Popular Science Monthly/Volume 47/July 1895/The Bowels of the Earth
|THE BOWELS OF THE EARTH.|
WHITHER man can not go his imagination the more fondly travels. Thus a most striking difference between man and the apes lies in the vast and boundless range of man's curiosity. Curiosity indeed becomes the mother of Science, while the collection of curiosities grows into the scientific museum. It is natural, therefore, that for generations the mysterious and inaccessible north pole and the bowels of the earth have been favorite dwellings for men's fancies. Since the abodes of the dead are equally mysterious and inaccessible to the living, we are not surprised to find these regions combined, and the dead consigned either to infernal—that is, inferior—regions, or, as did the Scandinavian saga, to the frozen north. But it was reserved for the fertile genius of an American naval officer to combine with one fell swoop the solution of all these mysteries into one, by supposing that the world was hollow, and that there was no north pole, but, instead, a vast annular cavity leading into interior and Arcadian regions, auroral glimpses and flashes of whose electric lights sometimes stream beyond the portals. Unfortunately, his solution is erroneous, and it is our aim in this paper to see what light science really has from the dark regions of Proserpine, and to consider why the world can neither be hollow nor stuffed with sawdust. Our light is, of course, indirect, as the depth below the surface of the earth to which man has burrowed is very small. The deepest mines are little over four thousand feet deep; and although, when one sees the rapid strides that the science of mining is making and the unexampled speed with which in the past four or five years shafts have been sunk over four thousand feet deep to tap the rich deposits of native copper on the south shore of Lake Superior, one may soon hope to see mines over a mile deep, yet, if we say that mines will never go down over two miles below the surface, we shall probably not live long enough to see our prediction proved false. The deepest mines, therefore, far from reaching the bowels of the earth, can not pierce so far in proportion as does the mosquito into the human epidermis. And yet we are not wholly without information concerning the deeper regions of the earth.
In the first place, man has succeeded in the weighing of the earth as a whole. In accordance with the law of gravity, if two balls of lead attached to elastic steel rods are placed close to each other, they must attract each other with a force increasing with their masses, but decreasing with the distance which separates them. The steel rods will be very slightly bent toward each other in consequence. But the same steel rods extended horizontally will be far more strongly bent downward, owing to the attraction of this great ball which we call the earth. If, then, we compare the size, the distance apart, and the density of the two balls, and the effect they produce, with the size of the earth, the distance of its center, and the effect it produces, we may find the average density and weight of the earth. We find that the earth weighs much more than would a ball of granite of like size, but less than a ball of iron. Its density is about halfway between the two, and it is about twice as heavy as, on the average, are the rocks at the surface.
Not only do we know the average weight and density of the earth, but we can form some idea as to how that density varies. It must, of course, increase toward the center, as the surface rocks are lighter than the average; but we can be even more precise than that. If we compare two tops of like mass which have similar conditions of support and are spinning away so as to make an equal number of turns a minute, that one will wabble least whose mass is farthest from the axis about which it turns. Therefore a top is often made in the shape of a light upright axis upon which it may turn, and this axis is connected by light spokes to a wheel in the rim of which, as far as possible from the axis, the mass is mainly collected, for we thus have the extra stability. If we have two such tops of exactly the same shape and size and weight, but the one having a wooden wheel spinning on an iron axis, the other having the iron in the rim of the wheel and the axis all wood, the latter will wabble least. Now, the earth is spinning like a top, and the axis about which she spins connects the north and south poles, and points at present nearly to the north star. But this axis wabbles also, and has not always pointed to the place to which it now points in the starry firmament. The time has been (since Egyptian monuments were built) when the pole star was other than the present one to which the lip of the Dipper points, and quite possibly our remote descendants may look to yet another star as the pole. The wabbling of the earth's axis in the heavens thus indicated is due to the attraction of the sun and the moon on the mass of the earth, and we can obtain, from its observed amount and from the forces known to be producing it, some idea as to how the mass of the earth must be distributed. Still, we can not, even with this help, be absolutely sure as to the law of the density, but we may rule out the idea of a hollow earth, and accept, as agreeing well with all the facts, the suggestion of Laplace that the condensing effect of pressure decreases as the density produced becomes greater. This increase of density with pressure is, of course, in part to be accounted for by the pressure of the outer layers of the earth on those beneath, which increases until it is something enormous, and, of course, tends to squeeze together the interior and thus render it more dense.
There are, nevertheless, limits to this squeezing effect; and there is another thing that we know about the earth's interior—namely, that it is hot. Hence, as the effect of heat is to expand, the increase of heat would tend to counteract the condensing effect of the increase of pressure. That the earth is really hotter within, and that thus the literally infernal regions are actually hot whatever may be said of the metaphorical inferno, is shown by various lines of reasoning.
In the first place, the astronomers tell us (although they are not quite so sure now that the earth may not be a lump of coagulated meteorites) that this world has cooled from a fluid mass. If so, of course it must be hotter inside. Further, although we have pierced but so little a way into the earth, yet everywhere we meet an increasing temperature. The rate of increase varies very much, however. In the deep copper mines of Lake Superior, for example, at a depth of three thousand feet the temperature has risen from a surface temperature of 40º F. only up to about 70º F., which is still quite a comfortable working temperature. This gives an increase of only one degree Fahrenheit per hundred feet. Beneath the peninsula of Lower Michigan there are brines and sheets of mineral water lying in basin form, and very rich in salt, bromides, etc., and of great medical and commercial value. They have been reached by numerous wells which run down to about three thousand feet near the center of the basin, as at Alma and Bay City. The water comes up from the bottom of these wells hot (over 90º), showing a decidedly more rapid increase in temperature than in the copper mines. But the famous Comstock lode, where fabulous wealth lured the miners on, showed perhaps the most rapid increase in temperature that man has ever dared to face. It was, however, doubtless due to the action of hot waters rising from still greater depths—probably the same waters that deposited the silver ores, still at work. In the mines of this region the miners, naked as savages, reeking with perspiration, drinking pailful after pailful of ice water (twenty tons of ice, or, in another case, ninety-five pounds per man, were used each day), could labor but ten minutes at the drift (in imminent danger of being scalded by striking a stream of hot water) before being overcome by the heat and reeling to a cooler place. Fainting, delirium, even death have been the effect of the reaction on coming to the surface. Verily the Cuban proverb, that a Yankee would be found to go after a sack of coffee though it were at the gates of hell, was not far from the literal truth.
However the rate of increase of temperature may vary, all indications thus agree that less than ten miles below us a red heat is attained and within twenty a white heat. Think of it! Ten miles below us it is red hot. Ten miles above we have the pitiless cold, far below zero, of interplanetary space. To what a narrow zone of delicately balanced temperature is life confined!
From the deeper zones of higher temperatures we have samples furnished us by the volcanoes, opened along great cracks in the earth, whence red or white hot foaming lava rises. They confirm our idea of the downward increasing heat of the earth. These outpourings of molten matter from volcanoes give us some idea also of the composition of the earth. To the path of investigation thus opened we shall return in a moment. They have also given rise to the very prevalent notion that the earth's surface is but a solid crust over a fluid interior of the consistence of lava. Observers on the Hawaiian Islands have even thought they could hear the dashing of the lava waves beneath. But it is not hard to see that the phenomena of volcanoes are far more complex than the mere welling up of a fluid interior. The lava is often more heavy than the crust, and it often stands at different heights in neighboring vents. Moreover, contemporaneous, not far distant vents sometimes furnish quite different material. This could hardly be possible if all volcanoes had a common source. The really essential and important part of a volcanic eruption is the escape of gases, which are or soon become largely steam. This forms the clouds which overhang a volcano, and descends in time of violent eruptions in torrential rains, such as buried Pompeii in mud. Hence, some have supposed that a volcanic eruption was due to the explosive action of sea water reaching the heated interior. But it is perhaps more probable that the gases which escape are originally contained in the lava and burst forth from the interior of the earth on their own account wherever a crack gives them a chance. According to this notion, the working of volcanoes is not unlike that of a bottle of ginger ale. All that is needed is the formation of some sort of a crack, to answer to drawing the cork, and fizz, away she goes!
Possibly the thought that we live on top of such effervescent stuff may not be comfortable, for it suggests that the whole earth will some time explode if the volcanoes allowing the gas to escape cease to act as safety valves. There are, indeed, astronomical traces of such catastrophes. Stars have suddenly burst into unwonted radiance, only to fade again almost as quickly; and the belt of asteroids around the sun, occupying as they do a place which naturally would be filled by one large planet, have been supposed to represent some disaster in the process of planet-making. Whether the course of life on this world is ended by the world suddenly exploding, or by a slow refrigeration, or whether the world finally drops into the sun, or is knocked staggering through space by some collision, makes little difference to us, however, so long as the inevitable end that none can foresee must some time come.
Something like this giving off of gas from within the earth is a curious "spitting," as it is called, of molten silver, which when melted absorbs much oxygen gas and gives it off again in cooling. Now, the spectroscope has shown us the kinship in composition of matter throughout the universe, so that stars millions of miles away are composed of the very same elements which make up our own earth and our own bodies. Thus, if we grant the possibility of the earth's exploding, we may expect to find fragments of similar explosions, in composition like that of the earth, scattered through space. And, in fact, every once in a while as we gaze into the starry heavens we see a flash and exclaim, "A shooting star!" It is in reality a bit of matter that has come into collision with our atmosphere at such a tremendous velocity that when so struck even the air resists almost like granite. Indeed, sometimes the shock of collisions dissolves these shooting stars into vapor or dust, but at other times they explode, and the fragments reach the earth, and are picked up. Such fragments are commonly known as meteorites, and we examine them with extreme interest to see if they can throw any light on the average composition of the earth. I think we find that they do, for they are closely allied in composition to some of the series of rocks in the earth's crust that have arisen from beneath and are associated with volcanic activity—the igneous rocks as they are called. In general the meteorites are much heavier than the average surface rocks, and their average weight is much nearer that of the whole earth. The heaviest meteorites are composed mainly of iron—native iron—not quite pure, but associated with some nickel and sulphur and also diamond. This last-named interesting component of meteorites was for years overlooked, but Foote's discovery of some sizable lumps of black diamond in the Cañon Diablo meteorite led Mr. Huntington to investigate further in the very extensive collection belonging to Harvard University, and he found, on dissolving sample chips as thoroughly as possible, that a powder remained whose resistance to corrosives and invincible hardness are signs manual of the sovereign of stones. We find, too, in these iron meteorites gases absorbed, such as those at whose door we have laid the responsibility for the production of volcanic eruptions.
Since the weight of our earth and the evidence of sample fragments of planetary matter point to its being mainly iron—if we may not only say that this is an iron age but also an iron world—is it any wonder that iron is so widely distributed, or that it is the universal pigment, even dyeing the blood of our veins? But there is further evidence on these lines at which we have as yet but hinted. We said that meteorites were connected in composition with terrestrial rocks. It is in fact true that native iron similar in structure to that of meteorites is found in some basaltic dikes in Greenland as large masses, and in microscopic quantities elsewhere, and it seems almost certain that it has been torn from the depths of the earth. The rock in which the diamonds occur in the Kimberly mine (and everywhere else where they occur originally, and not in sand and gravel, they are in similar connection) is very rich in iron, is composed of minerals common in meteorites, but is devoid of quartz and feldspar, the commonest minerals of the upper crust. Practically, all the minerals of the meteorites occur native in the earth's crust, but only sparingly, except in connection with rocks that have risen through fissures from beneath. They do not occur in connection with all these rocks, but only in connection with rocks like the Kimberly rock, which are darker and heavier and less siliceous. There are a number of reasons for supposing that these darker and heavier igneous rocks, containing more iron and less silica, have a deeper source than those composed mainly of quartz and feldspar, but we will mention only one. Our earth is wrapped with an atmosphere of oxygen, an element exceedingly ready to enter into combination—so much so that in all our ordinary surface rocks all the other elements are combined with oxygen as much as can be. Now iron, as is well known, has the power of combining with oxygen either in the proportion of three of oxygen to two of iron or in even proportions. The former compounds which have more oxygen are those found in ordinary rust, and are much more readily formed, being the so-called ferric compounds. They are often yellow or red in color. The other compounds containing less oxygen—the so-called ferrous compounds—very readily absorb more oxygen. In fact, their readiness to do so under the influence of light is at the basis of many photographic processes, notably those of making blue prints and tintypes. Now, of course, in the meteorites containing native iron, not all of the iron is oxidized, and the iron is contained in its less oxidized condition in the other associated minerals, such as the yellowish-green mineral known as chrysolite, sometimes used for a gem. In
Fig. 1.—Much-exaggerated Sections of the Earth through the Equator—illustrating (a) the tidal effect on a rigid earth with a fluid envelope, (b) on a yielding earth.
general, also (there are exceptions), the rocks which contain less silica and more iron have their iron less oxidized. By analogy, as we go from the oxidizing effect of the atmosphere toward the center of the earth, we may expect finally to encounter rocks not oxidized even in the less degree. To sum the argument up in a nutshell, we find among the rocks furnished us by volcanic and igneous agencies from various depths in the earth a series from quartzose and feldspathic rocks to those with less quartz and feldspar, more iron, less oxygen, and greater weight, in which the presence of a trace of nickel and the occasional occurrence of diamonds and native iron betoken a kinship to the meteorites. The latter in every way continue this series toward a goal which is nearly pure iron, and the weight of the earth as a whole is consistent with this idea that it is largely iron, almost purely so at the center, but gradually, perhaps not perfectly uniformly, growing more quartzose toward the crust.
One question still remains to us: In what condition is the interior of the earth? Is it a molten fluid or what? If we look at the downward increase in temperature alone it would seem as if within thirty miles a heat would be reached where even pure iron, which is much less fusible than cast iron containing carbon, would be quite fluid. If the earth were freely fluid, however, it would yield to the attraction of the sun and moon as the oceans now do. Some effects of this pull may indeed be seen in the distribution of earthquakes, which are more frequent at full moon than at other times, as though the strain produced by the attraction of the moon helped to produce these shocks by the cracking and giving way of the earth. But if the earth as a whole were anything like as fluid as water, it would yield as a whole and assume the same shape, bulging about as much toward the moon as the watery envelope, so that the water would not be perceptibly deeper toward the moon than elsewhere; whereas, if it were perfectly rigid, it would retain its shape unaltered, and the water about it alone would be drawn by the moon. It would be pulled up into tidal waves. These two different cases and effects are illustrated in Fig. 1. As a matter of fact, we find that the heights of the tides are nearly as great as though the earth were absolutely rigid. The earth, therefore, must be exceedingly rigid; we may say solid, so far as these tidal strains are concerned. These are, however,
Fig. 2.—P, point of origin of earthquake shock; E (epicentrum), point on the surface directly over it; A, B, limits of the area of vertical, simultaneous, and earliest shock.
so varying in their application, shifting their direction through all the points of the compass every twenty-four hours, that if the interior of the earth is very viscous—and we know that hot iron is in just this viscous condition when at welding heat—the yielding to forces so rapidly changing direction might be no greater than that which is observed.
Another argument for the solidity of the earth is based on the fact that the mountain ranges and continents are lifted so high above the normal level. To be sure, their weight is not so very great in comparison with that of the earth, nor the distance they project above the general level. But then the breadth of the base in comparison with the height is very great, and if we compute the thrust which so broad an arch as that of the Rocky Mountain plateau, for example, must exert on its abutments, we find that the earth, if not entirely solid, must have a solid crust some hundreds of miles thick; or else possibly that the density of the mountains and the part of the crust beneath them is much lighter than the average, so that they can rise by floating on a liquid interior to their present height. There are, in fact, some indications that these plateaus, and the continents generally, really have lighter matter beneath them than the sea basins do, so that the above argument against the fluidity of the earth has not much weight. Another more important argument for the solidity of the earth may be derived from earthquakes. Sometimes these convulsions of Nature are caused merely by the jar due to a giving way or cracking in the earth's crust. Such cracks we often find in studying the rocks, where on one side of the crack the beds do not match those on the other side, but a particular bed when it comes to the crack line is not found on the other side where we should expect it to come, but some distance to the right or left. Such cracks are technically known as faults, and the displacement produced is sometimes several thousand feet. Such faults or cracks have occurred in the red sandstone area of the Connecticut River, and are well marked. Similar faults have tilted the western plateaus in great blocks. Indeed, even the very line of displacement and sudden elevation have been sometimes noticed after earthquakes, notably in New Zealand and very recently in Japan, after the earthquake described by Koto, that cost so many lives (Fig. 3).
Now, these jars known as earthquakes spread with wavelike motion and decreasing intensity from their source, like the ripples from a pebble thrown into a pond. By careful study of the time at which the jar arrives at different points and of the direction of disturbance we can form some idea of its source, just as one can tell from the ripples at what point the stone was thrown in, even though too late to see the splash. In Japan, a country much afflicted with earthquakes—although, as a friend writes me, the shocks are commonly so slight that the only attention one pays to them is to stop shaving—their study has been so far advanced that one can actually tell what path a particle describes under the influence of a given quake, and what position it occupied at any moment, and a model of such a path was exhibited at the Chicago Exposition.Let us suppose, for example, that the shock started from the
Fig. 3.—View from Koto's report on the Cause of the Great Earthquake in Central japan, in the Journal of the College of Science of the Imperial University. Vol. V. The view shows the fault to which that earthquake is attributed. The displacement caused by it is very noticeable on the road.
From such considerations the depths of the sources of various earthquakes have been computed. For example, Schmidt computed that the Charleston earthquake started from a depth of no less than one hundred kilometres, say sixty miles. Unfortunately, there has been much difficulty in getting reliable facts enough for these estimates, and Dutton, who investigated the same earthquake for the United States, made it but twelve or eighteen miles deep. But whether it be one depth or the other does not affect what we wish to show—namely, that the earth is capable of cracking to a depth such that if the earth's heat increases at anything like the ratio that it does near the surface, it must there be more than white hot, and would be molten and freely fluid, except for the counteracting effect of pressure. If, then, the earth is solid at this depth, pressure has more effect than heat and keeps the earth solid. Barus has shown by experiment that for the basic rocks pressure tends to solidify. Moreover, the most basic rocks we know, those apparently from the greatest depths, contain fragments of chrysolite, etc., whose rounded and corroded outlines and often blackened edges show plainly that they have been in process of dissolving in the lava. They therefore may represent fragments of deep-seated rocks which have liquefied when pressure has been relieved by cracks and the eruption of lava following thereon.
The fact that we find the rocks in some places crumpled in folds and recrystallized has been by some taken to indicate that such rocks had been buried so deep beneath the surface as to be remelted. But recent investigations, by cutting thin sections of such rocks and studying them under the microscope, have shown that a rock may be thoroughly changed into different minerals, differently interwoven, and may be folded and contorted in most complex fashion, without for a moment being molten or ceasing to be crystalline. Recent experiments have also shown that we may account for the folds and crumplings without supposing a thin, flexible crust lying over a fluid interior; while, on the other hand, there are very numerous faults or cracks, where one part has slidden down on the other, that can hardly be accounted for except by supposing our earth solid (or very thick in crust), cooling and contracting unequally.
As to other arguments for the fluidity of the earth, we have seen that volcanic phenomena carefully studied go against the idea of one central reservoir for the lavas. It is, of course, natural to think of a cooling globe as having a solid crust and molten interior, but it is quite possible that solidification started at the center, just as even now in the nebulous stars the condensation from gaseous to liquid state proceeds from central points or nuclei.
We may say, then, in summing up, that there are no valid arguments against the conclusion to which all the facts point, that the earth is at heart an intensely hot but practically solid mass of iron.