Chapter XXV: Etna (2)

If, then, a conical or dome-shaped mass of volcanic materials was accumulated to the height of 4000, or perhaps 7000 feet, before the upward movement began, or, what is much more probable, during the continuance of the upward movement, that ancient mass would not be buried under the products of newer eruptions, because these last would then be poured out chiefly at a lower level.

Since I visited Etna in 1828, M. de Beaumont has published a most valuable memoir on the structure and origin of that mountain, which he examined in 1834;[575] and an excellent description of it has also appeared in the posthumous work of Hoffmann.[576]

In M. de Beaumont's essay, in which he has explained his views with uncommon perspicuity and talent, he maintains that all the alternating stony and fragmentary beds, more than 3000 feet thick, which are exposed in the Val del Bove, were formed originally on a surface so nearly flat that the slope never exceeded three degrees. From this horizontal position they were at length heaved up suddenly (d'un seul coup) into a great mountain, to which no important additions have since been made. Prior to this upthrow, a platform is supposed to have existed above the level of the sea, in which various fissures opened; and from these melted matter was poured forth again and again, which spread itself around in thin sheets of uniform thickness. From the same rents issued showers of scoriae and fragmentary matter, which were spread out so as to form equally uniform and horizontal beds, intervening between the sheets of lava. But although, by the continued repetition of these operations, a vast pile of volcanic matter, 4000 feet or more in thickness, was built up precisely in that region where Etna now rises, and to which nothing similar was produced elsewhere in Sicily, still we are told that Etna was not yet a mountain. No hypothetical diagram has been given to help us to conceive how this great mass of materials of supramarine origin could have been disposed of in horizontal beds, so as not to constitute an eminence towering far above the rest of Sicily; but it is assumed that a powerful force from below at length burst suddenly through the horizontal formation, uplifted it to a considerable height, and caused the beds to be, in many places, highly inclined. This elevatory force was not all expended on a single central point as Von Buch has imagined in the case of Palma, Teneriffe, or Somma, but rather followed for a short distance a linear direction.[577]

Among other objections that may be advanced against the theory above proposed, I may mention, first, that the increasing number of dikes as we approach the head of the Val del Bove, or the middle of Etna, and the great thickness of lava, scoriae, and conglomerates in that region, imply that the great centre of eruption was always where it now is, or nearly at the same point, and there must, therefore, have been a tendency, from the beginning, to a conical or dome-shaped arrangement in the ejected materials. Secondly, were we to admit a great number of separate points of eruption, scattered over a plain or platform, there must have been a great number of cones thrown up over these different vents; and these hills, some of which would probably be as lofty as those now seen on the flanks of Etna, or from 300 to 750 feet in height, would break the continuity of the sheets of lava, while they would become gradually enveloped by them. The ejected materials, moreover, would slope at a high angle on the sides of these cones, and where they fell on the surrounding plain, would form strata thicker near the base of each cone than at a distance.

What then are the facts, it will be asked, to account for which this hypothesis of original horizontality, followed by a single and sudden effort of upheaval, which gave to the beds their present slope, has been invented? M. de Beaumont observes, that in the boundary precipices of the Val del Bove, sheets of lava and intercalated beds of cinders, mixed with pulverulent and fragmentary matter evidently cast out during eruptions, are sometimes inclined at steep angles, varying from 15 degrees. to 27 degrees. It is impossible, he says, that the lavas could have flowed originally on planes so steeply inclined, for streams which descend a slope even of 10 degrees form narrow stripes, and never acquire such a compact texture. Their thickness, moreover, always inconsiderable, varies with every variation of steepness, in the declivity down which they flow; whereas, in several parts of the Val del Bove, the sheets of lava are continuous for great distances, in spite of their steep inclination, and are often compact, and perfectly parallel one to the other, even where there are more than 100 beds of interpolated fragmentary matter.

The intersecting dikes also terminate upwards in many instances, at different elevations, and blend (or, as M. de Beaumont terms it, articulate) with sheets of lava, which they meet at right angles. It is therefore assumed that such dikes were the feeders of the streams of lava with which they unite, and they are supposed to prove that the platform, on the surface of which the melted matter was poured out, was at first so flat, that the fluid mass spread freely and equally in every direction, and not towards one point only of the compass, as would happen if it had descended the sloping sides of a cone. This argument is ingeniously and plainly put in the following terms:--"Had the melted matter poured down an inclined plane, after issuing from a rent, the sheet of lava would, after consolidation, have formed an elbow with the dike, like the upper bar of the letter F, instead of extending itself on both sides like that of a T."[578] It is also contended that a series of sheets of lava, formed on a conical or dome-shaped mountain, would have been more numerous at points farthest from the central axis, since every dike which had been the source of a lava-stream, must have poured its contents downwards, and never upwards.

Dikes as they would now appear had they been originally perpendicular.]

In reference to the facts here stated, I may mention that the dikes which I saw in the Val del Bove were either vertical, or made almost all of them a near approach to the perpendicular, which could not have been the case had they been the feeders of horizontal beds of lava, and had they consequently joined them originally at right angles, for then the dikes, as at _a_, _b_, _c_, fig. 54, ought subsequently to have acquired a considerable slope, like the beds which they intersect. I may also urge another objection to the views above set forth, namely, that had the dikes been linear vents, or orifices of eruption, we must suppose the inter-stratified scoriae and lapilli, as well as the lavas, to have come out of them, and in that case the irregular heaping up of fragmentary matter around the vents would, as before hinted, have disturbed that uniform thickness and parallelism of the beds which M. de Beaumont describes.

If, however, some of the sheets of lava join the dikes in such a manner, as to imply that they were in a melted state simultaneously with the contents of the fissures,--a point not easily ascertained, where the precipices are for the most part inaccessible,--the fact may admit of a different interpretation from that proposed by the French geologists. Rents like those before alluded to (p. 399), which opened in the plain of S. Lio in 1669, filled below with incandescent lava, may have lain in the way of currents of melted matter descending from higher openings. In that case, the matter of the current would have flowed into the fissure and mixed with the lava at its bottom. Numerous open rents of this kind are described by Mr. Dana as having been caused, during a late eruption, in one of the volcanic domes of the Sandwich Islands. They remained open at various heights on the slopes of the great cone, running in different directions, and demonstrate the possibility of future junctions of slightly inclined lava-streams with perpendicular walls of lava.

To me, therefore, it appears far more easy to explain the uniform thickness and parallelism of so many lavas and beds of fragmentary matter seen in the Val del Bove, by supposing them to have issued successively out of one or more higher vents near the summit of a great dome, than to imagine them to have proceeded from lateral dikes or rents opening in a level plain. In the Sandwich Islands, we have examples of volcanic domes 15,000 feet high, produced by successive outpourings from vents at or near the summit. One of these, Mount Loa, has a slope in all directions of 6 degrees 30 minutes; another, Mount Kea, a mean inclination of 7 degrees 46 minutes. That their lavas may occasionally consolidate on slopes of 25 degrees, and even more, and still preserve considerable solidity of texture, has been already stated; see above, p. 383.

We know not how large a quantity of modern lava may have been poured into the bottom of the Val del Bove, yet we perceive that eruptions breaking forth near the centre of Etna have already made some progress in filling up this great hollow. Even within the memory of persons now living, the rocks of Musara and Capra have, as before stated, lost much of their height and picturesque grandeur by the piling up of recent lavas round their base (see fig. 51, p. 408), and the great chasm has intercepted many streams which would otherwise have deluged the fertile region below, as has happened on the side of Catania. The volcanic forces are now laboring, therefore, to repair the breach which subsidence has caused on one side of the great cone; and unless their energy should decline, or a new sinking take place, they may in time efface this inequality. In that event, the restored portion will always be unconformable to the more ancient part, yet it will consist, like it, of alternating beds of lava, scoriae;, and conglomerates, which, with all their irregularities, will have a general slope from the centre and summit of Etna towards the sea.

I shall conclude, then, by remarking that I conceive the general inclination of the alternating stony and fragmentary beds of the Val del Bove, from the axis of Etna towards its circumference or base, and the greater thickness of the volcanic pile as we approach the central parts of the mountain, to be due to the preponderance of eruptions from that centre. These gave rise, from the first, to a dome-shaped mass, which has ever since been increasing in height and area, being fractured again and again by the expansive force of vapors, and the several parts made to cohere together more firmly after the solidification of the lava with which every open fissure and chasm has been filled. At the same time the cone may have gained a portion of its height by the elevatory effect of such dislocating movements, and the sheets of lava may have acquired in some places a greater, in others a less, inclination than that which at first belonged to them.

Non-volcanic protuberance and valley of elevation.]

But had the mountain been due solely, or even principally, to upheaval, its structure would have resembled that which geologists have so often recognized in dome-shaped hills, or certain elevated regions, which all consider as having been thrust up by a force from below. In this case there is often an elliptical cavity at the summit, due partly to the fracture of the upraised rocks, but still more to aqueous denudation, as they rose out of the sea. The central cavity, or valley, exposes to view the subjacent formation _c_, fig. 55, and the incumbent mass dips away on all sides from the axis, but has no tendency to thin out near the base of the dome, or at _x_, _x_; whereas at this point the volcanic mass terminates (see fig. 56) and allows the fundamental rock _c_ to appear at the surface. In the last diagram, the more ordinary case is represented of a great hollow or crater at the summit of the volcanic cone; but instead of this, we have seen that in the case of Etna there is a deep lateral depression, called the Val del Bove, the upper part of which approaches near to the central axis, and the origin of which we have attributed to subsidence.

Volcanic mountain and crater.]

_Antiquity of the cone of Etna._--It was before remarked that confined notions in regard to the quantity of past time have tended, more than any other prepossessions, to retard the progress of sound theoretical views in geology;[579] the inadequacy of our conceptions of the earth's antiquity having cramped the freedom of our speculations in this science, very much in the same way as a belief in the existence of a vaulted firmament once retarded the progress of astronomy. It was not until Descartes assumed the indefinite extent of the celestial spaces, and removed the supposed boundaries of the universe, that just opinions began to be entertained of the relative distances of the heavenly bodies; and until we habituate ourselves to contemplate the possibility of an indefinite lapse of ages having been comprised within each of the modern periods of the earth's history, we shall be in danger of forming most erroneous and partial views in geology.

If history had bequeathed to us a faithful record of the eruptions of Etna, and a hundred other of the principal active volcanoes of the globe, during the last three thousand years,--if we had an exact account of the volume of lava and matter ejected during that period, and the times of their production,--we might, perhaps, be able to form a correct estimate of the average rate of the growth of a volcanic cone. For we might obtain a mean result from the comparison of the eruptions of so great a number of vents, however irregular might be the development of the igneous action in any one of them, if contemplated singly during a brief period.

It would be necessary to balance protracted periods of inaction against the occasional outburst of paroxysmal explosions. Sometimes we should have evidence of a repose of seventeen centuries, like that which was interposed in Ischia, between the end of the fourth century B.C., and the beginning of the fourteenth century of our era.[580] Occasionally a tremendous eruption, like that of Jorullo, would be recorded, giving rise, at once, to a considerable mountain.

If we desire to approximate to the age of a cone such as Etna, we ought first to obtain some data in regard to the thickness of matter which has been added during the historical era, and then endeavor to estimate the time required for the accumulation of such alternating lavas and beds of sand and scoriae as are superimposed upon each other in the Val del Bove; afterwards we should try to deduce, from observations on other volcanoes, the more or less rapid increase of burning mountains in all the different stages of their growth.

There is a considerable analogy between the mode of increase of a volcanic cone and that of trees of _exogenous_ growth. These trees augment, both in height and diameter, by the successive application externally of cone upon cone of new ligneous matter; so that if we make a transverse section near the base of the trunk, we intersect a much greater number of layers than nearer to the summit. When branches occasionally shoot out from the trunk, they first pierce the bark, and then, after growing to a certain size, if they chance to be broken off, they may become inclosed in the body of the tree, as it augments in size, forming knots in the wood, which are themselves composed of layers of ligneous matter, cone within cone.

In like manner, a volcanic mountain, as we have seen, consists of a succession of conical masses enveloping others, while lateral cones, having a similar internal structure, often project, in the first instance, like branches from the surface of the main cone, and then becoming buried again, are hidden like the knots of a tree.

We can ascertain the age of an oak or pine by counting the number of concentric rings of annual growth seen in a transverse section near the base, so that we may know the date at which the seedling began to vegetate. The Baobab-tree of Senegal (_Adansonia digitata_) is supposed to exceed almost any other in longevity. Adanson inferred that one which he measured, and found to be thirty feet in diameter, had attained the age of 5150 years. Having made an incision to a certain depth, he first counted three hundred rings of annual growth, and observed what thickness the tree had gained in that period. The average rate of growth of younger trees, of the same species, was then ascertained, and the calculation made according to a supposed mean rate of increase. De Candolle considers it not improbable that the celebrated Taxodium of Chapultepec, in Mexico (_Cupressus disticha_, Linn.), which is 117 feet in circumference, may be still more aged.[581]

It is, however, impossible, until more data are collected respecting the average intensity of the volcanic action, to make any thing like an approximation to the age of a cone like Etna; because, in this case, the successive envelopes of lava and scoriae are not continuous, like the layers of wood in a tree, and afford us no definite measure of time. Each conical envelope is made up of a great number of distinct lava-currents and showers of sand and scoriae, differing in quantity, and which may have been accumulated in unequal periods of time. Yet we cannot fail to form the most exalted conception of the antiquity of this mountain, when we consider that its base is about ninety miles in circumference; so that it would require ninety flows of lava, each a mile in breadth at their termination, to raise the present foot of the volcano as much as the average height of one lava-current.

There are no records within the historical era which lead to the opinion that the altitude of Etna has materially varied within the last two thousand years. Of the eighty most conspicuous minor cones which adorn its flanks, only one of the largest, Monti Rossi, has been produced within the times of authentic history. Even this hill, thrown up in the year 1669, although 450 feet in height, only ranks as a cone of second magnitude. Monte Minardo, near Bronte, rises, even now, to the height of 750 feet, although its base has been elevated by more modern lavas and ejections. The dimensions of these larger cones appear to bear testimony to _paroxysms_ of volcanic activity, after which we may conclude, from analogy, that the fires of Etna remained dormant for many years--since nearly a century of rest has sometimes followed a violent eruption in the historical era. It must also be remembered, that of the small number of eruptions which occur in a century, one only is estimated to issue from the summit of Etna for every two that proceed from the sides. Nor do all the lateral eruptions give rise to such cones as would be reckoned amongst the smallest of the eighty hills above enumerated; some of them produce merely insignificant monticules, which are soon afterwards buried by showers of ashes.

How many years then must we not suppose to have been expended in the formation of the eighty cones? It is difficult to imagine that a fourth part of them have originated during the last thirty centuries. But if we conjecture the whole of them to have been formed in twelve thousand years, how inconsiderable an era would this portion of time constitute in the history of the volcano! If we could strip off from Etna all the lateral monticules now visible, together with the lavas and scoriae that have been poured out from them, and from the highest crater, during the period of their growth, the diminution of the entire mass would be extremely slight: Etna might lose, perhaps, several miles in diameter at its base, and some hundreds of feet in elevation; but it would still be the loftiest of Sicilian mountains, studded with other cones, which would be recalled, as it were, into existence by the removal of the rocks under which they are now buried.

There seems nothing in the deep sections of the Val del Bove to indicate that the lava-currents of remote periods were greater in volume than those of modern times; and there are abundant proofs that the countless beds of solid rock and scoriae were accumulated, as now, in succession. On the grounds, therefore, already explained, we must infer that a mass so many thousand feet in thickness must have required an immense series of ages anterior to our historical periods for its growth; yet the whole must be regarded as the product of a modern portion of the tertiary epoch. Such, at least, is the conclusion that seems to follow from geological data, which show that the oldest parts of the mountain, if not of posterior date to the marine strata around its base, were at least of coeval origin.

Some geologists contend, that the sudden elevation of large continents from beneath the waters of the sea have again and again produced waves which have swept over vast regions of the earth.[582] But it is clear that no devastating wave has passed over the forest zone of Etna since any of the lateral cones before mentioned were thrown up; for none of these heaps of loose sand and scoriae could have resisted for a moment the denuding action of a violent flood. To some, perhaps, it may appear that hills of such incoherent materials cannot be of very great antiquity, because the mere action of the atmosphere must, in the course of several thousand years, have obliterated their original forms. But there is no weight in this objection; for the older hills are covered with trees and herbage, which protect them from waste; and, in regard to the newer ones, such is the porosity of their component materials, that the rain which falls upon them is instantly absorbed; and for the same reason that the rivers on Etna have a subterranean course, there are none descending the sides of the minor cones.

No sensible alteration has been observed in the form of these cones since the earliest periods of which there are memorials; and there seems no reason for anticipating that in the course of the next ten thousand or twenty thousand years they will undergo any great alteration in their appearance, unless they should be shattered by earthquakes or covered by volcanic ejections.

In other parts of Europe, as in Auvergne and Velay, in France, similar loose cones of scoriae, probably of as high antiquity as the whole mass of Etna, stand uninjured at inferior elevations above the level of the sea.