The phenomena attending an eruption can leave no doubt that below every active volcano a large subterranean cavity must exist in which melted lava accumulates. The partisans of the theory which supposes the earth to consist of a central fluid mass with a solid shell resting upon it, attribute the formation of volcanoes to rents or fissures in this crust through which the lava is cast forth; but the local development of heat by chemical action, or some other unknown cause, is quite sufficient to account for the existence of fiery lakes imbedded in a solid mass, and which, though insignificant when compared with the surface of the globe, may still be large enough to produce volcanic phenomena on the grandest scale.
The cause of the reaction of such a reservoir against the surface of the earth must in all probability be sought for in the expansive force of steam; for when water, penetrating through crevices or porous strata, comes in contact with the heated subterranean mass, it is evident that the steam thus generated must press upon the lava, and, when formed in sufficient quantity, ultimately forces it up the duct of the volcano. In other cases, we may suppose a continuous column of lava mixed with liquid water raised to a red-hot, or white-hot, temperature under the influence of pressure. A disturbance of equilibrium may first bring on an eruption near the surface, by the expansion and conversion into gas of the entangled water, so as to lessen pressure. More and more steam would then be liberated, bringing up with it jets of liquid rock, and ultimately ejecting a continuous stream of lava. Its force being spent, a period of rest succeeds, until the conditions for a new outburst (accumulation of steam and melted rock) are obtained, and another cycle of similar changes is renewed. The important part which water plays in volcanic action is moreover sufficiently proved by the enormous quantity of steam which is poured forth during every eruption, or is constantly escaping in the fumaroles of a crater. The various gases (carbonic, muriatic, sulphurous) which are likewise exhaled by volcanoes may also have been rendered liquid by pressure at great depths, and may assist the action of water in causing eruptive outbursts. The great number of active volcanoes on sea-coasts and in islands likewise points to the agency of water in volcanic operations; and in the few cases where eruptive cones are situated far inland, their situation on the borders of a lake, or their cavernous and porous structure, accounts for the absorption of a quantity of atmospheric water, sufficient for the production of volcanic phenomena.
ERUPTION OF VESUVIUS, BAY OF NAPLES.
CHAPTER VII.
DESTRUCTION OF HERCULANEUM AND POMPEII.
State of Vesuvius before the eruption in the year 79 A.C.—Spartacus—Premonitory Earthquakes—Letter of Pliny the Younger to Tacitus, relating the death of his Uncle, Pliny the Elder—Benevolence of the Emperor Titus—Herculaneum and Pompeii buried under a muddy alluvium—Herculaneum first discovered in 1713.
Of all the volcanic eruptions recorded in history there is none more celebrated than that which, on the 23rd of August, A.D. 79, buried the towns of Herculaneum and Pompeii under a deluge of mud and ashes. Many other eruptions have no doubt been on a grander scale, or may have spread ruin and desolation over a wider area, but never has a volcano, awakening from the slumber of a thousand years, devastated a more smiling paradise than the fields of happy Campania, or buried more beautiful cities.
Before that terrible catastrophe, Mount Vesuvius, now constantly smoking, even in times of rest, had, ever since the first colonisation of South Italy by the Greeks, exhibited no signs of volcanic activity. Even tradition knew of no previous disturbance. No subterranean thunder, or sulphurous steams, or cast-up ashes, gave token of the fires slumbering beneath its basis; and the real nature of the apparently so peaceful mountain could only be conjectured from the similarity of its structure to other volcanoes, or from the ancient lava-streams that furrowed its abrupt declivities. At that time also its shape was very different from its present form, for instead of two apices, it exhibited, from a distance, the regular outlines of a sharply truncated cone. Plutarch relates that rough rock walls, piled round its summit, and overgrown with wild vines, inclosed the waste of the crater.
When, in 73 B.C., Spartacus, with seventy of his comrades, broke the fetters of an insupportable slavery, he found a secure retreat in this natural stronghold, which could only be scaled by a single narrow and difficult path. By degrees 10,000 fugitive slaves gathered round his standard, and Rome began to tremble for her safety. The prætor Clodius led an army against the rebels, and surrounded the mountain; but Spartacus caused ropes to be made of the branches of wild vines, by means of which he, with the boldest of his followers, was let down from the rocks, where they were supposed to be totally inaccessible, and, falling unawares upon the prætor, put his troops to flight and took his camp. The declivities of the mountain, thus become historically renowned, were covered with the richest fields and vineyards, and at its foot, along the beautiful Bay of Naples, lay the flourishing towns of Herculaneum and Pompeii, the seats of luxury and refinement. Who could, then, have imagined that this charming scene was so soon to be disturbed in so terrible a manner, and that the time was nigh when the ancient volcanic channels, from which, in unknown ages, lava-streams and ashes had so frequently broken forth, were once more to be re-opened? The first sign which announced the awakening energies of the volcano was an earthquake, which, in 63 A.D. devastated the fertile regions of Campania. From that time to the crowning disaster of 79, slight tremors of the earth frequently occurred, until, finally, the dreadful eruption took place which Pliny the Younger so vividly describes in his celebrated letter to Tacitus.
‘My uncle,’ says the Roman, ‘was at Misenum, where he commanded the fleet. On the 23rd of August, about one o’clock in the afternoon, my mother informed him that a cloud of an uncommon size and form was seen to arise. He had sunned himself (according to the custom of the ancient Romans), and taken his usual cold bath, then dined, and studied. He asked for his sandals, and ascended an eminence, from which the wonderful phenomenon could be plainly seen. The spot from whence the cloud ascended, in a shape like that of an Italian pine-tree, could not be ascertained on account of the distance; its arising from Vesuvius only subsequently became known.