Volcanoes: Past and Present Part 4

CHAPTER II.

ETNA.

(_a._) _Structure of the Mountain._--Etna, unlike Vesuvius, has ever been a burning mountain; hence it was well known as such to cla.s.sic writers before the Christian era. The structure and features of this magnificent mountain have been abundantly ill.u.s.trated by Elie de Beaumont,[1] Daubeny,[2] Baron von Waltershausen,[3] and Lyell,[4] of whose writings I shall freely avail myself in the following account, not having had the advantage of a personal examination of this region.

_Structure of Etna._--So large is Etna that it would enclose within its ample skirts several cones of the size of Vesuvius. It rises to a height of nearly 11,000 feet above the waters of the Mediterranean,[5] and is planted on a floor consisting of stratified marine volcanic matter, with clays, sands, and limestones of newer Pliocene age. Its base is nearly circular, and has a circ.u.mference of 87 English miles. In ascending its flanks we pa.s.s successively over three well-defined physical zones: the lowest, or fertile zone, comprising the tract around the skirts of the mountain up to a level of about 2500 feet, being well cultivated and covered by dwellings surrounded by olive groves, fields, vineyards, and fruit-trees; the second, or forest zone, extending to a level of about 6270 feet, clothed with chestnut, oak, beech, and cork trees, giving place to pines; and the third, extending to the summit and called "the desert region," a waste of black lava and scoriae with mighty crags and precipices, terminating in a snow-clad tableland, from which rises the central cone, 1100 feet high, emitting continually steam and sulphurous vapours, and in the course of almost every century sending forth streams of molten lava.

The forest zone is remarkable for the great number of minor craters which rise up from the midst of the foliage, and are themselves clothed with trees. Sartorius von Waltershausen has laid down on his map of Etna about 200 of these cones and craters, some of which, like those of Auvergne, have been broken down on one side. Many of these volcanoes of second or third magnitude lie outside the forest zone, both above and below it; such as the double hill of Monti Rossi, near Nicolosi, formed in 1659, which is 450 feet in height, and two miles in circ.u.mference at its base. Sir C. Lyell observes that these minor crater-cones present us with one of the most delightful and characteristic scenes in Europe.

They occur of every variety of height and size, and are arranged in picturesque groups. However uniform they may appear when seen from the sea or the plains below, nothing can be more diversified than their shape when we look from above into their ruptured craters. The cones situated in the higher parts of the forest zone are chiefly clothed with lofty pines; while those at a lower elevation are adorned with chestnuts, oaks, and beech trees. These cones have from time to time been buried amidst fresh lava-streams descending from the great crater, and thus often become obliterated.

[Ill.u.s.tration: Fig. 10.--Ideal Section through Etna. (After Lyell.)--A.

Axis of present cone of eruption; B. Axis of extinct cone of eruption; _a._ Older lavas, chiefly trachytic; _b._ Newer lavas, erupted (with _a_) before origin of the Val del Bove; _c._ Scoria and lava of recent age; T. Tertiary strata forming the foundation to the volcanic rocks.

The position of the Val del Bove before its formation is shown by the lightly-shaded portion above B.]

(_b._) _Val del Bove._--The most wonderful feature of Mount Etna is the celebrated Val del Bove (Valle del Bue), of which S. von Waltershausen has furnished a very beautiful plate[6]--a vast amphitheatre hewn out of the eastern flank of the mountain, just below the snow-mantled platform.

It is a physical feature somewhat after the fas.h.i.+on of Monte Somma in Vesuvius, but exceeds it in magnitude as Etna exceeds Vesuvius. The Val del Bove is about five miles in diameter, bounded throughout three-fourths of its circ.u.mference by precipitous walls of ashes, scoriae, and lava, traversed by innumerable d.y.k.es, and rising inwards to a height of between 3000 and 4000 feet. Towards the east the cliffs gradually fall to a height of about 500 feet, and at this side the vast chasm opens out upon the slope of the mountain. At the head of the Val del Bove rises the platform, surmounted by the great cone and crater. It will thus be seen that by means of this hollow we have access almost to the very heart of the mountain.

What is very remarkable about the structure of this valley is that the beds exhibit "the _qua-qua_ versal dip"--in other words, they dip away on all sides from the centre--which has led to the conclusion that in the centre is a focus of eruption which had become closed up antecedently to the formation of the valley itself. Lyell has explained this point very clearly by showing that this focus had ceased to eject matter at some distant period, and that the existing crater at the summit of the mountain had poured out its lavas over those of the extinct orifice. This was prior to the formation of the Val del Bove itself; and the question remains for consideration how this vast natural amphitheatre came to be hollowed out; for its structure shows unquestionably that it owes its form to some process of excavation.

In the first place, it is certainly not the work of running water, as in the case of the canons of Colorado; the porous matter of which the mountain is formed is quite incapable of originating and supporting a stream of sufficient volume to excavate and carry away such enormous ma.s.ses of matter within the period required for the purpose. We must therefore have recourse to some other agency. Numerous ill.u.s.trations are to be found of the explosive action of volcanoes in blowing off either the summits of mountains, or portions of their sides. For example, there is reason for believing that the first result of the renewed energy of Vesuvius was to blow into the air the upper surface of the mountain.

Again, so late as 1822, during a violent earthquake in Java, a country which has been repeatedly devastated by earthquakes and volcanic eruptions, the mountain of Galongoon, which was covered by a dense forest, and situated in a fertile and thickly-peopled region, and had never within the period of tradition been in activity, was thus ruptured by internal forces. In the month of July 1822, after a terrible earthquake, an explosion was heard, and immense columns of boiling water, mixed with mud and stones, were projected from the mountain like a water-spout, and in falling filled up the valleys, and covered the country with a thick deposit for many miles, burying villages and their inhabitants. During a subsequent eruption great blocks of basalt were thrown to a distance of seven miles; the result of all being that an enormous semicircular gulf was formed between the summit and the plain, bounded by steep cliffs, and bearing considerable resemblance to the Val del Bove. Other examples of the power of volcanic explosions might be cited; but the above are sufficient to show that great hollows may thus be formed either on the summits or flanks of volcanic mountains. Chasms may also be formed by the falling in of the solidified crust, owing to the extrusion of molten matter from some neighbouring vent of eruption; and it is conceivable that by one or other of these processes the vast chasm of the Val del Bove on the flanks of Etna may have been produced.

(_c._) _The Physical History of Etna._--The physical history of Etna seems to be somewhat as follows:--

_First Stage._--Somewhere towards the close of the Tertiary period--perhaps early Pliocene or late Miocene--a vent of eruption opened on the floor of the Mediterranean Sea, from which sheets of lava were poured forth, and ashes mingled with clays and sands, brought down from the neighbouring lands, were strewn over the sea-bed. During a pause in volcanic activity, beds of limestone with marine sh.e.l.ls were deposited.

_Second Stage._--This sea-bed was gradually upraised into the air, while fresh sheets of lava and other _ejecta_ were acc.u.mulated round the vents of eruption, of which there were two princ.i.p.al ones--the older under the present Val del Bove, the newer under the summit of the princ.i.p.al cone.

Thus was the mountain gradually piled up.

_Third Stage._--The vent under the Val del Bove ceased to extrude more matter, and became extinct. Meanwhile the second vent continued active, and, piling up more and more matter round the central crater, surmounted the former vent, and covered its _ejecta_ with newer sheets of lava, ashes, and lapilli, while numerous smaller vents, scattered all over the sides of the mountain, gave rise to smaller cones and craters.

_Fourth Stage._--This stage is signalised by the formation of the Val del Bove through some grand explosion, or series of explosions, by which this vast chasm was opened in the side of the mountain, as already explained.

_Fifth Stage._--This represents the present condition of the mountain, whose height above the sea is due, not only to acc.u.mulation of volcanic materials round the central cone, but to elevation of the whole island, as evinced by numerous raised beaches of gravel and sand, containing sh.e.l.ls and other forms of marine species now living in the waters of the Mediterranean.[7] Since then the condition and form of the mountain has remained very much the same, varied only by the results of occasional eruptions.

(_d._) _Dissimilarity in the Const.i.tution of the Lavas of Etna and Vesuvius._--Before leaving the subject we have been considering, it is necessary that I should mention one remarkable fact connected with the origin of the lavas of Etna and Vesuvius respectively; I refer to their essential differences in mineral composition. It might at first sight have been supposed that the lavas of these two volcanic mountains--situated at such a short distance from each other, and evidently along the same line of fracture in the crust--would be of the same general composition; but such is not the case. In the lava of Vesuvius leucite is an essential, and perhaps the most abundant mineral.

It is called by Zirkel _Sanidin-Leucitgestein_. (See Plate IV.) But in that of Etna this mineral is (as far as I am aware) altogether absent.

We have fortunately abundant means of comparison, as the lavas of these two mountains have been submitted to close examination by petrologists.

In the case of the Vesuvian lavas, an elaborate series of chemical a.n.a.lyses and microscopical observations have been made by the Rev.

Professor Haughton, of Dublin University, and the author,[8] from specimens collected by Professor Guiscardi from the lava-flows extending from 1631 to 1868, in every one of which leucite occurs, generally as the most abundant mineral, always as an essential const.i.tuent. On the other hand, the composition of the lavas of Etna, determined by Professor A. von Lasaulx, from specimens taken from the oldest (voratnaischen) sheets of lava down to those of the present day, indicates a rock of remarkable uniformity of composition, in which the components are plagioclase felspar, augite, olivine, magnet.i.te, and sometimes apat.i.te; but of leucite we have no trace.[9] In fact, the lavas of Etna are very much the same in composition as the ordinary basalts of the British Isles, while those of Vesuvius are of a different type. This seems to suggest an origin of the two sets of lavas from a different deep-seated magma; the presence of leucite in such large quant.i.ty requiring a magma in which soda is in excess, as compared with that from which the lavas of Etna have been derived.[10]

[1] _Memoires pour Servir_, etc., vol. ii.

[2] Daubeny, _Volcanoes_, p. 270.

[3] Von Waltershausen, _Der aetna_, edited by A. von Lasaulx.

[4] Lyell, _Principles of Geology_, vol. ii., edition 1872.

[5] Its height, as determined by Captain Smyth in 1875 trigonometrically, was 10,874 feet, and afterwards by Sir J. Herschel barometrically, 10,872 feet.

[6] _Atlas des aetna_ (Weimar, 1858), in which the different lava-streams of 1688, 1802, 1809, 1811, 1819, 1824, and 1838 are delineated.

[7] Sir William Hamilton observes that history is silent regarding the first eruptions of Etna. It was in activity before the Trojan War, and even before the arrival of the "Sizilien" settlers. Diodorus and Thucydides notice the earliest recorded eruptions, those from 772 to 388 B.C., during which time the mountain was thrice in eruption. Later eruptions took place in the year 140, 135, 125, 122 B.C. In the year 44 B.C., in the reign of Julius Caesar, there was a very violent outburst of volcanic activity.--_Neuere Beobachtungen uber die Vulkane Italiens und am Rhein_, p. 173, Frankfurt (1784).

[8] "Report on the Chemical and Mineralogical Characters of the Lavas of Vesuvius from 1631 to 1868," _Transactions of the Royal Irish Academy_, vol. xxvi. (1876). In the lava of 1848 leucite was found to reach 44.9 per cent. of the whole ma.s.s. In that of Granatello, 1631, it reaches its lowest proportion--viz., 3.37 per cent.

[9] A. von Lasaulx, in Von Waltershausen's _Der aetna_, Book II., x. 423.

[10] The view of Professor Judd, that leucite easily changes into felspar, and that some ancient igneous rocks which now contain felspar were originally leucitic, does not seem to be borne out by the above facts. In such cases the felspar crystals ought to retain the forms of leucite. See _Volcanoes_, 4th edition, p. 268.

CHAPTER III.

THE LIPARI ISLANDS, STROMBOLI.

(_a._) A brief account of this remarkable group of volcanic islands must here be given, inasmuch as they seem to be representatives of a stage of volcanic action in which the igneous forces are gradually losing their energy. According to Daubeny, the volcanic action in these islands seems to be developed along two lines, nearly at right angles to each other, one parallel to that of the Apennines, beginning with Stromboli, intersecting Panaria, Lipari, and Vulcano; the other extending from Panaria to Salina, Alicudi, and Felicudi, and again visible in the volcanic products which make their appearance at Ustica. (See Map, Fig.

11.) The islands lie between the north coast of Sicily and that of Italy, and from their position seem to connect Etna with Vesuvius; but this is very problematical, as would appear from the difference of their lavas. The princ.i.p.al islands are those of Stromboli, Panaria, Lipari, Vulcano, Salina, Felicudi, and Alicudi. These three last are extinct or dormant, but Salina contains a crater, rising, according to Daubeny, not less than 3500 feet above the sea.[1] Vulcano (referred to by Strabo under the name of Hiera) consists of a crater which constantly emits large quant.i.ties of sulphurous vapours, but was in a state of activity in the year 1786, when, after frequent earthquake shocks and subterranean noises, it vomited forth during fifteen days showers of sand, together with clouds of smoke and flame, altering materially the shape of the crater from which they proceeded.

[Ill.u.s.tration: LIPARI ISLANDS.

Fig. 11.--Map to show the position of these islands, showing the branching lines of volcanic action--one parallel to that of the Apennines, the other stretching westwards at right angles thereto.]

The islands of Lipari are formed of beds of tuff, penetrated by numerous d.y.k.es of lava, from which uprise two or three craters, formed of pumice and obsidian pa.s.sing into trachyte. Volcanic operations might have here been said to be extinct, were it not that their continuance is manifested by the existence of hot springs and "stufes," or vapour baths, at St. Calogero, about four miles from the town of Lipari.

Daubeny considers it not improbable that this island may have had an active volcano even within the historical period, a view which is borne out by the statement of Strabo.[2]

[Ill.u.s.tration: Fig. 12.--Island of Vulcano, one of the Lipari Group, in eruption.--(After Sir W. Hamilton.)]

(_b._) But by far the most remarkable island of the group, as regards its present volcanic condition, is Stromboli, which has ever been in active eruption from the commencement of history down to the present day. Professor Judd, who visited this island in 1874, and has produced a striking representation of its aspect,[3] gives an account of which I shall here avail myself.[4] The island is of rudely circular outline, and rises into a cone, the summit of which is 3090 feet above the level of the Mediterranean. From a point on the side of the mountain ma.s.ses of vapour are seen to issue, and these unite to form a cloud over the summit; the outline of this vapour-cloud varying continually according to the hygrometric state of the atmosphere, and the direction and force of the wind. At the time of Professor Judd's visit, the vapour-cloud was spread in a great horizontal stratum overshadowing the whole island; but it was clearly seen to be made up of a number of globular ma.s.ses, each of which is a product of a distinct outburst of volcanic forces.

Viewed at night-time, Stromboli presents a far more striking and singular spectacle. When watched from the deck of a vessel, a glow of red light is seen to make its appearance from time to time above the summit of the mountain; it may be observed to increase gradually in intensity, and then as gradually to die away. After a short interval the same appearances are repeated, and this goes on till the increasing light of dawn causes the phenomenon to be no longer visible. The resemblance presented by Stromboli to a "flas.h.i.+ng light" on a most gigantic scale is very striking, and the mountain has long been known as "the lighthouse of the Mediterranean."

The mountain is built up of ashes, slag, and scoriae, to a height of (as already stated) over 3000 feet above the surface of the sea; but, as Professor Judd observes, this by no means gives a just idea of its vast bulk. Soundings in the sea surrounding the island show that the bottom gradually shelves around the sh.o.r.es to a depth of nearly 600 fathoms, so that Stromboli is a great conical ma.s.s of cinders and slaggy materials, having a height above its floor of about 6600 feet, and a base the diameter of which exceeds four miles.

The crater of Stromboli is situated, not at the apex of the cone, but at a distance of 1000 feet below it. The explosions of steam, accompanied by the roaring as of a smelting furnace, or of a railway engine when blowing off its steam, are said by Judd to take place at very irregular intervals of time, "varying from less than one minute to twenty minutes, or even more." On the other hand, Hoffmann describes them as occurring at "perfectly regular intervals," so that, perhaps, some variation has taken place within the interval of about forty years between each observation. Both observers agree in stating that lava is to be seen welling up from some of the apertures within the crater, and pouring down the slope towards the sea, which it seldom or never reaches.[5] The intermittent character of these eruptions appears to be due, as Mr.

Scrope has suggested, to the exact proportion between the expansive and repressive forces; the expansive force arising from the generation of a certain amount of aqueous vapour and of elastic gas; the repressive, from the pressure of the atmosphere and from the weight of the superinc.u.mbent volcanic products. Steam is here, as in a steam-engine, not the originating agent in the phenomena recorded; but the result of water coming in contact with molten lava constantly welling up from the interior, by which it is converted into steam, which from time to time acquires sufficient elastic force to produce the eruptions; the water being obviously derived from the surrounding sea, which finds its way by filtration through fissures, or through the porous ma.s.s of which the mountain is formed. Were it not for the access of water this volcano would probably appear as a fissure-cone extruding a small and continuous stream of molten lava. The advent.i.tious access of the sea water gives rise to the phenomena of intermittent explosions. The vitality of the volcano is therefore due, not to the presence of water, but to the welling up of matter from the internal reservoir through the throat of the volcano.

_Pantelleria._--This island, lying between the coast of Sicily and Cape Bon in Africa, is wholly volcanic. It has a circ.u.mference of thirty miles, and from its centre rises an extinct crater-cone to a height of about 3000 feet. The flanks of this volcano are diversified by several fresh craters and lava-streams, while hot springs burst out with a hissing noise on its southern flank, showing that molten matter lies below at no very great depth.

This island probably lies along the dividing line between the non-volcanic and volcanic region of the Mediterranean, and is consequently liable to intermittent eruptions. It was at a short distance from this island that the remarkable submarine outburst of volcanic forces took place on October 17th, 1891, for an account of which we are indebted to Colonel J. C. Mackowen.[6] On that day, after a succession of earthquake shocks, the inhabitants were startled by observing a column of "smoke" rising out of the sea at a distance of three miles, in a north-westerly direction. The Governor, Francesco Valenza, having manned a boat, rowed out towards the fiery column, and on arriving found it to consist of black scoriaceous bombs, which were being hurled into the air to a height of nearly thirty yards; some of them burst in the air, others, discharging steam, ran hissing over the water; many of them were very hot, some even red-hot. One of these bombs, measuring two feet in diameter, was captured and brought to sh.o.r.e. It was observed that after the eruption the earthquake shocks ceased. A vast amount of material was cast out of the submarine crater, forming an island 500 yards in length and rising up to nine feet above the surface, but after a few days it was broken up and dispersed over the sea-bed by the action of the waves.

[1] _Volcanoes_, p. 262. These islands are described by Hoffmann, _Poggendorf Annal._, vol. xxvi. (1832); also by Lyell, _Principles of Geology_, vol. ii., and by Judd, who personally visited them, and gives a very vivid account of their appearance and structure.