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.
Res. 1, 332 (1985). (5.) P. Papale and F. Dobran, J. Volcanol. Geotherm. Res.
58,101 (1993). (6.) P. Gasparini, M. S. M. Mantovani, R. Scandone, Bull.
Volcanol. 44, 317 (1981). (7.) Hoffer W (1982) Volcano: the search for Vesuvius.
New York: Summit Books, p189 (8) Lirer L, Munno R, Postoglione I, Vinci A and
Vitelli L (1997) The A.D. 79 eruption a future explosive scenario in the
Vesuvian area: eveluation of associated risk. Bulletin of Volcanology 59:
112-124. (9) Barberi F, Rosi M, Santacroce R and Sheridan MF (1983) Volcanic
hazard zonation at Vesuvius. In: Tazieffn H and Sabroux JC (eds) Forecasting
volcanic events. Developments in Volcanology I. Elsevier Amsterdam: 149-161 (10)
Sigurdsson H, Carey S, Cornell W and Pescatore T (1985) The eruption of Vesuvius
in 79 AD. National Geographic Research 1: 332-387 (11)Scandone R, Arganese G and
Galdi F (1993b) The evaluation of volcanic risk in the Vesuvian area. Journal of
Volcanology and Geothermal Research 58: 263-271 (12) Rosi M and Santacroce R
(1983) The A.D. 472 "Pollena" eruption: Volcanological and
petrological for this poorly-known, Plinian-type event at Vesuvius. Journal of
Volcanology and Geothermal Research 17: 237-248 (13)Rolandi G, Barrella AM and
Borrelli A (1993a) The 1631 eruption of Vesuvius. Journal of Volcanology and
Geothermal Research 58: 183-201 (14)Scandone R, Giacomelli L and Gasparini
(1993a) Mount Vesuvius: 2000 years of volcanological observations. Journal of
Volcanology and Geothermal Research 58: 5-25 (15)Mastrolorenzo G, Munno R and
Rolandi G (1993) Vesuvius 1906: a case study of a paroxysmal eruption and its
relation to eruptive cycles. Journal of Volcanology and Geothermal Research 58:
217-237 (16) Santacroce R (1983) A general model for the behaviour of the
Somma-Vesuvius volcanic complex. Journal of Volcanology and Geothermal Research
237-248 (17) Albitino Elio, Vesuvio; a volcano and its history. Naples Usmate
Press.3-24 Barberi F, Macedonio G, Pareschi MT, Santacroce R (1990) Mapping the
tephra fallout risk: an example from Vesuvius, Italy. Nature 344: 142-144 (18)
Sigurdsson H, Cashdollar S and Sparks RSJ (1982) The eruption of Vesuvius in
A.D. 79: Reconstruction from historical and volcanological evidence. American
Journal of Archaeology 86: 39-51 Mount Vesuvius Tami Gaudette Febuary 29,2000
Dr. Rode - Tice