CHAPTER VII

THE FIRE-HARDENED ROCKS

So far we have been considering the deposits laid down, for the most part, in a leisurely and orderly manner, by the action of air and water; by floods, rivers, lakes, the sea, or by the slow movements of ice. If these, however, had been the only agents by which the earth's strata were accumulated, then it is clear that for the most part these deposits and these strata would lie evenly, one on top of the other, like the lines of print on this page. But as a matter of observation the earth's strata do not lie like that. If we were to tear this page out and crumple it up in a ball, first having torn it in half and shredded a few irregular pieces out of it, we should get a truer picture of the way in which many of the earth's strata are contorted, crumpled, and displaced. They have not been so distorted by the action of the sea, violent as are some of the sea's assaults on the land; nor would the heat of the sun at its greatest ever produce such effects. They must have taken place from some causes which arise in the earth itself. These causes can be summed up in one word—fire. Some of the strata of which we have spoken, and which are called sedimentary strata, although they were composed of soft materials to begin with, have become very hard since, in some cases owing to the enormous pressure of the accumulated deposits above them, in other cases because of chemical action. In a few cases they have become hardened not so much by losing their water, as by direct heat. But the hardest of them is not so hard as another class of rocks with which we are all acquainted—rocks like granite, or quartz, or basalt. And it will be evident to any one who thinks about the subject for a moment that no amount of pressure would make a rock as hard as a diamond. Now how have these rocks been made? The answer is that they have been made in some interior furnace of heat deep down in the earth. Sometimes they have boiled up, and we can trace them bursting their way through the sedimentary strata above them. We do not know very much about the furnaces or cauldrons whence they have come; in fact, we know very little about the depths of the earth. The deepest mine-shaft known is near Lake Superior, and is only 5000 feet in depth. In Silesia a bore-hole has been made by the Austrian Government of a mile and a quarter in depth. It would be by no means an easy task to sink a great boring. The Hon. Charles Parsons has described some of the difficulties.

The shaft would have to be sunk in a neighbourhood where it would not be likely to encounter water on its way down, because otherwise there would be the necessity of pumping operations. In order to be of value for purposes of observation, the shaft would be of the size usual in ordinary mines and coal-pits. It would be sunk in stages each of about half a mile in depth, and at each stage there would be placed the hauling and other machinery for dealing with the next stage below. This machinery, in order to economise space and limit the heat of the workings, would be electrical. Even so there would have to be special arrangements for cooling; and the depth of each stage in the boring would be restricted to half a mile in order to avoid great cost in the hauling arrangements, great weight of rope, and the great cost of keeping the machinery and workings cool. At each second or third mile down there would be air-locks to prevent the air-pressure from becoming excessive, owing to the weight of the superincumbent air. For when we got between two and three miles down below the surface of the earth the atmospheric pressure there would be double what it is at the earth's surface, or, therefore, about thirty pounds to the square inch. It would not be easy to work under greater air-pressure than that, firstly because of the strain on the workmen, and secondly because of the rise of temperature which this increased air-pressure would cause. Therefore special chambers would have to be constructed to relieve the pressure, as well as special pumps to provide ventilation, and other machinery to carry the superfluous heat to the surface. This last-named machinery would be of the nature of brine-filled pipes, in which a freezing mixture would always be kept circulating. (The arrangements suggested by Mr. Parsons for keeping the underground workings cool are rather too complicated for description here; but no doubt the means he suggests would be effective, and it would be possible, though with great difficulty, to keep the workings cool.)

When the borings extended to a depth of some miles it would be necessary to freeze the bottom of the shaft. This is a thing which is sometimes now done when a shaft is being sunk through quicksands that may be encountered on its way down. Round the circle of the main shaft a number of small bore-holes are driven, and into them is poured very cold brine, which freezes the material through which the shaft is to be driven. In the case of the great boring we are considering this would have to be done not only at the bottom of the shaft but also for some time on the newly pierced shaft sides, until the surrounding rock has been cooled for some distance from the face.

What would such a shaft cost? How long would it take to build? What would the temperature be that it encountered on the way down? The following is the estimate offered by Mr. Parsons:—

Cost
£		Time in
Years		Temperature
of Rock
For2miles depth from the surface500,00010122° F.
"4"""1,100,00025152°
"6"""1,800,00040182°
"8"""2,700,00055212°
"10"""3,700,00070242°
"12"""5,000,00085272°

But this estimate does not include the cost of cooling the shaft or of providing it with air-locks. Mr. Parsons in delineating the scheme remarked on the vast amount of information with which such a boring would furnish engineers, miners, and geologists; but the point that we wish to make is that even with this enormous expenditure of time, industry, and money we should be as far as ever from knowing anything about the core of the earth. We should have only gone about a third of the way through what geologists call the earth's crust.

Here, again, we are in a condition of difficulty. How thick is the earth's crust? and what is there beneath it? Well, as we are still such a long way from exploring it we can only give a rather doubtful answer; and we must therefore try to show not only what is thought about the earth's interior but why we think it. From Mr. Charles Parsons' table it will be seen that he calculates that as the boring went deeper it would find a higher and higher temperature among the rocks. At two miles down it would be hotter than the hottest summer's day at the earth's surface; at eight miles down water would boil by itself; at twelve miles down, unless the cooling arrangements were extremely good, the workmen would die like flies. How does Mr. Parsons know that there would be these temperatures, seeing that the deepest boring hitherto made is only a mile? He bases his calculations on what we know already of the ascending temperature at deepening levels.

For ten years Professor Agassiz took observations concerning a very deep mine in the United States called the Calumet and Hecla Mine. He and Professor Chamberlin, after examining all the observations very carefully, came to the conclusion that in going down from the earth's surface the temperature rose at a rate of about 1° of heat (Fahrenheit) for every 125 feet.

At the North Garden Gully Mine, Bendigo, Australia, and at the New Chum Mine a temperature of 99° F. was reached at 3000 feet, and 107° at 3645 feet. The rate of increase of temperature was reckoned to be 1° of heat (Fahrenheit) for every 80 feet.

This rate of 1° for 80 feet was also found at a South German mine, Maldon, as well as at a Ballarat mine, and at a mine near Port Jackson.

In a French mine more than 3000 feet deep, at the collieries of Ronchamp, the rate of increase was as high as 1° in 50 feet.

In the North Staffordshire mines Mr. Atkinson, H.M. Inspector of Mines, found the increase to be on the average 1° in 65 feet; whereas in the South Staffordshire Hamstead Colliery Mr. F. G. Meachem found that the increase was 1° F. for every 110 feet. The same rate was obtained at the Baggeridge Wood Colliery, South Staffordshire.

In South Wales, in the neighbourhood of Rhondda and Aberdare, the rate is 1° for 95 feet; at Dowlais, in the Merthyr coalfield, it was 1° in 93 feet; at the Niddrie Collieries, near Edinburgh, the increase is at the rate of 1° in 99 feet.

It will thus be seen that all over the world there is an increase of temperature at a rate which, on the average, is about 1° for every 100 feet. There are 5280 feet in a mile; therefore, if this rate of increasing temperature were maintained, at a depth of 100 miles the temperature would be perhaps 5000° F.; a temperature at which steel would melt and boil away into vapour. At a depth of 200 miles the heat would be greater than that of the surface of the sun.[2]

[2] According to the calculations made by the late Mr. W. E. Wilson, F.R.S., in Ireland, 5773° Centigrade above the lowest temperature which is possible in space, or about 10,500° F.

Now at temperatures like that everything we know on the surface of the earth would melt. Something else would happen to it besides that. Those of our readers who have ever seen experiments at the Royal Institution in London by Sir James Dewar or Sir William Crookes will know that if metals are made hot enough they will not only melt but will boil away into vapour as water boils into steam. And perhaps we need tell no one that air, if it be subjected to a low enough temperature, can be made a solid like ice. In fact, everything in nature, whether we generally know it as a solid (like iron), or a liquid (like water), or a gas (like air), can be made to assume either of the two other forms. Thus the solid iron can be turned into a liquid or a gas, and the liquid water can be turned into a gas by boiling, or into ice by freezing. The gaseous air can be turned into a liquid by lowering its temperature to some 300° F. or more below the point at which water turns into ice; while if we lower the temperature to about 390° F. below freezing, it will turn into a solid. At a temperature of about 490° F. below freezing everything in nature, whether gaseous or liquid, would become a solid, and that temperature, which is the lowest that can possibly exist, is called Absolute Zero. But just as every gas becomes a solid at that temperature, so there are temperatures at which every solid becomes a gas. Gold, for instance, begins to be a liquid at about 1900° F., and if we heat it to 2000° it will become a gas.

Therefore it will be seen that if we were to suppose that the earth grew steadily hotter all the way down to its centre, we should comparatively soon come to a point when everything would be trying to turn into a gas. But there is one other thing to be thought of. Imagine what the pressure of the weight of the rocks themselves must be. At a depth of a mile pressure from above arising from the weight of the overlying rock is about 6000 lb. to the square inch. At three miles the weight has increased to 18,000 lb., at four miles to about 24,000 lb., and at five miles to about 30,000 lb. to the square inch. Now the average strength required to crush rocks has been shown to be about 25,000 lb. to the square inch for granite, for limestones about 16,000 lb. to the square inch, and for the sandstones about 6000 lb. to the square inch. At a depth of five miles, therefore, the weight above must be equal if not greater than the resisting power of the rock. What will happen lower than that? An experiment shown some years ago by Sir William Roberts Austen at the Royal Institution gives us some idea of what might happen. He subjected iron to very great hydraulic pressure, and he arranged the experiment in such a way that the spectators could see an image of what was happening projected by a beam of light on to a kind of magic-lantern screen. The iron began to move like slowly melting pitch, or very thick gum. In fact, at depths of about six, seven, or eight miles, it is supposed by many geologists that if the lower rocks had room to move they would have a tendency to flow.[3]

[3] Geology: Earth History, p. 127. Chamberlin and Salisbury.

Suppose, however, they cannot flow, that there is no room for them to flow, and that the pressure is not merely thirteen or fifteen tons to the square inch, as it would be at depths between five and six miles, but a hundred times that amount, as it might be between five and six hundred miles down. What would happen then? We can only imagine what does happen by stating what does not happen. It used to be supposed as late as half a century ago that the earth consisted of a crust of hard rocks perhaps thirty to fifty miles in thickness, and that below this crust the whole earth was a mass of red-hot or white-hot molten stuff with flaming gases mixed with it. If that were the case it would explain a good deal of what we see around us. It would explain the volcanoes, for instance, which belch out fire and lava and ashes and molten rock, and sometimes great fragments of rock. Perhaps some of our readers may remember the great eruption of Mount Pelée, which took place in Martinique some years ago. At one stage of the eruption a great obelisk of rock a thousand feet high was pushed upwards out of the crater, and eventually sank back again. It came out of the depths of the earth. It was like a vent-peg plugging some boiling mass below. Similarly we might suppose that all volcanoes were vent-holes for the tremendous commotion of boiling fiery rocks below the earth's surface. The only thing we can urge is that they do not seem big enough for the purpose, if the earth were indeed all molten except for a thin crust—thirty miles thick. For that would leave a molten ocean more than 7900 miles across any way it was measured: 7900 miles deep, 7900 miles broad, 7900 miles long, if we take the diameter of the earth to be 8000 miles. We all know what great tides the Moon and Sun by their attraction raise in the earth's outer ocean of water. Think what tides they would raise in this inner ocean of molten rock and metal. The earth's crust would not be able to hold such tides in. The molten stuff would be always breaking through the flimsy thirty miles of outer solid rock as if it were egg-shell. Twice a day there would be outbreaks of lava vast enough to submerge continents.

No, that will not do. We will not confuse our readers by telling them all the theories that have been formed, but will only state what the late Lord Kelvin believed, and most of the present generation of geologists believe. It is that the heat of the earth's crust continues to increase only for a certain distance of the way down, and that owing to pressure the earth is solid (though very hot except towards the surface) for two thousand miles down. There remains a thickness of another four thousand miles on either side of the earth's centre to be considered. That might be molten, but the pressure would be so great that it would behave as if it were a solid. We know the earth cannot be solid all through because it does not weigh enough. The earth cannot, of course, be weighed in any scales, but there are methods of weighing it nevertheless. One of two methods is by seeing how strongly it attracts bodies to itself. But these things belong rather to the romance of astronomy than to that of geology. We need only trouble ourselves at present about the results.

One word more about the deep interior of the earth. Dr. J. J. See, an American astronomer, has found how heavy and how hard the earth is, taken as a whole. He finds that if it were built from surface to surface of hardened steel it would be just about as heavy and as hard—or as rigid. The steel would be like that used for the armour-plate of battleships. Dr. See is not prepared, however, to discard the idea that the earth has a large fluid interior. If it were fluid, yet it would be subjected to such enormous pressure by its own weight, that if there were a moderately thick earth-crust, its tidal surgings would be so "cabin'd, cribbed, confined," that they would be comparatively ineffectual. We must not run away with the idea (against which Dr. See specially warns us), that there is any free circulation of currents within the fluid interior. The rigidity produced by pressure (or weight) is too great for that. Indeed, this pressure is so great that, as another scientific authority, Professor Arrhenius, has pointed out, the matter at the core of the earth might even be gaseous; and yet would be so compressed by pressure that it would possess a rigidity equal to the hardest steel. The earth may be partly solid, partly liquid, partly gaseous, but for all practical purposes Professor See would have us regard it as a solid sphere having an average hardness and weight and "rigidity" greater than that of ordinary steel.

We are still some way off an explanation of how the many igneous rocks which were and are being "boiled up" in some inner molten cauldron came to the surface; but the better to understand that we must ask our readers to carry their imagination back to the very beginning of the world when it was "without form and void."