Volcanoes: Past and Present Part 3

VESUVIUS.

Having now dealt in a necessarily cursory manner with volcanoes of distant parts of the globe, we may proceed to the consideration of the group of active volcanoes which still survive in Europe, as they possess a special interest, not only from their proximity and facility of access, at least to residents in Europe and the British Isles, but from their historic incidents; and in this respect Vesuvius, though not by any means the largest of the group, stands the first, and demands more special notice. The whole group rises from the sh.o.r.es of the Mediterranean, and consists of Vesuvius, Etna, Stromboli, one of the Lipari Islands, and Vulcano, a mountain which has given the name to all mountains of similar origin with itself.[1] Along with these are innumerable cones and craters of extinct or dormant volcanoes, of which a large number have been thrown out on the flanks of Etna.

(_a._) _Prehistoric Ideas regarding the Nature of this Mountain._--Down to the commencement of the Christian era this mountain had given no ostensible indication that it contained within itself a powerful focus of volcanic energy. True, that some vague tradition that the mountain once gave forth fire hovered around its borders; and several ancient writers, amongst them Diodorus Siculus and Strabo, inferred from the appearances of the higher parts of the mountain and the character of the rocks, which were "cindery and as if eaten by fire," that the country was once in a burning state, "being full of fiery abysses, though now extinct from want of fuel." Seneca (B.C. 1 to A.D. 64) had detected the true character of Vesuvius, as "having been a channel for the internal fire, but not its food;" nevertheless, at this period the flanks of the mountain were covered by fields and vineyards, while the summit, partially enclosed with precipitous walls of the long extinct volcano, Somma, was formed of slaggy and scoriaceous material, with probably a covering of scrub. Here it was that the gladiator Spartacus (B.C. 72), stung by the intolerable evils of the Roman Government, retreated to the very summit of the mountain with some trusty followers. Clodius the Praetor, according to the narration of Plutarch, with a party of three thousand men, was sent against them, and besieged them in a mountain (meaning Vesuvius or Somma) having but one narrow and difficult pa.s.sage, which Clodius kept guarded; all the rest was encompa.s.sed with broken and slippery precipices, but upon the top grew a great many wild vines; the besieged cut down as many as they had need of, and twisted them into ladders long enough to reach from thence to the bottom, by which, without any danger, all got down except one, who stayed behind to throw them their arms, after which he saved himself with the rest.[2] "On the top" must (as Professor Phillips observes) be interpreted the summit of the exterior slope or crater edge, which would appear from the narrative to have broken down on one side, affording an entrance and mode of egress by which Spartacus fell upon, and surprised, the negligent Clodius Glabrus.

[Ill.u.s.tration: Fig. 6.--Probable aspect of Vesuvius as it appeared at the beginning of the Christian era; seen from the Bay of Naples.]

In fancied security, villas, temples, and cities had been erected on the slopes of the mountain. Herculaneum, Pompeii, and Stabiae, the abodes of art, luxury, and vice, had sprung up in happy ignorance that they "stood on a volcano," and that their prosperity was to have a sudden and disastrous close.[3]

(_b._) _Premonitory Earthquake Shocks._--The first monitions of the impending catastrophe occurred in the 63rd year after Christ, when the whole Campagna was shaken by an earthquake, which did much damage to the towns and villas surrounding the mountain even beyond Naples. This was followed by other shocks; and in Pompeii the temple of Isis was so much damaged as to require reconstruction, which was undertaken and carried out by a citizen at his own expense.[4] These earthquake shakings continued for sixteen years. At length, on the night of August 24th, A.D. 79, they became so violent that the whole region seemed to reel and totter, and all things appeared to be threatened with destruction. The next day, about one in the afternoon, there was seen to rise in the direction of Vesuvius a dense cloud, which, after ascending from the summit of the mountain into the air for a certain height in one narrow, vertical trunk, spread itself out laterally in such a form that the upper part might be compared to the cl.u.s.ter of branches, and the lower to the stem of the pine which forms so common a feature in the Italian landscape.[5]

(_c._) _Pliny's Letters to Tacitus._--For an account of what followed we are indebted to the admirable letters of the younger Pliny, addressed to the historian Tacitus, recounting the events which caused, or accompanied, the death of his uncle, the elder Pliny, who at the time of this first eruption of Vesuvius was in command of the Roman fleet at the entrance to the Bay of Naples. These letters, which are models of style and of accurate description, are too long to be inserted here; but he recounts how the dense cloud which hung over the mountain spread over the whole surrounding region, sometimes illuminated by flashes of light more vivid than lightning; how showers of cinders, stones, and ashes fell in such quant.i.ty that his uncle had to flee from Stabiae, and that even at so great a distance as Misenum they enc.u.mbered the surface of the ground; how the ground heaved and the bed of the sea was upraised; how the cloud descended on Misenum, and even the island of Capreae was concealed from view; and finally, how, urged by a friend who had arrived from Spain, he, with filial affection, supported the steps of his mother in flying from the city of destruction. Such being the condition of the atmosphere and the effects of the eruption at a point so distant as Cape Misenum, some sixteen geographical miles from the focus of eruption, it is only to be expected that places not half the distance, such as Herculaneum, Pompeii, and even Stabiae, with many villages and dwellings, should have shared a worse fate. The first of these cities, situated on the coast of the Bay of Naples, appears to have been overwhelmed by volcanic mud; Pompeii was buried in ashes and lapilli, and Stabiae probably shared a similar fate.[6]

(_d._) _Appearance of the Mountain at the Commencement of the Christian Era._--At the time of the first recorded eruption Vesuvius appears to have consisted of only a single cone with a crater, now known as Monte di Somma, the central cone of eruption which now rises from within this outer ruptured casing not having been formed. (Fig. 6.) The first effect of the eruption of the year 79 was to blow out the solidified covering of slag and scoriae forming the floor of the caldron. Doubtless at the close of the eruption a cone of fragmental matter and lava of some slight elevation was built up, and, if so, was subsequently destroyed; for, as we shall presently see by the testimony of the Abate Guilio Cesare Braccini, who examined the mountain not long before the great eruption of A.D. 1631, there was no central cone to the mountain at that time; and the mountain had a.s.sumed pretty much the appearance it had at the time that Spartacus took refuge within the walls of the great crater.

(_e._) _Destruction of Pompeii._--Pompeii was overwhelmed with dry ashes and lapilli. Sir W. Hamilton found some of the stones to weigh eight pounds. At the time of the author's visit, early in April 1872, the excavations had laid open a section about ten feet deep, chiefly composed of alternating layers of small pumice stones (lapilli) and volcanic dust. It was during the sinking of a well in 1713 upon the theatre containing the statues of Hercules and Cleopatra that the existence of the ancient city was accidentally discovered.

(_f._) _More recent eruptions._--Since the first recorded eruption in A.D. 79 down to the present day, Vesuvius has been the scene of numerous intermittent eruptions, of which some have been recorded; but many, doubtless, are forgotten.

In A.D. 203, during the reign of Severus, an eruption of extraordinary violence took place, which is related by Dion Ca.s.sius, from whose narrative we may gather that at this time there was only one large crater, and that the central cone of Vesuvius had not as yet been upraised. In A.D. 472 an eruption occurred of such magnitude as to cover all Europe with fine dust, and spread alarm even at Constantinople.

(_g._) _Eruption of 1631._--In December 1631 occurred the great convulsion whose memorials are written widely on the western face of Vesuvius in ruined villages. This eruption left layers of ashes over hundreds of miles of country, or heaps of mud swept down by hot water floods from the crater; the crater itself having been dissipated in the convulsion. Braccini, who examined the mountain not long before this eruption, found apparently no cone (or mount) like that of the modern Vesuvius. He states that the crater was five miles in circ.u.mference, about a thousand paces deep (or in sloping descent), and its sides covered with forest trees and brushwood, while at the bottom there was a plain on which cattle grazed.[7] It would seem that the mountain had at this time enjoyed a long interval of rest, and that it had reverted to very much the same state in which it was at the period of the first eruption, when the flanks were peopled by inhabitants living in fancied security. But six months of violent earthquakes, which grew more violent towards the close of 1631, heralded the eruption which took place in December, accompanied by terrific noises from within the interior of the mountain. The inhabitants of the coast were thus warned of the approaching danger, and had several days to arrange for their safety; but in the end, a great part of Torre del Greco was destroyed, and a like fate overtook Resina and Granatello, with a loss of life reported at 18,000 persons. During the eruption clouds condensed into tempests of rain, and hot water from the mountain, forming deluges of mud, swept down the sides, and reached even to Nola and the Apennines. Nor was the sea unmoved. It retired during the violent earthquakes, and then returned full thirty paces beyond its former limits.

Not indeed until near the close of the seventeenth century is there any evidence that the central cone of Vesuvius was in existence; but in October 1685 an eruption occurred which is recorded by Sorrentino, during which was erected "a new mountain within, and higher than the old one, and visible from Naples," a statement evidently referable to the existing cone--so that it is little more than two centuries since this famous volcanic mountain a.s.sumed its present form.

(_h._) _Eruptions between the years 1500 and 1800._--Since A.D. 1500 there have been fifty-six recorded eruptions of Vesuvius; one of these in 1767 was of terrific violence and destructiveness, and is represented by Sir William Hamilton in views taken both before and during the eruption. A pen-and-ink drawing of the appearance of the crater before the eruption is here reproduced from Hamilton's picture, from which it will be seen that the central crater contained within itself a second crater-cone, from whence steam, lava, and stones were being erupted (Fig. 7). Thus it will be seen that Vesuvius at this epoch consisted of three crater-cones within each other. The first, Monte di Somma; the second, the cone of Vesuvius; and the third, the little crater-cone within the second. During this eruption, vast lava-sheets invaded the fields and vineyards on the flanks of the mountain. A vivid account of this eruption, as witnessed by Padre Torre, is given by Professor Phillips.[8] We shall pa.s.s over others without further reference until we come down to our own times, in which Vesuvius has resumed its old character, and in one grand exhibition of volcanic energy, which took place in 1872, has evinced to the world that it still contains within its deep-seated laboratory all the elements of destructive force which it exhibited at the commencement of our era.

[Ill.u.s.tration: Fig. 7.--View of the crater of Vesuvius before the eruption of 1767, showing an interior crater-cone rising from the centre of the exterior crater.--(After Sir W. Hamilton.)]

(_i._) _Structure of the Neapolitan Campagna._--But before giving a description of this terrific outburst of volcanic energy, it may be desirable to give some account of the physical position and structure of this mountain, by which the phenomena of the eruption will be better understood. Vesuvius and the Neapolitan Campagna are formed of volcanic materials bounded on the west by the Gulf of Naples, and on the east and south by ranges of Jura.s.sic limestone, a prolongation of the Apennines, which send out a spur bounding the bay on the south, and forming the promontory of Sorrento. The little island of Capri is also formed of limestone, and is dissevered from the promontory by a narrow channel.

The northern side of the bay is, however, formed of volcanic materials; it includes the Phlegraean Fields (Campi Phlegraei), and terminates in the promontory of Miseno. Lying in the same direction are the islands of Procida and Ischia, also volcanic. Hence it will be seen that the two horns of the bay are formed of entirely different materials, that of Miseno on the north being volcanic, that of Sorrento on the south being composed of Jura.s.sic limestone, of an age vastly more ancient than the volcanic rocks on the opposite sh.o.r.e. (Map, p. 52.)

The general composition of the Neapolitan Campagna, from which the mountain rises, has been revealed by means of the Artesian well sunk to a depth of about 500 metres (1640 feet) at the Royal Palace of Naples, and may be generalised as follows:--

{ Recent beds of volcanic tuff (1) From surface to depth of { with marine sh.e.l.ls, and containing 715 feet { fragments of trachytic { lava, etc. (_Volcanic Beds_).

{ Bituminous sands and marls (2) From 715 to 1420 { with marine sh.e.l.ls of recent { species(?) (_Pre-Volcanic Beds_).

(3) From 1420 to 1574 { EOCENE BEDS. Micaceous sandstone { and marl (_Macigno_).

(4) From 1574 to bottom { JURa.s.sIC BEDS. Apennine { Limestone.

[Ill.u.s.tration: Fig. 8.--Map of the district bordering the Bay of Naples, with the islands of Capri, Ischia, and Procida.]

From the above section, for which we are indebted to Mr. Johnston-Lavis, the most recent writer on Vesuvius, it would appear that the first volcanic explosions by which the mountain was ultimately to be built up took place after the deposition of the sands and marls (No. 2), while the whole Campagna was submerged under the waters of the Mediterranean.

By the acc.u.mulation of the stratified tuff (No. 1), the sea-bed was gradually filled up during a period of volcanic activity, and afterwards elevated into dry land.[9]

[Ill.u.s.tration: Fig. 9.--View of Vesuvius from the Harbour of Naples at the commencement of the eruption of 1872.--(From a sketch by the author.)]

(_j._) _Present Form and Structure of Vesuvius and Somma._--The outer cone of Vesuvius, or Monte di Somma, rises from a circular platform by a moderately gentle ascent, and along the north and east terminates in a craggy crest, with a precipitous cliff descending into the Atria del Cavallo, forming the wall of the ancient crater throughout half its circ.u.mference; this wall is formed of scoriae, ashes, and lapilli, and is traversed by numerous d.y.k.es of lava. Along the west and south this old crater has been broken down; but near the centre there remains a round-backed ridge of similar materials, once doubtless a part of the original crater of Somma, rising above the slopes of lava on either hand. On this has been erected the Royal Observatory, under the superintendence of Professor Luigi Palmieri, where continuous observations are being made, by means of delicate seismometers, of the earth-tremors which precede or accompany eruptions; for it is only justice to say that Vesuvius gives fair warning of impending mischief, and the instruments are quick to notify any premonitory symptoms of a coming catastrophe. The elevation of the Observatory is 2080 feet above the sea.

On either side of the Observatory ridge are wide channels filled to a certain height with lavas of the nineteenth and preceding centuries, the most recent presenting an aspect which can only be compared to a confused mult.i.tude of black serpents and pachyderms writhing and interlocked in some frightful death-struggle. Some of this lava, ten years old, as we cross its rugged and black surface presents gaping fissures, showing the ma.s.s to be red-hot a few feet from the surface, so slow is the process of cooling. These lava-streams--some of them reaching to the sea-coast--have issued forth from the Atria at successive periods of eruption.

From the midst of the Atria rises the central cone, formed of cinders, scoriae, and lava-streams, and fissured along lines radiating from the axis. This cone is very steep, the angle being about 40-45 from the horizontal, and is formed of loose cindery matter which gives way at every step, and is rather difficult to climb. But on reaching the summit we look down into the crater, displaying a scene of ever-varying characters, rather oval in form, and about 1100 yards in diameter. From the map of Professor Guiscardi, published in 1855, there are seen two minor craters within the central one, formed in 1850, and an outflow of lava from the N.W. down the cone. At the time of the author's visit the crater was giving indications, by the great quant.i.ty of sulphurous gas and vapour rising from its surface, and small jets of molten lava beginning to flow down the outer side, of the grand outburst of internal forces which was presently to follow.

(_k._) _Eruption of 1872._--The grand eruption of 1872, of which a detailed account is given by Professor Palmieri,[10] commenced with a slight discharge of incandescent projectiles from the crater; and on the 13th January an aperture appeared on the upper edge of the cone from which at first a little lava issued forth, followed by the uprising of a cone which threw out projectiles accompanied by smoke, whilst the central crater continued to detonate more loudly and frequently. This little cone ultimately increased in size, until in April it filled the whole crater and rose four or five metres above the brim. At this time abundant lavas poured down from the base of the cone into the Atria del Cavallo, thence turned into the Fossa della Vetraria in the direction of the Observatory and towards the Crocella, where they acc.u.mulated to such an extent as to cover the hillside for a distance of about 300 metres; then turning below the Canteroni, formed a hillock without spreading much farther.

In October another small crater was formed by the falling in of the lava, which after a few days gave vent to smoke and several jets of lava; and towards the end of October the detonations increased, the smoke from the central crater issued forth more densely mixed with ashes, and the seismographical apparatus was much disturbed. On the 3rd and 4th November copious and splendid lava-streams coursed down the princ.i.p.al cone on its western side, but were soon exhausted; and in the beginning of 1872 the little cone, regaining vigour, began to discharge lava from the summit instead of the base as heretofore.

In the month of March 1873, with the full moon, the cone opened on the north-west side--the cleavage being indicated by a line of fumaroles--and lava issued from the base and poured down into the Atria as far as the precipices of Monte di Somma. On the 23rd April (another full moon) the activity of the craters increased, and on the evening of the 24th splendid lava-streams descended the cone in various directions, attracting on the same night the visits of a great many strangers. A lamentable event followed on the 26th. A party of visitors, accompanied by inexperienced guides, and contrary to the advice of Professor Palmieri, insisted on ascending to the place from which the lava issued.

At half-past three on the morning of the 26th they were in the Atria del Cavallo, when the Vesuvian cone was rent in a north-west direction and a copious torrent of lava issued forth. Two large craters formed at the summit of the mountain, discharging incandescent projectiles and ashes.

A cloud of smoke enveloped the unhappy visitors, who were under a hail-storm of burning projectiles. Eight were buried beneath it, or in the lava, while eleven were grievously injured.[11] The lava-stream, flowing over that of 1871 in the Atria, divided into two branches, the smaller one flowing towards Resina, but stopping before reaching the town; the larger precipitated itself into the Fossa della Vetraria, occupying the whole width of 800 metres, and traversing the entire length of 1300 metres in three hours. It dashed into the Fossa di Farone, and reached the villages of Ma.s.sa and St. Sebastiano, covering a portion of the houses, and, continuing its course through an artificial foss, or trench, invaded cultivated ground and several villages. If it had not greatly slackened after midnight, from failure of supply at its source, it would have reached Naples by Ponticelli and flowed into the sea. The eruption towards the end of April had reached its height. The Observatory ridge was bounded on either side by two fiery streams, which rendered the heat intolerable. Simultaneously with the opening of the great fissure two large craters opened at the summit, discharging with a dreadful noise an immense cloud of smoke and ashes, with bombs which rose to a height of 1300 metres above the brim of the volcano.[12] The torrents of fire which threatened Resina, Bosco, and Torre Annunziata, and which devastated the fertile country of Novelle, Ma.s.sa, St.

Sebastiano, and Cerole, and two partially buried cities, the continual thunderings and growling of the craters, caused such terror, that numbers abandoned their dwellings, flying for refuge into Naples, while many Neapolitans went to Rome or other places. Fortunately, the paroxysm had now pa.s.sed, the lava-streams stopped in their course, and the great torrent which pa.s.sed the shoulders of the Observatory through the Fossa della Vetraria lowered the level of its surface below that of its sides, which appeared like two parallel ramparts above it. Had these streams continued to flow on the 27th of April as they had done on the previous night, they would have reached the sea, bringing destruction to the very walls of Naples. During this eruption Torre del Greco was upraised to the extent of two metres, and nearly all the houses were knocked down.

The igneous period of eruption having terminated, the ashes, lapilli, and projectiles became more abundant, accompanied by thunder and lightning. On the 28th they darkened the air, and the terrific noise of the mountain continuing or increasing, the terror at Resina, Portici, and Naples became universal. It seemed as though the tragic calamities of the eruption of A.D. 79 were about to be repeated. But gradually the force of the explosions decreased, and the noise from the crater became discontinuous, so that on the 30th the detonations were very few, and by the 1st May the eruption was completely over.

Such is a condensed account of one of the most formidable eruptions of our era. In the frontispiece of this volume a representation, taken (by permission) from a photograph by Negretti & Zambra, is given, showing the appearance of Vesuvius during the final stage of the eruption, when prodigious ma.s.ses of smoke, steam, and illuminated gas issued forth from the summit and overspread the whole country around with a canopy which the light of the sun could scarcely penetrate.

It will be noticed in the above account that, concurrently with the full moon, there were two distinct and special outbreaks of activity; one occurring in March, the other in the month following. That the conditions of lunar and solar attraction should have a marked effect on a part of the earth's crust, while under the tension of eruptive forces, is only what might be expected. At full moon the earth is between the sun and the moon, and at new moon the moon is between the sun and the earth; under these conditions (the two bodies acting in concert) we have spring tides in the ocean, and a maximum of attraction on the ma.s.s of the earth. Hence the crust, which at the time referred to was under tremendous strain, only required the addition of that caused by the lunar and solar attractions to produce rupture in both cases, giving rise to increased activity, and the extrusion of lava and volatile matter. It may, in general, be safely affirmed that low barometric pressure on the one hand, and the occurrence of the syzygies (when the attractions of the sun and moon are in the same line) on the other, have had great influence in determining the crises of eruptions of volcanic mountains when in a state of unrest.

_Contrast between the Northern and Southern Slopes._--Before leaving Vesuvius it may be observed that throughout all the eruptions of modern times the northern side of the mountain, that is the old crater and flank of Somma, has been secure from the lava-flows, and has enjoyed an immunity which does not belong to the southern and western side. If we look at a map of the mountain showing the direction of the streams during the last three centuries,[13] we observe that all the streams of that period flowed down on the side overlooking the Bay of Naples; on the opposite side the wall of Monte di Somma presents an unbroken front to the lava-streams. From this it may be inferred that one side, the west, is weaker than the other; and consequently, when the lava and vapours are being forced upwards, under enormous pressure from beneath, the western side gives way under the strain, as in the case of the fissure of 1872, and the lava and vapours find means of escape. From what has happened in the past it is clear that no place on the western side of the mountain is entirely safe from devastation by floods of lava; while the prevalent winds tend to carry the ashes and lapilli, which are hurled into the air, in the same westerly direction.

[1] For an excellent view of this remarkable volcanic group see Judd's _Volcanoes_, 4th edition, p. 43.

[2] Plutarch, _Life of Ca.s.sius_; _ed. Reiske_, vol. iii. p. 240.

[3] Strabo gives the following account of the appearance and condition of Vesuvius in his day:--"Supra haec loca situs est Vesuvius mons, agris cinctus optimis; dempto vertice, qui magna sui parte pla.n.u.s, totus sterilis est, adspectu sinereus, cavernasque ostendens fistularum plenas et lapidum colore fuliginoso, utpote ab igni exesorum. Ut conjectarum facere possis, ista loca quondam arsisse et crateras ignis habuisse, deinde materia deficiente restricta fuisse."--_Rer. Geog._, lib. v.

[4] A tablet over the entrance records this act of pious liberality, and is given by Phillips, _loc. cit._, p. 12.

[5] The stone pine, _Pinus pinea_, which Turner knew how to use with so much effect in his Italian landscapes.

[6] Bulwer Lytton's _Last Days of Pompeii_ presents to the reader a graphic picture of the terrible event here referred to:--"The eyes of the crowd followed the gesture of the Egyptian, and beheld with ineffable dismay a vast vapour shooting from the summit of Vesuvius, in the form of a gigantic pine tree; the trunk--blackness, the branches--fire! A fire that s.h.i.+fted and wavered in its hues with every moment--now fiercely luminous, now of a dull and dying red that again blazed terrifically forth with intolerable glare!... Then there arose on high the shrieks of women; the men stared at each other, but were speechless. At that moment they felt the earth shake beneath their feet; the walls of the theatre trembled; and beyond, in the distance, they heard the crash of falling roofs; an instant more and the mountain-cloud seemed to roll towards them, dark and rapid; at the same time it cast forth from its bosom a shower of ashes mixed with vast fragments of burning stone. Over the crus.h.i.+ng vines--over the desolate streets--over the amphitheatre itself--far and wide, with many a mighty splash in the agitated sea, fell that awful shower." A visit to the disinterred city will probably produce on the mind a still more lasting and vivid impression of the swift destruction which overtook this city.

[7] Quoted by Phillips, _loc. cit._, p. 45.

[8] _Vesuvius_, p. 72 _et seq._

[9] Johnston-Lavis, "On the Geology of Monti Somma and Vesuvius,"

_Quart. Jour. Geol. Soc._, vol. 40 (1884).

[10] Palmieri, _Eruption of Vesuvius in 1872_, with notes, etc., by Robert Mallet, F.R.S. London, 1873.

[11] Those who lost their lives were medical students, and an a.s.sistant Professor in the University, Antonio Giannone by name.

[12] Involving, as Mr. Mallet calculates, an initial velocity of projection of above 600 feet per second.

[13] Such as that given by Professor Phillips in his _Vesuvius_.