OTHER VOLCANOES OF THE MEDITERRANEAN
The relative positions of the other volcanic mountains of the
Mediterranean Sea; i. e., Etna, Stromboli, and the volcanoes of the
Santorin group of the Grecian Archipelago, are shown in the map, Fig.
12.
We will begin with the volcanic mountain of Etna, under which,
according to mythology, the angry gods had buried the rebellious
Typhoon.
Etna is situated on the island of Sicily, immediately southwest of
Italy. It is a much larger mountain than Vesuvius, rising, as it
does, from a circular base about eighty-seven miles around, to a
height of 10,840 feet above the level of the Mediterranean. It forms
a conspicuous object when seen either from the Mediterranean, or from
distant parts of Italy.
The height of Etna is so great that its slopes can be divided into
three distinct climatic zones or belts. The lowest of these lies
between the sea and a height of 2,500 feet. In this zone the mountain
slopes are covered with cultivated fields, olive groves, orchards,
and vineyards. The middle zone lies between 2,500 feet and 6,270
feet. This zone is covered with forests of chestnuts, oaks, beeches,
and cork trees. The third and highest zone includes the rest of the
mountain, and may be called the desert zone, since it is a sterile
region, covered with huge blocks of lava and scoriæ, and terminating,
in the higher portions, in a snow-covered plain, from which the
central cone rises.
Etna is continually sending up columns of steam and sulphur vapor.
Every now and then it starts in eruption, throwing out large
quantities of lava either from the crater on its summit, or from
some of the 200 smaller cones or craters that occupy portions of its
slopes. On account, probably, of its height the eruptions are most
frequently on the sides. Etna affords a magnificent example of a huge
volcanic pile of the Vesuvian type, which has been slowly built up
by the gradual accumulation of materials that have escaped from its
craters.
One of the most interesting features of the higher regions of Etna is
an immense chasm rent in a side of the cone near the summit, and known
as the Val del Bove. This chasm forms a vast amphitheater.
The great force that removed such an immense mass of matter from
the cone could not have been the eroding power of water, since the
materials of the cone are too porous to permit streams of any size to
rush down the slopes. The force is most probably to be found in some
explosive eruption of the mountain, when a portion of the crater was
suddenly blown off, just as was done in Vesuvius when a large part of
the old crater of Somma was blown away. What is especially interesting
about the Val del Bove is the opportunity it affords for studying the
interior structure of the mountain, for it practically enables one to
enter to almost the heart of this great volcano.
The Val del Bove has the shape of a great pit five miles in diameter.
It has almost vertical walls, the height of which varies with their
position. Those which reach highest up the mountain vary from 3,000 to
4,000 feet in height.
Like Vesuvius, Etna has been split or fissured into great crevices
that have been filled with lava during the many eruptions of its
central crater. On hardening, these lava streams form what are known
as dikes. As the sides of the mountain are worn away by erosion, the
dikes, being harder than the rest of the cone, project from its sides
like huge walls. An excellent opportunity for seeing them is afforded
in the walls of the Val del Bove.
Sir Charles Lyell, the English geologist, who has carefully studied
Mt. Etna, asserts that this mountain began to be formed during a
geological period known as the Tertiary Age, through a crater that
opened on the floor of the Mediterranean Sea. The material thus
thrown out, collected around the crater and produced a mountainous
pile that gradually emerged above the level of the sea, and on fresh
materials continuing to be thrown out, at length reached its present
height. It would appear that at some former time in its history, there
were two vents near the top of the mountain, the second crater being
formed immediately under the Val del Bove. Soon, however, the second
and lower crater was closed, the upper one alone remaining active.
The mountain, therefore, continued to be slowly raised in the air by
the materials brought out through this opening. Then came the great
explosive eruption during which the side of the mountain was blown off
to form the great chasm of the Val del Bove.
Because of its almost constant activity, Mt. Etna must have been
well known to the ancients, who described some of its most violent
eruptions. The following brief notes concerning these eruptions have
been taken from Lyell.
According to Diodorus Siculus, an eruption that occurred before the
Trojan war, caused the people living in districts near the mountain to
seek new homes. Thucididies, the Greek historian, states that in the
sixth year of the Peloponnesian war, which would be about the spring
of 425 B. C., a lava stream caused great destruction in the
neighborhood of Campania, this being the third eruption that had
occurred in Sicily since it had been settled by the Greeks.
Seneca, during the first century of the Christian Era, calls the
attention of Lucullus to the fact that during his time Mt. Etna had
lost so much of its height that it could no longer be seen by boatmen
from points at which it had before been readily visible.
But passing by these very early eruptions of Etna we come to the great
eruption of 1669. This eruption was preceded by an earthquake that
destroyed many houses in a town situated in the lower part of the
forest zone, about twenty-five miles below the summit of the mountain,
and ten miles from the sea at Catania. During this eruption two deep
fissures were opened near Catania. From these such quantities of sand
and scoriæ were thrown out, that, in the course of three or four
months, a double cone was formed 450 feet high, which is now known as
Monte Rosso. But what was most curious was the sudden opening, with
a loud crash, of a fissure six feet broad reaching down to unknown
depths that extended in a somewhat crooked course to within a mile
of the summit of Etna. This great fissure was twelve miles in length
and emitted a most vivid light. Five other parallel fissures of
considerable length opened, one after another, throwing out vapor,
and emitting bellowing sounds which were heard at a distance of forty
miles. These fissures were afterwards filled with molten rock, and in
this manner were formed the long dikes of porphyry and other rocks
that are seen to be passing through some of the older lavas of Mt.
Etna.
[Illustration: FIG. 14. MT. ETNA _From Map of State and
Government_]
The great lava streams which flowed down the side of the mountain
during this eruption, destroyed fourteen towns and villages, and
at length reached Catania. A great wall had been raised around this
city to prevent the lava from entering it. The molten rock, however,
accumulated, until it rose to the top of the wall, which was sixty
feet high, and then pouring over it in a fiery cascade, overwhelmed
part of the city. It is said that during the first part of its
journey, the lava streams moved over thirteen miles in twenty days,
or at the rate of 162 feet an hour. Beyond this, after the lava had
thickened by cooling, it had a velocity of only twenty-two feet per
hour.
Fig. 14 represents a plan of Mt. Etna reduced from a map by the
Italian Government. During the eruption of 1865, a rent was made in
the mountain extending from Mount Frumento (B in the preceding map)
for one and one-half miles, and six cones from 300 to 350 feet in
height were formed along the fissure.
During the eruption of 1874, great fissures three miles in length were
formed in the mountain.
There exists on the slopes of Mt. Etna vast subterranean grottoes
formed by the sudden conversion into steam of great quantities of
water that were overwhelmed by the molten mass. These immense volumes
of steam produced enormous bubbles in the molten lava. When the lava
hardened irregular grottoes were left. Lyell describes one of these as
follows:
"Near Nicolosi, not far from Monte Rosso, one of these
great openings may be seen, called the _Fossa della
Palomba_, 625 feet in circumference at its mouth and
seventy-eight deep. After reaching the bottom of this, we
enter another dark cavity, and then others in succession,
sometimes descending precipices by means of ladders. At
length, the vaults terminate in a great gallery ninety feet
long, and from fifteen to fifty broad, beyond which there
is still a passage, never yet explored, so that the extent
of these caverns remains unknown. The walls and roofs of
these great vaults are composed of rough bristling scoriæ
of the most fantastic forms."
Besides the eruptions mentioned there have been many others, such as
those of 1811, 1819, and 1852. The last of these was greater than any
eruption except that of 1669. It began in August, 1852, and continued
until May, 1853, and was remarkable for the immense quantity of lava
thrown out.
[Illustration: FIG. 15. STROMBOLI, VIEWED FROM THE NORTHWEST,
APRIL, 1874]
We come now to the volcano of Stromboli. Stromboli, one of the Lipari
islands, is situated about sixteen miles west of the Straits of
Messina. Its general appearance is shown in Fig. 15. The form of the
mountain is that of an irregular four-sided pyramid, which rises about
3,090 feet above the level of the Mediterranean, and stands on the
bottom of the sea in water about 3,000 feet deep.
If you carefully examine the appearance of Stromboli, as shown in the
preceding figure, you will notice that the flat cloud which hangs over
the island is made up of a number of globular masses of vapor, formed
during the peculiar action of the volcano.
When examined by night Stromboli presents a still more curious
appearance. Since the mountain stands alone, its height permits it to
be seen readily at sea for distances of at least a hundred miles. At
night a curious glow of red light may be seen on the lower surfaces of
the cloud. This light is not continuous, but increases in intensity
from a faint glow to a fairly bright red light, then gradually
decreases, and finally dies away completely. After awhile the light
again appears, again gradually decreases, and disappears, and this
continues until the rising sun prevents the red glow from being any
longer visible. Stromboli, therefore, acts not unlike the flashing
lighthouses so common on the sea coasts of all parts of the world.
Indeed, it is actually used by sailors in the Mediterranean for the
purpose of showing them their direction. For this reason Stromboli is
commonly called "The Lighthouse of the Mediterranean."
As Judd remarks, from whom much of the information concerning some of
the volcanic districts of the Mediterranean has been obtained, the
flashing light of Stromboli differs from that of the ordinary flashing
light in two important respects; viz., in the intervals that elapse
between the successive flashes, and in the intensity of the light
emitted. As you know, it is necessary that the different lighthouses
placed near one another on a coast must have their lights of such a
nature that they can be readily distinguished. In order to do this,
the flashing light has been devised. In flashing lighthouses, the
lights only appear at intervals, one lighthouse being distinguished
from another in its neighborhood by the intervals between successive
flashes, or, sometimes, indeed, by the color of some of the flashes.
Now, in the case of Stromboli, the intervals between the successive
glowings of the red lights are very irregular, varying between one and
twenty flashes per second. Moreover, the intensity of the light also
varies greatly from time to time.
You naturally inquire as to the cause of these flashes of light that
are emitted by Stromboli. If, as Judd suggests, you should climb to
the summit of the mountain, during the daytime, and look down the
inside of the crater, you could see its black slag bottom crossed by
many cracks and fissures. From most of the smaller fissures the vapor
of water is quietly escaping. This vapor rises in the air in which it
soon disappears. There are, however, larger cracks on the bottom of
the crater from which, at more or less regular intervals, masses of
steam are emitted with loud snorting puffs not unlike those produced
by a locomotive. From some of the openings molten matter is seen
slowly oozing out, collecting in parts of the crater and moving up and
down in a heaving motion. Every now and then a bubble is formed on
the surface of this liquid. The bubble swells to a gigantic size, and
suddenly bursts. The steam it contained escapes, carrying fragments of
scum which are thrown high into the air. The masses of steam, formed
below the surface of the sticky, boiling, lava, in endeavoring to
escape, force their way through the mass, blow huge bubbles, which,
on bursting, produce the roaring sounds that are heard, and throwing
great columns of vapor in the air, produce the rounded masses of
clouds you can see floating high up in the air over the mountain. At
the same time the scum is partially removed from the red hot surface,
its light illumines the lower surface of the overhanging cloud, which
flings it back again to the earth. With the bursting of each bubble,
and the clearing of the scum from the surface of the red hot mass,
the light begins, increases in intensity, and then as the scum again
begins to collect on the surface, decreases, and finally disappears,
and not until the bursting of the next bubble is it again visible.
But let us make a study of some of the peculiarities of Vulcano,
another of the Lipari islands, which lies north of Sicily.
Vulcano affords a curious example of a volcano that has been harnessed
by man, or made to do work for him. All volcanoes bring from inside
of the earth different kinds of chemical substances, in the form of
vapors, gases, or molten materials. Now, these materials acting on
one another, produce chemical substances some of which, such as sal
ammoniac, sulphur, and boracic acid, possess commercial value. This is
especially true in the case of Vulcano, and since the eruptions are
not generally violent, a chemical works has actually been erected by
a Scotch firm on the side of the mountain, where the materials are
collected from the crevices.
This effort to harness a volcano was for a time so successful that
the same people contemplated the building of great leaden chambers
over the principal fissure at the bottom of the crater, so that the
large volumes of ejected vapors might be condensed and collected. But
Vulcano, like all other volcanoes, could not be relied on continually
to keep the peace. One day it suddenly burst forth more fiercely than
usual, so that the workmen were compelled to abandon the factory and
fly down the mountain for their lives, but not, however, before some
of them were severely injured by the explosions.
Vulcano is an instance of a volcano in an almost exhausted or dormant
condition. It has had, however, many eruptions during the past few
centuries, some of which have been very violent, for example, that of
1783, and that of 1786.
There still remains to be considered the volcanic region of the
Santorin group of the Grecian Archipelago. The island of Santorin
or Thera, is the southernmost of the Cyclades. It is an exceedingly
curious island, being a submerged volcano, with most of the top of the
crater remaining above the waters, so that the entire island has the
shape of an irregular circle or crescent broken at several points.
Its formation is, probably, due to the gradual sinking of a volcanic
mountain until its crater has been almost completely submerged, only
the higher parts of the edges of the crater being left above the
surface of the waters. Suppose, for example, a mountain like Vesuvius
at the time the crater Somma existed, was sunk below the level of the
Mediterranean until only the highest parts of the crater remained
above the waters. If, now, one or more volcanic eruptions occurred,
producing craters or volcanic islands inside the submerged rim, you
would have a condition of affairs seen in the island of Santorin.