Vesuvius, the most striking object seen from the Bay of Naples, a mountain of dense tufa (pumice-stone and ashes), is supposed to have been heaved up from the submarine level where it was formed. Besides the shells which indicate its sea-origin, it contains erratic blocks of limestone from the higher Apennine-offshoot, Monte Somma, which, in an irregular semicircle, surrounds it on the north and east. The latter—the Mons Summums of antiquity, once crowned by a temple of Jupiter—was the seat of volcanic activity long before Vesuvius, which first (63 A.D.) became convulsed by earthquakes, repeated at intervals till 79, in which year occurred its earliest known eruption (see POMPEII). This was followed by others, of which the more memorable are that in 472, when its ashes alighted in Constantinople; in 512, when they were wafted to Tripoli; in 1036; and in 1500; after which ensued a period of inaction, broken in December 1631 by a destructive outbreak which denuded the mountain of the forest-growth with which it had become clothed. The 18th century witnessed many of its eruptions, the most remarkable being that of 1793, when a lava-stream 12 to 40 feet thick swept over Torre del Greco and penetrated the sea to a distance of 380 feet, by which time its volume was 1204 feet wide and 15 feet high. This stream was so liquid that to leave the crater and enter the sea—a journey of 4 miles—it took only six hours. Another memorable outbreak was that of 1822, when the so-called 'smoke' from the crater rose to a height of 10,000 feet, emitting flashes of lightning, raining torrents of hot water, and flooding the villages of S. Sebastiano and Massa. In 1855 occurred a terrible eruption, in which the summit of the cone discharged a lava-stream which ravaged the fertile and highly-cultivated region below. On December 8, 1861, Torre del Greco suffered severely from another visitation, surpassed in turn by that of 1871-72, when the sudden emission of lava from a crater of 1855 killed twenty spectators on the spot. S. Sebastiano and Massa were again greatly damaged, the cone threw up fragments of rock to a height of 4000 feet, and the explosions were so loud that the whole country-side fled panic-stricken to Naples. The activity of the volcano, accompanied by distinct shocks of earthquake, lasted for a week. From the observations of many years the following characteristics of the volcanic activity of Vesuvius have been summarised by Professor Palmieri. (1) The filling up of the crater portends an imminent eruption, and its full discharge is followed by a period of repose. (2) The narrowing of the mouth of the crater by accumulated debris impedes the flow of the lava, and this impediment leads to the outburst of lateral openings which from their greater proximity to the source of heat emit the lava in a more liquid condition, whereby its flow becomes that of a continuous stream. (3) When the internal channel is blocked by solid debris, the effort of the elastic vapour to clear it is supposed to cause the earthquakes by which the greater eruptions are preceded and accompanied. (4) What is called 'smoke' from the crater is simply steam more or less blackened with incinerated dust. When this dust is in excess it accelerates the fall of the steam, which, having become water by condensation, descends like a mud-torrent, flooding the ground. This was a notable feature of the visitation in which Pompeii perished. (5) During an eruption what appears as flame shooting out of the crater is really the reflection of the molten lava within the crater upon the steam and upon the ashes suspended in the steam accumulated above it. (6) The rapid condensing of vapour into water, and the conversion of this into steam, generates electricity, which explains the lightning-effects visible on the edges of the clouds overhanging the crater. Vesuvius is reckoned by geologists the most instructive object-lesson on volcanoes in general, and the university of Naples, by an admirable assortment of specimens of its structure, has greatly facilitated its study. Professor Sacchi numbers forty species of minerals found in it, of which augite, hornblende, mica, sodalite, breisakite, magnetic iron, and leucite are the most abundant. The fertility of its slopes, since Martial's famous epigram on the destruction of Pompeii, has passed into a proverb, its chief product being the wine called Lacrima Christi, red and white, the latter superior in bouquet. Its observatory (1844) has acquired a European reputation from the meteorologist Melloni, and still more from his successor, Professor Palmieri, who directed it with equal sagacity, skill, and daring from 1854 till his death in 1882. The so-called railway, but rather cable-tram, from the base to near the summit, was opened in 1880.
See the late Prof. Phillips' Description of Vesuvius, the papers of Dr Johnston-Lavis, and J. S. Lolley, Mount Vesuvius: Historical and Geological Account (1889).