Popular Science Monthly/Volume 18/April 1881/The Origin and Structure of Volcanic Cones I
By H. J. JOHNSTON-LAVIS, F. G. S.
OUR general ideas of its appearance, if we have never seen a volcano, differ considerably from what we find when actually brought in contact with one.
We always have the tendency to associate a mountain as the site of volcanic outbursts. Such is the case in general rule, though with many exceptions. In fact, the variations are so great that in many cases we should be inclined to attribute the extreme forms to totally different origins, were there not existing intermediate ones which demonstrate that they are all varied modifications of one almost uniform series of physical effects.
Thus to one looking first at the vast volcanic cone of Cotopaxi, almost perfect in form, and comparing it with the ring-like cavity of Astroni in the Phlegrean field, it would be almost incomprehensible to believe that these two extremes are the result of identical forces acting much in the same manner and producing such widely different effects. But in the latter district we have not to travel far to find other vents that act as interpreters in explaining these variations of forms. In the present paper it will be my endeavor to explain the building up of what we will call a normal volcanic cone, and then afterward to point out the extreme variations to which such a mass is liable.
Given a large volume of heated vapors and liquid rock that has burst its way upward through the subjacent strata, in what way will it manifest its presence, and what traces will it leave behind? This vapor does not seem to exist separately from the molten rock or lava at any great depth, but as it approaches the surface the enormous pressure is reduced, the water and other gaseous matter expand, separate themselves into little bubbles scattered through the highly heated liquid magma. These will collect, to a certain extent, and from their lightness will float to the surface of the lava and there burst. The vapor may have commenced to form at great depths, and in its upward journey have become exceedingly bulky, so when it reaches the surface it would escape with a loud explosion. If we watch lava in the crater of a volcano in a quiescent state, such as Vesuvius, we see these great bubbles, so to speak, continually forming and bursting. As they burst, the surface of the vesicle is blown up as soft, pasty fragments, to the height of many feet. These masses appear black by day, but red-hot by night; they may cool or not, before falling; if the latter, when they strike the ground, they adapt themselves to the irregularities of the surface, and form, as it were, a cast thereof. This condition is much exaggerated at the first outbreak of an eruption; the vast column of fragments often reaches an altitude of two and three thousand feet. There the pieces ascending meet those descending, and so there is a continual grinding going on between them; the fine dust is taken by the wind and transported often many miles, forming the so-called clouds of volcanic ash. The larger fragments (or lapilli, as they are named) may again fall back into the opening or around its edge, thus building up an annular bank. This is really the foundation of the cone.
If we speculate for a moment on the formation of such a heap, we shall see that the first strata deposited will be horizontal, but somewhat thicker toward the axis of explosions. (See D, diagram.) This, however, as the action continues, will begin to arrange itself in a direction slanting away from the axis, until the beds reach the maximum angle of repose of the rock-fragments in question as the beds (D D) on diagram. Thus we have constructed a conical mass in the center of which is the volcanic chimney (B), and, dipping away on all sides at angles, varying generally between 20° and 45°, we find the strata composing the cone (D, E). This arrangement is often called
Diagrammatic bird's-eye view of a volcanic cone.—The upper part is supposed to be removed by an horizontal section and one half of the remaining base by another longitudinal one. A, vent; B, chimney; C, basement rock, compressed downward at C' and upward at C"; D, ash-beds; E, lava streams, one of which, E' is seen to have run down the slopes, G, of the cone, and spread over plain F.
periclinal. The funnel, or chimney, which has been mentioned as occupying the center, has the form of an inverted cone, the inclination of its sides and its diameter necessarily being proportional to the volume and force of the escape of vapor, and also to the nature, form, and size of the surrounding fragments, forming the growing cone, which have already been ejected. The upper, or basin part, is technically called the crater. The vapor only may have made its appearance at the surface, and in fact may have parted company with the lava at very considerable depths.
Or the latter may have been forced up almost simultaneously with the vapor, and poured out over the edge of the primitive cone. This, however, is not the general rule, for an escape of much gaseous material nearly always precedes for a variable period the appearance of the lava. In fact, when a volcanic outburst has forced a convenient passage for the vapor, the exit of liquid rock seems of secondary importance, for generally the terrific explosions, earthquakes, and subterranean thunder that accompany the first stage of eruption are more or less absent, or at least much diminished during the welling up of the fluid rock. If, as in the latter case, a cone of some considerable size has been formed, the lava will rise and occupy the whole of the crater-cavity. Two things may happen: If the cone which now forms, as it were, an embankment around the lava is of sufficient strength to withstand the pressure of the fluid mass contained within it and the continual explosive vibrations, the liquid rock pours out over the edge of the crater down the side of the cone, and may continue its course for variable distances from its starting-point; or if, on the other hand, the cone is too weak to support the strain, it may break away and give free passage to the lava through the breach. This condition is well illustrated in many of the Puys of central France. There is another series of events, that is to say, the formation of dikes, about which we shall have more to say anon.
The lava may form a series of little streams over the cone sides, changing their situation according to the point at which the crater is lowest. Here it will cool, forming a buttress of rock on the slopes of the cone. These masses will be covered again by lapilli, other buttresses formed in the same manner, and thus the cone built up higher and made stronger. If we see it in section, as in the diagram, it will present a stratification of alternate beds of rock and cinders. This, however, is misleading. The lava-streams do not form a continuous sheet surrounding the cone—see diagram, where they are seen cut through in transverse section. When a mountain of some height has been formed, it then becomes liable to fracturing, and the formation of so-called volcanic dikes. Mr. Mallet, in a communication to the Geological Society, thoroughly explained this condition of things. As we have seen, the cone may form an embankment around the column of lava occupying the chimney and crater, consequently there is an enormous pressure put upon the supporting wall of loose material. Let us begin by taking the pressure of a column of water thirty-two feet high, then let us say another four thousand feet, roughly the altitude of Vesuvius, and compare that with a column of molten lava, whose specific gravity is two or three times that of water. This would be an interesting calculation: given the specific gravity of Etnean lava, the height of the crater, what is the unit of pressure at the sea-level?
The outward pressure of the lava will increase in proportion to the depth. Also the cone wall necessarily increases in thickness from above downward. This, therefore, tends to counteract the augmentation of pressure from within. Nevertheless, when this is so great inside that the inner layer of the chimney must necessarily be compressed outward, and therefore the circumference made larger, the consequence is that at one point it begins to yield, forming the commencement of a perpendicular fissure, radiating from the central axis, and, by the same course of circumstances, this will gradually spread outward. Mr. Mallet, in his paper describing these mechanical effects, aptly compares them to the bursting of a gun where the greatest strain is on the inner lining, and consequently the fissure commences in this and radiates outward. In a volcano, as the fissure is formed, it is immediately occupied by the fluid lava. If the fracture extends far enough it may reach the surface, where it may form one or more parasitic cones. By the explosion of vapor from the lava, these cones are generally formed in a row, radiating from the mountain axis, and in a step-like arrangement. This is attributed to the fact that, as the lava and vapor escape, the former reaches a lower level, and here forms the second, third, fourth, and so on in succession. This was well illustrated in 1861 at Vesuvius, where seven such hollow mounds were formed, the first being the largest, and gradually diminishing downward, as the igneous forces became exhausted. The pressure of the contained fluids may be so great that the entire side of the mountain may be rent asunder with the rapid escape of the contained lava, thus forming a breached cone. In the above-mentioned paper, in fact, it is supposed by the author that all such have originated in this manner. A third condition of things may be brought about: this fissure may only extend a certain distance from the chimney, never showing itself superficially, and the lava occupying the fissure will gradually become cooled and consolidated, forming a perpendicular sheet of rock or dike, as it is called, radiating from the mountain axis. These are well illustrated in the Val de Bove of Etna and the escarpment of Monte Somma. In the former, Sir Charles Lyell adopted the plan of endeavoring to find-the orientation or point of convergence of these dikes, to localize the site of the old crater supposed to have produced this curious cavity. This was followed by the untiring work of Mr. Mallet in the latter locality, to determine where the axis of Somma should be placed. In the latter case twenty-seven of the largest were chosen, but, when their directions were taken by a careful survey, they were found not to converge at one point, but in some there were discrepancies of upward of two kilometres between the points of melting. This we can well understand when we know how irregularly the cone is constructed, and how buried coulées of lava may derange the direction of the fracture, such as we exaggerately see illustrated in some old denuded trap dikes, threading their way along planes of least resistance. There is another source of error—that is, that so little of the projecting edge of the dike is exposed to accurately take its strike, thus rendering us unable to determine by this means the locality of an old volcanic axis.
If we look at the figure, at the surface C' C'' of the subjacent rock, we observe it forms a wave-like line in section. It is again to Mr. Mallet that credit is due for the explanation of this somewhat anomalous appearance. It is known that the ground under high towers and other heavy structures is gradually compressed by the immense superincumbent weight. At the same time a corresponding elevation takes place around the base of the structure. This is just what occurs in a volcanic mountain. The immense pressure of superposed material compresses, to a variable degree, the subjacent rock, according to its yielding power. This will be greatest where the column of materials is highest, that is to say, exactly under the crater edge as at C', in the diagram. This causes a corresponding rim-like elevation around the base, or at the toe of the cone as at C'', in the diagram.
The materials which go to form the cone are the subjects of our next consideration.
Taking as our standpoint the old but useful division of lavas into basaltic or basic, and trachytic or acidic, let us look at the characters presented by these two great classes of rocks. Basalt and its congeners are generally heavy, compact, dark-colored, more or less crystalline. Very rarely vitreous in structure, and only in small patches. Excessively fluid in the molten state, losing heat and fluidity slowly, and then passing rapidly from the liquid to the solid state, the liquid fragments of which, when ejected from the crater, generally fall still plastic, and, when cold, form an excessively ragged, hard, angular mass. The surface or scoria of the lava-stream also is hard, and not easily broken, the main mass itself being very apt to form the well-known columnar structure. On the other hand, the trachytic or acidic lavas, when molten, are very viscid, which condition increases rapidly as it loses its heat, so that it flows very short distances, often stopping midway down the steep side of the cone, as in the island of Vulcano, or forming a large, boss-shaped mass around the vent. When cooled slowly it crystallizes, but it is much more liable to form a vitreous mass or obsidian than the basaltic rocks, resulting probably from its high percentage of silica. In fact, it behaves very much like glass or slag in its physical transformations. As on the surface of the glass pot is formed a frothy-like mass which cools as a light, spongy, vesicular material, so by the explosions from a trachytic volcano, similar masses are formed and thrown out, well known as the useful pumice-stone. This variety of lava produces often a very ragged surface, much less durable to mechanical agents than that of the other class. Again, it is very light, often more so than water. These differences, of course, merge into one another, lavas often occurring that are not easy to classify; but for our purposes the extremes are more suitable of illustration. Also, the same volcano may at different periods have yielded successively each of the varieties of igneous matter. Vesuvius, for instance, has ejected materials of each of the classes, and many distinct varieties of the basic. Obviously the discordance of these physical characters must necessarily produce considerable distinction in the physical conformation of a volcanic region in general, and of the cone in particular. It may be our want of a thorough examination, but it is apparently the rule that dikes are much less common among the trachytic volcanoes than the basaltic, whereas, apparently the largest number of breached cones belong to the former, thus contradicting to some small extent Mr. Mallet's dike theory already referred to. Thus we see that all the solids so far derived from a volcano, lava, scoria, lapilli, ash, etc., are all mechanical modifications of the one molten rock. There is, however, another important factor of which we have not spoken, the so-called ejected blocks. These are nothing more than fragments of the solid rock walls of the volcanic chimney or vent. They, therefore, vary according to the rock through which the igneous outburst has occurred. Thus we find among the constituents of the Vesuvian slopes a great variety of such blocks, among which the beautiful minerals yielded by Somma are found. These may be roughly divided into three classes:
1. Limestone variously metamorphosed, derived from that like Castellamare, which dips under and forms the Vesuvian platform. These fragments are sometimes so altered, by the intense heat, pressure, and chemical action to which they have been subjected, that it is only by studying the intermediate varieties that their origin can be detected. It is these blocks that are richest in the Vesuvian minerals.
2. Calcareous mudstones containing late pleistocene fossils, these being in a very perfect condition, containing generally a great number of well-preserved leaves. This rock is curious, as being of apparently (though not real) volcanic origin, and containing marine fossils without submergence.
3. Trachytic and corresponding tufa, also basaltic tufa. These are also masses of highly micaceous feldspathic rocks, that probably are nothing more than the excessive metamorphosed condition of the first class.—Science Gossip.