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 principal 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 quantities 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.
[Illustration: 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
dykes of lava, from which uprise two or three craters, formed of pumice
and obsidian passing 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]
[Illustration: 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 masses 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 masses,
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 "flashing 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 shores to a depth of nearly 600 fathoms, so
that Stromboli is a great conical mass 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
superincumbent 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 mass 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 adventitious 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 circumference 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
shore. 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.
[2] Strabo, lib. vi.
[3] Judd, _Volcanoes_, p. 8.
[4] Stromboli has also been described by Spallanzani, Hoffmann, Daubeny,
and others. The account of Judd is the most recent. Of this island
Strabo says, "Strongyle a rotundate figurae sic dicta, ignita ipsa
quoque, violentia flammarum minor, fulgore excellens; ibi habitasse
Aecolum ajunt."--Lib. vi.
[5] _Poggend. Annal._, vol. xxvi., quoted by Daubeny.
[6] Communicated by Captain Petrie to the Victoria Institute, 1st
February 1892. See also a detailed and illustrated account of the
eruption communicated by A. Ricco to the _Annali dell' Ufficio centrale
Meteorologico e Geodonamico_, Ser. ii., Parte 3, vol. xi. Summarised by
Mr. Butler in _Nature_, April 21, 1892.