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)