The Subterranean World

by G. Hartwig

As the point where a lava-current finds a vent is often situated at a considerable distance below the surface of the liquid column in the internal chimney of the volcano, the pressure from above not seldom causes the lava to spout forth in a jet, until its level in the crater shaft has been reduced to that of the newly-formed orifice. Thus, when Vesuvius was rent by the dreadful paroxysmal eruption of 1794, the lava was seen to shoot up in magnificent fountains as it issued from the openings along the fissure.

Further on, the lava flows down the mountain’s side according to the same laws which regulate the movements of any other stream, whether of water, mud, or ice: more rapidly down an abrupt declivity, slower where the slope is more gradual; now accumulating in narrow ravines, then spreading out in plains; sometimes rushing in fiery cascades down precipices, and, where insurmountable obstacles oppose its progress, not seldom breaking off into several branches, each of which pursues its independent course.

At the point where it issues, the lava flows in perfect solution, but, as its surface rapidly cools when exposed to the air, it soon gets covered with scoriæ, which are dashed over each other in wild confusion, by successive floods of liquid stone, so as to resemble a stormy sea covered with ice-blocks. But the liquefied stone not only hardens on its external surface; it also becomes solid below, where it touches the colder soil, so that the fluid lava literally moves along in a crust of scoriæ, which lengthens in the same proportion as the stream advances.

The movements of the lava-current are of course considerably retarded by the formation of scoriæ, so that, unless where a greater inclination of the soil gives it a new impulse, it flows slower and slower. Thus the lava-stream which was ejected by Etna during the great eruption of 1669, performed the first thirteen Italian miles of its course in twenty days, or at the average rate of 162 feet per hour, but required no less than twenty-three days for the last two miles. While moving on, its surface was in general a mass of solid rock; and its mode of advancing, as is usual with lava streams, was by the occasional fissuring of the solid walls. Yet, in spite of the tardiness of its progress, the inhabitants of Catania watched its advance with dismay, and rushed into the churches to invoke the aid of the Madonna and the Saints. One citizen only, a certain Baron Papalardo, relied more upon his own efforts than upon supernatural assistance, and set out with a party of fifty men, dressed in skins to protect them from the heat, and armed with iron crows and hooks for the purpose of breaking open one of the solid walls of scoriæ that flanked the liquid current, so as to divert it from the menaced city. A passage was thus opened for a rivulet of melted matter, which flowed in the direction of Paterno; but the inhabitants of that town being alarmed for their safety, took up arms against Papalardo, whose fifty workmen would hardly have been able to cope with the powers of nature. Thus, slowly but irresistibly, the lava advanced up to the walls of Catania, which, being formed of huge Cyclopean blocks, and no less than sixty feet high, at first stemmed the fiery stream. But the glowing floods, pressing against the rampart, rose higher and higher, and finally reaching its summit, rushed over it in fiery cataracts, and destroying part of the town, at length disgorged themselves into the sea, where they formed a not inconsiderable promontory.

A truly gigantic conflict might naturally be expected from the meeting of two such powerful and hostile bodies as fire and water. This, however, is by no means the case, for as soon as the lava enters the sea, the rapid evaporation of the water that comes into immediate contact with it accelerates the cooling of the surface and thickens the hard external crust to such a degree that very soon all communication is cut off between the water and the fiery mass. While the lava continues to advance from the land, the crust of scoriæ is prolonged in the same proportion, and should it be rent here and there, steam is at once developed with such violence as to prevent all further access of the water into the interior of the fissures. Thus, Breislak informs us that, in 1794, the eruption of a lava-stream into the Bay of Naples, near Torre del Greco, took place with the greatest tranquillity, so that he himself was able to observe the advancing of the lava into the sea while seated in a boat immediately near it, without being disturbed by explosions or any other violent phenomenon.

As the crust of scoriæ is so bad a conductor of heat, it occasions a very slow cooling and hardening in the interior of the lava-stream, forming as it were a vessel in which the liquid fire can be retained and preserved for a long time. When Elie de Beaumont visited the lava-stream of Etna, nearly two years after its eruption in 1832, its interior was still so warm that he could not hold his finger in the hot steam issuing from its crevices. It has also been proved, on trustworthy evidence, that after twenty-five and thirty years, many lava-streams of Etna still continued to emit heat and steam; and after twenty-one years it was possible to light a cigar in the crevices of the lava that issued from Jorullo in 1759.

Another extremely curious effect of the scoriæ being such bad conductors of heat is, that masses of snow will remain unmelted, though a lava-stream rolls over them. Thus, in 1787, the lava of Etna flowed over a large deposit of snow, which, however, was by no means fully liquefied, but remained for the greatest part entire, and gradually changed into a granular and solid mass of ice. This was traced in 1828, by the geologist Gemellaro, for a distance of several hundred feet under the lava, and most likely still reposes under it as in an ice-cellar. The cliffs which form the vast crater-ring of the Isle of Deception, in the extreme Southern Atlantic, are likewise composed of alternate layers of ice and lava. Probably in both these cases the ice-beds had been covered before the lava flowed over them, by a rain of scoriæ and volcanic sand, which is so well known among the shepherds in the higher regions of Etna as a bad conductor of caloric, that, to obtain a supply of water for their herds during the summer, they cover some snow a few inches deep with volcanic sand, which entirely prevents the penetration of solar heat.

Most of the recent lava-streams evolve from all their fissures and rents a quantity of vapour, so as to be dotted with innumerable fumaroles, and to exhibit, as they flow along, a smoking surface by day and a luminous one by night. At first these fumaroles are so impetuous that they frequently puff up the lava-crust around their orifices into little cones or hillocks, consisting of blocks of scoriæ irregularly piled up over each other, and from whose summit the vapours continue to ascend. As the mass cools, they are naturally lessened in numbers and in power; but in 1803 Humboldt still saw fumaroles from twenty to thirty feet high, rising from the small cones which covered by thousands the great lava-stream of Jorullo of the year 1759.

The vast dimensions of single lava-streams give proof of the enormous powers which forced them out of the bowels of the earth. The lava-stream of Vesuvius which destroyed Torre del Greco in 1794, is 17,500 French feet long, and when it reached the town was more than 2,000 feet wide and forty feet deep. While this mighty mass of molten stone, the volume of which has been reckoned at about 457 millions of cubic feet, was descending towards the sea, another stream, whose mass is computed at about one-half of that of the former, was flowing in the direction of Mauro. This single eruption has therefore furnished more than 685 millions of cubic feet of lava, equal to a cube of 882 feet, in which at least a dozen of the largest churches, palaces, and pyramids on earth might conveniently find room. If to the solid lava we add the astonishing quantities of scoriæ, sand, and ashes thrown out by this same eruption, we may form some idea of the masses of matter which were in this one instance ejected from the interior of the earth.

The volume of the lava-stream which flowed from the volcano of the Isle of Bourbon in the year 1787 is estimated at 2,526 millions of cubic feet; but even this astonishing ejection of molten stone is surpassed by that which took place during the eruption of Skaptar Jökull[^[9]] in 1783, when the lava rolled on to a length of fifty miles, and, on reaching the plain, expanded into broad lakes, twelve and fifteen miles in diameter and a hundred feet deep.