The occurrences of these "faults" are most frequent when the process of mountain building has been going on over the earth. In other words, if we imagine a period in which some great continental area of the world's surface was slowly rising above the oceans, then at that time we should expect that there would have been great "faults" occurring in the strata, and great earthquakes following them. To quote his words: "If therefore we wish to know when earthquakes were greatest and most frequent, we must turn to those periods in geological history when mountain ranges were built, when volcanic activity was pronounced, and when great 'faults' were made. The first of these periods would be coincident with the creation of the Ural Mountains, the Grampians, and other mountain ranges. This took place in the earliest geologic times. Another period of mountain formation was when the Himalayas and the Alps were slowly but intermittently brought into existence. In both these periods volcanic activity was pronounced, and beds of coal were formed. When the crust of the earth was crumbling, mountains grew spasmodically—(they sprang up, as it were, from out of the giant forces which we have described earlier in this book)—'faults' gave rise to earthquakes, volcanic forces found their vents, and conditions existed which gave rise to the accumulation of materials to form coal."
But, the reader will naturally inquire, if "faults" gave rise to earthquakes, and faults are the result of pressure, what produces the pressure? And what produces the mountains? Before we answer that we must again have recourse to examples taken from common experience. A sheet of glass or of marble we usually regard as a thing that may break but does not bend. But all of us have seen glass strips, if they are long enough, bend under their own weight; and there are even marble mantelpieces which, if examined, show that the slab of marble has bent. Thus we can readily imagine that sometimes the strata of the earth will bend by reason of the weight put upon them. If that weight is not put on them quite evenly the strata will be still more likely to bend; it will go from bending to buckling, and from buckling to breaking. As soon as it breaks there will be a "fault" formed. Those who recollect what we have said about the enormous weight of the rocks one above the other will not have to search far for the cause of weight sufficient to bend or buckle the rock strata of the earth. And those who have followed carefully all that has been written about the shifting of materials from the land to the sea by the processes of denudation and erosion set up by rain and wind and carried out by streams and rivers, will easily discover where and how the shifting of great weights of the earth's surface goes on. Little by little great weights are taken from one place on the earth's surface to another, as we might shift the weights in balanced scale-pans grain by grain, till at last the heavier scale-pan goes down quickly, or it may be with a crash, and the "fault" occurs, while the earthquake follows.
There remains another question, however, to be answered, and it is, How were the mountains formed? Mountains are very closely connected with earthquakes, for nearly all the regions of the earth where the great disturbances take place are in the neighbourhood of great mountain ranges; and many, indeed most, of the students of earthquakes believe this to be the case, because the great weight of the mountains, especially when near the deep sea, induces pressure on the rocks, and consequently slipping and "faults." But the causes of mountain formation are very difficult to show with certainty. One such explanation, that of the continual shifting of portions by weight of the earth, we have already given. There is another one which may perhaps supply an additional cause. It is that just as "faults" produce earthquakes, perhaps in some cases earthquakes produce "faults." In the illustration we gave of the bottle-neck being broken by continual pressure, or the slide of a microscope suddenly breaking with a crash, because the increasing pressure of a screw was greater than it could bear, we have considered cases where the pressure was slowly applied. But a tap with a hammer or any sudden shock would also produce a breakage—and "faults," and an earthquake on a small scale—and it is possible that some of the convulsions of nature and some of their permanent effects are caused by sudden and violent causes.
One such cause might be the violent and sudden formation of steam by the contact of water with rock at a very high temperature. Everybody knows what happens to the kitchen boiler after a severe frost. The frost clogs the pipes with ice, so that the kitchen boiler becomes dry because no water is reaching it; but it continues to grow hotter and hotter till the iron plates or iron lining become red-hot. Then the frost perhaps gives way and a small amount of water finds its way into the red-hot boiler. The water is converted instantly into steam; and, as a result, the boiler, if it is a weak one, is blown out into the kitchen, causing grave personal inconvenience to the cook. If the boiler is a strong one, it may merely crack. Now, apply these considerations to the instance of the earth, its oceans, its thin crust, and its hot rocks situated at a depth of not more than thirty miles, and perhaps at a good deal less depth than that. What would happen if the ocean leaked through into the strata of red-hot or molten rocks? There would be enormous quantities of steam formed; and if, owing to the vast pressure of strata and water above, these quantities of steam did not instantly produce violent explosions, yet underneath there would be imprisoned, peak, forces of tremendous power which would only await a favourable opportunity in order to manifest themselves. The hot steam until this favourable opportunity arose would be absorbed by the rocks, just as hot steel can be shown to absorb gases.
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A Geyser at Rest in Yellowstone Park, U.S.A.
The encrusted deposits of mineral salts should be observed.
We are thus face to face with the following situation, as it is expressed by Dr. T. J. J. See, the American physicist:—The internal temperature of the earth is extremely high with heated rocks quite near the surface, while the crust is fractured and leaky everywhere, and especially where the depth of the sea is greatest. The sea covers three-fourths of the earth's surface, and earthquakes are found to be most violent where the sea is deepest, and volcanoes most numerous on the adjacent shores. Can we then suppose that both earthquakes and volcanoes depend on the explosive power of steam which has developed in the heated rocks of the earth's crust? We have said that earthquakes and volcanoes are most common in regions where high mountains are near deep oceans—as on the westward of South and Central America, the Aleutian Islands, the Kurile Islands, Japan, Sumatra, Java, and other islands of the East Indies bordering on the deep waters of the Indian Ocean, New Zealand, and the Lesser Antilles in the West Indies, Iceland, Italy, and Greece. These are also the regions where, owing to the existence of high mountains, the weight, the pressure, the tear and stress on the underlying strata are greatest, and where consequently there is the greater chance of strata slipping or bending or giving way. Mr. John Milne divides the world into eleven such great "world-earthquake" districts; and he has endeavoured to show that all the great convulsions of the earth have their origin in one or other of these areas—where usually a great mountain range slopes steeply down to the sea. There are eleven such world-earthquake districts. There is the Alaskan region, where on the shore Mount Elias rises to a height of 18,000 feet and where the water is 7000 feet deep sixty miles from the shore—altogether a drop of 25,000 feet from the top of the mountain to the bed of the ocean in 200 miles. This drop, without going into measurements, may be taken as typical of the rest, which are classified as the Cordillerean region, the Antillean region (in the earthquake district of which Mount Pelée at Martinique and the Soufrière in St. Vincent are situated); the Andean district; the Japan district; the Javan district; the Mauritius district; the Antarctic district; three submarine districts of sunken ridges in the North-Eastern Atlantic, the North-Western Atlantic, and the Northern Atlantic; and one great land district, distant from the sea, which lumps together all the mountains of the Alps, the Balkans, the Caucasus, and the Himalayas. In thirteen years, from the time in which earthquake investigation has become a science, 750 great earthquakes originated in these districts. On the average, about sixty great earthquakes occur every year, or a little more than one a week. In addition to these world-shaking effects there are about 30,000 small earthquakes every year, England's annual contribution to this number being about half a dozen.