Vesuvius is the most famous volcano of the earth, the only active in continental Europe and is also one of the most dangerous because the vast territory that extends to its slopes has seen the construction of houses up to 700 meters of height. It is a typical example of an enclosed volcano consisting of an outer trunk cone, Monte Somma (1133 meters), with a largely demolished crater wall surrounded by a smaller cone represented by Vesuvius (1281 meters), separated by a depression called Valle del Gigante, part of the ancient caldera, where later, presumably during the eruption of 79 AD, the Great Cono or Vesuvius was formed.
The Valle del Gigante is divided in turn into Atrio del Cavallo to the west and Valle dell’Inferno to the east. The enclosure of the Somma is well preserved throughout its northern part, in fact it was less exposed in historical times to the devastating fury of the volcano, because it was sheltered from the height of the internal wall that prevented the outflow of lava on its slopes. The slopes, variously degrading, are furrowed by deep radial valleys produced by the erosion of rainwater. Its walls from part of the cone appear to peak.
The whole section is then strewn with spikes and ditches of dark volcanic rock. The old crater rim is a succession of tops called cognoli. While the height of the Sum and its profile have been preserved the same over the centuries, the height and profile of Vesuvius have undergone considerable variations, due to subsequent eruptions, with rises and lowerings. Vesuvius is a characteristic polygenic and mixed volcano, that is, composed of lavas of different chemical composition (for example trachyte, tephrite, leucitite) and formed both by lava flows and pyroclastic deposits. All the areas on the slopes of the mountain are to be considered formed by lands transported by mud lava that descend from steep slopes in the rainy seasons through deep and narrow valleys called riverbeds or more commonly spiders.
The high banks are formed by heaps of lava slag, which precipitated in the incandescent state and spread to the low slopes, are now revealed because of their fertile material, rich in silicon and potassium, precious for the vegetation. On colder days the condensation of the vapors makes visible the fumaroles present in numerous points of the inner wall of the crater. Continuing along the edge of the crater, looking towards the sea you can see the entire extension of the southern part of the volcano and, on days with good visibility, all the Gulf of Naples from the Sorrento Peninsula and Capri to Capo Miseno, Procida and Ischia. It is also inevitable to note the reckless urban expansion that goes back along the slopes of the volcano.
The eruption of 79 AD
The eruption began on August 24, 79 AD around noon. The first eruptive phase was characterized by strong phreatomagmatic explosions. After this phase, magmatic explosions followed one another until the morning of the following day, feeding a column made up mainly of gas, pumice and ashes that rose up to 30 kilometers. The upper part of the column expanded, assuming the shape of a pine tree, and was pushed by the winds towards the south-east. The particles contained in it fell to the ground, forming a thick layer of pumice that reached Pompeii and Oplonti reached 2-3 m. thick. Partial collapse of the eruptive column generated pyroclastic flows that spotted at high speed along the sides of the volcano, reached and destroyed Herculaneum.
The city of Pompeii, much further away, it was not reached and most of its inhabitants survived. During the last hours of the night the intensity of the eruptive activity diminished.
In the early hours of the morning of August 25, a phreaticomagmatic explosion generated pyroclastic, turbulent flows – the terrible “base-surge” – which, traveling at the speed of a hurricane, descended the slopes of the volcano, devastated the surrounding areas up to distances of 15 kilometers and caused numerous victims even among the inhabitants of Pompeii who had survived the first phase of the eruption. During the day the explosions decreased in intensity and, in the evening, ceased altogether, leaving a large blanket of ashes and pumice on a vast area. The heavy rains, also caused by the introduction of enormous quantities of steam and fine particles into the atmosphere, mobilized this material.
The eruption of 1631
The 1631 eruption was the most violent and destructive in the history of Vesuvius in the last millennium. After a long period of quiescence, about 5 centuries, preceded by a series of precursory phenomena, such as earthquakes and earth lifts, the volcano reawakened causing the death of about 6,000 people and the devastation of an area of almost 500 Km.2
The eruption began at 7 am on December 16th, with the formation of an eruptive column of about 15 Km., From which pumice and ashes began to fall in the area east of the Vesuvius. At 10 am on December 17, pyroclastic flows were generated from the central crater, clouds of gas laden with fragments of magma that, running at great speed along the western and southern flanks of the volcano, destroyed everything they encountered on their way. In the night between 16 and 17, and in the afternoon of the 17th, the abundant rains mobilized the incoherent ash cover causing the formation of mudslides. The castings descended from both sides of the volcano and from the slopes of the Apennine foothills to the north and north-east.
The paroxysmal phase of the eruption lasted three days, causing a huge panic among the population. On the streets of Naples there were public confessions of sins, accompanied by extraordinary manifestations of penance, and processions were organized with the statue and blood of St. Gennaro, so that the patron might placate that divine wrath of which the explosion of Vesuvius undoubted 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 a few months, deeply disturbed by the event, he made a plaque in Portici that urges posterity not to forget the nature of the mountain, and to readily recognize the precursors of a volcanic eruption.
The eruption of 1944
On March 18, 1944, during the occupation of the allied troops, the last eruption of Vesuvius began, which ended a period of activity begun in 1914, during which there had been only modest eruptions from the central crater.
Between 1914 and 1944, the lava and the slag produced by the volcano had filled the crater, 720 m wide. and 600 m deep, which had formed during the previous eruption of 1906.
A cone of slag emerged from the crater.
The slag cone begins to collapse and the seismic activity becomes more intense. A new cone of slag is formed and immediately collapsed.
The eruption starts in the afternoon with slag shots. At 4.30pm a lava flow flows over the northern part of the crater and reaches the Valle dell’Inferno at 10.30pm. Almost at the same time another casting spills over from the southern part of the crater. At 11 pm there is also a lava flow from the western part of the crater: the casting runs along the funicular track and interrupts the railway.
At 11 am the lava flows along the Fosso della Vetrana.
Between afternoon and night, new flows overflow from the northern part of the crater. All effusive activity is accompanied by seismic tremor with increasing amplitude up to half of the day.
The southern casting stops at an altitude of about 300 m. above sea level. In the night, the northern flow reaches S. Sebastiano and Massa di Somma and is divided into two branches that advance towards Cercola, from which in the evening about 1.5 km away. S. Sebastiano and Massa di Somma are evacuated and the 10,000 inhabitants transferred to Portici. Around 17, spectacular lava fountains begin to form, the last of which lasts about 5 hours and reaches a height of almost 1,000 m. Fragments of lava and ashes moved from the high winds, settle on the south-eastern areas of the volcano, between Angri and Pagani. The smallest fragments reach distances of over 200 km. towards the south-east. Scoriae up to one kilogram of weight reach the inhabited area of Poggiomarino, about 11 km. from the crater. Large quantities of still hot slag accumulate on the sides of the Gran Cono. Continue the seismic tremor, with maximum amplitude coinciding with the emission of lava fountains.
At around 1 pm the eruption reaches its maximum intensity. A column of gas and ash rises to a height of about 6 km. The upper part of the column is pushed by the wind towards the south-east, ashes and slag fall on the south-eastern slopes of the volcano. Partial collapse of the eruptive column form small pyroclastic flows that flow along the sides of the cone. An intense seismic tremor accompanies this whole phase, during which the crater gradually widens.
A series of explosions are caused by the entry of water into the volcanic conduit and earthquakes occur. The explosions generate columns of ash, which are pushed by the wind towards the south-west, and small pyroclastic flows flow along the sides of the cone.
The eruption ends. The morphology of the summit area of the cone is profoundly modified with a new large crater depression, the same visible today.
The eruption of the ’44, although of moderate energy, caused the death of some tens of people due to the collapse of the roofs and caused serious damage to S. Sebastiano and Massa di Somma.