Volcano Mount Vesuvius
Mount Vesuvius is a volcano located in
southern Italy, near the bay of Naples
and the city of Naples. It is the only
active volcano on the European mainland.
Vesuvius rises to a height of
1277 m (4190 ft). Vesuvio (Vesuvius) is probably
the most famous volcano on
earth, and is one of the most dangerous. Mount
Vesuvius is a
strato-volcano consisting of a volcanic cone (Gran Cono) that was
built
within a summit caldera (Mount Somma). The Somma-Vesuvius complex has
formed
over the last 25,000 years by means of a sequence of eruptions of
variable
explosiveness, ranging from the quiet lava outpourings that
characterized
much of the latest activity (for example from 1881 to 1899 and
from 1926 to
1930) to the explosive Plinian eruptions, including the one that
destroyed
Pompeii and killed thousands of people in 79 A.D. At least seven
Plinian
eruptions have been identified in the eruptive history of Somma-Vesuvius
(1).
Each was preceded by a long period of stillness, which in the case of
the
79 A.D. eruption lasted about 700 years. These eruptions were fed by
viscous
water-rich phonotitic to tephritic phonolitic magmas that appear to
have
differentiated in shallow crustal conditions. They are believed to have
slowly
filled a reservoir where differentiation was driven by compositional
convection.
A minimum depth of about 3 km was inferred for the top of the
magmatic reservoir
from mineral equilibria of metamorphic carbonate ejecta
(2). Fluid inclusions
([CO.sub.2] and [H.sub.2]O-[CO.sub.2]) in
clinopyroxenes from cumulate and
nodules indicate a trapping pressure of 1.0
to 2.5 kbar at about 1200 [degrees]C,
suggesting that these minerals
crystallized at depths of 4 to 10 km (3). The
differentiated magma fraction
was about 30% of the total magma in the reservoir,
and a volume of about 2 to
3 [km.sup.3] was inferred for the reservoir (4). The
magma ascent to the
surface occurred through a conduit of possibly 70 to 100 m
in diameter (5). A
thermal model predicts that such a reservoir should contain a
core of
partially molten magma (6) that can be detected by high-resolution
seismic
tomography. The earliest outcropping volcanic deposits date back to
about
25,000 years ago. The lavas observed at a -1125 m bore-hole are
about
0,3-0,5 million years old. It is known for the first eruption of
which an
eyewitness account is preserved, in 79 AD. Geologically, Vesuvio is
unique for
its unusual versatility. Its activity ranging from Hawaiian-style
release of
liquid lava, fountaining and lava lakes, over Strombolian and
Vulcanian activity
to violently explosive, plinian events that produce
pyroclastic flows and
surges. Vesuvius is a complex volcano. A complex
volcano is "an extensive
assemblage of spatially, temporally, and genetically
related major and minor
[volcanic] centers with there associated lava flows
and pyroclastic flows."
Vesuvius has a long history. The oldest dated
rock from the volcano is about
300,000 years old. It was collected from a
well drilled near the volcano and was
probably part of the Somma volcano.
After Somma collapsed about 17,000 years
ago, Vesuvius began to form. Four
types of eruption have been documented: a)
Plinian (AD 79, Pompeii type)
events with widespread air fall and major
pyroclastic surges and flows; b)
sub-Plinian to Plinian, more moderately sized
eruptions (AD 472, 1631) with
heavy tephra falls around the volcano and
pyroclastic flows and surges; c)
small to medium-sized, Strombolian to Vulcanian
eruptions (numerous events
during the 1631-1944 cycle, such as 1906 and 1944)
with local heavy tephra
falls and major lava flows and small pyroclastic
avalanches restricted to the
active cone itself. The fourth type it is the
smallest of all eruption types
observed at Vesuvio. It is the persistent
Strombolian to Hawaiian style
eruption that characterizes almost all of an
eruptive sub-cycle, such as was
the case during the period 1913-1944. Activity
of this kind is mainly
restricted to the central crater where one or more
intracrateral cones form,
and to the sides of the cone. Lava flows from the
summit crater or from the
sub terminal vents extend beyond the cone's base. A
somewhat particular kind
of persistent activity is the slow release of large
amounts of lava from sub
terminal fractures to form thick piles of lava with
little lateral extension,
such as the lava cupola of Colle Umberto, formed in
1895-1899. (7)
Vesuvius lies over a subduction zone. The two plates are the
African
plate and the Eurasian plate. The African plate is moving northward at
about
one inch (2-3 cm) per year and is slowly closing the Mediterranean
basin.
As it moves to the north, the African plate is pushed beneath the
Eurasian
plate. The rocks at Vesuvius are called tephrite. A tephrite is
basaltic in
character and contains the following minerals: calcic
plagioclase, augite, and
nepheline or leucite. (8) Eruptive activity of
Vesuvio noticeably occurs in
cycles that last several centuries and alternate
with repose periods lasting
several centuries. Each repose period ends with a
major (Plinian) eruption,
initiating an active cycle. One of the problems
researchers of Vesuvio have to
deal with is that the cycles do not always
repeat the same patterns and
phenomena. The cycle or cycles following the 79
A.D. eruption seem to have been
different from the most recent one, lasting
from 1631 until 1944. The most
recent Plinian eruption of major magnitude was
that of August 79 A.D. The 79
A.D. eruption of Vesuvius was the first
volcanic eruption ever to be described
in detail. From 18 miles (30 km) west
of the volcano, Pliny witnessed the
eruption and later recorded his
observations in two letters. He described the
earthquakes before the
eruption, the eruption column, air fall, the effects of
the eruption on
people, pyroclastic flows, and even tsunami. (9) Volcanologists
now use the
term "plinian" to refer to continued explosive eruptions,
which generate
high-altitude eruption columns and blanket large areas with ash.
It is
estimated that at times during the eruption the column of ash was 20
miles
(32 km) tall. About 1 cubic mile (4 cubic kilometers) of ash was
erupted in
about 19 hours. It is world-famous for the destruction of the
Roman towns of
Pompeii and Herculaneum that has inspired of generations
of poets, philosophers
and scientists. (10) Two more very strong eruptions
have occurred since 79 AD, a
very poorly known one in 472 AD and another one
in December 1631. It's argued
whether this eruption has been purely explosive
or mixed explosive-effusive. It
is clear that it was the second most
devastating eruption of Vesuvio next to the
eruption of 79 AD. Numerous
villages and towns were devastated by pyroclastic
flows, tephra falls and
lahars, and at least 3000 people died. Compared with the
AD 79 eruption,
the event of 1631 was of minor size regarding eruptive magnitude
and erupted
volumes but not in terms of destruction and fatalities. Beginning
on
December 16, 1631 and culminating the day after, it destroyed all
towns and
villages around the volcano and killed between 3000 to 6000 people.
(9) It was
the worst volcanic disaster in the Mediterranean during the past
1800 years.
Like the AD 79 eruption, the 1631 event had been purely
explosive but was
characterized by the emplacement of devastating pyroclastic
surges and flows.
The eruption occurred after a calm period lasting
between 130 to 500 years. Only
recently (starting in the late 1980's) has
there been modern volcanological
research on this important event that has
significant implications for volcanic
hazard assessments. When Vesuvius
became active again, Vesuvio had no
significant eruptions since 1139; an
eruption recorded for the year 1500 was a
minor phreatic event, increased
fumarolic activity, or a major rock fall. (11).
Before the eruption of
1631, Vesuvio was densely vegetated except at the summit
of the active cone
which by then had an elevation of about 1187 m about 100 m
less than its
present elevation, and 55 m higher than Monte Somma. The crater
had a
diameter of about 480 meters; it was funnel-shaped, had a few fumaroles
on
the rim and in its deepest part. Small ponds were present in the crater,
but
they probably existed on the caldera floor rather than within the active
crater.
(7) Increased fumarolic activity and nocturnal glow that was visible
on the
north side of the Vesuvian cone as early as August 1631. Strongly
increased
local seismicity began to be perceived after December 10, 1631. The
strongest
tremors were felt as far away as Napoli. (12) The other warning
signs were
repeated subterranean rumblings in the night that preceded the
outbreak and the
drying up of wells around the volcano; some other wells
reportedly became muddy.
Among the somewhat stranger happenings is the
reported filling to the rim of the
crater with a steaming "bituminous mass"
the nature of which was not
further detailed, during the first days of
December. During the 24 hours before
the eruption, earthquakes were felt more
and more frequently. (9) The population
must have become extremely nervous,
but there was no major evacuation from the
area. Chronology of the eruption
Following several strong earthquakes, a series
of vents became active between
6:00 and 7:00 on December 16, 1631. They were
situated along an eruptive
fracture on the west-southwest side of the active
cone, splitting it open
from the summit to the base. This initial activity
ejected fresh magma along
with material torn from the walls of the fissure, i.e.
older volcanic rocks.
Blocky, nonvesicular fragments of juvenile fragments point
to some
magma-water interactions at this stage (13). The eruption rapidly
gained
energy as more vents opened on the flanks of the cone ejecting
pyroclastics at a
growing mass eruption rate. Soon after the beginning of the
eruption, a large
eruption column rose up, attaining the famous shape of a
pine tree. The height
of the eruption column at this stage exceeded 20 km and
may have reached up to
28 km, thus the eruption was Plinian. Ash began to
fall around the volcano about
one hour after the start of the activity, but
heavy block and scoria fall began
at about 1000 in the direction of Ottaviano
(north east side of Monte Somma), a
village that later was to suffer from
many other eruptions of Vesuvio. (12)
During the morning of December 16,
a continuous tremor began to be felt in
Napoli, it did not cease until
8-10 hours later. Darkness fell over the area
around the volcano and reached
Napoli at 4:00 on that fatal day. (13) The main
portion of the eruptive plume
was blown towards the east, causing darkness and
tephra falls over southern
Italy and over the Balkan. Slight asfalls are
reported to have occurred as
far as Constantinople, W Turkey, about 1250 km from
the volcano. (12) The
proximal maximum thickness of the initial pumice deposit
is 1.5 m at Canale
dell'Arena. (8) After the initial plinian phase, between
7:00and 10:00 on
December 16 the eruption took on a pulsating character,
accompanied by
strongly increased seismicity. During the night of 16-17
December, strong
earth shocks occurred at intervals lasting 1-15 minutes. At
about 2:00 on
December 17 the first glowing avalanche that was observed to
descend into the
Atrio del Cavallo. At around the same time, strong rainfalls
saturated large
amounts of already fallen ash to form lahars that caused damage
and
disruption on the north and northeast sides of Mount Somma.(14) On
December
17 the activity changed with occasional surges of sub-Plinian to
Plinian
activity that caused tephra falls around the volcano. On the 17th,
the summit of
the volcano was partially destroyed by the activity. (13)
Within an active
cycle, smaller sub cycles can be observed, starting with
minor intracrateral
(effusive and Strombolian) activity with some
fluctuations until a strong
eruption produces tall eruption columns, more
voluminous, rapidly moving lava
flows, and heavy tephra falls. This
culminating, sub cycle-ending eruption is
followed by a brief (max. 7 years
during the most recent, and well-documented,
cycle, 1631-1944) repose, then
intracrateral activity starts again. (15).
Typical eruptions closing
Vesuvian sub cycles were those of 1767, 1779, 1794,
1822, 1872, 1906, and
1944. Each of them caused damage in the towns around the
volcano and the
people suffered partial or total destruction at least once
during the
1631-1944 cycle. Torre del Greco, on the coast west of Vesuvio, was
destroyed
three times in that period. Lava flows entered populated areas also
during
some more intense activity in the course of a sub cycle, most recently
in
1929. Eruptions of this type have been seriously disruptive for life
near
Vesuvio in the past and would be extremely disturbing, were they to
occur today.
To cite one example: the 1906 eruption caused heavy tephra
falls in the
northeastern sector of Vesuvio, causing the collapse of almost
all roofs in the
towns of that area. Up to 500 people were killed in that
event. 26 People died
much the same way during the most recent eruption in
1944. (13) After that
event, the volcano has most obviously entered one of
the longer periods of
repose that is maybe to last much longer - up to
several centuries - until a new
eruptive cycle will begin with a major
explosive eruption. Such spastic
eruptions produce heavy tephra falls,
pyroclastic flows, surges, and lahars.
Lava flows are uncommon during
these events. As the next eruption will probably
be a paroxysmal one, primary
volcanic hazards are tephra falls and pyroclastic
flows and surges. They form
a significant threat for a zone including parts of
Napoli and the entire
belt of towns around the volcano. It is certain phenomena,
such as increasing
seismicity, deformation, and others, will warn of an
impending eruption, as
has been the case before the AD 79 and 1631 eruptions.
There are,
however, serious logistical problems regarding the evacuation of
maybe up to
a million people in the areas endangered by tephra fall and
pyroclastic flows
and surges. Vesuvio has a long and complex record of
eruptions. Eruptions
before AD 79 have neither been recorded in historical
documents nor are there
any folklore of previous activity. For the first
millennium after Christ the
record is incomplete and only with the late 17th
century it becomes
reasonably adequate. We can say that the most recent eruptive
cycle, lasting
from 1631 until 1944, has been very well documented and gives an
idea of the
behavior of the volcano during such a cycle. Understanding of the
volcano in
longer terms of cycles is now beginning to form. It is known that
eruptive
cycles begin after non-active periods that may last centuries to
millennia,
and their opening eruptions are devastatingly violent, Plinian
events. The
most famous one is the AD 79 eruption that has been so well
described in the
letters by the Pliny the Younger. His description inspired
volcanologists in
the late 19th century to call eruptions like that of AD 79
"Plinian"
eruptions. Certainly the most notable aspect of Vesuvio's
eminence among
Earth's volcanoes is the dense population surrounding it and
climbing higher
and higher up its slopes. In an enchanting landscape with
beautiful islands,
magnificent mountain ranges, marvellous coasts and
historically famed cities,
Vesuvio is the focus, lying in the center of a plain
on the east north
eastern side of the Gulf of Napoli. It is the steepness, the
sudden way it
rises from its peaceful surroundings, which make it so impressive.
(16)
Vesuvius is a very dangerous and deadly volcano. Mudflows and lava flows
from
the eruption in 1631 killed 3,500 people.(13) About 3,360 people died in
the
79 A.D. eruption from ash flows and falls.(9) Studies of past eruptions
and
their deposits continue. These studies help volcanologists understand
the
hazards associated with future eruptions. The population density in some
areas
of high risk is 20,000 to 30,000 per square km. About 3 million people
could be
seriously affected by future Eruptions. In the first 15 minutes of a
medium- to
large-scale eruption an area with a 4 mile (7 km) radius of the
volcano could be
destroyed (Dobran and others, 1994). About 1 million people
live and work in
this area immediately threatened by future eruptions. There
are no signs of
volcanic unrest at Vesuvius at the present time.
(11)
References
(1.) V. Arno et al., in Somma-Vesuvius, R. Santacroce,
Ed. (Quaderni de La
Ricerca Scientifica, Rome, 1987), pp. 53-103. (2.) F.
Barberi et al., Bull.
Volcanol. 44, 295 (1981); L. Civetta, R. Galati, R.
Santacroce, ibid. 53, 517
(1991). (3.) H. E. Belkin and B. De Vivo, J.
Volcanol. Geotherm. Res. 58, 89
(1993). (4.) H. Sigurdsson, S. Carey, W.
Cornell, T. Pescatore, Natl. Geogr.
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and F. Dobran, J. Volcanol. Geotherm. Res.
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Gasparini, M. S. M. Mantovani, R. Scandone, Bull.
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Journal of Archaeology 86: 39-51 Mount Vesuvius Tami Gaudette
Febuary 29,2000
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