Chapter VI Geology and Mineralogy of the Mountain
Elie de Beaumont’s classification 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.—Analcime.—Titaniferous 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 masses, 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 accumulation of these products around the vent forms the mass of the volcano and the crater of eruption. Lyell favours the latter view; Von Buch, Dufrénoy, 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 accumulated at first in layers; afterwards the whole mass 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 masses 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 shells, nearly all of living Mediterranean species, and with associated 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.
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 space 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 mass of the surface of the mountain, not specially mentioned above, is volcanic tuff.
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 analysed 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 multitude of obliterated monticules exist.
The strata which surround Mount Etna on the south are of Newer Pliocene date, and contain shells 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 passing 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.
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 scoriæ 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 assumed 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 scoriæ 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 masses 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 scoriæ do not accumulate 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 scoriæ. “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 circumference; so that it would require ninety flows of lava, each a mile in breadth at their termination, to raise the present foot of the volcano as much as the average height of one lava current.” If all the minor cones now visible on Etna could be removed, with all the lava and scoriæ 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 titaniferous iron ore.
Elie de Beaumont affirms that the lavas of Etna consist of labradorite, pyroxene (augite), peridot (olivine), and titaniferous 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 quantity of white crystals from the interior of a lava found between Giarre and Aci Reale; these were analysed by M. Auguste Laurent with the following results in 100 parts:—
Silica
Alumina
Oxide of Iron
Soda (Na2O)
Potash (K2O)
Lime
Magnesia
47·9
34·0
2·4
5·1
·9
9·5
·2
100·0
Von Waltershausen gives the following as the composition of two specimens of Labradorite from Etna:—
Silica
Alumina
Sesquioxide of Iron
Magnesia
Lime
Soda
Potash
Water I.
53·56
25·82
3·41
·52
11·69
4·09
·54
·95
100·58 II.
55·83
25·31
3·64
·74
10·49
3·52
·83
—
100·36
Specimens of Augite from Etna have been examined by Von Waltershausen and Rammelsberg, with the following results:—
Silica
Alumina
Protoxide of Iron
,, Manganese
Magnesia
Lime
Sesquioxide of Iron
Water
Black.
47·63
6·74
11·39
·21
12·90
20·87
—
·28
100·02 Greenish
Black.
51·70
4·38
4·24
—
21·11
18·02
—
·49
99·94 From
Mascali.
49·69
5·22
10·75
—
14·74
18·44
—
·51
99·35 From
Monti Rossi.
47·38
5·52
7·89
·10
15·29
19·10
3·85
·43
99·56
Olivine is generally met with in the lavas of Etna. It has an olive, or bottle-glass green colour, and is disseminated through the lavas in the form of small crystalline grains, sometimes of some magnitude. Specific gravity 3·334. A specimen from Etna gave the following results on analysis:—
Silica
Protoxide of Iron
Magnesia
Alumina
Oxide of Nickel
Water
41·01
10·06
47·27
·64
·20
1·04
100·22
The titaniferous iron of Etna is found disseminated through the mass of the lavas, and is plainly distinguished when a thin section is examined under the microscope. It is sometimes met with in masses. A specimen from Etna, analysed by Von Waltershausen, was found to contain:—
Titanic Acid
Sesquioxide of Iron
Protoxide of Iron
11·14
58·86
30·00
100·00
The basalts of the Isole de’ Ciclopi enclose beautiful transparent crystals of Analcime, the zeolite dure of Dolomieu. The word is derived from ανα’λκις weak, in allusion to the weak electric power which the mineral acquires when heated or rubbed. Dana prefers the term analcite. Specimens from the Cyclops Islands have been analysed by Von Waltershausen and Rammelsberg, with the following results:—
Silica
Alumina
Lime
Soda (Na2O)
Potash (K2O)
Water
Magnesia
Sesquioxide of Iron
I.
53·72
24·03
1·23
7·92
4·46
8·50
·05
—
99·91 II.
55·22
23·14
·25
12·19
1·52
7·68
—
—
100·00 III.
54·34
23·61
·21
12·95
·66
8·11
—
·12
100·00
The minerals of Etna are not nearly as numerous as those of Vesuvius. It has been remarked that no area of equal size on the face of the globe furnishes so many different species of minerals as Vesuvius and its immediate neighbourhood. Out of the 380 species of simple minerals enumerated by Hauy, no less than 82 had been found on and around Vesuvius, as long ago as 1828, and many have been since found.
Of other common products of Etna, there are sulphur in various forms, sulphurous acid gas, ammonia salts, hydrochloric acid gas, and steam. A curious white mass, which we found near the summit, proved to be the result of the decomposition of lava by hot acid vapours. In the different lavas, the crystals of labradorite, and of olivine, vary in size considerably. Magnetic oxide of iron is very visible in thin slices of the lavas when placed under the microscope; and iron appears to be a constant constituent in nearly all the products of the mountain.
Within the last few months Prof. Silvestri has detected a mineral oil in the cavities of a prehistoric doleritic lava found near Paterno.21 The lava is in close contiguity to the clay deposits of a mud volcano, and when examined under the microscope is seen to consist mainly of augite, together with olivine and transparent crystals of labradorite. It contains numerous cavities coated with arragonite, and filled with a mineral oil which constitutes about one per cent of the whole weight of the lava. It was taken from the lava at a temperature of 24° C., (75·2° F.), and solidified at 17° C. (62·6° F.) to a yellowish green mass, which on analysis gave the following percentage composition:—
| Liquid hydrocarbons boiling at 79° C. | = 17·97 |
| Hydrocarbons solidifying below 0° C., boiling between 280° and 400° C. | = 31·95 |
| Paraffine melting between 52° and 57° C. | = 42·79 |
| Asphalt containing 12 per cent of ash | = 2·90 |
| Sulphur | = 4·32 |
| 99·93 |
Prof. Silvestri has recently made some interesting determinations of the specific gravity and chemical composition of the different products of Etna. They are given in full in his work entitled, “I Fenomeni Vulcanici presentati dall’ Etna, nel 1863, 1864, 1865, 1866,” which was published in Catania in 1867. The following table gives the specific gravity of various ancient and modern forms of lava, ashes, etc. of Etna:—
| Sp. Gr. | |
| Ashes ejected in 1865 | 2·644 |
| Sand,,,,,, | 2·715 |
| Scoriæ,,,,,, | 2·633 |
| Compact lava,,,, | 2·771 |
| Scoriæ ejected in 1669 | 2·622 |
| Compact lava,,,, | 2·697 |
| Lapilli ejected in 1444 | 2·420 |
| Compact lava ejected in prehistoric times | 2·854 |
A very decided change in the specific gravity was found to take place after fusion. This can only be accounted for on the supposition that a chemical change is effected during the fusion:—
| Sp. Gr. before fusion. | Sp. Gr. after fusion. | |
| Pyroxene of Etna | 3·453 | 2·148 |
| Felspar,,,, | 2·925 | 1·361 |
| Olivine,,,, | 3·410 | 2·290 |
| Lava of 1865 | 2·771 | 1·972 |
| Ancient basaltic lava from the Scogli de’Ciclopi | 2·854 | 2·000 |
| Ancient basaltic lava from Aci Reale | 2·795 | 1·947 |
It will be seen from the following analyses that the sand, ashes, scoriæ, and compact lava have virtually the same composition—indeed they consist of the same substance in different states of aggregation.
| Ashes. | Sand. | Scoriæ. | Compact lava. | |
| Silica Alumina Protoxide of iron Protoxide of Manganese Lime Magnesia Potash Soda Water | 50·00 19·08 12·16 ·40 9·98 4·12 ·60 3·72 ·36 | 49·80 18·20 12·42 ·45 11·00 4·00 ·49 3·60 ·29 | 50·00 19·00 11·70 ·50 10·28 4·20 ·69 3·40 ·33 | 49·95 18·75 11·21 ·49 11·10 4·05 ·70 3·71 ·23 |
| Phosphoric acid Titanic acid Vanadic acid Sesquioxide of iron | traces | traces | traces | traces |
| 100·42 | 100·25 | 100·10 | 100·19 |
With these we may compare the composition of the lava which issued from Monti Rossi in 1669, and was analysed by Lowe, and of an ancient lava of Etna ejected during an unknown eruption, and analysed by Hesser.
| Ancient lava. | Lava of 1669. | |
| Silica Alumina Protoxide of Iron Protoxide of Manganese Lime Magnesia Soda Potash | 49·63 22·47 10·80 ·63 9·05 2·68 3·07 ·98 99·31 | 48·83 16·15 16·32 ·54 9·31 4·58 3·45 ·77 99·95 |
The sublimations from the fumaroles are chiefly chloride of ammonium, perchloride of iron, and sulphur. An analysis of the gases of the fumaroles of 1865 gave the following results:—
Carbonic acid
Hydrosulphuric acid
Oxygen
Nitrogen
50·5
11·9
7·1
30·5
100·0
An account of microscopic analysis of some of the lavas of Etna, for which I am indebted to Mr. Frank Rutley, will be found appended to this chapter. He considers that they are Plagioclase-basalts, and occasionally Olivine-basalts; and that they consist of Plagioclase, Augite, Olivine, Magnetite, Titaniferous iron, and a residuum of glass.
Near the summit of the great crater I found a mass of perfectly white, vesicular, and very friable substance, somewhat pumiceous in appearance. It proved to be a decomposed lava, and was found elsewhere on the sides of the crater. Mr. Rutley examined a section of it, and reports: “Under the microscope a tolerably thin section shows the outlines of felspar crystals, lying in a hazy milk-white semi-opaque granular matrix. The felspar crystals are lighter and more translucent than the matrix, but are of much the same character, having a granulated or flocculent appearance, somewhat like that of the decomposed felspars in diabase. There are numerous roundish cavities in the section which may once have contained olivine, or some other mineral, or they may be merely vesicles.”
A qualitative analysis of this substance, made by Mr. H. M. Elder, has proved that it contains a large quantity of Silica (about 70 per cent.), and smaller proportions of Alumina, Iron, Magnesium, Calcium, and Potash; together with very small amounts of Sulphuric Acid and a trace of Ammonia. Lithium is absent, and Sodium is only present in very minute quantity. Water is present to the extent of nearly 20 per cent.
During the eruption of Etna in 1869 Von Waltershausen noticed on some of the lava blocks which were still hot and smoking, silver-coloured particles, which rapidly underwent change. An insufficient quantity for analysis was collected, but during the eruption of 1874, Silvestri found a quantity of the substance and analysed it. (Poggendorff’s Annalen, CLVII. 165, 1876.) It possesses a specific gravity of 3·147, and shows a metallic lustre similar to that of steel. On analysis it was found to consist of:—
| Iron | 90·86 |
| Nitrogen | 9·14 |
| 100·00 |
which corresponds with the formula Fe5N2,—a formula assigned by Fremy to Nitride of iron. It has been named Siderazote. This new mineral species appears to be formed by the action of hydrochloric acid, and of ammonia on red-hot lava containing a large percentage of iron. It was formed artificially by exposing fragments of lava alternately to the action of hydrochloric acid and ammonia in a red-hot tube. At a high temperature Siderazote undergoes decomposition, nitrogen being evolved. In contact with steam at a red heat it forms magnetite and ammonia.
Notes
20 “Récherches sur la structure et sur l’origine du Mont Etna.” 1836.
21 “Atti Accademia Gioenia,” serie iii., vol. xii.