Etna Part 6

75. Near the end of the following year, fifteen mouths of fire opened near the crater of 1832, at a height of 7000 feet above the sea. They began by discharging scoriae and sand, and afterwards lava, which divided into three streams, the two outer ones soon came to a standstill, while the central stream continued to flow at the rapid rate of 180 feet a minute, the descent being an angle of 25. The heat at a distance of 120 feet from the current was 90 F. A new crater opened just above Bronte, and discharged lava which threatened the town, but it fortunately encountered Monte Vittoria and was diverted into another course. While a number of the inhabitants of Bronte were watching the progress of the lava, the front of the stream was suddenly blown out as by an explosion of gunpowder; in an instant red-hot ma.s.ses were hurled in every direction; and a cloud of vapour enveloped everything. Thirty-six persons were killed on the spot, and twenty survived but a few hours.

The great crater showed signs of disturbance, by emitting dense volumes of smoke, and loud bellowings, also quant.i.ties of volcanic dust saturated with hydrochloric acid, which destroyed the vegetation wherever it fell.

76. A very violent eruption which lasted more than nine months, commenced on the 21st of August, 1852. It was first witnessed by a party of six English tourists, who were ascending the mountain from Nicolosi in order to see the sunrise from the summit. As they approached the Casa Inglesi the crater commenced to give forth ashes and flames of fire. In a narrow defile they were met by a violent hurricane, which overthrew both the mules and their riders, and urged them towards the precipices of the Val del Bove. They sheltered themselves beneath some ma.s.ses of lava, when suddenly an earthquake shook the mountain, and their mules in terror fled away. They returned on foot towards daylight to Nicolosi, fortunately without having sustained injury. In the course of the night many _bocche del fuoco_ opened in that part of the Val del Bove called the Balzo di Trifoglietto, and a great fissure opened at the base of the Giannicola Grande, and a crater was thrown up from which for seventeen days showers of sand and scoriae were ejected. During the next day a quant.i.ty of lava flowed down the Val del Bove, branching off so that one stream advanced to the foot of Monte Finocchio, and the other to Monte Calanna. Afterwards it flowed towards Zaffarana, and devastated a large tract of woody region. Four days later a second crater was formed near the first, from which lava was emitted together with sand and scoriae, which caused cones to rise around the craters. The lava moved but slowly, and towards the end of August it came to a stand, only a quarter of a mile from Zaffarana: on the second of September, Gemellaro ascended Monte Finocchio in the Val del Bove in order to witness the outburst. He states that the hill was violently agitated, like a ship at sea. The surface of the Val del Bove appeared like a molten lake; scoriae were thrown up from the craters to a great height, and loud explosions were heard at frequent intervals. The eruption continued to increase in violence. On October 6th two new mouths opened in the Val del Bove, emitting lava which flowed towards the Valley of Calanna, and fell over the Salto della Giumenta, a precipice nearly 200 feet deep. The noise which it produced was like that of the clash of metallic ma.s.ses. The eruption continued with abated violence during the early months of 1853, and it did not finally cease till May 27. The entire ma.s.s of lava ejected is estimated to be equal to an area six miles long by two miles broad, with an average depth of about twelve feet.

I am indebted to M. Antonin Moris of Palermo for the following account of the eruption of 1852:

The eruption of 1852 commenced on the 21st of August. The earthquakes, the jets of flame from the great crater, and the subterranean rumblings which usually precede an eruption, did not herald the approach of this one. An English family, who were then making the ascent of the mountain, together with a poor shepherd of Riposto, were the only witnesses of the first outburst. The latter was asleep in the midst of his flocks, and was awakened by violent shakings of the ground; he fled in haste, and some seconds afterwards the earth opened with a loud noise, vomiting a terrible column of fire, at the very spot which he had just abandoned.

An enormous creva.s.se opened on the north side of Trifoglietto in the direction of the great crater. On its summit near the opening called the Piccolo Teatro, several openings were produced at the very first, but they only emitted feeble currents of lava. All the force of the eruption was concentrated at the foot of the escarpment of the Serra di Giannicola, 4 kilometres, (2-1/2 miles) from the summit of Etna. To the west of, and somewhat above the princ.i.p.al crater, a second one was formed, but its activity was of short duration. The liquid lava issued with such violence that in 24 hours it had reached the base of Monte Calanna, a distance of 3 kilometres, (nearly 2 miles). After surrounding this hill, it divided into two currents, one of which ran towards Zaffarana, and the other towards Milo. At a distance they seemed to present a united front of 2 kilometres, (1-1/4 mile), which threatened to destroy all the villages below. The Val del Bove was already entirely overrun; Isoletta dei Zappinelli in the midst of the lavas of 1811 and 1819 was overwhelmed; the valley of Calanna was buried under the fire with lava, when on the 28th of August the lava hurled itself into the narrow pa.s.sage of the Portella di Calanna. A frightful cascade of lava was then seen to precipitate itself from a height of 60 metres, with a harsh metallic noise, accompanied by loud cracking. Zaffarana was on the eve of total ruin; the fire had taken the direction of the ravine which terminates there, when suddenly, in the beginning of September, the devastating stream stayed its march against the ill-fated district.

On the contrary that which had taken the direction of Milo, reinforced by a new current on the 10th of September, destroyed the hamlet of Caselle del Milo; and afterwards divided itself into two branches, which left the village of Caselle in safety between them.

The inhabitants of La Macchia and Giarre gave themselves up for lost; for it seemed that the lava would be obliged to follow the valley of Santa Maria della Strada; happily, however, from the 20th of September onward, it ceased to advance perceptibly. The eruption did not totally subside till March 1853; but the lava-flows did no more than travel by the side, or on the top of the older, without extending beyond them.

The crater of 1852 was called the Centenario, from its having been formed at the time of the centenary of the fete of S. Agatha. Santiago, in the island of Cuba, was destroyed by an earthquake on the very day of the eruption.

During the whole period of the eruption, only one explosion proceeded from the great crater of Etna. By it an enormous column of ashes and scoriae was cast into the air.

On the 9th of September white ashes were seen on the summit, which at a distance appeared like snow. When pressed together by the hand they took the consistence of clay, but they hardened in the fire, and could then be reduced to powder. They have been considered to be the _debris_ of felspathic rocks, disintegrated by the heat of the lava, and blown out by the expansive power of disengaged gas.

The eruption of 1852 was one of the grandest of the recorded eruptions of Etna. More than 2,000,000,000 cubic feet of red hot lava was spread over three square miles. This eruption was minutely described by Carlo Gemellaro, in a memoir ent.i.tled, "_Breve ragguaglio della eruzione dell' Etna del 21 Agosto, 1852_."

77. In October 1864, frequent shocks of earthquakes were felt by the dwellers on Etna. In January clouds of smoke were emitted by the great crater, and roaring sounds were heard. On the night of the 30th a violent shock was felt on the north-east side of the mountain, and a mouth opened below Monte Frumento, from which lava was ejected. It flowed at a rate of about a mile a day, and ultimately divided into two streams. By March 10th the new mouths of fire had increased to seven in number, and they were all situated along a line stretching down from the summit. The three upper craters gave forth loud detonations three or four times a minute. Professor Orazio Silvestri has devoted a quarto of 267 pages to an account of _I Fenomeni Vulcanici presentati dall'Etna nel 1863-64-65-66_.

78. In August 1874 the inhabitants of the towns situated on the north, west, and east sides of the mountain, were awakened by loud subterranean rumblings. Soon afterwards a formidable column of black smoke issued from the crater, accompanied by sand, scoriae, and ignited matter (_infuocata materia_). Severe shocks of earthquake were felt, the centre of impulsion being apparently situated on the northern flank of the mountain, at a height of 2450 metres above the level of the sea. Some small _bocche eruttive_ opened near the great crater, and ejected lava, but the quant.i.ty was comparatively small, and but little damage was done. An account of this eruption was given by Silvestri in 1874, in a small pamphlet ent.i.tled, _Notizie sulla eruzione dell' Etna del 29 Agosto, 1874_. Since 1874 the mountain has been in a quiescent state.

The centre of disturbance was at an elevation of 2450 metres (7600 feet) above the sea, on the north side of the crater, and between the minor cones known as the _Fratelli Pii_ and _Monte Grigio_. A new crater, having an elliptical contour, and a diameter of about 100 metres, was formed at this point. It is composed of a prehistoric grey labradorite, and of doleritic lava. Downwards from the main crater, in the direction of Monte di Mojo, a long fissure extended for 400 metres, and along the line of this fissure no less than _thirty-five_ minor cones opened, with craters of from thirty to three metres in diameter. The stream of lava ejected from the various _boccarelle_ was 400 metres long, 80 wide, and 2 metres in thickness, and the bulk of volcanic material brought to the surface, including the princ.i.p.al cone and its thirty-five subordinates and their ejectamenta, was calculated to amount to 1,351,000 cubic metres. The lava is of an augitic character, and magnetic; it possesses a specific gravity of 23636 at 25 C.

It will be seen from the account of the foregoing eruptions that there is a great similarity in the character of the eruptions of Etna.

Earthquakes presage the outburst; loud explosions follow, rifts and _bocche del fuoco_ open in the sides of the mountain; smoke, sand, ashes, and scoriae are discharged, the action localises itself in one or more craters, cinders are thrown up, and acc.u.mulate around the crater and cone, ultimately lava rises, and frequently breaks down one side of the cone, where the resistance is least. Then the eruption is at an end.

Smyth says, "The symptoms which precede an eruption are generally irregular clouds of smoke, ferilli, or volcanic lightnings, hollow intonations, and local earthquakes that often alarm the surrounding country as far as Messina, and have given the whole province the name of Val Demone, as being the abode of infernal spirits. These agitations increase until the vast cauldron becomes surcharged with the fused minerals, when, if the convulsion is not sufficiently powerful to force them from the great crater (which, from its great alt.i.tude and the weight of the candent matter, requires an uncommon effort), they explode through that part of the side which offers the least resistance with a grand and terrific effect, throwing red-hot stones and flakes of fire to an incredible height, and spreading ignited cinders and ashes in every direction." After the eruption of ashes, lava frequently follows, sometimes rising to the top of the cone of cinders, at others breaching it on the least resisting side. When the lava has reached the base of the cone, it begins to flow down the mountain, and being then in a very fluid state, it moves with great velocity. As it cools the sides and surface begin to harden, its velocity decreases, and in the course of a few days it only moves a few yards in an hour. The internal portions, however, part slowly with their heat, and months after the eruption, clouds of steam arise from the black and externally cold lava beds after rain, which, having penetrated through the cracks, has found its way to the heated ma.s.s within.

Of the seventy-eight eruptions described above, it will be noticed that not more than nineteen have been of extreme violence, while the majority have been of a slight and comparatively harmless character.

[Ill.u.s.tration: Geological Map of Etna]

CHAPTER VI.

GEOLOGY AND MINERALOGY OF THE MOUNTAIN.

Elie de Beaumont's cla.s.sification of rocks of Etna.--Hoffman's geological map.--Lyell's researches.--The period of earliest eruption.--The Val del Bove.--Two craters of eruption.--Antiquity of Etna.--The lavas of Etna.--Labradorite.--Augite.--Olivine.-- a.n.a.lcime.--t.i.taniferous iron.--Mr. Rutley's examination of Etna lavas under the microscope.

The opinion of geologists is divided as to the manner in which a volcano is first formed. Some hold that the volcanic forces have upraised the rocks from beneath, and at last finding vent have scattered the lighter portions of such rocks into the air, and have poured out lava through the rent ma.s.ses, thus forming a _crater of elevation_. Others maintain that the volcanic products are ejected from an aperture or fissure already existing in rocks previously formed, and that the acc.u.mulation of these products around the vent forms the ma.s.s of the volcano and the _crater of eruption_. Lyell favours the latter view; Von Buch, Dufrenoy, and Elie de Beaumont the former.

According to M. Elie de Beaumont, Etna is an irregular crater of elevation. The original deposits were nearly horizontal, and lavas were poured through fissures in these, and acc.u.mulated at first in layers; afterwards the whole ma.s.s was upheaved and a crater formed.[20] The upheaving force does not appear to have acted at one point, but along a line traversing the Val del Bove. The latter he refers to a subsidence of a portion of the mountain. He divides the rocks of Etna into six orders: 1. The lowest basis of the mountain would appear to consist of granite, because ma.s.ses of that rock have from time to time been ejected. 2. Calcareous and arenaceous rocks, of which the mountains surrounding Etna are composed, and which appear capped with lava near Bronte and elsewhere. 3. Basaltic rocks, which are met with near Motta S. Anastasia, Paterno, Licodia, and Aderno, and in the Isole de'Ciclopi.

4. Rolled pebbles, which form a range of slightly rising ground between the first slopes of Etna on the southern side and the plain of Catania.

(Lyell speaks of this rising ground as consisting of "argillaceous and sandy beds with marine sh.e.l.ls, nearly all of living Mediterranean species, and with a.s.sociated and contemporaneous volcanic rocks.") 5.

Ancient lavas forming the escarpments around the Val del Bove; and 6th, Modern lavas. He considers that the fissures which abound on Etna are shifts or faults produced by dislocation, and that the minor cones are points along such fissures from which ashes and lava have been ejected.

He admits the existence of two cones. The geological map of Etna prepared by M. Elie de Beaumont to accompany his memoir can scarcely be regarded as a great addition to our knowledge of the mountain. For although in the main points it is correct, so many details have been omitted that the map must be considered to have now been quite superseded by those of Von Waltershausen and Friedrich Hoffmann.

[20] "Recherches sur la structure et sur l'origine du Mont Etna." 1836.

The most convenient geological map of the mountain is without doubt that of Hoffmann, given in the _Vulkanen Atlas_ of Dr. Von Leonhard; and here reproduced. Von Waltershausen's geological map has been the foundation of all others which have subsequently appeared. It is a marvel of accurate work, and patient industry. The form however is inconvenient, as it nowhere appears as a whole, but in separate portions, which are scattered through the folio sheets of the very expensive _Atlas des Aetna_. It is accurate, and at the same time very clear and intelligible. By reference to the map it will be seen that from Capo di Schiso westward, to near Paterno, Etna is surrounded by sandstone hills; at the south we have recent clays, and, at intervals, chalk. A large triangular s.p.a.ce having the two angles at its base, respectively near Maletto and Aderno, and its apex at the great crater, is covered with new lava; while around Nicolosi there is volcanic sand. At the Isole de'Ciclopi, Motta S. Anastasia, and a few other places, basalt is seen; on each side of the Val del Bove, dolerite; and near Misterbianco and Piedemonte, small deposits of clay slate. The great ma.s.s of the surface of the mountain, not specially mentioned above, is volcanic tuff.

[Ill.u.s.tration: Map of the Val del Bove, to ill.u.s.trate the theory of a double axis of eruption. (_Lyell_).]

Among the more important and recent additions to our knowledge of the geology of Etna may be mentioned Lyell's paper on the subject, communicated to the Royal Society in 1858, the matter of which is incorporated in a lengthy chapter on Etna in the "Principles of Geology." Lyell visited the mountain in 1828, 1857, and 1858, and he then collected together a great number of personal observations; he also made use of the maps and plans of Von Waltershausen, and he has a.n.a.lysed the views of Elie de Beaumont and other writers. He alludes at the outset to the numerous minor cones of Etna produced by lateral eruption, and points out the fact that they are gradually obliterated by the lava descending from the upper part of the mountain, which flows around them and heightens the ground on which they stand. In this way the crater of Monte Nocilla is now level with the plain, and the crater of Monte Capreolo was nearly filled by a lava stream in 1669. Thus without doubt beneath the sloping sides of Etna a mult.i.tude of obliterated monticules exist.

[Ill.u.s.tration: Ideal section of Mount Etna]

The strata which surround Mount Etna on the south are of Newer Pliocene date, and contain sh.e.l.ls which are nearly all of species still living in the Mediterranean. Out of sixty-five species collected by Lyell in 1828, sixty-one were found to belong to species still common in the Mediterranean. These strata are about the age of the Norwich crag; and the oldest eruptions of Etna must have taken place during the glacial period, but before the period of greatest cold in Northern Europe.

Before visiting Etna, Lyell had been told by Dr. Buckland that in his opinion the Val del Bove was the most interesting part of Etna, accordingly he specially and minutely examined that part of the mountain. This vast valley is situated on the eastern flanks of the mountain, and it commences near the base of the cone, stretching for nearly five miles downwards. It is a large oval basin formed in the side of the mountain, and surrounded by vast precipices, some of which at the head of the valley are between three and four thousand feet in height.

The surface is covered with lava of various dates, and several minor cones, notably those of 1852, are within its boundaries. The abrupt precipices reveal the presence of a large number of vertical dikes, radiating from a point within the valley, some of them, according to Von Waltershausen, being of ancient greenstone. Other dikes of more modern doleritic lava radiate from the present crater. From the slope of the beds in the Val del Bove, Lyell and Von Waltershausen have independently inferred that there was once a second great centre of eruption in the Val del Bove between the Sierra Giannicola, and Zoccolaro (_vide_ the Figure on p. 117). The axis of eruption pa.s.sing through this point Lyell calls the _Axis of Trifoglietto_; while he distinguishes the present centre of eruption as the _Axis of Mongibello_. These centres probably existed simultaneously, but were unequal as regards eruptive violence; the crater of Mongibello was the more active of the two, and eventually overwhelmed the crater of Trifoglietto with its products, by which means the whole mountain became a fairly symmetrical cone, having the crater of Mongibello at its apex (_vide_ the Figures on pp. 119 and 121).

Subsequently the Val del Bove was formed, probably by some paroxysmal explosion, caused by pent-up gases escaping from fissures. Possibly also subsidence may have occurred.

[Ill.u.s.tration: Profile of Etna]

We must then in the first place think of Etna as a submarine volcano of the Newer Pliocene age; when it reached the surface it increased rapidly in bulk by pouring out scoriae and lava from its two centres of eruption--the centre of Mongibello, and the centre of Trifoglietto,--general upheaval of the surrounding district followed, and ultimately the crater of Trifoglietto was obliterated by the discharges from the crater of Mongibello. Afterwards the Val del Bove was blown out by sudden eruptive force from beneath, and the mountain a.s.sumed its present aspect. Then the historical eruptions commenced, and of these we have given an account in the preceding chapter.

The most obvious method of obtaining some idea as to the age of Etna, is to ascertain the thickness of matter added during the historical period to the sides of the mountain, and to compare this with the thickness of the beds of ancient lava and scoriae exposed at the abrupt precipices of the Val del Bove. There is reason for believing, however, that none of the ancient lavas equalled in volume the lava streams of 1809 and 1852, and the question is much complicated by other considerations. Lyell compares the growth of a volcano to that of an exogenous tree, which increases both in bulk and height by the external application of ligneous matter. Branches which shoot out from the trunk, first pierce the bark and proceed outwards, but if they die or are broken off they become inclosed in the body of the tree, forming knots in the wood.

Similarly the volcano consists of a series of conical ma.s.ses placed one above the other, while the minor cones, corresponding to the branches of the tree, first project, and then become buried again, as successive layers of lava flow around them. But volcanic action is very intermittent, the layers of lava and scoriae do not acc.u.mulate evenly and regularly like the layers of a tree. A violent paroxysmal outbreak may be succeeded by centuries of quiescence, or by a number of ordinary eruptions; or, again, several paroxysmal outbreaks may occur in succession. Moreover, each conical envelope of the mountain is made up of a number of distinct currents of lava, and showers of scoriae. "Yet we cannot fail to form the most exalted conception of the antiquity of this mountain, when we consider that its base is about 90 miles in circ.u.mference; 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." If all the minor cones now visible on Etna could be removed, with all the lava and scoriae which have ever proceeded from them, the mountain would appear scarcely perceptibly smaller. Other cones would reveal themselves beneath those now existing. Since the time when, in the Newer Pliocene period, the foundations of Etna were laid in the sea, it is quite impossible even to hint at the number of hundreds of thousands of years which have elapsed.

We collected specimens of lava from various points around and upon the mountain. They presented a wonderful similarity of structure, and a mineralogist to whom they were shown remarked that they might almost all have come from the same crater, at the same time. A specimen of the lava of 1535 found near Borello, was ground by a lapidary until it was sufficiently transparent to be examined under the microscope by polarised light. It was found to contain good crystals of augite and olivine, well striated labradorite, and t.i.taniferous iron ore.

Elie de Beaumont affirms that the lavas of Etna consist of labradorite, pyroxene (augite), peridot (olivine), and t.i.taniferous iron. Rose was the first to prove that the lavas of Etna do not contain ordinary felspar (or potash felspar), but labradorite (or lime felspar.) (_Annales des Mines_, 3 serie, t. viii., p. 3.) Elie de Beaumont detached a quant.i.ty of white crystals from the interior of a lava found between Giarre and Aci Reale; these were a.n.a.lysed by M. Auguste Laurent with the following results in 100 parts:--

Silica 479 Alumina 340 Oxide of Iron 24 Soda (Na{2}O) 51 Potash (K{2}O) 9 Lime 95 Magnesia 2 ----- 1000

Von Waltershausen gives the following as the composition of two specimens of Labradorite from Etna:--

I. II.

Silica 5356 5583 Alumina 2582 2531 Sesquioxide of Iron 341 364 Magnesia 52 74 Lime 1169 1049 Soda 409 352 Potash 54 83 Water 95 -- ------ ------ 10058 10036

Specimens of Augite from Etna have been examined by Von Waltershausen and Rammelsberg, with the following results:--

_Greenish_ _From_ _From_ _Black._ _Black._ _Mascali._ _Monti Rossi._

Silica 4763 5170 4969 4738 Alumina 674 438 522 552 Protoxide of Iron 1139 424 1075 789 " Manganese 21 -- -- 10 Magnesia 1290 2111 1474 1529 Lime 2087 1802 1844 1910 Sesquioxide of Iron -- -- -- 385 Water 28 49 51 43 ------ ----- ----- ----- 10002 9994 9935 9956

Olivine is generally met with in the lavas of Etna. It has an olive, or bottle-gla.s.s green colour, and is disseminated through the lavas in the form of small crystalline grains, sometimes of some magnitude. Specific gravity 3334. A specimen from Etna gave the following results on a.n.a.lysis:--

Silica 4101 Protoxide of Iron 1006 Magnesia 4727 Alumina 64 Oxide of Nickel 20 Water 104 ------ 10022