The Eruption of Vesuvius in 1872 Part 6

On the 29th, with a strong wind blowing from the east, scoriae of such a size fell at the Observatory, that the gla.s.s of the windows unprotected by external blinds was broken. The noise from the crater continued, but the projectiles rose to a less height, indicating a diminution in the dynamic power of the eruption. Towards midnight the noise of the craters was no longer continuous, and recurred with less force and for shorter intervals. Almost at the same hour a tempest burst over the Campania with loud thunder and a little rain. The gra.s.s, the seeds, the vine tendrils, the leaves and tops of the trees dried up immediately, and the country was changed from spring to winter. The storm, although repeated on the following days, pa.s.sed away by degrees, and thus the floods, which I strongly feared, did not occur. Almost always after great eruptions of Vesuvius, storms of heavy rain have followed, and the ground being covered with ashes, the water could not filtrate through into the soil, but descended in muddy torrents over the adjacent country, occasioning as much damage as the fire itself.

On the 30th, the detonations were very few, and the smoke issued only at intervals, and by the 1st May the eruption was completely over.

When the smoke had cleared off the figure of the cone was seen to be changed. (_Vide_ Plate 5a.)

The ground was perpetually disturbed whilst the Volcano raged, so that the Observatory oscillated continually. Some shocks were felt not only in the adjacent territory, but at a greater distance, at Montovi and elsewhere. The oscillations at the Observatory were chiefly undulatory, from N.E. to S.W. They were observed for some days after the termination of the eruption, but not continuously, although they maintained some intensity.

If we refer to January, 1871, we shall find that that eruption was preceded by several earthquakes, among which were those of the months of October, November and December, in the previous year, that wrought such destruction in Calabria, and especially in the province of Cosenza; if we consider that as only the last phase, we shall find that it was preceded by great shocks of earthquake that devastated some regions of Greece.[3]

The great quant.i.ty of lapilli which fell buried the scoriae with which the Vesuvius cone was covered, so that it became somewhat more difficult to ascend to the summit, and much less difficult to descend. Having reached the top of the mountain, I found a large crater divided into two parts by what seemed a cyclopean wall. The two abysses had vertical sides, and revealed the internal structure of the cone. Their vertical depth was 250 metres; and beyond that I observed a sort of tunnel perforated in the rock, with a covering arch raised above the bottom of the eastern abyss about 12 metres, judging by the eye. The interior walls of the crater showed neither the usual stalact.i.tic scoriae nor sublimations, nor fumaroles, but alternate beds of scoriae and of compact lava. The fumaroles and sublimations abounded, only about the brims of the craters. Hydrochloric and sulphuric acid and sometimes sulphuretted hydrogen affected respiration, and the temperature rose sometimes to 150 degrees. Various fissures about the brim of the double crater indicated prolongations downwards, which allowed me to descend with a rope, in order to examine the interior of the tunnel to which I have just alluded. The highest brim of the crater was fissured for a distance of 80 metres, and the greatest depth of fissure was at that place.

By measurement with the barometer, we ascertained approximately (for only one barometer was used) that the height of the Vesuvian cone was somewhat diminished.

Not only the Vesuvian cone, but the whole adjacent country appeared white for many days, as if covered with snow, when exposed to sunlight.

This was due to the sea-salt contained in the ashes with which the surface was strewn.

A great quant.i.ty of coleoptera a.s.sembled on the flat roof of the Observatory, where the ashes and lapilli were heaped up two decimetres in height. I found the same species on the cone, where many insects were observed on other occasions, such as the _Cuccinella septempunctata_; the crysomela populi, etc., were wanting. This phenomenon of the extraordinary concourse of insects on the top of Vesuvius, in order to die in some of the fumaroles, especially noted previous to and after great eruptions, is a circ.u.mstance for which I cannot account.[4] The whole of the lava emitted in this eruption occupies a surface of about five square kilometres; allowing an average thickness of four metres, we obtain a ma.s.s of twenty millions of cubic metres. About three-fifths of this lava did no injury, being deposited upon other pre-existing lava.

However, the lava in the Novelle, which was deposited upon the lava of 1858, covered quarries of the best stone which had been worked at the time, covered many paths that had been cleared, and buried the new Church of St. Michele, with some houses that surrounded it, which had been rebuilt on the site of the former church, which was covered by the lava of 1868. The destruction of land in occupation, of buildings and of crops, exceeded three million francs in value. Many proposals for relieving the sufferers have been received. Wishing to aid in this benevolent work, I gave a public lecture, admission for each person being one franc; and this lecture, from notes badly taken, was printed by private speculation, and I was compelled to repudiate the report of it through the public papers.

The evolutions of carbonic acid (_mofette_), which usually appear at the end of great Vesuvian eruptions at low-situated spots or hollows, with very rare exceptions, were observed on this occasion a few days after the eruption had completely ceased. They appeared in the direction of Resina. I found the most elevated at Tironi, and the most numerous between La Favorita and the Bosco Reale di Portici.

The water in wells was on this occasion neither deficient nor scarce previous to the eruption, but was very acid after the appearance of the carbonic acid evolutions in those neighbourhoods in which they abounded. Having stated that the disastrous conflagration of the 26th April ought, in my opinion, to be regarded as the last phase of a long period of eruption, which commenced at the beginning of 1871, I consider it right to discuss the question at somewhat greater length.

Not only from twenty years' personal observations, but from the attentive study of accounts of previous eruptions, I have found that when the central crater awakens with small eruptions after a certain time of previous repose, these almost always have a long duration, and, after various phases of increase and decrease, terminate in a great eccentric eruption, that is to say, with the production of an aperture from which a copious lava stream issues. The eruptions of 1858, 1861, 1868 and 1872, furnish the most recent examples of what I affirm. I might cite many others of earlier date, but I shall content myself with recording the greatest conflagration of this century, that of October, 1822.

Before the erection of the Vesuvian Observatory, it was impossible to obtain a consecutive account of all the phases which the Volcano presented; but we generally obtained the description of the more splendid phases of the eruption which arrested the attention of everyone. Hence, notices of the small phenomena which preceded a great eruption are frequently wanting. We cannot always ascertain whether the fumaroles of the craters became active and at what periods, what was their temperature and what the diverse nature of their emanations, etc.: whether and when any change in the crater with slight eruptive manifestations occurred; discharges which sometimes commenced in the bottom of a crater becoming active, and so are invisible at Naples.

But it may be asked whether the inverse proposition be equally true, that is, whether all the great eruptions of our Volcano were preceded by small fiery manifestations of long duration? There have undoubtedly been great eruptions not preceded by small central eruptions, but these also had their period of preparation or precursory signs. After the great eruption of 1850, Vesuvius remained in apparent repose until the end of May, 1855, when there was an eccentric eruption and a great flow of lava lasting twenty-seven days. But for a year before the fumaroles on the top of the mountain had acquired great activity, their temperature increased, and hydrochloric and sulphuric acid became more abundant, and generated the usual coloured products on the adjacent scoriae. Finally, in the month of January, a crater was formed by falling in of the ground, and although it did not discharge fire, yet it poured forth dense smoke. This was the beginning of the fissure manifested four months afterwards.

Ign.a.z.io Sorrentino, who spent a long life in the study of Vesuvius, and frequently ascended it, considered the increase of those yellow products--which are chiefly chlorides of iron, but were, at that time, mistaken for sulphur--as the sign of an approaching eruption.

The only grave objection that can be alleged is that of the memorable eruption of 1631, which surprised the neighbouring population so suddenly that many perished miserably, surrounded or covered with lava.

But that terrible conflagration occurred after centuries of repose, so that trees had grown in the interior of the crater. No one suspected the possibility of danger. It took place, too, at the end of autumn, when the cone is usually covered with clouds, and, therefore, no one had an opportunity of observing any precursory phenomena.

When the Observatory was established, I was able--in the first instance, at my own expense, and afterwards with some slight a.s.sistance from Government--to undertake studies more a.s.siduous than any previously made. I had two instruments adjusted to indicate the internal efforts of the Volcano, viz., M. Lamond's apparatus of variations, which, by means of finely-balanced needles and methods of amplification proposed by Gauss, indicates the slightest trepidation of the ground, and my own electro-magnetic seismograph, a self-registering instrument of exquisite delicacy. These instruments, when attentively observed, give the most valuable information with respect to the activity of the adjacent Volcano.

If the very slightest eruption occurs, these instruments manifest slight perturbation, increasing with the activity of the mountain. When the Volcano attains a certain degree of activity, and the instruments are proportionately disturbed, it is impossible to foresee a new phase of increase without constantly watching the changes in the intensity of the perturbations; and to effect this it is requisite to have upon the spot a staff of a.s.sistants sufficiently numerous, scientific and intelligent.

If, therefore, on the night preceding the 26th of April the instruments had been properly watched, they would have undoubtedly indicated the great increase in the activity of the Volcano. The perturbations on the 23rd were steadily increasing, and on the evening of the 25th they were much stronger than on the 24th, but on the morning of the 26th they had become extraordinarily strong; they must, therefore, have increased considerably during the night.

II.

NATURE OF THE LAVAS.

When the observer is near the source of the lava, he sees matter in a state of fusion, which, like a torrent of liquid fire, runs along, with more or less impetuosity, between two banks formed by itself. But as soon as the surface of the torrent cools to the point of congelation, it loses the splendour of its first incandescence. The part which begins to harden breaks readily in some lavas into fragments which float on the viscous fluid beneath; these, increasing in number with distance from the source, conceal the molten matter beneath and r.e.t.a.r.d its progress, and at last nothing is seen but the more or less red-hot scoriae moving along. These lavas I shall call "_Lavas with fragmentary scoriae_."

On other occasions, a skin forms on the surface of the lava, which, gradually thickening, keeps flexible for some time, and then wrinkles or swells or extends and breaks to give egress to the hot fluid within, which, in its turn, skins over and repeats the same phenomena. This I shall call "_Lavas with a united surface_."

These, in their course, discharge less smoke than the first, draw out more easily into threads, and, when cold, have a dark colour, something like bitumen or pitch. _The lava with fragmentary scoriae_, when stretched, breaks easily, discharges smoke copiously, and, when hardened, has a more bluish tint, like clods of upturned earth (_formato di zolle_). It is noisy in its course, because the incoherent scoriae that it carries along strike and crunch against each other; the other lava flows silently, except for a sort of crackling arising from the actual fracturing up of the solid skin by distension from the liquid matter within. If required to give the mineralogical characteristics of this lava, I would say that it was rich in leucite and contained little or no pyroxene; the fragmentary lava, on the contrary, is poor in leucite and rich in pyroxene. The lavas of 1871 were of the "united surface" character; those of 1872 were "fragmentary," with some characteristics which I shall describe:

1. They were of the clearest tint I have ever seen, when regarded superficially, but, when broken, the fracture was darker than any other lava.

2. They had very little leucite and abounded in pyroxene and olivine, and sometimes contained a few crystals of amphibole.

3. Their specific gravity varied with their porosity; the most compact attained 275.

4. These lavas carried along in their course a quant.i.ty of scoriae which had long been subjected to the action of the acids of the fumaroles close to the craters, and also a great many bombs (_bombe_)--that is, round ma.s.ses similar to those ejected from craters. These varied in size, some having a diameter of four to five meters. They frequently contained a large nucleus of very leucitic lava, like that of 1871, with a larger or smaller quant.i.ty of feroligiste (peroxide of iron). Others contained lavas changed by the action of the acid vapours near the craters. These bombs must have flowed out with the lava, for they are found through its whole course, and they were certainly not ejected from the crater; for not only are they found on the lava exclusively, but ma.s.ses so enormous were not thrown up from the craters during the eruption; those lying on the cone near the craters seldom exceed a decimetre in diameter.

As to the qualitative chemical a.n.a.lysis of the lavas, it always presents the same elements, with the exception of small quant.i.ties of some metals, lead for example, which have escaped the researches of good chemists, but which I have constantly found in the sublimations of the fumaroles of the lava. With respect to the quant.i.tative a.n.a.lysis, two specimens of the same lava appear indeed to have their const.i.tuents in different proportions. To arrive at any conclusion a long and patient investigation, requiring means and a.s.sistance which the Observatory does not possess, would be necessary.

Professor Fuchs, of Heidelberg, has devoted himself to this work for years past, and if he continue it with well-selected and sufficiently large specimens we may hope some day to obtain satisfactory results.

5. Every specimen of lava which I examined with a very sensitive magnetoscope improved by myself, was invariably magneto-polar, not excepting the pieces of the bombs, whether rejected from the crater or carried along with the lava.

III.

FUMAROLES OF THE LAVAS.

Smoke generally issues from all lava when it cools down to a certain degree, hence it is more abundant at the edges of the fiery torrent, or is liberated from the scoriae that form on its surface. But when the lava stops, the smoke issues only from certain vent-holes, through which we can still see the fire, and at the edge of which different amorphous or crystallized matters collect by sublimation. These centres of heat, of more or less duration, are the fumaroles of the lavas. I believe I have on other occasions shown that a fumarole is nothing but a communication between the more or less cooled and hardened surface of the lava and the interior, which is still incandescent. Some fumaroles last but a day, others preserve their activity for weeks, months or years, according to the depth of lava through which they penetrate; and when they cease to be active, that is, when the sublimations are formed, or smoke or other aeriform matters issue from them, they still retain a rather elevated temperature. In the lavas of 1858, in a place where they had a transverse width of 150 metres, a vent-hole may still be found where the thermometer registers 60 and the scoriae are warm. Sometimes, while the lava is in process of cooling, new fumaroles appear, in which the fire is visible. This phenomenon, which appeared marvellous and inexplicable when I first observed it in 1855, is now very easily understood; the cooled and hardened crust of the lava fractures with noise and suddenly, and so a new communication is opened with the incandescent lava below, thus creating a new fumarole.

As the smoke of the fluid lava is perfectly neutral, that is, neither acid nor alkaline, so the fumaroles at the first period of their existence with sublimations of sea-salt, mixed frequently with oxide of copper either in black powder or in shining laminae, ought also to be neutral. But if the fumarole continues active, hydrochloric acid issues with the smoke, and often some time after sulphuric acid. Then the sublimations turn first yellow, then green, and more rarely azure. The chemical reactions show that these sublimations are chlorides or sulpho-chlorides, and sometimes sulphides, and they afford reactions, indicative of soda, magnesia, copper, lead, and traces of other substances, not excluding ammonia, which I must speak of separately.

This, I have observed, is the general law with the fumaroles of the tranquil lavas, which occur with long and moderate eruptions--for instance, the lavas of 1871, and even those of 1872, preceding the 26th April.

But in the great lavas of the great conflagrations of Vesuvius, chloride of iron more or less in combination with all the other substances above mentioned changes the appearance of the sublimations. The fumaroles in the lava of the 26th April frequently indicated chloride of iron.

Sulphuretted hydrogen, by reaction of sulphurous acid, is decomposed, and sulphur sublimed, having a particular aspect, collects on the scoriae. This is never found but in fumaroles of the smaller lavas; it was therefore absent in those of 1871, but frequently occurred in those of 1872.

Although the sublimations are generally mixtures, yet sometimes distinct and crystallized chemical or mineral species are found, such as sulphur, sal ammoniac, _tenorite_, _cotunuite_, etc. Micaceous peroxide of iron (feroligiste), so common near eruptive cones, is very scarce on lava; any found in it has been carried down from the craters, and proofs of this transport are very abundant and striking in the lavas of this last eruption. Even the iron found in the bombs is evidently transported; there is a fumarole on the ridge of the lava in the Fossa di Faraone which contains micaceous peroxide of iron, and this, at first sight, appears to oppose what I have affirmed; nevertheless, it gives additional force to my statement. This fumarole is only a bomb or rounded ma.s.s of enormous size, four or five metres in diameter. Smoke and hydrochloric acid issued from the aperture in its envelope, and being partly broken it was seen to contain lapilli and pieces of antecedent lava, covered with micaceous peroxide of iron. The internal temperature of this ma.s.s was very high; the hydrochloric acid which it discharged had, in some places, covered the micaceous iron with a yellow coating of chloride of iron. From small apertures, on the lower side of the ma.s.s, white and green stalact.i.tes of chloride of calcium were visible. In one spot only of lava I found a fumarole, with a small quant.i.ty of micaceous peroxide of iron, evidently in a state of formation; but this was the very spot where the lava became eruptive, and whence issued the column of smoke which was so well photographed--the place under the hill of Apicella. (See Plate 4a.)

I have enumerated the products which are constantly collected in fumaroles, although they are not all found at the same time or place, in order to show that the sublimations follow a certain law in their appearance. _Tenorite_, for instance, was formerly considered an accidental product of certain eruptions, and I have always found it; but if you visit the fumarole when the acids have had time to transform it, you will no longer see it. I found the crystallized chloride of lead, or "cotunuite," as it is called, for the first time in the lavas of 1855, and thought it a singular circ.u.mstance; but from that time I recognised it in all the lavas, though not always so beautiful and abundant; and even when not found as a distinct substance, I observed it in combination with chloride of copper. In the lavas of the 26th April _cotunuite_ and _tenorite_[E] were not very abundant, because the chloride of iron disturbed the greater number of the sublimations. I found sal ammoniac very abundantly on the fumaroles of the lavas that invaded the cultivated ground. Although chloride of ammonia, contrary to opinion, was not wanting in the sublimations of the fumaroles of the lavas deposited on other lavas, yet it was neither abundant nor crystallized, but combined in small quant.i.ties with other substances. It appeared in great abundance in all the fumaroles of lavas which covered cultivated or woody ground. At first it was scarce enough, and mixed with chloride of sodium; but when the rains came the sea-salt was washed away, and sal ammoniac formed beautiful crystals, nearly free from advent.i.tious matters, as was the case with the fumaroles of the last lava. Afterwards, when chloride of iron was produced, ferro-chloride of ammonia was found. Crystals of sal ammoniac were sometimes found of a beautiful amber yellow. This colour was, in the opinion of my colleague, Professor Scacchi, produced by such small traces of chloride of iron that neither Professor Guiscardi nor I, nor indeed any other chemists to whom I submitted specimens for examination, could detect any. What I can affirm with certainty is, that these limpid crystals of a yellow colour were almost always attached to an amorphous substance, soluble in water, composed of various chlorides, in which iron was often detected.

From these remarks, it is evident that in the tranquil lavas the sublimations appear with a certain order of succession, and in the violent lavas, and those which flow most copiously, they are more complicated, and render both chemical a.n.a.lysis and spectroscopic researches more difficult. Notwithstanding, I observed traces of lithium and thallium, which I had previously perceived in some sublimations of 1871. I purpose submitting many sublimations which I have collected to more complete spectroscopic investigation, although I am persuaded that the discovery of traces of certain bodies in the sublimations or in the lavas is a matter of small importance to the science of volcanoes. I must say, however, that calcium was discovered on this occasion in great abundance, not only by the spectroscope, but also by chemical a.n.a.lysis.

Sulphate of lime has often been found in larger or smaller proportions, but this was the first time I had observed chloride of calcium both close to the craters, and also in the sublimations of the fumaroles upon the lavas. The white stalact.i.tes which I collected beneath the great ma.s.s or bomb above described were almost exclusively composed of chloride of calcium, and only a few green drops manifested, with the usual re-agents, the presence of iron.

I did not fail to look often at the spectrum of the flowing lavas covered with the smoke which issued from them, but I always had a continuous spectrum. The spectroscope employed was Hoffmann's construction, with direct vision; but I think it would be better on other occasions to use a spectroscope combined with a telescope, like those used by astronomers.

But avoiding minute particulars of these sublimates, let us see what is the general direction and the order of their appearance. Sublimations are generally oxides, chlorides and sulphates, sometimes sulphides.

Among the oxides, we must enumerate in the first place "tenorite" and _feroligiste_ or micaceous peroxide of iron. The first is almost always found at the commencement of activity in the fumaroles, simultaneously with the sublimation of chloride of sodium; the second--which is, perhaps, never wanting in eruptive cones that are often found lined with it inside--is seldom generated in the fumaroles of the lava, and therefore it is not easy to define the moment of its appearance.

Sometimes one collects micaceous peroxide of iron on the lava, but it is often transported there from the mouths of eruption, as happened on this occasion.

Trustworthy writers are of opinion that all the oxides are derived from the decomposition of the chlorides, but I think I have clearly demonstrated that, with regard to copper and lead, the opposite statement may be affirmed; for the oxides are changed into chlorides, and hydrochloric acid liberated. Oxide of copper forms sublimates at the beginning, at the same time as the sea-salt; and if the fumarole be anhydrous or, as Deville would say, _dry_, this oxide does not change into either a chloride or a sulphate; but if the fumarole gives watery vapour, after a little hydrochloric acid is formed, which changes the oxide into a chloride, and if whilst this is going on oxide of lead be developed, it is changed into the chloride of lead, so frequently found in combination with chloride of copper. Then the sublimations change from white to red or yellow, and specimens when carried away gradually turn light blue, but when heated on platinum over a spirit lamp they resume their yellow tint. Sometimes the yellow colour remains longer, and in time changes to green; this also happens on the fumarole itself, the green commencing at the zones furthest removed from the centre, where the temperature is highest. When these sublimations are greenish, they become far less soluble than at first. The yellow, so common at a certain period on the fumaroles of the tranquil lavas, never attracted attention before I first examined it, doubtless, because it was considered chloride of iron, and yet in small eruptions this is only found close to the discharging mouths, and never in the sublimations of the fumaroles of the lava; but, on the other hand, it is the most copious and common product on the lavas of the great eruptions. This probably also accounts for the fact that lead, which is so obvious in the fumaroles of the lavas, had never previously been observed. In 1855, I noticed the crystallized chloride of lead in a fumarole in the Fossa della Vetrana, and this induced me always to look for it on the fumaroles of the later lavas; and I ascertained that, if it did not always appear as a distinct mineral, it was easily discovered in combination with other chlorides. The specimens which I have collected are not the most beautiful, but the presence of lead in the sublimations is not less common.

Micaceous peroxide of iron, when found on the lava, has been mostly conveyed from the eruptive mouths, as I have already stated, and perhaps never so abundantly and evidently as on this occasion. The lava of the 26th of April carried along a large quant.i.ty of round ma.s.ses or bombs, varying in size, among which were found antecedent lava more or less covered with micaceous iron, either collected in the cavities of the lava, or incorporated with its ma.s.s. Sometimes the micaceous iron appears like little veins in the paste of new lava enveloping the exterior of these rounded ma.s.ses, an exterior compact and lithoidal, and not resembling scoriae. Among these spherical ma.s.ses I found one of enormous size, four to five metres in diameter, which, having broken up where the exterior envelope was thinnest, I found filled with a great ma.s.s of lapilli and fragments of other lavas covered with micaceous iron. This bomb still preserves (June 5th) an elevated temperature within, and emits smoke and hydrochloric acid, which, meeting the micaceous iron discovered by breaking the envelope with blows of a hammer, transforms it superficially into chloride of iron, showing most clearly how, on some occasions at least, chloride of iron is formed from the oxide which precedes it. That those lapilli and the pieces of lava were solid when enveloped in the paste of the new lava, we infer from seeing the impressions on the inside of the said envelope. The chloride of calcium, which I found in this spherical ma.s.s almost pure, caused me to suspect that the sulphate of lime which is so often found on Vesuvius is a transformation of the chloride produced by the contact of sulphurous acid, which easily becomes transformed into sulphuric acid.