ETNA.
(_a._) _Structure of the Mountain._--Etna, unlike Vesuvius, has ever
been a burning mountain; hence it was well known as such to classic
writers before the Christian era. The structure and features of this
magnificent mountain have been abundantly illustrated 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 circumference of 87 English miles. In ascending its
flanks we pass 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 circumference 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.
[Illustration: 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 fashion 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 circumference by precipitous walls of ashes,
scoriae, and lava, traversed by innumerable dykes, 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 canyons 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
masses of matter within the period required for the purpose. We must
therefore have recourse to some other agency. Numerous illustrations 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 shells were
deposited.
_Second Stage._--This sea-bed was gradually upraised into the air, while
fresh sheets of lava and other _ejecta_ were accumulated round the vents
of eruption, of which there were two principal ones--the older under the
present Val del Bove, the newer under the summit of the principal 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 accumulation 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
shells 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 Constitution 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
analyses 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 constituent. On the
other hand, the composition of the lavas of Etna, determined by
Professor A. von Lasaulx, from specimens taken from the oldest
(voraetnaeischen) 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, magnetite, and
sometimes apatite; 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
quantity 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 ueber 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 mass. 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.