have heavier basaltic rocks beneath them, and that the difference in specific gravity amounts to about 3 per cent. We must now ask how these differences have come about.
Barrell says that originally the fluid earth had a surface as level as that of the ocean. The problem of the origin of the ocean basins and of the continental platforms resolves itself into one of the origin of the density differences in the lithosphere and the maintenance of the heated and weak condition of the asthenosphere. It is thought that the disintegration of the radium-bearing minerals has acted as a permanent generator of heat in the rocks that contain them (see Pt. I, p. 264). Near the surface, this heat is lost through conduction, but that generated within the asthenosphere can not so escape but must slowly transform some of the solid rock into liquid form. In this way, reservoirs of molten rock arise that may melt themselves through to the surface. It is this deepest seated and heaviest magma that, through rising into the lighter subcrust, weights it and thus drags down into basin form parts of the original granitic lithosphere. The forms and relations of the ocean basins suggest that in the earliest times, following the solidification of the earth, such dense molten matter from the depths of the earth broke into or through the outer crust , on a gigantic scale, eruption following eruption until the widespread floods of rock had weighted down broad areas and caused them to subside into ocean basins.
As seen in the lava plains of the moon, such an action, once started at a certain point, is conceived to have gone forward with widening radius, leading to the origin of the many rudely circular outlines characteristic of the ocean basins. The process left great angular segments of the original lighter crust as continental platforms standing in relief between the coalescent basins. The waters gathered naturally into the basins and the continents were left standing emerged as elevated areas.
Eegional crustal subsidence was especially characteristic of Azoic time, but the process did not cease then. In later chapters we shall see how the same process during the later Paleozoic and Mesozoic continued to break down great lands permanently into the ocean basins.
Source of the Salts of the Oceans. — The composition of ocean waters is discussed on p. 91 of Pt. 1 of this book. As all of this saline matter has been leached out of the rocks of the dry land since the earth has had rains, and as very little of it, comparatively, [ p. 133 ] has been taken out of the ocean by the accumulating rocks, it has been estunated that it represents the brea kin g down of a of average igneous rock equal to at least 6900 feet in thickness over all the continental platforms. Probably it is more correct to state that the continents have suffered erosion of igneous rocks amounting to between 1 and 2 miles of average depth. Of course all erosion throughout geologic time was far greater, perhaps from 50 to even 100 per cent higher, since it included the reworking of older materials, igneous and sedimentary. Furthermore, “ more than a half, perhaps four fifths, of the erosion of igneous rocks was accomplished before the beginning of the Paleozoic ” (Barrell).
¶ Eozoic Time
Eozoic time is the final hypothetic interval between the Azoic, when the earth was being prepared as the abode for life, and the Archeozoic, when life is known to have been on the earth.
Evolution of the Primordial Atmosphere. — With the separation of the lands from the oceans. Barrel! states, erosion began, carbon dioxide was abstracted from the atmosphere to make carbonates, and a further cause of atmospheric depletion was initiated. Thinner, rarer, and colder grew the gaseous envelope, until an oscillating balance was established between the supplies of new gases from the uprising molten rocks and the loss involved in the weathering of their solid forms. Nitrogen was at first relatively small in quantity and oxygen not present in more than a trace. An evolution in atmospheric composition had still to go forward through the following ages to transform it into a gaseous medium for the support of the higher land-living plants and animals.
Even early in the times following the gathering of the oceans and the emergence of the lands, the sun warmed the atmosphere and the earth. An environment suitable for the original and most primitive life had probably arisen in the oceanic waters of Eozoic time, since very low forms of marine plants, algse and bacteria, are known in Archeozoic rocks.
The chlorophyl-bearing plants, now as ever, are using the carbon of the carbon dioxide of the air and water and freeing the oxygen. In this way, through plant life, the amount of free oxygen in the atmosphere and hydrosphere has constantly been made. On the other hand, much free oxygen is consumed in the conversion of igneous rocks into other kinds through the weathering processes, and more is lost to the atmosphere through the oxidation of sulphides. The great amount of oxygen in the atmosphere is therefore [ p. 134 ] entirely due to the dissociation of the carbon dioxide by the green plants.
If all of the water of the oceans and the gaseous emanations of the earth, including the salts and chlorine of the waters and strata, could be put back into the present atmosphere, the pressure, according to Barrell, would be equal to 3756 pounds per square inch at the surface of the earth. It is now 14.7 pounds. This calculation gives some idea of the vast quantity of materials that the earth has belched out of its interior.
In the primordial atmosphere, there must have been but a trace of free oxygen, since the extensive lava flows at the time were consuming it. The ocean waters were then almost fresh and the chlorine was combined with calcium and iron. Oxygen in notable amounts seems not to have been present until some time in the Proterozoic, since it is at this tune that the first oxidized or red rocks appear (Animikian formation).
We see accordingly that the first plants must have been such that they could live without free oxygen, and they may have been like the living anaerobic bacteria. The photosynthesizing plants of the oceans, however, made free oxygen, and with its existence animal life was possible. Carbonaceous strata occur and beds of graphite are common in the Archeozoic.
Origin of Life. — Life has been propagating life with change (evolution) probably ever since the earth has had a hydrosphere and an atmosphere. How, where, and when it began is geologically unknown, though the theory of its origin has been discussed in Chapter II. We note the presence of life in the Archeozoic, both directly, and indirectly through its chemical action upon the elemental substances, as shown in the accumulation of carbonaceous deposits (black shales, graphite) and iron-ores. Early in the Archeozoic occur limy precipitates of algae, and later, bacteria, and in the late Proterozoic we meet for the first time with the actual remains of higher animals, radiolarians, sponges, trails and tubes of annelids.
¶ Collateral Reading
Joseph Bahrbll, The Origin of the Earth. Chapter I in “The Evolution of the Earth and its Inhabitants.” New Haven (Yale University Press), 1918.
T. C. Chamberlin, The Origin of the Earth. Chicago (University Press), 1916.
L. J. Henderson, The Fitness of the Environment. New York (Macmillan), 1913 .
| VIII. The Evolution of the Stars and the Origin of the Solar System. | Title page | X. The Changing Aspect of North America, or the Geosynclines, Borderlands, and Geanticlines |