Popular Science Monthly/Volume 24/November 1883/Ischia and its Earthquakes
|ISCHIA AND ITS EARTHQUAKES.|
By M. CH. VÉLAIN.
THE island of Ischia, which has recently been so terribly rent by an earthquake, is situated in the northwestern part of the Bay of Naples, and near the Phlegrean fields, with which the little island of Procida, likewise volcanic, constitutes a connecting link. It forms a part of the Neapolitan volcanic region, which may be considered as still in a state of solfatarian activity, which is exemplified by the well known solfatara of Puzzuoli, where the sulphur is redeposited, as far as it is mined, by numerous gaseous emanations, and by the escape of carbonic acid in the Grotto del Cane near Lake Agnano. All of these exhalations, which are the mark of a declining volcanic activity, attest that this region, situated on a great line of fracture running northwest and southeast from Vesuvius to Vultura, is still in direct communication with the subterranean sources. The ancients fully recognized this, and regarded all those explosive craters, now transformed into a chain of remarkably picturesque lakes across the Phlegrean fields, as so many doors of Tartarus through which the infernal divinities took souls to the banks of the Acheron. The most celebrated of them, Lake Avernus, "Atri Janua Ditis" (the gate of black hell), now smiling and salubrious, then exhaled torrents of suffocating gases which well justified its name, and rendered a stay there mortal to the birds that ventured into its neighborhood.
The Neapolitan volcanic region extends from Vesuvius to Vultura, on the eastern edge of the Apennines, and includes the Phlegrean fields and the connected islands of Ischia and Procida. The volcanic activity of this whole space is now concentrated at Vesuvius, and is manifested at other places in the vicinity only by the emanations and thermal springs of which we have spoken, and from time to time, during periods when the volcano is inactive, by violent shocks, of which the terrible disaster of the 28th of July, at Ischia, has just given an impressive example.
Previous to the Christian era, Vesuvius, covered with a rich vegetation, was wholly inactive. Nothing except the form of the mountain could give a suspicion of the intensity of the fires that were raging beneath it. Volcanic activity, then localized in the Phlegrean fields,
Fig. 1.—Bay of Naples. Geological Map showing the Relations of Ischia with the Phlegrean Fields.
attained its maximum in Ischia, which was its escape-valve during the entire period of Vesuvian quiet. It produced then, through the action of a large number of eruptions taking place within a period of several thousand years, a considerable island, which now rises more than eight hundred metres, or two thousand six hundred feet, above the level of the sea. It is eighty kilometres, or a little less than fifty miles, in circumference at the level of the sea, eight kilometres, or not quite five miles, long from east to west, and eight kilometres, or about three miles, broad. From its center rises Mount Epomeo, which, crowned by an abrupt, semicircular rampart, which is nothing else than the eastern edge of the grand crater, whence have issued all the trachytic projections that now form the greater part of the island, presents the somber aspect of a fire-vomiting mountain. This crater has never given out lavas. Built on masses of pumiceous tufas of slight consistency, the lava-flows have always been produced upon the slope or at the base of the mountain. At each of the orifices of issue the projections forced out by tumultuous jets of gas have formed adventitious cones of dimensions often considerable, like those of il Toppo, il Trippiti, and il Garifoli; and we may count some ten such cones around Epomeo, all of which have been centers of activity and furnished large flows.
The appearance of Ischia was relatively of recent date; it is not placed farther back than the older quaternary. The foundation of the island was begun by submarine eruptions, above which opened the crater of Epomeo, at first appearing above the surface of the sea as an annular reef, from which were thrown out jets of trachytic scoria. The island was raised up in successive stages by the accumulation of the projected matter around the orifice of issue. The proof of this is drawn from the fact that we may still find on the sides of Mount Epomeo, carried to a height of four hundred and seventy metres, masses of marine shells of species yet living in the Mediterranean, encased in clays that have resulted from the decomposition of trachytic tufas under water. The whole of this trachytic mass is itself established on marls and clays, including numerous remains of Mediterranean shells, and has evidently acquired its present relief within the historical epoch.
The most ancient of the recorded eruptions in Ischia was that of Montagnone, to which is ascribed the origin of the vast crater of regular form that still existed before the recent earthquake, in a state of perfect preservation, in the northwestern part of Ischia. About 470 B. C., successive eruptions at Point Comacchia gave rise to the vast flows of Manecoco and Bale, which extended far into the sea and prolonged the point to the north. Numerous efforts have been made since these ancient times to plant colonies on this unstable land, even then fertile and covered with a luxuriant vegetation.
Lyell, who made a long exploration of the island in 1828, relates that first the Erythreans and afterward the Chalcideans, who had settled in the island before the Christian era, were driven away by the incessant earthquakes and the mephitic exhalations escaping from every point. At a later time, 280 B. C., Hiero, king of Syracuse, tried to found a colony there, but it was soon driven away by a formidable explosion preceding the great flows of lava which gave rise to the masses now forming the promontories of Zaro and Camso.
The same fate befell the Grecian colonies which afterward tried at different times to occupy the island. The eruption that forced the retreat of the first Grecian colony gave rise to Monte Rosato, that cone of projections the sudden formation of which is comparable to that of Monte Nuovo. The last-named mountain was raised in September, 1538, in forty-eight hours, at Puzzuoli, after a succession of formidable shocks which occasioned great disasters in the Phlegrean fields and destroyed a great number of Roman buildings. These two mountains of volcanic erection, formed under similar conditions, at two distinct epochs corresponding in each case with a period of repose in Vesuvius, are distinguished by their regular form, which may be compared with that of the classic volcanoes of the chain of the puys of Auvergne. Both, terminating in a vast crater, have emitted, like the volcanoes of Auvergne, only a single flow of lava, which seems to have exhausted all their energy. A long period of repose followed. During more than a century "Ischia the Joyous," as it was called, rested in perfect tranquillity. The pleasure-loving Romans made of it the most enchanting resort in the world; all their magnates had villas there.
It is to be remarked that this period of repose was correspondent with a resumption of activity on Vesuvius. The first symptom of an awakening of energy in that volcano was an earthquake, which in the year 68 occasioned considerable damage in the neighboring towns. We know well how, eleven years later, in 79, the hitherto peaceful mountain, covered at the time with rich plantations and forests nearly to its crater, revealed by a sudden explosion the terrible force that was sleeping in its depths. La Somma, reduced to powder, was projected into the air; then a column of thick smoke was seen to rise vertically from the summit of the mountain, and to spread horizontally, covering the country under its immense shadows. The sun was obscured even as far as to Rome, and it was believed that the "great night of the earth" was about to begin. When light was restored, the dismantled mountain had changed its form; the luxuriant forests that had covered it had disappeared, and so had the populous cities of Herculaneum, Pompeii, and Stabise, buried, with their inhabitants, under ashes and volcanic débris. From this time, Vesuvius does not appear to have emitted any eruption of lava for several hundred years; and this period of quiet at that center seems to have been marked at Ischia by a resumption of the fires of Epomeo, which had enjoyed so long a rest that large forests had grown up to the very edge of its crater. In 1302, after the island had been shaken with a succession of earthquakes during the previous year, the lava gushed out by a new opening near the city of Ischia, and in less than four hours reached the sea, having destroyed everything in its passage as if it had been a torrent of fire. The city was terribly afflicted; large houses and numerous villas were buried, with their inhabitants. The rough surface of this lava stream has resisted all weathering, and still refuses to bear any vegetation. The new eruptive phase was of long duration, and it is remarked that while it continued Vesuvius was quiet. The alternations between the eruptive movements of lava in the two volcanic centers find a natural explanation in the facts that they are both on the same line of fracture, and a subterranean communication probably exists between them.
Epomeo became tranquil after Vesuvius resumed its eruptions; and for long series of years the island of Ischia had no other outlets for the escape of the gases generated in its depths than its thirty or forty thermal springs, which have contributed, together with the pure air and the beauty of the situation, to increase every year the crowd of visitors.
Every indication tends to support the belief that Ischia, a rival to Vesuvius in the height of its volcano, is an ancient cone composed of the matter thrown up by extremely violent submarine eruptions which took place before the present epoch. As the mountain increased in height through the successive accumulations of the trachytic projections from the central crater, the weaker parts of its flanks, yielding to the height of the liquid column in the vent, were cleft in every direction; the injection of lavas into all the fissures thus formed giving rise to the flows we have just mentioned, melted in with and consolidated the structure, which is thus the result of a protracted alternation of projected débris and flows of compact lavas. We can in this manner account for the disposition of the grand ravines which, descending from Epomeo, plow the flanks of the mountain to a great depth.
Fig. 2.—Coast of Ischia, been from the West, Point Comacchia.
The island has, therefore, been progressively raised above the waters, and has grown laterally during the historic period, as is testified by the flows of lava still visible on the Arso and on Monte Tabor, which are prolonged to the sea, and by the numerous secondary cones scattered over its plateaus. It definitely acquired its present relief toward the beginning of this century. Since that time, Mount Epomeo has not given any other signs of its volcanic character than those which the scientific observer might deduce from the analogy of its form with the forms of other volcanoes. Its arid, slashed summit, looking up to the sky, served as the end of the promenade for the numerous visitors who every summer frequented the thermal stations at Casamicciola, Castiglione, and San Lorenzo. Its springs, highly endowed with thermal qualities, and the exceptional fertility of its volcanic soil, on which small shrubs became arborescent, would have sufficed to give to the fortunate, healthful, and gay island great wealth, had not its earthquakes always caused apprehensions.
These disturbances of the earth, the relations of which with the volcanic structure are most evident have repeatedly brought frightful disasters upon Ischia. Hardly a trace of the splendid Roman structures once built upon it now remains; without mentioning specifically all the recorded earthquakes, that of 1881, which is still comparatively-fresh in memory, partly destroyed the city of Casamicciola, which has now been obliterated. It gave a warning by which no one knew how to profit. The constitution of the soil of the island, which is composed
Fig. 3.—Castle of Ischia.
chiefly of trachytic tufas and unconsolidated loose matters, is a considerable element in promoting these disasters.
The Ischian earthquakes are narrowly localized. Their origin is not doubtful, but is readily traceable to the efforts which the lavas and the gases, strongly compressed under the earth, make to escape. Their effects never extend to great distances. The catastrophe which has just consummated the destruction of Casamicciola, already severely shaken in 1881, is a striking example of them. A violent shock, quick as the firing of a cannon, was enough to unsettle and partly destroy the whole northern slope of the island. Procida, which was near it, was shaken, but only a few rumblings in the earth were felt on the neighboring coast. The phenomena are marked by vertical shocks, acting only upon a definite point, and violent in proportion as they are limited in extent. These shocks are propagated irregularly, without continuity, by sudden starts, across the trachytic tufas forming the sub-soil of the island. Slides of the ground are thus produced, which carry off with them cultivated fields and buildings. One is sometimes tempted to compare them, on account of the formidable subterranean sounds that accompany them, and of their suddenness, to mine explosions; but the illustration would be badly chosen, for these movements have never caused a sudden rising of the soil, and there is nothing about them comparable to the disturbances produced by an explosion.
They are rather sinkings down, into a soil already cracked and partly disintegrated by the thermal waters, that have produced all these disasters which we now know have been greater in the neighborhood of the points where these springs are most active and most abundant. Casamicciola, where the hydro-thermal activity of the island is concentrated, has been destroyed forever, for prudence will demand that it never be rebuilt. A single house remains standing in the midst of that disorder of ruins and that accumulation of dead bodies that now cover the site of a watering-place once so prosperous and so thronged. The city of Ischia itself has suffered severely; Loco Ameno exists no more; Forio is almost in ruins; Porto d'Ischia has also been very much tried; and we might say that there is not one of those picturesque villas, hung upon the mountain-side, or hidden in the verdure of the valleys, that has not been damaged; and the number of victims buried under the mass of ruins will probably never be fully ascertained.
We shall have to go very far back in the history of the Neapolitan volcanoes to find an example of another such catastrophe. Since Herculaneum and Pompeii were buried under a cover of ashes and lava, the most recent great disaster we can at all compare with the destruction of Ischia is that of Potenza, which, in December, 1857, cost the lives of more than ten thousand persons. This was in Calabria—that is, in one of the provinces between Vesuvius and Etna, which have frequently been subjected to terrible disturbances.—Translated for the Popular Science Monthly from La Nature.