Volcanoes: Past and Present

by Edward Hull

Fig. 8.—Map of the district bordering the Bay of Naples, with the islands of Capri, Ischia, and Procida.

From the above section, for which we are indebted to Mr. Johnston-Lavis, the most recent writer on Vesuvius, it would appear that the first volcanic explosions by which the mountain was ultimately to be built up took place after the deposition of the sands and marls (No. 2), while the whole Campagna was submerged under the waters of the Mediterranean. By the accumulation of the stratified tuff (No. 1), the sea-bed was gradually filled up during a period of volcanic activity, and afterwards elevated into dry land.[9]

Fig. 9.—View of Vesuvius from the Harbour of Naples at the commencement of the eruption of 1872.—(From a sketch by the author.)

(j.) Present Form and Structure of Vesuvius and Somma.—The outer cone of Vesuvius, or Monte di Somma, rises from a circular platform by a moderately gentle ascent, and along the north and east terminates in a craggy crest, with a precipitous cliff descending into the Atria del Cavallo, forming the wall of the ancient crater throughout half its circumference; this wall is formed of scoriæ, ashes, and lapilli, and is traversed by numerous dykes of lava. Along the west and south this old crater has been broken down; but near the centre there remains a round-backed ridge of similar materials, once doubtless a part of the original crater of Somma, rising above the slopes of lava on either hand. On this has been erected the Royal Observatory, under the superintendence of Professor Luigi Palmieri, where continuous observations are being made, by means of delicate seismometers, of the earth-tremors which precede or accompany eruptions; for it is only justice to say that Vesuvius gives fair warning of impending mischief, and the instruments are quick to notify any premonitory symptoms of a coming catastrophe. The elevation of the Observatory is 2080 feet above the sea.

On either side of the Observatory ridge are wide channels filled to a certain height with lavas of the nineteenth and preceding centuries, the most recent presenting an aspect which can only be compared to a confused multitude of black serpents and pachyderms writhing and interlocked in some frightful death-struggle. Some of this lava, ten years old, as we cross its rugged and black surface presents gaping fissures, showing the mass to be red-hot a few feet from the surface, so slow is the process of cooling. These lava-streams—some of them reaching to the sea-coast—have issued forth from the Atria at successive periods of eruption.

From the midst of the Atria rises the central cone, formed of cinders, scoriæ, and lava-streams, and fissured along lines radiating from the axis. This cone is very steep, the angle being about 40°-45° from the horizontal, and is formed of loose cindery matter which gives way at every step, and is rather difficult to climb. But on reaching the summit we look down into the crater, displaying a scene of ever-varying characters, rather oval in form, and about 1100 yards in diameter. From the map of Professor Guiscardi, published in 1855, there are seen two minor craters within the central one, formed in 1850, and an outflow of lava from the N.W. down the cone. At the time of the author's visit the crater was giving indications, by the great quantity of sulphurous gas and vapour rising from its surface, and small jets of molten lava beginning to flow down the outer side, of the grand outburst of internal forces which was presently to follow.

(k.) Eruption of 1872.—The grand eruption of 1872, of which a detailed account is given by Professor Palmieri,[10] commenced with a slight discharge of incandescent projectiles from the crater; and on the 13th January an aperture appeared on the upper edge of the cone from which at first a little lava issued forth, followed by the uprising of a cone which threw out projectiles accompanied by smoke, whilst the central crater continued to detonate more loudly and frequently. This little cone ultimately increased in size, until in April it filled the whole crater and rose four or five metres above the brim. At this time abundant lavas poured down from the base of the cone into the Atria del Cavallo, thence turned into the Fossa della Vetraria in the direction of the Observatory and towards the Crocella, where they accumulated to such an extent as to cover the hillside for a distance of about 300 metres; then turning below the Canteroni, formed a hillock without spreading much farther.

In October another small crater was formed by the falling in of the lava, which after a few days gave vent to smoke and several jets of lava; and towards the end of October the detonations increased, the smoke from the central crater issued forth more densely mixed with ashes, and the seismographical apparatus was much disturbed. On the 3rd and 4th November copious and splendid lava-streams coursed down the principal cone on its western side, but were soon exhausted; and in the beginning of 1872 the little cone, regaining vigour, began to discharge lava from the summit instead of the base as heretofore.