CHAPTER XVI
VOLCANOES AND MOUNTAIN FORMATION
The great prominence which we have given in the preceding pages to earthquakes is owing to the growing belief in the influence of earthquakes on the appearance and structure of those portions of the world's crust which are known to us. There are two views which we can take of earthquakes. One is to regard the larger number of them as being caused by slipping movements of the earth's crust. Looking at things in this way we should say that whenever there was a sudden break in the earth's strata, such as might occur (in accordance with an illustration given in a previous chapter) if all the level strata were broken up like a crumpled page of type—then that an earthquake would result. So that whenever we saw what geologists call a "fault" in strata we should know that an earthquake had occurred there. And why did it occur? Well, if we had a massive column of steel or of granite five miles high, the steel or granite at the bottom of the column would have to sustain such an enormous weight of material above it that it would begin to spread. If we had a pyramid of the same materials five miles high, the tendency to spread would not be so great, but still it would be there. Consequently, wherever there are high mountains there is a tendency of the earth strata beneath them to spread, perhaps slowly, but inevitably; and if there is any weakness in the structure of the rocks near the base of the mountain, then these will give way with a crash. A great "fault" will be produced, and with it an earthquake.
People living on the earth will only see the results of the earthquake on the ground just immediately below their feet; and there these results are often very destructive to life and property; yet if they were all that happened, we should expect them to be covered up in time, and the "geological record" of an earthquake would not be a very important or even discernible thing a million years after it had happened. But are these things, which the eye of man can perceive, the only things that are happening during an earthquake? Is nothing happening underneath the earth which will leave its mark thousands of years after man has left the spot where the earthquake took place? May it not be that the earthquake is the outcome of some mighty force deep down in the earth; and may not this force cause both the earthquake and the geological "fault" which remains as the witness of its occurrence? If this be the case then the earthquake may be of enormous importance in geology.
We regard an earthquake, as we see it, as a destructive force. That is because it destroys the works of man. But earthquakes are doing constructive work as well; or, at any rate, they are usually present when constructive work is being done. Destructive forces, such as erosion, are wearing down the structure of the globe, while earthquakes are the only known forces that are building it up. It is true that when an earthquake occurs rocks often fall, loose sediment is shaken down, and other settlements occur, but the real constructive work consists in upheavals, little by little, as it may be, of beaches, islands, coasts, plateaux, and perhaps larger areas. These elevations are actually witnessed in certain earthquakes.
Many islands in the sea have been raised from time to time within even living memory.
The south-western part of the island of Crete has been elevated within the historical period.
The region about Pozzuoli and the Bay of Naples has suffered both elevation and depression. There is the famous instance cited by Sir Charles Lyell nearly eighty years ago of the Temple of Jupiter Serapis. This temple had many columns; and they are now situated on dry land. The pillars are forty-two feet in height, and for twelve feet upwards they are of smooth undisfigured marble. Then for another twelve feet they are pitted with the holes made by a marine shell-fish called Lithodomus, the stone-dweller. What are we to judge from this? The temple was first built on dry land. Then the land sank taking the temple with it, and the columns were submerged in sea sediment to a depth of some thirty feet above their pedestals. The lower portions of these pedestals were preserved intact, but the marine shell-fish found a home in the upper part of the marble columns, and pierced them with the channels and grooves. After this had gone on for a number of years the elevation of the land lifted the temple and its columns clean out of the sea again, and the marine shell-fish could no longer live in the columns. But the traces of their habitation remain.
The elevation of this coast was actually witnessed at the time of the eruption of Monte Nuovo in 1538. Moreover, the raising of the land was perceived on a larger scale round the whole of the Bay of Naples during the eruption of Vesuvius in April, 1706. Professor Lorenzo found the elevation of the land at Pozzuoli to be six inches, and at Portici one foot.[14] The foundations of both Etna and Vesuvius were ages ago laid in the sea.
[14] The coast about Pozzuoli is now sinking again.
In almost every part of the world there are raised beaches, such as we have already mentioned in the neighbourhood of Valparaiso, on the Chilian coast. The idea has been put forward by Dr. T. J. See that the same cause which produces earthquakes produces these elevations of the land and produces also volcanoes. There are many circumstances which favour this idea. Let us consider what is happening at the bed of the sea. Some years ago, when certain officers of the United States Navy were making ocean surveys, it was found that if hollow balls of thick glass were sunk to great depths in the ocean, they came up more and more completely filled with water in proportion as the depth increased, though no breakage or cracking of the glass had occurred, and no holes in it could be discovered even by the best microscopes. In other words, it became evident that the water had been slowly but bodily forced through the thick walls of the glass (under a pressure of less than 15,000 lb. to the square inch) in less than an hour's time. Evidently, then, even such a substance as glass will be penetrated by water if the pressure is great enough.
To make a practical application of these principles, what shall we now say with respect to the ocean bottoms? In deep places the pressure of the sea-water on them is very great, sufficient to force water through glass. Obviously most of these bottoms will leak, and leak at a rapid rate under the enormous pressure operating in the greatest depths of the sea. The bed of the ocean will not leak with equal rapidity in all places; but almost universal leakage will certainly develop, and the water will be driven back into the earth at various rates. Where the rock is volcanic and badly fractured, or sandy, the leakage will be most rapid; and where the bed is made of clay or unbroken granite the leakage will be much more gradual. It will also depend on the depth of the sea, and will be greatest where the ocean is deepest, and quite insignificant in shallow water. A rapid rate of leakage would mean that large quantities of water quickly come in contact with the heated rock, and develop correspondingly great steam pressure in the crust which underlies that part of the ocean. One case in which we may suppose a rapid leakage to be taking place is in the case of volcanoes near the sea. In the case of lava pouring from a volcano, it is observed that the molten rock emits vast quantities of vapour, of which, according to Sir Archibald Geikie, 999 parts in 1000 are steam. The enormous volume of these has been brought home to us in recent years by the behaviour of the volcano Mount Pelée, from which for several years after the great eruption which devastated Port au Prince the vapours rose in clouds that were to be measured in cubic miles. Similar observations about the quantities of vapour ejected by volcanoes have been made in Japan.
While speaking of Mount Pelée we may recall another phenomenon connected with it, which also appears to bear out the supposition that in the volcano's activity the action of steam takes a very large share. After its first outburst Mount Pelée continued to pour out lava and great quantities of vapour, as if like some gigantic cauldron it were being fed with fresh supplies of water; and there in the early March of the following year a most amazing thing took place, under the very eyes of a celebrated investigator of volcanoes, now dead, Professor Angelo Heilprin, who was remaining on the island. A great obelisk of andesite (a stone not unlike basalt) was forced up from the crater. It rose rapidly, as much as five feet a day; and it reached altogether a height of 840 feet above the crater's lip. It was calculated to be about 300 feet in diameter at its base. It continued to push itself up for some months, sometimes sinking a little, sometimes rising like a colossal piston above a steam boiler. Its greatest height was 1100 feet above the height of Mount Pelée, and therefore at a height of 5143 feet above the sea-level.
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The New Spine of Mont Pelée, showing Fissures and Vertical Grooves Photographed on March 15th, 1903. The spine was then 82 feet lower than it became ten days later. | The New Spine of Mont Pelée, viewed from the Basin of the Lac des Palmistes The apex, 1174 feet above the rim directly in front; the remains of Morne la Croix on the edge of the crater at the right. |
A violent eruption would reduce its mass and its steeple-like pinnacle; but after its losses it generally pushed up again. Professor Heilprin at last got near enough to observe it, and the obelisk was found to be not of pumice stone, as had at first been suspected, but of the hard rock we have mentioned. It had, in fact, been comparable to a Titanic cork of rock which had closed up some vent far down in the crust of the earth, and which had at last been lifted by the steam pressure beneath it. It finally sank back into the crater, but it was replaced by a dome of rock which underwent similar changes in height, though on a smaller scale, to those of the obelisk. The dome of rock was, however, on a more massive scale even than the obelisk, and at one period of its career a spine, 100 feet in height, like a smaller obelisk, was pushed up through its middle. This dome was examined by the explorers, the Abbé Yvon and M. Beaufroy, who found that the dome was a great mass of andesite, while about it were fragments of the rock of which the obelisk had been composed. They wrote at the time:—
"It is an error to suppose that there exists in the bottom of Mount Pelée a hole from which lava and gases have come out. At present there is a tremendous cork of andesite, which is called the 'Dome,' and which must have as its dimensions a diameter half a mile across at its base and a height of about 1200 feet. On all sides of the dome there are fumaroles (small cone-like craters), some of which throw out a reddish smoke, others of which discharge white smoke, and others are still surrounded with a carpet of sulphur several yards in depth."
After the great eruption of Mount Pelée in 1902 it was found by measurement that a considerable portion of the adjacent sea bottom had sunk down many fathoms. It is impossible to believe that this sinking had been caused by the mere shaking of the earthquakes accompanying that eruption. We must, therefore, suppose that after the dreadful explosions which destroyed St. Pierre and devastated Martinique a subsidence near the roots of the mountain (which is just by the sea) took place. What we should judge to have happened is that by some means an explosion took place below the sea bottom; that parts of the molten rock, moved by the forces of the explosion, were moved towards the mountain (Mount Pelée), which thereupon broke into eruption, acting as an outlet for the imprisoned rocks. When these molten rocks were thus removed a great cavity was formed in the bed of the sea, which accordingly caved in.
A similar explanation would account for the raising of the Chilian coast-line after the great earthquakes of 1835, of which we have already spoken. The coast and, indeed, the whole country back to the Andes was slightly raised. This could only be explained by the pushing in or forcing in of a corresponding bulk of lava under the land; and this lava could come from nowhere except from under the bed of the great trough in the adjacent sea. After an explosion (which is caused by the sea penetrating through to the molten rocks) the trough, where the "accident" first took place, would naturally deepen.
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The Dead City of St. Pierre, Martinique
The town of St. Pierre was perhaps the most beautiful in the West Indies. The volcano of Mont Pelée, which is seen in the background, and which is five miles away, suddenly belched out deadly gases, dust, steam, and boiling mud, which overwhelmed the town and completely destroyed it. The houses were reduced to ruins, and the people were killed by the wave of hot gases sweeping down from the volcano.
Moreover, if we suppose that after a time the water again penetrates, and again comes into contact with rocks that have again become heated up, there will be another explosion, and yet others. Each of these explosions will push along the ejected streams of lava, step by step, till they reach the land, and even till they reach the mountains bordering the sea. The forces thus arising would cause upheavals, even if they did not cause earthquakes. Such forces might bend strata, contort the rocks, and cause "faults."
But why, the reader may ask, do you suppose that all these explosions of lava are directed to the land? We do not suppose that they are. The lava may be forced away from the land. Then if that occurs a ridge may be upheaved, or possibly a submarine volcano.
"At the Hawaii Islands on 25th February, 1877," writes Sir Archibald Geikie, "masses of pumice during a submarine volcanic explosion were ejected to the surface, one of which struck the bottom of the boat with considerable violence and then floated. At the same time, when we reflect to what a considerable extent the bottom of the great ocean basin is dotted over with volcanic cones, rising often solitary from profound depths, we can understand how large a proportion of the actual eruptions may take place under the sea. The foundations of these volcanic islands doubtless consist of submarine lavas and fragmentary materials, which in each case continue to accumulate to a height of two or three miles, till the pile reaches the surface of the water and appears above it. The immense abundance and wide diffusion of volcanic ash, pumice, etc., over the bottom of the Pacific and Atlantic oceans, even at distances remote from land, as has been made known by the voyage of the Challenger, may indicate the prevalence and persistence of submarine volcanic action."
It is fairly clear, therefore, that the sea bottom is leaky, and that volcanoes which are a consequence of it are scattered freely over the deep ocean floor. In some places, of course, very few eruptions occur, either because the underlying rocks are less leaky or the sea is too shallow for pressure.
We have pictured the water of the oceans thus sinking down into the hot rocks. It will not always cause an explosion at once. The steam may not immediately become free, but will become absorbed in the hot rocks till the lava grows so fully saturated by the hot vapour that it swells and requires more space. When the tension becomes great enough the crust begins to shake and the paroxysm continues till the steam-saturated lava moves along the nearest break or "fault" or vent. When the underlying molten rock has thus obtained more space the agitation ceases till the tension again becomes too powerful for the crust to withstand, when another readjustment takes place. A familiar illustration of this process is seen in the lid of a tea-kettle when the steam pressure accumulates till it sets the lid quivering. As the steam escapes at the sides the agitation slowly dies down and the lid then remains quiet till the accumulating pressure again requires relief, when the shaking is renewed. Thus the process is periodic, and the period depends on the rapidity with which the steam is developed. In the case of earthquakes, as already remarked, the steam is not free, but absorbed in the molten rock, and when the agitation begins this gives a similar quivering motion to the block of the earth's crust overlying it, and ceases only when readjustment occurs—usually by the neighbouring "fault" slipping in some way so as to give more space to the swelling lava beneath. Of course, many of the cracks caused by this swelling are never seen; and the molten lava seldom reaches the surface except when through volcano vents or cracks in mountains that are near the sea-shore; but such outbreaks are probably more common in the deep sea.
To see how effective the pressure arising from the depths of the ocean may be in driving water into the crust of the earth, we may observe that the tendency to penetrate is everywhere proportional to the depth of the sea. Now everybody knows that if a cistern be placed at the top of a house and connected with a fountain in the garden the fountain ought to throw a jet as high as the cistern because water, as the saying goes, always rises to its own level. As a matter of practice the water does not rise so high because of the resistance of the air. But for theoretical purposes we may consider the proportion true, and we might similarly say that the pressure in a sea one mile deep would thus throw a stream a mile high; in a sea two miles deep, two miles high; and so on. Now some of the ocean depths exceed five miles, the greatest, near Guam, being 5269 fathoms, almost exactly six miles. Is it therefore any wonder that the deeps east of Japan, near the Aleutian Island, west of South America, near Guam, between Samoa and New Zealand, give rise to enormous leakage of the sea bottom, and consequently many world-shaking earthquakes? A comparatively feeble pressure of water, such as hydraulic engineers use in mining, rapidly cuts away hills and washes out all their gold; in the same way the waters of Niagara, falling through only 160 feet, slowly wear away the solid rock over which they pour. What, then, may be expected of a constant water pressure which will throw a jet five miles high? Such is the pressure all over the bed of the Tuscarora Deep, and it continues from year to year, century to century. It is this pressure which forces the water so rapidly into the earth, and gives rise to all the great earthquakes and sea-waves with which Japan is afflicted. No stone on earth, however thick its layers, could withstand such a pressure; nay, under it the water would go through the hardest metals, and sink down deeper and deeper into the bowels of the earth. Thus subterranean steam would arise beneath the crust and accumulate till relief was afforded by a shaking of the earth.
Thus we see how immensely important the same causes that give rise to earthquakes may be in moulding the outlines and contours of the rocks and the "everlasting hills." In the present state of geological knowledge we cannot say that these steam explosions are the sole causes of mountain building, but it is evident that they must play a great part in them. The action of the submarine explosions may be compared to a man digging out a trench. As he digs along the trench, the earth that he excavates he throws on to either side of the trench, so that a ridge appears on each side of the excavation. The result is the same in the case of the continuous lava explosions in deep seas, especially in those deep seas like the waters off the west coast of South America, where a great range of mountains runs parallel to an ocean that is of great depth only a short distance from the land. A trough or trench is cut downwards by successive explosions and expulsions of lava. As the trough is arched downwards like a broad letter U, the steam pressure from beneath cannot easily force it upwards. What will therefore happen? Imagine what would happen in a steam saucepan or kettle if the vapour could not get out at the top or lid. It would tend to blow out at the sides. Or if you think of a slab of dough rising under the effect of yeast. Suppose the baker presses a flat board on the top of the rising dough, and presses down on it so that it cannot force its way upwards. It will then naturally spread out to the sides. Similarly the rising yeasty lava under the curved ocean bed has to force its way sideways under the crust. It forces its way partly towards the land—where the mountains run along the coast as in the case of the Andes—or farther out underneath the ocean. Generally the movement of the lava will be towards the mountains till the trough gets broad and deep and the mountains very far away, and so high that their weight offers unexpectedly great resistance to the underground stream of lava. Then the release will at length become easier towards the ocean by the forcing up of ridges or volcanoes along the other margin of the trough. Ridges with peaks in them will usually result, and this is the beginning of a new range of mountains in the sea, which are destined to rise slowly from it parallel to the great range of mountains on the shore. There may thus be two parallel ridges, perhaps hundreds of miles apart, with a valley between them. This valley may be drained in the course of ages, or filled in by the processes of erosion which we have described in the earliest chapters of this volume.
It will be of interest to quote at this point what Pliny nearly two thousand years ago said in his Natural History (Book II) on islands which have been uplifted from the Mediterranean, evidently as the result of volcanic causes:—
"Land is sometimes formed in a different manner, rising suddenly out of the sea, as if nature was compensating the earth for its losses, restoring in one place what she had swallowed up in another. Delos and Rhodes, islands which have now been long famous, are recorded to have risen up in this way. More lately there have been some smaller islands formed: Anapha, which is beyond Melos; Nea, between Lemnos and the Hellespont; Halone, between Lebedos and Teos; Thera and Therasia, among the Cyclades, in the fourth year of the 135th Olympiad. And among the same islands, 130 years afterwards, Hiera, also called Automate, made its appearance; also Thia, at the distance of two stadia from the former, 110 years afterwards, in our own times, when M. Junius Silanus and L. Balbus were consuls, on the 8th of the Ides of July.
"Opposite to us, and near to Italy, among the Æolian Isles, an island emerged from the sea; and likewise one near Crete, 2500 paces in extent, and with warm springs in it; another made its appearance in the third year of the 163rd Olympiad, in the Tuscan Gulf, burning with a violent explosion. There is a tradition, too, that a great number of fishes were floating about the spot, and that those who employed them for food immediately expired. It is said that the Pithecusan Isles rose up in the same way in the Bay of Campania, and that shortly afterwards the mountain Epopos, from which flame had suddenly burst forth, was reduced to the level of the neighbouring plain. In the same island it is said that a town was sunk in the sea; that, in consequence of another shock, a lake burst out, and that, by a third, Prochytas was formed into an island, the neighbouring mountains being rolled away from it."
There are, no doubt, other causes which warp and bend strata. We have compared the earth to the core of a tightly wound golf ball—always in a state of strain. The strain at great depths below the surface might amount to several tons to the square inch, and it can easily be understood that breaks might occur in consequence, especially if some slight additional shock set the rocks into vibration. In the deep copper mines of the northern peninsula of Michigan the behaviour of the whole earth, with respect to earthquakes and stresses due to other causes, is well illustrated on a small scale. At certain times during each day blasts are set off in the solid rock at various places in each mine. Each battery of blasts is a miniature earthquake. In that particular spot, the earthquake centre, the rock is fractured within a space limited by a radius of a few feet. Within a large space, limited by a radius of a few hundred feet, elastic vibrations are set up in the solid rock which are sufficiently violent to be perceptible to the touch and to the hearing. Within this larger space no fracture of the rock occurs. Feebler vibrations doubtless extend out for miles from the point of fracture, just as vibrations extend over the whole earth from an earthquake centre. Now it also happens that in the lower levels of these deep mines, at a mile below the surface of the earth, the solid rock is slowly yielding, in a non-elastic manner, under the influence of the great weight above it, so that the larger openings are gradually closing up. This is so clearly recognised and progresses so rapidly that it is proposed as routine practice, at the deep levels in these mines, to take out the ore at the distant end of each drift first. The miners will then work back slowly toward the shaft from which the drift is entered, while the spaces in which they have recently laboured gradually close up behind them. The gradual collapse known to be in progress occurs apparently by imperceptible flow and by minor fracturing, but not, as a rule, by catastrophes which close up any opening suddenly. In this respect it is an epitome of what is taking place every year in the failing earth as it yields under such stresses.
There may be local tremors due to causes which are less immense and world-wide. One such cause might be the collapse of cavities in the earth. We are well acquainted with some such caves near the surface of the earth. These caves, especially in limestone, are commonly caused by the action of springs. Even pure water will dissolve a minute quantity of the substance of many rocks, and rain water is far from being chemically pure water. It takes oxygen and carbonic acid out of the air as it falls, and it abstracts acids out of the soil through which it sinks. The presence of this acid gives the water a greatly increased power of attacking carbonate of lime. Now limestone is a rock almost entirely composed of carbonate of lime. It occurs in most parts of the world, covering sometimes tracts of hundreds or thousands of square miles, and often rising into groups of hills and ranges of mountains. The abundance of this rock offers ample opportunity for the display of the dissolving action of subterranean water. The water trickles down the vertical fissures along the planes below the limestone beds. As it flows on it dissolves and removes the stone till in the course of centuries these passages are gradually enlarged into clefts, tunnels, and caverns. The ground becomes honeycombed with dark subterranean chambers, and running streams fall into these chambers and continue their course underground.
In England there are famous "pot-hole" caverns in Yorkshire and the west of England. The Peak Cavern in Derbyshire is believed to be 1200 feet long, and in some places 120 feet high. The caverns of Adelsberg near Trieste have been explored to a distance of several miles. The River Poik has broken into one part of the labyrinth of chambers through which it rushes before emerging again to the light. "Narrow tunnels," says Sir Archibald Geikie, "expand into spacious halls, beyond which egress is again afforded by low passages into other lofty recesses. The most stupendous chamber measures 669 feet in length, 630 feet in breadth, and 111 feet in height. From the roof hang white stalactites which uniting with the floor form pillars showing endless varieties of form and size." Still more gigantic is the system of subterranean passages in the Mammoth Cave of Kentucky, the accessible parts of which are believed to have a combined length of about 150 miles. The caverns of Luray, in Virginia, are scarcely less wonderful; and in their case American ingenuity has hit on the idea of sucking the pure, dustless air out of these caverns in order to ventilate a sanatorium. Indeed, a book might easily be written on the wonders of the limestone caverns of the world, but our only purpose in mentioning them in this chapter is to indicate how the rocks of the earth may be made unstable, so that a slight shock may precipitate a catastrophe in them—a kind of subterranean landslip which in its turn may give rise to some of the symptoms of earth tremors.
A Yorkshire Pot-Hole: showing the Effects which can be Produced in Limestone by Underground Water
The immense depth may be better realised by comparing the pot-hole with the Nelson Monument, which is 162 feet in height.