Popular Science Monthly/Volume 20/November 1881/Volcanoes, their Action and Distribution
|VOLCANOES, THEIR ACTION AND DISTRIBUTION.|||
"WHAT is a volcano?" This is a familiar question, often addressed to us in our youth, which "Catechisms of Universal Knowledge" and similar school manuals have taught us to reply to in some such terms as the following: "A volcano is a burning mountain, from the summit of which issue smoke and flames." This description, says Professor Judd, is not merely incomplete and inadequate as a whole, but each individual proposition of which it is made up is grossly inadequate and, what is worse, perversely misleading. In the first place, the action which takes place at volcanoes is not "burning," or combustion, and bears, indeed, no relation whatever to that well-known process. Nor are volcanoes necessarily "mountains" at all; essentially, they are just the reverse—namely, holes in the earth's crust, or outer portion, by means of which a communication is kept up between the surface and the interior of our globe. When mountains do exist at centers of volcanic activity, they are simply the heaps of materials thrown out of these holes, and must, therefore, be regarded not as the causes but as the consequences of volcanic action. Neither does this action always take place at the "summits" of volcanic mountains when such exist, for eruptions occur quite as frequently on their sides or at their base. That, too, which popular fancy regards as "smoke" is really condensing steam or watery vapor, and the supposed raging "flames" are nothing more than the glowing light of a mass of molten material reflected from these vapor-clouds. The name of volcano has been borrowed from the mountain Vulcano, in the Lipari Islands, where the ancients believed that Hephæstus, or Vulcan, had his forge. Volcanic phenomena have been at all times regarded with a superstitious awe, which has resulted in the generation of such myths as the one just mentioned, or of that in which Etna was said to have been formed by the mountains under which an angry god had buried the rebellious Typhon. These stories changed their form, but not their essence, under a Christian dispensation, and Vulcano became regarded as the place of punishment of the Arian Emperor Theodosius, and Etna as that of Anne Boleyn, who had sinned by perverting the faith of King Henry VIII.
Volcanic phenomena can be studied to great advantage in Stromboli, whose crater, the edge of which is easily accessible, is in a state of constant moderate action, and can be watched for hours together without having the judgment warped either by an excited imagination or the sense of danger. It is an island of rudely circular outline and conical form, and rises to the height of three thousand and ninety feet above the level of the Mediterranean (Fig. 1). From a point on the side of the mountain masses of vapor are seen to issue, which unite to form a cloud over the summit, the outline of which varies continually according to the hygrometric state of the atmosphere and the direction and force of the wind. At the time the sketch was made, April 20, 1874, the vapor-cloud was spread in a great horizontal stratum overshadowing the whole island, but was clearly seen to be made up of a number of globular masses, each of which was a product of a distinct outburst of the volcanic forces.
The mountain is visible over the sea for a hundred miles. When it is watched from the deck of a vessel anywhere within this distance, "a glow of red light is seen to make its appearance from time to time above the summit of the mountain; this glow of light may be observed to increase gradually in intensity, and then as gradually to die away. After a short interval the same appearances are repeated, and this goes on till the increasing light of the dawn causes the phenomenon to be no longer visible. The resemblance presented by Stromboli to a 'flashing-light' on a most gigantic scale is very striking, and the mountain has long been known as the 'lighthouse of the Mediterranean.'"
The island appears, if we land upon it, to be entirely built up of such materials as we know to be ejected from volcanoes; "indeed, it
resembles, on a gigantic scale, the surroundings of an iron-furnace, with its heaps of cinders and masses of slag. The irregularity in the form of the island is at once seen to be due to the action of the wind, the rain, and the waves of the surrounding sea, which have removed the loose, cindery materials at some points, and left the hard, slaggy masses standing up prominently at others." This pile stands in a sea of about six hundred fathoms depth; as it rises to more than three thousand feet above the surface, it represents a conical mass of cinders and slaggy materials, six thousand feet high and more than four miles in diameter at the base. The crater may be approached by a flat slope called the Sciarra, which rises at an angle of 35° with the horizon, and ends abruptly at its edge. The accompanying sketch (Fig. 2) was
made from this point at the moment of an outburst. "Before the out-burst, numerous light, curling wreaths of vapor were seen ascending from fissures on the sides and bottom of the crater. Suddenly, and without the slightest warning, a sound was heard like that produced when a locomotive blows off its steam at a railway-station; a great volume of watery vapor was at the same time thrown violently into the atmosphere, and with it there were hurled upward a number of dark fragments, which rose to the height of four or five hundred feet above the crater, describing curves in their course, and then falling back upon the mountain. Most of these fragments tumbled into the crater with a loud, rattling noise, but some of them fell outside the crater, and a few rolled down the steep slope of the Sciarra into the sea."
Spallanzani and other later investigators carried on their observations from a point whence they could look down into the bottom of the crater, and, with a wind that would blow the vapors away, sit and watch for hours the wonderful scene. From this point, the blacky slaggy bottom of the crater is seen to be traversed by many fissures or cracks, from most of which curling jets of vapor issue quietly and gradually mingle with and disappear in the atmosphere. Besides, there are several larger openings, varying in number and position at different periods, from some of which "steam is emitted with loud, snorting puffs, like those produced by a locomotive-engine, but far less regular and rhythmical in their succession"; from others, masses of molten material well out and sometimes flow outside of the crater, with steam escaping, often in considerable quantities; in some a viscid or semi-liquid substance is seen seething, swelling up and forming gigantic bubbles, which burst under occasional great rushes of steam, carrying fragments of the scum-like surface of the liquid high up into the atmosphere, almost precisely as takes place in a pot of boiling mush. At night, "the smaller cracks and larger openings glow with a ruddy light. Every time a bubble bursts, and the crust is broken up by the escape of steam, a fresh, glowing surface of the incandescent material is exposed. If at these moments we look up at the vapor-cloud covering the mountain, we shall at once understand the cause of the singular appearances presented by Stromboli when viewed from a distance at night; for the great masses of vapor are seen to be lit up with a vivid, ruddy glow, like that produced when an engine-driver opens the door of the furnace and illuminates the stream of vapor issuing from the funnel of his locomotive."
These phenomena differ only in degree from those presented by volcanoes which, like Vesuvius, are spasmodically more active. Occasionally, the violence of the outbursts at Stromboli is temporarily increased; and at Vesuvius a series of small explosions, quite similar to those occurring at Stromboli, were observed for some months before the great eruption of 1872. French geologists, in fact, define the conditions of activity of volcanoes by speaking of the "Strombolian" and the "Vesuvian" stage as two degrees between which the passage is by insensible gradations. The eruption of 1872, of which the accompanying view (Fig. 3) is from a photograph, afforded a fine example of the intense Vesuvian stage. The activity of the forces at work within the mountain had been on the increase for more than a year, and reached its climax on the day the photograph was taken. During the eruption the bottom of the crater was entirely broken up, and the sides of the mountain were rent by fissures in all directions whence liquid matter appeared to be oozing as from every part of the surface, or as Professor Palmieri expressed it, "Vesuvius sweated fire." Enormous volumes of steam rushed out from the crater and from some of the fissures, to the height, as shown by the picture, of twenty thousand feet, or nearly four miles, with a prodigious roaring sound that frightened the inhabitants of Naples into leaving their houses and seeking refuge in the streets. The roaring was produced by explosions or detonations rapidly following one another, each of which sent up a globe of white vapor, out of a mass of which globes the overhanging cloud was formed. Lava, or molten rock, rushed down the sides of the mountain in great streams, whence enormous volumes of steam continually rose, forcing the congealing rock as it escaped from it into great bubbles and blisters, thus giving rise to the formation of innumerable miniature volcanoes.
Other phenomena accompanying this eruption were the prevalence of earthquake-shocks in the country around, the vivid lightnings and thunders arising from the electrical action engendered by the column of steam, and the excessive rainfall which followed the condensation of the steam.
In both these volcanoes the active cause of all the phenomena exhibited is found to be the escape of steam from the midst of masses
of incandescent liquefied rock. The violence, the grandeur, and the destructive effects of an eruption depend upon the abundance and tension of this escaping steam.
The manner in which volcanic cones are built up is ascertained from the examination of sections of them. This was first attempted at the suggestion of Goethe, in the case of the Kammerbühl, a small hill in Bohemia, concerning which it was disputed whether its materials had been derived from the combustion of coal or from aqueous precipitation, or whether they were of volcanic origin. An excavation to the core of the hill, finished in 1837, showed that the center of the mass was filled with a plug of basalt, which was connected with a small lava-stream flowing down the side of the hill; while the bulk of the hill was composed of volcanic scoriæ and lapilli (Fig. 4). Natural
sections are not rare. A very fine one is afforded by the peninsula of Vulcanello, in the Island of Vulcano (Fig. 5). The peninsula consists of three volcanic cones, united at their base, with the lava-streams
which have flowed from them. One half the cone on the left side of the picture has been completely washed away by the sea, so that a perfect section of the internal structure is exposed, as in the accompanying figure (Fig. 6). This section shows—1. The loose scoriæ and lapilli, d, which, in falling through the air, have arranged themselves in tolerably regular layers on the sides of the cone; 2. streams, b, which have been ejected from the crater or from fissures, and flowed down the sides of the cone; and, 3. Masses of lava, c, filling up cracks in the cone, called "dikes." Most volcanic mountains are built up of these three kinds of material, but with varying arrangement and proportions. One kind very often predominates
almost to the exclusion of the others. The materials falling through the air upon the surface of the mountain assume a stratified arrangement, in which the finer matters are sorted out and carried to a greater distance than the others, the same as when the deposit is made from water. When materials of a different character are thrown out by different eruptions, the distinctions are very plainly marked.
An opportunity was given for observing the formation of a volcanic cone through all its stages in the case of the eruption of Monte Nuovo, or New Mountain, on the shores of the Bay of Naples, in 1538 (Fig. 7.)
After the neighborhood had been affected by earthquakes for more than two years, on the 29th of September, at eight o'clock in the morning, to be exact, water, first cold, afterward becoming tepid, was observed to issue from a depression which was noticed on the site of the future hill. Four hours afterward the ground was seen to swell up and open, forming a gaping fissure, within which incandescent matter was visible. Masses of stone and vast quantities of pumice and mud were then thrown up to a great height for two days and nights, and, falling on the sides of the vent, formed a great mound, which was climbed up by adventurous persons on the third day, a quiet one. The ejections were resumed on the next day, when several persons who undertook to climb the hill were killed or injured, but ceased on the seventh or eighth day. The mass of the hill, which is four hundred and forty feet above the sea, was chiefly composed of the materials which were thrown out during the first two days and nights. It consists of scoriæ, lapilli, and dust, and is now covered with a thick growth of pines. The crater is marked by a steep, cup-shaped depression, the bottom of which is but little above the level of the sea. The district in which this mountain is situated contains a great number of hills, strikingly resembling it, some of which are larger, some smaller than it, but all so similar that "no stranger visiting the district, without previous information on the subject, would suspect the fact that, while all the other hills of the district have existed from time immemorial, and are constantly mentioned in the works of Greek and Roman writers, this particular hill of Monte Nuovo came into existence less than three hundred and fifty years ago."
The form of the cones is modified by the character of the materials thrown out, by the action of the weather, and by repeated eruptions. Loose material, scoriæ and lapilli, roll till they reach a position of rest, and leave a more or less regular cone. Very liquid lavas flow to great distances, resting at a very slight slope, as in the volcanoes of Hawaii, where, with a slope of only six or eight degrees, the mountains have a diameter of seventy miles at their base, and reach a height of
fourteen thousand feet. If, on the other hand, the lava is only imperfectly liquid, it tends to accumulate around the vent and form a more or less steep-sided bulbous mass, as in number two of the figure (Fig. 8). The shape of the cone may undergo changes during an eruption, as in the accompanying outlines of Vesuvius (Fig. 9). Most of the great volcanic mountains belong to the class of "composite cones," and are built up by alternate ejections of fluid lava and fragmentary materials. The slopes of their sides are subject to a wide range of variation, corresponding with the varying character and degree of liquidity of these materials. The sides of the cones are liable to be rent asunder and traversed with fissures, through which liquid lava forces its way and
gives rise to new subsidiary cones, or series of cones, along the lines of the clefts. These cones are called parasitic cones, and frequently attain considerable dimensions, some of those on the flanks of Etna being nearly eight hundred feet high. A typical example of a group of such cones is given in the Island of Ischia, where several parasitic cones have been formed around the main cone. Another example is furnished on the slopes of Etna, where the scoria-cones raised in 1865 along a fissure were so close together as to make a long, irregular ridge (Fig. 10). Seven of these cones were formed along this fissure,
thirty-six along another fissure during the eruption of 1874. Similar phenomena were witnessed upon the slopes of Vesuvius in 1760, when a fissure opened on the south side of the mountain, and fifteen scoria-cones, which are still visible, were thrown up along it.
The center of eruption has sometimes shifted itself along a line of fissure, as has taken place at Etna, where the present center is four miles from the old one, and in the Island of Vulcano with the peninsula of Vulcanello, which affords the best possible example of such a shifting. Whole systems of volcanoes appear to be built up along the lines of such fissures, as is shown by the linear arrangement of volcanoes in different parts of the earth, and strikingly in the Lipari Islands, where the volcanoes are arranged along a series of lines which doubtless mark rents in the earth's crust, and which radiate from a center at which we have proofs of the former existence of a volcano of enormous dimensions.
There is also good ground for believing that the great linear bands of volcanoes which, as we shall see, stretch for thousands of miles over the earth, have had their positions determined by great lines of fissure in the earth's crust. While, however, the smaller fissures, upon which rows of scoria-cones are thrown up, seem to have been in many cases opened by a single effort of the volcanic forces, the enormous fissures which traverse so large a portion of the surface of the globe are doubtless the result of numerous manifestations of energy extending over vast periods of time.
It is very hard accurately to estimate the number of volcanoes in the world. They vary in size from immense mountains like Chimborazo and Cotopaxi to mere holes in the ground, letting escape barely perceptible columns of vapor. The history of a large proportion of them has been known only for a brief period, and many which are considered extinct because they have never been seen in action may be merely dormant, ready to burst out at any time, as Vesuvius did in the year 79. If we include only habitual vents of considerable importance, which we have reason to believe may still be in active condition, the number may be put at between three hundred and three hundred and fifty. Most of these are marked by more or less considerable mountains formed of the matters ejected from them. If we include the mountains which exhibit the main features of volcanoes, but concerning the activity of which we have no record or tradition, the number will not fall much short of one thousand. Then there are other "ruined volcanoes," the cones of which have been worn away and of which the "ground-plans" only are left, still more numerous. The smaller temporary openings, usually subordinate to the habitual vents, known to ancient and modern history and tradition, may be counted by thousands and tens of thousands. The still feebler manifestations—steam-jets, geysers, thermal and mineral waters, fumaroles, mud-volcanoes, and the like—must be numbered by millions. The latter class seem to play a small part as we contemplate them singly, but their force in the aggregate probably far exceeds that of all the great habitual vents. These volcanoes, in all their classes, are very unequally distributed over the globe. Vesuvius is the only habitual vent on the Continent of Europe, and it is on the shores of the Mediterranean; the Mediterranean islands contain six; Africa has ten—four on the western, six on the eastern coast; Asia, so far as is known, twenty-four, twelve of which are on the peninsula of Kamtchatka. None are known in Australia. North America has twenty volcanoes, Central America twenty-five, and South America thirty-seven. In all, one hundred and seventeen volcanoes are situated on the great continental lands, leaving nearly twice that number distributed over the islands of the oceans.
In nearly all cases, the volcanoes are either close to the shores of the continent or at no very great distance from them. The only known exceptions are in the Central Asian plateau and Chinese Mantchooria, concerning which more accurate information is needed. All the oceanic islands that are not coral reefs are of volcanic origin, and many of them contain active volcanoes. A ridge running through the midst of the Atlantic Ocean and embracing the Islands of Jan Mayen, Iceland, the Azores, Canaries, and West Indies, contains forty active volcanoes and a greater number of extinct ones. A similar line in the Pacific Ocean, including the group of islands southeast of the Asiatic Continent, "the grandest focus of volcanic activity on the globe," contains no less than one hundred and fifty active volcanoes; and, if we include those on lines branching from the main one, half the habitually active vents of the globe. A third series of volcanoes starts from near the last one in the neighborhood of Behring Strait, and stretches along the whole western coast of the American Continent, with about eighty active vents.
The volcanoes of the globe thus usually assume a linear arrangement, and are nearly all situated along three well-marked bands and the branches proceeding from them. The volcanoes of the eastern coast of Africa, with Mauritius, Bourbon, Rodriguez, and the vents along the line of the Red Sea, may be regarded as forming a fourth and subordinate band. Nearly all of them are situated near the limits which separate the great land and water masses of the globe, either on the parts of continents not far removed from their coast-lines, or on islands in the ocean not very distant from the shores. Two conspicuous exceptions to this rule are the volcanoes of the Thian-Shan range in Asia, in the center of the largest unbroken land-mass of the globe; and the group of the Sandwich Islands, almost in the center of the largest ocean, and rising almost from the greatest depths of that ocean. Geological researches have, however, shown that the Thian-Shan Mountains in Pliocene times stood on the southern borders of a great inland sea. A regular parallelism seems to exist between volcanic bands and the great mountain-chains; and the researches of Mr. Darwin have shown that "nearly all the active volcanoes are situated upon rising areas, and that volcanic phenomena are conspicuously absent from those parts of the earth's crust which can be proved at the present day to be undergoing depression."
Inferences are sometimes hastily drawn from the fact that most volcanoes are near the ocean which the facts themselves will hardly warrant. Thus, it is frequently assumed that we may refer all the phenomena of volcanic action to the penetration of sea-water to a mass of incandescent lava in the earth's crust and to the chemical and mechanical actions which result from the meeting. This argument, however, as Mr. Scrope has shown, involves a reasoning in a circle. "It is assumed, on the one hand, that the heaving subterranean movements which give rise to the fissures by which steam and other gases escape to the surface are the result of the passage of water to the heated masses in the earth's crust. But, on the other hand, it is supposed that it is the production of these fissures which leads to the influx of waters to the heated materials. If it is the passage of water through these fissures which produces the eruptions, it may be fairly asked, What is it gives rise to the fissures? And if, on the other hand, there exist subterranean forces competent to produce the fissures, may they not also give rise to the eruptions through the openings which they have originated?"
Many of the various theories which have been proposed to account for volcanic action—depending upon the supposed presence of active forces within the earth; upon the contact of water with the hot solid or liquid matter of the interior of the globe; upon the chemical actions that may be taking place within the earth; upon the heat that may be developed by the contraction of the earth's crust; or upon the occlusion of gases by the metallic elements of which many suppose the core of the earth to be composed—have a certain probability. They are not definitely contradicted by anything that we know, but are as yet still further from being fully sustained by facts; and they all prove to be beset by difficulties when they are subjected to a critical analysis. Thus, "it must be admitted that we do not at present appear to have the means for framing a complete and consistent theory of volcanic action, but we may hopefully look forward to the time when further observation and experiment shall have removed many of the existing difficulties which beset the question, and when by the light of such future researches untenable hypotheses shall be eliminated and the just ones proved and established."
Modern speculation, recognizing that the worlds of our system are bound by the same laws and had the same origin, now tends to look to the study of what is going on in the sun and planets as a valuable aid in ascertaining the reason of the operations to which our planet is and has been subjected.
- Volcanoes, what they Are, and what they Teach. By John W. Judd, F.R.S. With Ninety-six Illustrations. "International Scientific Series." In press of D. Appleton & Co.