Popular Science Monthly/Volume 19/May 1881/The Origin and Structure of Volcanic Cones II
|THE ORIGIN AND STRUCTURE OF VOLCANIC CONES.|
By H. J. JOHNSTON-LAVIS, F. G. S.
IT is observable in certain volcanoes that the lava frequently strewed around after an eruption contains more or less perfect spheres, consisting of a hard external coat and more scoriaceous contents, and these from their resemblance are known as volcanic bombs. Their contents may be divided into two classes:
1. Scoriaceous vesicular lava, identical in composition with the external shell.
2. Miscellaneous, such as altered masses of lapilli, loose blocks of foreign materials caught up in the current of lava. These balls are generally considered to be formed by the masses being ejected to great heights, and cooling as they whirl through the atmosphere.
This seems improbable, as on falling they would inevitably smash into a thousand fragments. It would appear more likely that they are simply concretionary in structure around a nucleus of low temperature, solidifying on the surface a layer forming a crust of lava. Let us now direct our attention to the minor particulars, such as the changes of the crater, and metamorphism, or alteration of the already ejected materials. If the volcano has already reached some considerable dimensions, effected by one or many eruptions, we shall find that certain definite changes have taken place in the chimney. The eruption is reduced in force, there are spasmodic puff-like ejections of lapilli, and occasionally small streams of lava emitted. Before entering further into our subject, we must return a step or two. It has been mentioned that the inclination of the outer slope of the cone is that of the "angle of repose" of the rock-fragments. We should, therefore,
conclude that the inner or chimney side would be much the same. This, however, is not generally the case, the inner retaining a greater slope than the outer. It is due chiefly to the fusion, or cementing together, of the fragments by the intense heat and the presence of lava, which, so to speak, solders each mass to its neighbor. Each is retained in position by the fluid column occupying the internal cavity, and when this has disappeared the temperature is necessarily lowered, and thus there is formed a lining to the tube by the semifusion of its superficial components. Nevertheless, the upper edges crumble away, falling into the vents, thence to be again ejected. This process continually repeated will result in the majority of the lapilli falling on the outer slope, leaving the chimney of the form of a true funnel, that is to say, a cavity whose sides descend for a certain distance at a moderate angle, say roughly 45°, and then suddenly increasing to nearly a perpendicular. The consequence of this is, a basin-like cavity of sloping walls, with the volcanic vent situated at its center. The materials now ejected by the volcano, supposing it to be in a comparatively quiescent state, will tend to build up a fresh cone occupying this basin. It is not a thing unknown for such a concentric arrangement of cones and craters to be extended to many repetitions. Let us take for example the crater of Somma (Fig. 1) occupied by the cone of Vesuvius, and this again inclosing within its own walls the little cone of eruption, A. We may perhaps represent it thus: A: B:: B: C. Such a repetition is recorded as being quadruple, thus giving to the mountain, near its apex, a step-like appearance.
From various irregularities and accidents, the vent may shift its position and become eccentric, and thus produce an overlapping of the newer cone upon the older. This is well illustrated by the Island of Vulcano at this moment. In fact, the little hill of scoria surrounding the active bocca or mouth of Vesuvius is situated right away to the east-northeast of the crater, and consequently the lava-streams are more abundant on that side of the mountain (Fig. 2).
The escape of the lava and vapor is the next thing to require our attention. Little more, however, has to be said. The lava rarely mounts the edge of the cone of eruption, generally escaping near its foot, by forcing itself a passage through the loose materials or some preëxisting fissure according to hydrostatical laws. The vapor is the real agent in keeping a vent clear, as the vast bubbles rise through the viscid mass, bursting at its surface, thus keeping up the temperature of the lava-column which it has traversed by the heat brought up from below, and at the same time preventing any permanent stagnation therein. The vapor is generally to be seen carried away by the wind in beautiful white clouds. When, however, the eruption is of a more intense kind, these vast volumes mount into the air at great heights appearing like a column of fire by night, carrying with them lapilli and ash often thousands of feet above the mouth of the volcano;
then, suddenly spreading out, these vast clouds give rise to the well-known appearance of the Italian pine-tree. At the same time, from every available fissure are seen to issue little columns of vapor, adding their small share to the grandest visible display of force that Nature has provided for our amusement or peril.
This brings us to the next point of interest, the formation of fumaroles, which may be considered as the effect of these two agents last spoken of acting together. In Fig. 2 are seen two varieties, one assuming a spire-like form. These may make their appearance anywhere on the volcano, but are usually situated in close proximity to the vent. Their size varies from twelve inches in height upward; generally from three to thirty feet. They are commenced in the major number of cases in the fissured crust recently formed over still-flowing lava. Here the vapor escapes in spasmodic puffs, and by its force a small quantity of lava is forced up and spread out around the aperture, which rapidly cools. It is followed by another puff and another oozing of lava above and around the aperture of the first. In this manner layer by layer is built up, thus giving an irregular, imbricated, roll-like appearance to the exterior. The surface is rapidly covered by brilliantly-colored sublimates, and the fumarole then presents a very pretty spectacle. The author lately was able to thoroughly watch the formation of such a fumarole some twenty feet high, its decadence and disintegration extending over a period of eight months. On passing the arm down the central tube. (i. e., the fumarole was extinct), it could be felt very regular and smooth, and having a pretty uniform bore of about nine inches.
After one slight eruption, the fumarole in question presented a very curious phenomenon. Immediately (about two or three seconds) after the explosion from the main vent, there came three terrific bangs, with a spout of vapor from its apex, the last one shooting out small fragments of still liquid lava.
This continued without variation for six hours that the author remained in the crater. The spire-like form may be varied according to
surrounding circumstances. If the escape take place along a fissure, it will assume on occasions a miter-like form. There are many other varieties in form, depending on the variability of surrounding circumstances.
It is now necessary to draw attention to the great difference of opinion which has been expressed upon a point for which we have very little data to support either of two views of the question.
Vulcanologists were for a long time divided into two schools, which often waged war against each other with considerable fierceness. The so-called upheavalists were led by such eminent men as Von Buch, Elie de Beaumont, and Humboldt; whereas those who held the opposite view, which will be immediately explained, claimed as their adherents Sir Charles Lyell, Poulett Scrope, and others.
In Figs. 3 and 4 are diagrammatic representations of the two theories.
The upheavalists believed that the earth-crust actually surrounding the vent was bodily lifted up by the subterranean igneous forces into a dome-shaped or bubble-like mass, thus forming the main mass of the
Fig. 4.—Crater of Upheaval.
cone, of which the center was the point of fracture, and therefore the vent. The ejecta were therefore considered to form only a thin superficial crust covering this. The subjacent rock which had been elevated would thus have a quaquaversal or periclinal dip away on all sides from the chimney (Fig. 4).
The opponents to this view attribute the entire bulk of the mountain to the ejecta, as seen in Fig. 3, the only change in the basement beds being those produced by pressure and excavation, both of which tend to make them dip toward the vent, thus producing quite a converse effect to the former.
This latter view certainly seems the most feasible, and, after a careful examination of many of the old craters brought forward by the upheavalists as evidence, one becomes satisfied that they have wrongly interpreted facts, which the more advanced state of knowledge at the present day and the collected experience of subsequent observers make easy to our perception. On the other hand, it would be undoubtedly rash to conclude that all craters were formed entirely on one or the other model. Jorullo, in Mexico, for instance, has many points about it to support the upheaval theory. David Forbes, that clear observer, mentions many facts about South American volcanoes that should deter us from admitting the formation of cones and craters by the deposition of ejecta only.
The rapidity with which a volcanic cone may be raised is a point of great interest. We hear every now and then of some small island appearing and again disappearing below the sea almost as rapidly as it rose. Probably, however, the best illustration is that of Monte Nuovo, four hundred and fifty-six feet high, situated in the Campi Phlegraci, about eight miles west of Naples. This was raised from a marshy plain almost level with the sea in about four days, commencing on September 30, 1538.
The whole hill is, therefore, the product of one eruption. It is an interesting fact that no stream of lava was developed. It would seem that the explosions were so intense that the fluid rock was entirely broken up and ejected in a fragmentary condition, of which there are great quantities forming the slopes.
The cone of Vesuvius proper, fifteen hundred feet high above the lowest edge of the crater of Somma, has entirely been built up of the ejecta thrown out at the time of and since the memorable eruption of a. d. 79, in which Herculaneum, Pompeii, Stabia, etc., were destroyed. Besides the bulk of the mountain now seen, we must not forget the vast quantity that has been required to fill up the crater of Somma, much enlarged by the eruption spoken of. It is said that no lava ran from Vesuvius till the tenth century; this probably would be explained by the fact that all the earlier streams were occupied in filling up the great crater.—Science-Gossip.