As an evidence of the widespread and far-reaching influences set in motion by the eruptions of Vesuvius it should be noted that Father Odenbach of St. Ignatius" college in Cleveland, O., the noted authority on seismic disturbances, reported that his microseismograph, the most delicate instrument known for detecting the presence of earthquakes in any part of the globe, had plainly recorded the disturbances caused by the eruption of Vesuvius. The lines made by the recorder, he said, had shown a wavy motion for several days, indicating a severe agitation in the earth"s surface at a remote point.
CHAPTER XXIII.
VOLCANOES AND EARTHQUAKES EXPLAINED.
BY TRUMBULL WHITE.
=The Theories of Science on Seismic Convulsions--Volcanoes Likened to Boils on the Human Body through Which the Fires and Impurities of the Blood Manifest Themselves--Seepage of Ocean Waters through Crevices in the Rock Reach the Internal Fires of the Earth--Steam is Generated and an Explosion Follows--Geysers and Steam Boilers as Ill.u.s.trations--Views of the World"s Most Eminent Scientists Concerning the Causes of Eruption of Mount Pelee and La Soufriere.=
The earth, like the human body, is subject to const.i.tutional derangement. The fires and impurities of the blood manifest themselves in the shape of boils and eruptions upon the human body. The internal heat of the earth and the chemical changes which are constantly taking place in the interior of the globe, manifest themselves outwardly in the form of earthquakes and volcanoes. In other words, a volcano is a boil or eruption upon the earth"s surface.
Scientists have advanced many theories concerning the primary causes of volcanoes, and many explanations relating to the igneous matter discharged from their craters. Like the doctors who disagree in the diagnosis of a human malady, the geologists and volcanists are equally unable to agree in all details concerning this form of the earth"s ailment. After all theories relating to the cause of volcanoes have been considered, the one that is most tenable and is sustained by the largest number of scientific men is that which traces volcanic effects back to the old accepted cause of internal fires in the center of the earth. Only in this way can the molten streams of lava emitted by volcanoes be accounted for.
The youngest student of familiar science knows that heat generates an upward and outward force, and like all other forces that it follows the path of least resistance. This force is always present in the internal regions of the earth, which for ages upon ages has been gradually cooling from its poles toward its center. When conditions occur by which it can outwardly manifest itself, it follows the natural law and escapes where the crust of the earth is thinnest.
But something more than the mere presence of internal fire is necessary to account for volcanic action, although it may in a large degree account for minor seismic convulsions in the form of an earthquake. The elements which enter into the source of volcanic eruption are fire and water. The characteristic phenomenon of a volcanic eruption is the steam which issues from the crater before the appearance of the molten lava, dust, ashes and scoria. This accepted theory is plainly ill.u.s.trated in the eruption of a geyser, which is merely a small water volcano. The water basin of a geyser is connected by a natural bore with a region of great internal heat, and as fast as the heat turns the water into steam, columns of steam and hot water are thrown up from the crater.
One form of volcanic eruption, and its simplest form, is likewise ill.u.s.trated in a boiler explosion. Observations of the most violent volcanic eruptions show them to be only tremendous boiler explosions at a great depth beneath the earth"s surface, where a great quant.i.ty of water has been temporarily imprisoned and suddenly converted into steam. In minor eruptions the presence of steam is not noticeable in such quant.i.ties, which is simply because the amount of imprisoned water was small and the amount of steam generated was only sufficient to expel the volcanic dust and ashes which formed between the earth"s surface and the internal fires of the volcano. The flow of lava which follows violent eruptions is expelled by the outward and upward force of the great internal heat, through the opening made by the steam which precedes it.
The two lines of volcanoes, one north and south, the other east and west, which intersect in the neighborhood of the West Indies, follow the courses where the crust of the earth is thinnest and where great bodies of water lie on the shallowest parts of the ocean bed.
The terrific heat of the earth"s internal fires is sufficient to cause crevices leading from these bodies of water to the central fires of the volcano, and the character of the volcanic eruption is determined largely by the size of the crevices so created and the amount of water which finds its way through them. The temperature of these internal fires can only be guessed at, but some idea may be formed of their intense heat from the streams of lava emitted from the volcano. These will sometimes run ten or twelve miles in the open air before cooling sufficiently to solidify. From this it will be seen that the fires are much hotter than are required merely to reduce the rock to a liquid form. From this fact, too, may be seen the instantaneous action by which the water seeping or flowing into the volcano"s heart is converted into steam and a tremendous explosive power generated.
The calamity which befell Martinique and St. Vincent will unquestionably lead to a fresh discussion of the causes of volcanic disturbance. Not all of the phenomena involved therein are yet fully understood, and concerning some of them there are perceptible differences of opinion among experts. On at least one point, however, there is general agreement. At a depth of about thirty miles the internal heat of the earth is probably great enough to melt every known substance. Confinement may keep in a rigid condition the material which lies beneath the solid crust, but if an avenue of escape is once opened the stuff would soften and ooze upward. There is a growing tendency, moreover, to recognize the importance of gravitation in producing eruptions. The weight of several miles of rock is almost inconceivable, and it certainly ought to compel "potentially plastic" matter to rise through any crevice that might be newly formed. Russell, Gilbert and some other authorities regard this as the chief mechanical agent in an eruption, at least when there is a considerable outpouring of lava.
As to the extent to which water operates there is some lack of harmony among volcanists. Shaler, Milne and others hold that substance largely, if not entirely, responsible for the trouble. They point to the fact that many volcanoes are situated near the coast of continents or on islands, where leakage from the ocean may possibly occur.
Russell, on the other hand, regards water not as the initial factor, but as an occasional, though important, reinforcement. He suspects that when the molten rock has risen to a considerable distance it encounters that fluid, perhaps in a succession of pockets, and that steam is then suddenly generated. The explosive effects which ensue are of two kinds. By the expansion of the moisture which some of the lava contains the latter is reduced to a state of powder, and thus originate the enormous clouds of fine dust which are ejected. Shocks of greater or less violence are also produced. The less severe ones no doubt sound like the discharge of artillery and give rise to tremors in the immediate vicinity. In extreme cases enough force is developed to rend the walls of the volcano itself. Russell attributes the blowing up of Krakatoa to steam. The culminating episode of the Pelee eruption, though not resulting so disastrously to the mountain, would seem to be due to the same immediate cause. To this particular explosion, too, it seems safe to a.s.sign the upheaval which excited a tidal wave.
The precise manner in which the plastic material inside of the terrestrial sh.e.l.l gets access to the surface, is not entirely clear.
Nevertheless, it is possible to get some light on the matter. It is now well known that in many places there are deep cracks, or "faults,"
in the earth"s crust. Some of them in the remote past have been wide and deep enough to admit molten material from below. The Palisades of the Hudson are believed to have been formed by such an intrusion, the adjacent rock on the eastern face having since been worn away by the weather or other agents. It has been observed that many volcanoes are distributed along similar faults.
The existence of a chain of volcanic islands in the West Indies suggests the probability that it follows a crack of great antiquity, though the issue of lava and ashes for several centuries may have been limited to a few isolated points. Just how these vents have been reopened is one of the most difficult questions still left for investigation. Given a line of weakness in the rocks, though, and a susceptibility to fresh fracture is afforded. Professor McGee suggests that the overloading of the ocean bed by silt from the Mississippi river or other sources may have been the immediately exciting cause of the recent outbreaks. Other geologists have found a similar explanation acceptable in the case of eruptions elsewhere. The theory has much to commend it to favor.
The Martinique disaster already has drawn from geologists and volcanists many expressions of opinion, and explanations of volcanic phenomena which set forth in detail the causes and effects of volcanic eruptions, in particular, and seismic convulsions, in general.
Dr. A. R. Crook, a professor in Northwestern University, has made a special study of volcanoes. He has made an ascent of the two highest in the world, and has climbed many others for purposes of study. He is an authority upon volcanography.
"There are two great circles of volcanoes about the earth," said Professor Crook. "One girdles the earth north and south, extending through Tierra del Fuego (called "land of fire" because of its volcanoes), Mexico, the Aleutian islands and down through Australia; the other east and west through Hawaii, Mexico, West Indies, Italy (including Mount Vesuvius) and Asia Minor.
"These two circles intersect at two points. One of these is the West Indies, which include Martinique, the scene of this terrible disaster; the other is in the islands of Java, Borneo and Sumatra. On the latter islands there are extinct volcanoes. On the former is the terrible Pelee. It is just at these points of intersection of the two volcanic rings that we expect unusual volcanic activity, and it is there that we find it.
"There has been more or less theorizing as to volcanic disturbances moving in cycles, but it cannot be proved. One fact is established, and that is that a volcano is an explosion caused by water coming in contact with the molten ma.s.s below the surface of the earth. This is proved by the great clouds of steam that accompany the action.
"The old theory that the very center of the earth is a molten ma.s.s,"
he says, "is no longer held." He a.s.serts the latest idea is that the center of the earth is more rigid than gla.s.s, though less rigid than steel. About this there is more or less molten matter, and over all the surface crust of the earth. This molten matter causes the surface of the earth to give, to sag, and form what is called "wrinkling."
When water comes in contact with the heated ma.s.s an explosion follows that finds its outlet through the places where there is least resistance, and the result is a volcano.
"There is no part of the earth"s surface which is exempt from earthquakes," said Professor Crook, "and there is no regularity in their appearance. Volcanic eruptions are almost always preceded by earthquakes somewhere in the circle. Recently there were earthquakes in the City of Mexico in which many lives were lost. As it is impossible to predict when the next will take place, it is also impossible to tell where it will be. It will certainly be somewhere in the line of the two circles.
"All this is of interest as showing that the earth is still in process of formation just as much as it was a billion years ago. We see the same thing in Yellowstone Park. There most decided changes have taken place even in the last eight years. Old Faithful, which used to play regularly every sixty minutes, now does so only once in twice the time."
With reference to contributions to science, which might be expected from investigations at Martinique, Professor Crook said:
"Even new elements might be discovered, and seismic theories either confirmed or disproved. A volcano always throws off a great variety of materials, hydrochloric and sulphuric acids, iron, silica (sand), sulphur, calcium and magnesium. The lava is of two kinds. That which is easily fusible flows more rapidly than a horse can trot. A more viscous kind cools into shapes like ropes. The latter is common in Hawaii.
"The danger of living in proximity to a volcano is usually well known, but the iron oxides render the soil extremely fertile. This is seen in Sicily about aetna and Vesuvius. It is seen also in Martinique, where an area of forty miles square was occupied by 160,000 people.
"Owing to the presence of the fumes of chlorine it is probable that many of the victims in St. Pierre were asphyxiated, and so died easily. Others doubtless were buried in ashes, like the Roman soldier in Pompeii, or were caught in some enclosed place which being surrounded by molten lava resulted in slow roasting. It is indeed a horrible disaster and one which we may well pray not to see duplicated. Science, however, has no means of knowing that it may not occur again."
Professor Robert T. Hill, of the United States Geological Survey, who visited the French West Indies on a tour of scientific inspection, says:
"Across the throat of the Caribbean extends a chain of islands which are really smoldering furnaces, with fires banked up, ever ready to break forth at some unexpected and inopportune moment. This group, commencing with Saba, near Porto Rico, and ending with Grenada, consists of ancient ash heaps, piled up in times past by volcanic action. For nearly one hundred years there has been not the slightest sign of explosion and we had grown to cla.s.s these volcanoes as extinct.
"Volcanism is still one of the most inexplicable and profound problems which defy the power of geologists to explain, and one of its most singular peculiarities is the fact that it sometimes breaks forth simultaneously in widely distant portions of the earth. A sympathetic relation of this kind has long been known between Hecla and Vesuvius, and it is very probable that the Carib volcanoes have some such sympathetic relation with the volcanoes of Central America and southern Mexico. At the time of the explosion of St. Vincent other explosions preceded or followed it in northern South America and Central America.
"The outburst of Mount Pelee, in Martinique, is apparently the culmination of a number of recent volcanic disturbances which have been unusually severe. Colima, in Mexico, was in eruption but a few months previous, while Chelpancingo, the capital of the State of Guerrero, was nearly destroyed by earthquakes which followed.
"Only a few days before Mount Pelee erupted, the cities of Guatemala were shaken down by tremendous earthquakes."
Professor N. S. Shaler, of Harvard University, a world authority on volcanic disturbances, says:
"Volcanic outbreaks are merely the explosion of steam under high pressure--steam which is bound in rocks buried underneath the surface of the earth and there subjected to such tremendous heat that when the conditions are right its pent up energy breaks forth, and it shatters its stone prison walls into dust.
"The common belief is that water enters the rocks during the crystallization period, and that these rocks, through the natural action of rivers and streams, become deposited in the bottom of the ocean. Here they lie for many ages, becoming buried deeper and deeper under ma.s.ses of like sediment, which are constantly being washed down upon them from above. This process is called the blanketing process.
"When the first layer has reached a depth of a few thousand feet the rocks which contain the water of crystallization are subjected to a terrific heat. This heat generates steam, which is held in a state of frightful tension in its rocky prison.
"It is at these moments that volcanic eruptions occur. They result from wrinkling in the outer crust of the earth"s surface--wrinklings caused by the constant shrinking of the earth itself and by the contraction of the outer surface as it settles on the plastic center underneath. Fissures are caused by these foldings, and as these fissures reach down into the earth the pressure is removed from the rocks and the compressed steam in them and it explodes with tremendous force.
"The rocks containing the water are blown into dust, which sometimes is carried so high as to escape the power of the earth"s attraction and float by itself through s.p.a.ce. After the explosions have occurred lava pours forth. This is merely melted rock which overflows like water from a boiling kettle. But the explosion always precedes the flow, and one will notice that there is always an outpouring of dust before the lava comes."
Professor W. J. McGee, of the Smithsonian Inst.i.tution at Washington, says: "It may be that a violent earthquake tremor came after the volcanic eruption, but it does not necessarily follow that the two travel together. Oftentimes we hear of earth tremors with no apparent accompaniment. This was true of the Charleston earthquake in 1886.
Earthquakes are caused by mysterious disturbances in the interior of the earth. The most commonly accepted belief is that ma.s.sive rock beds away down in the earth, at a depth of twelve miles or more, become disturbed from one cause or another, with the result that the disturbance is felt on the earth"s surface, sometimes severely, sometimes faintly.
"Probably the most violent earthquake in history occurred about ten years ago at Krakatoa. The explosion could be heard for more than one thousand miles, and the earth"s tremors were felt for thousands of miles. The air was filled with particles of earth for months afterward. The air-waves following the explosion are believed to have pa.s.sed two and one-half times around the globe. The face of the land and sea in the vicinity of the eruption was completely changed."
Dr. E. Otis Hovey, professor in the Museum of Natural History, New York, offers the following explanation of the Martinique disaster:
"A majority of volcanic eruptions are similar in cause and effect to a boiler explosion. It is now the accepted belief that sudden introduction of cold water on the great molten ma.s.s acts as would the pouring of water into a red hot boiler. It causes a great volume of steam, which must have an outlet. You can readily see how water could get into the crater, located as this one was--on an island, and not far from the coast. The volcanic chains crossed at that point. Such crossing would cause a tension of the crust of the earth, which might cause great fissures. If water were to search out those fissures and reach the great molten ma.s.s below it is not hard to imagine what the result would be. There are two cla.s.ses of volcanoes--those which have explosive eruptions, like Vesuvius and Krakatoa, and this latest one, and those of no explosive nature, like Mauna Loa and Kilauea, in Hawaii, which boil up and flow over. It is the explosive eruption which brings widespread destruction, and it is astonishing to learn of the tremendous power one of those eruptions unleashes."
Professor John Milne, of London, the highest authority in the world on volcanic explosions, cla.s.sifies eruptions into two grades: Those that build up very slowly. Those that destroy most rapidly.