Henry James Johnston-Lavis

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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, [1] 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 ”

“ 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 ”

“ 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. ”

“ 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. ”