Volcanoes: What They are and What They Teach

by John W. Judd

In other cases, like Vesuvius, we find that great volcanic cones of trachytic tuff have been built up, and that these masses of fragmentary trachytic materials have been surrounded and enclosed by the ejection, at a later date, of great outbursts of basaltic lavas. In still other cases, of which Rocca Monfina in Southern Italy constitutes an excellent example, we find that a great crater-ring of trachytic tuffs has been formed in the first instance, and in the midst of this a cone, composed of more basic materials, has been thrown up.

EXCEPTIONS TO THE GENERAL LAW.

In all these volcanoes we see the tendency towards the eruption of intermediate lavas in the first instance, and of basaltic and acid lavas at a later date. Valuable, however, as are the early generalisations of Scrope, and the more precise law enunciated by Von Richthofen concerning the 'natural order of succession of volcanic products,' we must not forget that there are to be found a considerable number of exceptions to them. There are some volcanic centres from which only one kind of lava has been emitted, and this may be either acid, basic, or intermediate in composition; and on the other hand, there are districts in which various kinds of lava have been ejected from the same vents within a short period of time, in such a way as to defy every attempt to make out anything like a law as to the order of their appearance. Nevertheless the rules which we have indicated appear to hold good in so great a number of cases that they are well worthy of being remembered, and may serve as a basis on which we may reason concerning the nature of the action going on beneath volcanic vents.

From the study of the external appearances of volcanic mountains, combined with investigations of those which have been dissected by denudation, we are able to picture to our minds the series of actions by which the great volcanic mountains of the globe have been slowly and gradually built up.

In the first instance the eruptions appear to have taken place at several points along a line of fissure, but gradually all of these would become choked up except one which became the centre of habitual eruption. From this opening, ejections, firstly of lavas of intermediate composition, and afterwards of basic materials, would take place, until a volcano of considerable dimensions was built up around it. But at last a point would be reached in the piling up of this cone, when the volcanic forces below would be inadequate to the work of raising the liquid lava through the whole length of the continually upward-growing tube of the volcano. Under these circumstances the expansive force of the imprisoned steam would find it easier to rend asunder the sides of the volcanic cone than to force the liquid material to the summit of the mountain. If these fissures reached the surface explosive action would take place, in consequence of the escape of steam from the glowing mass, and scoria-, tuff-, and lava-cones would be formed above the fissure. In this way, as we have already pointed out, the numerous 'parasitic cones' which usually abound on the flanks of the greater volcanic mountains have been formed. The extrusion of these masses of scoriæ and lava on the flanks of the mountain tends, not only to increase the bulk of the mass, but to strengthen and fortify the sides. For by the powerful expansive force at work below, every weak place in the cone is discovered and a fissure produced there; but by the extrusion of material at this fissure, and still more by the consolidation of the lava in the fissure, the weak place is converted into one of exceptional strength.

INTRUSIVE MASSES BENEATH VOLCANOES.

As the sides of the cone are thus continually repaired and strengthened they are rendered more capable of withstanding the heaving forces acting from below, and these forces can then only find vent for themselves by again raising the liquefied lava to the central orifice of the mountain. Many volcanoes, like Etna, exhibit this alternation of eruptive action from the crater at the summit of the mountain, and from fissures opened upon its flanks, the former tending to raise the height of the volcanic pile, the latter to increase its bulk.

But at last a stage will be reached when the volcanic forces are no longer able either to raise the lava up the long column of the central vent on the one hand, or to rend asunder the strongly-built and well-compacted flanks of the mountain on the other. It is probably under these conditions, for the most part, that the lavas find their way between the masses of surrounding strata and force them asunder in the way that we have already described.

In the case of the more fluid basaltic lavas, as was pointed out so long ago by Macculloch, the liquefied materials may find their way between the strata to enormous distances from the volcanic centre. Such extended flat sheets of igneous rock retain their parallelism with the strata among which they are intruded over large areas, and did not probably produce any marked phenomena at the surface.

But in the case of less fluid lavas, such as those of intermediate or acid composition, for example, the effect would be far otherwise. Such lavas, not flowing readily from the centre of eruption, would tend to form great bulky lenticular masses between the strata which they forced asunder, and, in so doing, could not fail to upheave and fissure the great mountain-mass above. Vast lenticular masses of trachytic rock, thus evidently forced between strata, have been described by Mr. G. K. Gilbert, as occurring in the Henry Mountains of Southern Utah, and by him have been denominated 'laccolites,' or stone-cisterns. Whether the great basaltic sheets, like those described by Macculloch, and those more bulky lenticular reservoirs of rock of which Mr. Gilbert has given us such an admirable account, were in all cases connected with the surface, may well be a matter for doubt. It is quite possible that, in some cases, liquefied masses of rocky materials in seeking to force their way to the surface only succeeded in thus finding a way for themselves between the strata, and their energy was expended before the surface was reached and explosive action took place. But it is an undoubted fact that beneath many of the old volcanoes, of which the internal structure is now revealed to us by the action of denuding forces, great intrusive sheets and laccolites abound; and we cannot doubt that beneath volcanoes now in a state of eruption, or in those which have but recently become extinct, similar structures must be in course of formation.