CHAPTER XII

HOW THE EARTH MAY HAVE ORIGINATED

The problem of the origin of the earth is essentially astronomical rather than geological, because geological history is considered to have begun when common earth processes, such as erosion, deposition, and transportation of sediments, etc., were brought into play. It is quite certain, however, that the earth in its pregeologic state gradually merged into its geological condition. For this reason the geologist is interested in the more important doctrines or hypotheses which have been put forth to account for the origin of the earth. In fact, one of the few hypotheses which must be taken seriously is largely the work of a geologist. The most acceptable hypothesis not only best satisfies the facts regarding the earth's astronomical relationships, but also best harmonizes with our knowledge of the oldest known rocks and their history.

Since the problem of the origin of the earth is an essential part of the problem of the origin of the solar system, the following well-known facts should be clearly in the mind of the reader. Eight planets, including the earth, revolve in nearly circular paths around the central sun, whose diameter is 866,000 miles. The radius of the solar system is at least 2,800,000,000 miles, this being the distance of the outermost known planet (Neptune) from the Sun. Neptune requires 164 years for a trip around the sun, while the earth, which averages about 93,000,000 miles from the sun, makes its circuit once a year. The planets all revolve around the sun in the same direction, and in nearly the same plane. The sun and all eight planets rotate on their axes in the same direction, the earth's rotation being accomplished every twenty-four hours. Most of the planets have one or more smaller bodies called satellites revolving about them, such as Earth, with its one satellite (the moon), and Saturn, with its eight satellites, etc. It is well known that this solar system is only a very small part of the vast universe, as shown by the facts that no star is nearer the earth than several trillion miles, and that some stars are so far away that light traveling at the rate of 186,000 miles per second requires a thousand years to reach the earth!

Toward the end of the eighteenth century the famous nebular or ring hypothesis was set forth by the astronomer named Laplace. This assumes an original very hot incandescent mass of gas spheroidal in shape and greater in diameter than the present solar system. This mass rotated in the direction of rotation of our sun and its planets. Loss of heat by radiation caused the mass to shrink, and this in turn not only made it rotate faster, but also caused the centrifugal force (i.e., the force whose direction was from the center) in its equatorial portion to gradually become stronger. Finally a time came when the force of gravity (i.e., the force whose direction was toward the center) and the centrifugal force became equal and a ring was left (not thrown) off, while the rest of the mass of gas continued to shrink. After a time the material of the ring collected to form the outermost planet. The other planets were similarly formed from other rings which were left off as contraction of the great mass of gas went on. The sun represents the remainder of the great mass of rotating gas.

What is the bearing of this nebular hypothesis upon the early geological history of the earth? According to the hypothesis the earth must once have been much more highly heated and larger than now. It condensed to a liquid and then it cooled enough to permit the formation of a solid crust over a liquid interior. It then had a hot dense atmosphere containing all the water of the earth in the form of vapor, and this atmosphere steadily became thinner due to absorption by the earth. When the pressure and temperature conditions became favorable, much of the water vapor condensed to form the ocean and the atmosphere gradually changed to its present condition. According to this view the oldest rocks of the earth must have been igneous because they resulted from the solidification of the outer part of the molten globe.

Within recent years certain serious objections to the nebular hypothesis have been raised, and Chamberlin and Moulton have formulated the planetesimal or spiral hypothesis as an attempt at a more rational explanation of the origin of the solar system. Some of the objections to the older doctrine are that among the many thousands of known nebulæ in the universe very few only are of the Laplacian or ring type, while spiral forms are abundant. Spectroscopic study shows that the nebulæ are not gaseous, but made up of either liquid or solid particles, and that the leaving off of rings would necessitate the assumption of an intermittent process which could scarcely have operated under the conditions of the hypothesis.

Anything like a full understanding of the planetesimal hypothesis would be difficult to obtain, and, in the brief space at our disposal, we shall attempt to make clear only a few of the salient points. According to this hypothesis the solar system was, during a previous stage of its evolution, a great, flat, spiral nebula, made up of finely divided solid or possibly liquid particles called planetesimals, among which were scattered some larger "knots" or masses. Each tiny particle and larger mass or knot is considered to have traveled in its own particular orbit or path about a central very large mass--the future sun. It is even suggested that the spiral nebula originated by disruption of one star by a swift-moving passing star. Each disrupted particle and large mass at first started straight for the large passing star, but because of change of position of the latter the particles and larger masses were gradually pulled around so that their paths curved into spirals. Because of crossing of paths, the larger masses or knots gradually increased in size by accretion of the small particles or planetesimals. Meteors (so-called "shooting stars") which now strike the earth are thought to be disrupted materials still being gathered in, though very slowly at present. After the passing star got well out of range, the spiral paths of the disrupted masses gradually changed to nearly circular, due to a wrapping-up process around the central body (sun) which then controlled the movements of the both larger masses (future planets) and small masses (planetesimals).

Let us now inquire briefly into the bearing of this planetesimal hypothesis upon the early geological history of the earth. According to this doctrine the earth was never in the form of a highly heated gas, nor was it ever necessarily hotter than now. Instead of beginning as a much larger body which has gradually diminished in size, the earth steadily grew, up to a certain stage, by ingathering of planetesimals. Increase in size caused the force of gravity to increase and this caused not only steady contraction of the earth's matter, but also a development of greater internal heat. The earth has been getting smaller ever since the force of compression has predominated over the building-up process, because of the diminishing supply of planetesimals. Due to steadily increasing internal pressure and heat the various gases, including water vapor, have been driven out of the earth to form an atmosphere which has gradually become larger and denser. After sufficient accumulation of water vapor, condensation and rainfall took place; the waters of the earth began to gather to form the oceans; and the ordinary geologic processes of erosion and deposition of strata were initiated. According to this view stratified rocks could have been formed very early in the history of the earth, and in this connection it is interesting to note that the oldest known rocks are actually of sedimentary origin.