Mt. Vesuvius
is the best known volcano on earth; it dominates the Bay of Naples with its
characteristic cone. It is a typical example of a volcano in a volcano made
by an outer broken cone, Mt. Somma (1133 metres) with a crateric belt most
of which is destroyed. In it there is a smaller cone, the Mt. Vesuvius (1281
metres), divided by lowering named Valle del Gigante (Giants Valley), a part
of the ancient caldron where in a later period, perhaps during the 79 A.D.
eruption, the Gran Cono (Great Cone) or Mt. Vesuvius arose. The Valle del
Gigante is still divided in Atrio del Cavallo on the west and the Valle
dell'Inferno on the east. The Somma's ancient crater is well preserved as
far as its entire northern part is concerned, in fact in historic times it
was less exposed to the volcano's devastating violence, because it was well
protected by the height of the internal face that has prevented the downflow
of lava on its slopes. The slopes, which vary in their steepness, are
furrowed by profound radial grooves produced by the erosion of the
meteoric waters. The whole section is then characterized by dikes and
fringes of dark volcanic rock. The old crater edge is a stream of summits
called cognoli. While the height of mount Somma and its profile have
remained the same for centuries, the height and the profile of the mount
Vesuvius have suffered considerable variation, because of the following
eruptions, with raisings and lowerings. Mt. Vesuvius is a characteristic
polygenic mixed volcano, meaning that it is constituted by lava of different
chemical composition (for example trachytes, tephrites, leucitites) and
formed either by casting
of lava or pyroclastic deposits. All the zones at the slopes of the mountain
are formed by transported earth of lava mud which goes down from the steep
slopes in the rainy seasons through deep and narrow grooves called channels
or more commonly "lagni". The high embankments are formed by piles of lavic
scoriae, which precipitated in incandescent state and spread towards the low
slopes, proving precious for the vegetation thanks to their fertile
material, rich in silicon and potassium. Proceeding along the rim of the
crater, one can observe the whole extent of the southern part of the volcano
and, during days with good visibility, it is possible to see the entire gulf
of Naples, from the Sorrento peninsula to Cape Miseno, Procida and Ischia.
It is also possible to note the large number of buildings which have been
built on the vulnerable flanks of the mountain.
The eruption
The eruption of the 79 A.D.
The
eruption began on 24 August of the 79 A.D. towards noon. The first eruptive
phase was characterized from strong "freatomagmatiche" outbreaks. After this
phase, magmatic outbreaks were followed until the morning of the following
day, feeding a column constituted mostly from gas, pomici and ashes that
were raised until 30 kilometers. The high part of the column expanded,
assuming the shape of a pine, and was pushed from the twenty towards
south-east. The contained particles in it often fell to the ground, forming
a layer of pomici that to Pompei and Oplonti caught up 2-3 m. of thickness.
Partial collapses of the eruptive column generated piroclastic flows that
noticed to high long speed the flanks of the volcano, caught
up and
destroyed Ercolano. The city of Pompei, the much farthest one, did not come
caught up and the greater part of its inhabitants survived. During the last
hours of the night the intensity of the eruptive activity diminished.
At the first hours in the 25 morning, a "freatomagmatica" outbreak
generated piroclastici, turbulent flows - terrible "the base-surge " - that,
travelling at the speed of a hurricane, came down along the slopes of the
volcano, devastated the surrounding areas until distances of 15 kilometers
and caused numerous victims also between the inhabitants of Pompei that were
survivors to the first phase of the eruption. In the course of the day the
outbreaks diminished of intensity and, in evening stopped of all, leaving
one large pall of ashes and pomici on the huge area. The abundant rains,
provoked also from the breaking in in the atmosphere of enormous fine
particle and vapor amounts, mobilized this material, forming dense mud taps
that came down from the flanks of the volcano and of the Appennine relieves
along it goes them to them, ulteriorly having the territory of the vesuvian
area.
The eruption of 1631
The
eruption of 1631 has been most violent and destructive of the history of the
Vesuvio in the last millenium. After along period of quiescence,
approximately 5 centuries, preceded from one series of premonitory phenomena,
which earthquakes and raisings of the ground, the volcano waked causing the
death of approximately 6.000 persons and the devastation of an area nearly
500 km. 2
The eruption began at the 7 in the morning of 16 December, with the
formation of an eruptive column of approximately 15 km., from which they
began to fall pomici and ashes in the area to east of the Vesuvio. At the 10
in the morning of 17 December, from the central crater generated
piroclastici flows, gas clouds loaded with magma fragments that, sliding to
long high velocity the flanks western and southern of the volcano, they
destroyed all that they met in their way. In the night between
the 16 and the 17, and in the afternoon of the 17, the abundant rains
mobilized the incoherent ash cover causing the formation of mud taps. The
taps came down from the flanks of the volcano, from the slopes of the
Appennine to north and the northeast.
The phase of paroxysm in the eruption lasted three days, provoking an
enormous panic between the population. There were on the roads of Naples
public confessions of sins, accompanied from extraordinary manifestations of
penance, and were organized processions with the statue and the blood of S.
Gennaro, so that the patron appease that divine temper of which the outbreak
of the Vesuvio it seemed the indubitabile sign.
The Count of Monterrey, viceroy of Naples from January of that year, sent
some ships to collect the survivors of Torre del Greco and Torre Annunziata.
After some months, deeply upsetting from the event, it made to affix in
Portici a tablet that it exhorts the descendants not to forget the nature
about the mountain, and to recognize ready the premonitory ones of a
volcanic eruption.
The eruption of 1944
On 18
March of 1944, during the occupation of the allied troops, began the last
eruption of the Vesuvio, that concluded a period of activity begun in 1914,
during which it had been taken place the only modest eruptions from the
central crater .
Between 1914 and 1944, lave and the slag produced from the volcano had
filled up crater, wide 720 m. and deep 600 m., than it had been formed
during the previous eruption of 1906.
A little cone of slag it emerged from the crater.
13-17 March
The little cone of slag it begins to collapse and the sismic activity
becomes more intense. A new slag cone born and collapsed again.
18 March
The eruption begins in the afternoon with slag launch. At the 16,30 a lavic
strained overflowed from the northern part of the crater and catches up the
Valle dell'Inferno at the 22.30. Nearly at the same time an other tap
overflows from the southern part of the crater. At the 23 there was also a
spillage of washes from the western part of the crater: the tap follows the
railroad of the funicular and interrupts the railroad.
19 March
At the 11 the washes flows along the Fosso della Vetrana.
20 March
Between the afternoon and the night, new strained overflowed from the
northern part of the crater. All the effusive activities are accompanied by
sismic tremor with increasing amplitude until half of the day.
21 March
The southern
tap arrests at a quota approximately 300 m. on the sea level. In the night,
the northern tap catches up S. Sebastiano and Massa di Somma and is divided
in two coppers that are left over in direction of Cercola, from which in
evening they are distant approximately 1,5 km. S. Sebastiano and Massa di
Somma come evacuate and the 10,000 inhabitants transferred to Portici.
Around the 17, the spectacular fountains of wash begin to form, the last of
which hard approximately 5 hours and catch up a height nearly 1,000 m.
Fragments of wash and ashes moved from the winds in quota, are deposited on
the Southeastern areas of the volcano, between Angri and Pagani. The smaller
fragments catch up distances than beyond 200 km. towards south-east. Slag
until a kilogram of weight catch up the city of Poggiomarino, at
approximately 11 km. from the crater. Great still warm slag amounts are
accumulated on the flanks of the Great Cone. The sismic tremor continues,
with maximums of amplitude in coincidence with the emission of the fountains
of wash.
22 March
Towards the 1 p.m. the eruption was in its maximum activity. A column of
gas and ash rises at a 6 Km. of height. Ashes and slag fall on the
south-eastern slope of the volcano. A big seismic tremor accompanies all the
phase, during which the crater becomes wide.
23 March
A series of explosions are caused by the entrance of water in the volcanic
duct and there was a swarm of earthquakes.
29 March
The eruption end. It caused the death of some persons, the collapse of the
roofs and serious damages in S. Sebastiano and Massa di Somma.
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