CHAPTER IX
THE CHILDHOOD OF THE EARTH
Let us now sum up the various stages in the early growth of the earth, as most geologists believe them to have occurred.
The first stage was that when the earth, shining like a star, existed as a fluid globe surrounded by heavy vapours of great thickness, which contained the future waters of the globe.
Then began the second era of the earth's existence, when it was a hot solid globe—solid at any rate at the surface, and with a temperature of about 2500° F. The globe's atmosphere still contained all the waters of the earth. It contained all the carbonic acid gas which now exists in the limestones and in coal and other minerals containing carbon. It contained also all the oxygen since shut up in the rocks and in vegetation and in animal substances. Such an atmosphere was probably at least two hundred times as great as the atmosphere which now surrounds the earth.
Then followed an epoch when great volcanic action set in. We have partially described it already. Just a thin solid crust was all that covered the molten interior of the globe. It was too thin always to contain the boiling liquid, and Titanic explosions, followed by enormous overflows of lava, continually broke up the crust. The pressure was relieved by these explosions, and gradually the earth would settle down again to its process of consolidation. Another explosion would follow; again a great flow of lava; and again the effects of the catastrophe would subside. After each explosion and outflow the earth's crust would grow a little thicker. All this time, and for long succeeding ages, the earth was attracting to itself, as a magnet attracts iron filings, all the small bodies which it encountered on its path round the Sun. These little rocks or masses of matter, some great and some small, would each add something to the size of the earth, and, by the shock of collision with it, something to the earth's heat—just as a bullet flattening itself against a target melts in the heat of collision. Just also as the bits of matter which we call "shooting-stars" are set on fire by friction as they rush into the earth's atmosphere. These meteorites, as they are called when they are comparatively small bodies, or "planetismals," as they are called when they are large, still exist. But the earth, in the millions on millions of years which it has been coursing round the Sun, has swept up all the large ones that are likely to fall into it, and there remain only the small ones which occasionally cross its path. These are called "Leonids" or "Lyrids" or "Perseids," and these meteor showers occur at nearly the same time every year when the earth runs through a swarm of them on its pathway round the Sun. But they are very small. Some of them are no bigger than a slate pencil. Few are as big as brickbats, and nearly all are burnt up by the air-friction before they reach the earth's surface. Larger ones still fall on the earth, however, and it is calculated that many hundreds reach us in fragments every year. But when the earth was young many thousands fell every day.
To this era, or immediately before it, belongs the birth of the Moon. It is a subject of interest to geologists, because it is admitted that the materials of which the Moon is constituted are similar to those of the earth; and it is believed that its history up to a certain point was very like that of the earth. It had its great volcanic era such as we have described; but its development closed shortly afterwards. We are considering, however, at this moment its origin. It was once part of the earth. All of us have read of those little animals, of which one form is the amœba and another form the white corpuscles of the blood. If we watch them under the microscope we may see one of them slowly lengthen out, then break in two, and each part go swimming away by itself, a perfect animal. It was Sir G. H. Darwin, F.R.S., who proved mathematically in 1879 that the origin of the Moon was such that we may properly compare it to the splitting up of the little animals just described. The date of this event cannot be fixed even approximately—beyond saying that as astronomical events go it must have been rather recent, though not less than fifty million years ago. The Moon is therefore one of the younger members of the Solar System. But no other catastrophe, either before or since, has occurred on the earth to compare with its prodigious birth. Five thousand million cubic miles of material left the earth's surface never again to return to it. Whether it all left at once or whether the action was prolonged we do not know, but we may try in vain to imagine the awful uproar and fearful volcanic phenomena exhibited when a planet was cleft in twain and a new moon was born into the Solar System.
Then, life still being absent from the earth, the Oceanic Era began. The waters condensed into an ocean over the earth, or else collected in some great oceanic depression. Lands presently emerged from it. It was a hot ocean, steaming no doubt, for its temperature was perhaps about 500° F. Some one may ask—Why, then, did it not steam away into clouds? The answer is that the atmosphere was still very heavy in that past era, probably still exerting a pressure as much as fifty times as great as to-day. The pressure of the atmosphere at the earth's surface to-day is usually about fifteen pounds to the square inch. In such circumstances water boils rapidly away at the temperature of 212° F. But if the water be taken up to the top of Mont Blanc, where the air pressure is less than that at the sea-level (or if, which amounts to the same thing, we reduce the pressure on the water's surface by placing it under the receiver of an air-pump and partially exhaust the air), it will boil at a lower temperature than this. If, on the other hand, we increase the pressure on the surface of the water by any means, such, for example, as by placing it in a chamber of compressed air, the water can be heated to a higher temperature without boiling away. In the bygone era of which we are speaking the pressure of the atmosphere on the water's surface was 700 lb. or 800 lb. to the square inch; and therefore it could be heated up to a high temperature without evaporating rapidly.
Another thing began to happen in those days. All bodies in space attract one another; the Sun attracts its planets; the planets attract the Sun and their satellites; and the satellites in their turn attract the planets. Ages before the earth had a moon these forces were at work. But the attractions of solid bodies for one another do not bring about any very perceptible alterations in their shapes; though if the bodies are spinning they effect slow changes in their speed of rotation. It is different when the bodies are liquid, or if they have liquid surfaces. Then the attractions of a sun or a moon on a planet begin to draw up the waters of the planet and produce tides. The attraction of the earth would produce tides on the Moon if an ocean existed there; and, it is suspected, do produce something resembling tides on the present surface of the Sun.[8] As soon, therefore, as oceans appeared on the earth the waters began to ebb and flow in tides. (Another consequence of this constant ebb and flow was that the friction of these movements began to diminish the speed of the earth's rotation—just as a string that was placed round the circumference of a spinning-top would, if constantly pulled backwards and forwards, gradually help to slow down the top.) Then oceanic waves and currents would begin to eat a way into the land that was on their borders, or which was emerging from their depths. Rivers would begin to arise, and they would carry on the work of erosion. Other causes tending to break up the rocks would be the gases in the air—the excessive quantities of carbonic acid and oxygen would be active chemical agents in this work. Before the close of this era the limestones and iron carbonates began to form; sediments arose in the lifeless oceans, and thus began the first formation of those sedimentary rocks and strata which have been dealt with in the earlier chapters.
[8] A paper read by Mr. E. W. Maunder before the Royal Astronomical Society in 1907 gave reasons for believing that the earth has perceptible effects on the movements of sun-spots.
After the lifeless era began the age when the lowest forms of life came into existence. The initial stage was perhaps the Era of the First Plants, Algæ, and later still aquatic fungi or bacteria. This began when the general temperature of the ocean may have been as high as 150° F. (some water plants can now live in waters up to and above 180° F.). Limestones began to form from the secretions of plants, and deposits of silica from silica secretions. Also where the conditions were favourable there were large sedimentary deposits and accumulations. In the second part of this æon the earth, still continuing to cool, and going down in temperature to 115°, gave opportunity to animal life. At the end of this era the general temperature of the earth and its oceans was as low as 90° F. The first animal life had begun to appear; its activity greatly increased under what were favourable conditions for it. This increase of animal life had its effect on the earth's crust. We have already spoken of the formation of limestones from the bodies of sea animals. This was going on in those ages millions of years ago before any of the higher forms of life had appeared on the earth, and though it was not going on so rapidly, still it must be remembered that at some point of the world's history the oceans were of greater extent than now, and consequently the deposits of lime and the accumulations of sediment were more widespread. The sedimentary rocks grew faster and faster, especially on the floors of the oceans.
It will be understood by those who have read the foregoing two chapters closely that the "igneous" or fire-born rocks must lie underneath the sedimentary ones. But that is only true in general terms, for a double reason. In the first place, owing to the inestimable forces which for millions of years were still continually effective below the earth's crust, the igneous rocks over and over again were able to burst their way through the slow-forming sediment of other rocks laid down above them. In the second place, the igneous rocks, owing to their composition and superior hardness, were much less worn by wind and weather than the less compact "sedimentary rocks," and these remained, showing themselves at the surface as coast-lines of oceans and in mountain ranges, after the sandstones and shales and limestones had disappeared.
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The Crater of an Extinct Volcano
This is the entrance to the long extinct volcano of Red Mountain, Arizona.
The memorials of volcanic action remain, we had almost said permanently, among the decay of other rocks, though, of course, even hard volcanic or igneous rocks will be worn down in time. In many cases volcanoes themselves are left, though they may have been for ages extinct. In some volcanic regions where no great central cones have existed the vast floods of lava that were poured forth extend to-day as vast black plains of naked rock, mottled with shifting sand-hills, or as undulating tablelands carved by running water into valleys and ravines, between which the successive sheets of lava are exposed in terraced hills. Beyond the limits over which the lava sheets have spread there are often great veins or parapets or sunken walls of lava to which are given the names of igneous or volcanic "dykes." Dykes vary from less than a foot to one hundred feet in breadth, and often run in nearly straight courses sometimes for many miles. They consist usually of very hard rock like basalt, "andesite," or "diabase."
They were fissures in the earth's surface, and, after the manner we have described, the molten rock welled forth through these fissures, and spread out sheet after sheet, till like a rising lake it has not only overflowed the lower grounds but even buried all the minor hills.
Lava eruptions of this kind have taken place in recent years in Iceland. On a small scale they can be seen to take place in the island of Hawaii, where the outflows of lava reproduce for us, like models in miniature, the great outbreaks of the past. On the largest possible scale similar effects may be seen on the great lava plains of the Moon, where the giant craters that we can see through telescopes are not the mouths of extinct volcanoes, but the banked-up edges and shores of lava outflows.
On a much smaller scale than this, but still on a gigantic scale, the same thing took place in Western North America, where there are vast tracts of land which are best to be explained by supposing that there were once great outbreaks and overflows. The area which has there been flooded with lava has been estimated to be larger than that of France and Great Britain put together, and the depth of the total mass erupted reaches in some places as much as 3700 feet. Some rivers have cut gorges in this plain of lava, laying bare its component rocks to a depth of 700 feet or more. Along the walls of these ravines we see that the lava is arranged in parallel beds or sheets often not more than ten or twenty feet thick, each of which, of course, represents a separate outpouring of molten rock.
These are comparatively modern lava plains, although, of course, the outpourings in North America occurred ages before historic time, or, indeed, before there are any traces of man's existence on the earth. Such lava outflows can only occasionally be examined—as in the instance just quoted when rivers have cut deep into them. Consequently we have to speculate on the connection between the dykes and fissures and the lava flood itself. But in various parts of the world lava plains of much older date have been so eaten into and worn by the action of the elements that not only the successive sheets of lava are exposed but the rock floor over which they poured. Exposed also are the abundant dykes which served as the channels by which the lava rose to the surface. In Western Europe important examples of this structure occur from the north of Iceland through the inner Hebrides and the Faroe Islands to Iceland. This volcanic belt presents a succession of lava sheets, which even yet, in spite of enormous waste, are in some places more than 3000 feet thick. These sheets are nearly flat and rise in terraces over one another into green grassy hills or into the dark fronts of lofty sea-washed precipices. Where sheets have been stripped off or worn down by wind and weather thousands of volcanic dykes are exposed. These dykes are, as it were, the roots of which the lava sheets were the branches; and even where the whole of the material that gushed up to the surface has been worn away the dykes remain as evidence of the vigour and energy of the volcanic forces.