« PreviousContinue »
islands, where a train of volcanic mountains exists, nine of which are known to have been in eruption; and elevations of the bed of the sea from earthquakes have occurred several times since the middle of the last century. The line is next continued through the Japanese group, which contains a number of active volcanoes and is continually liable to earthquakes. Proceeding southward, the chain is continued through the islands of the East Indian Archipelago. Mountain ranges of a volcanic character traverse almost all these, some rising upwards of 12,000 feet in height. In Sumatra, four volcanoes occur, and also several in Java. The largest of the Mollucca group, Celebes, contains a number of volcanoes in a state of activity, and one of the most terrible eruptions ever recorded happened on the island of Sumbawa another of this group. Here the chain branches off eastward and westward, passing to the west through New Guinea, New Britain the Solomon group, and the New Hebrides, thence through the Friendly and Society Islands nearly east. Indeed the Pacific Ocean in the equatorial regions seems to have been one vast theatre of igneous action, its innumerable archipelagos being composed of volcanic rocks, or coralline limestones with active vents here and there. To the westward, the chain passes through Borneo, and Sumatra, to Barren Island in the Bay of Bengal. From Java southward, the chain may be traced along the coast of New Holland and Van Diemens land, and thence probably is a submarine connection with Freeman's Peak, in the Ballerny Isles, on the Antarctic continent. Still farther south we have the chain extending along Victoria land, between 800 and 70° of south latitude, connecting with Mounts Erebus and Terror before mentioned. Another great chain of mountains runs nearly east and west from the shores of the Caspian sea to the Atlantic, passing through Turkey, Austria, Italy, Switzerland, France, and Spain. The whole region along this chain, which sends off many lateral branches, is subject to earthquakes and other volcanic phenomena; the well known volcanoes Etna, and Vesuvius, are a part of this chain. In addition to the volcanic chains we have named, there are some cases of isolated volcanic action, such as Mount Hecla in Iceland, and the volcanoes of Madagascar.
IGNEOUS CAUSES OF CHANGE.
“But, even then, the ground
The number of active volcanoes, and solfatares or vents, from which sulphureous and acid vapors and gases are given off, is about 305; of these, 196 are in islands, and the other 109, are on continents. It is however, a remarkable fact that a majority of them are located near the ocean, or large bodies of water; and even submarino volcanoes are not of unfrequent occurrence. Besides the volcanoes now in action, there are many undisputable extinct volcanoes, i. e., volcanoes which at some period of the earth's existence, but before the historic era, have been in the state of active eruption. In no country is there better evidence of this than in France. There are in the districts of Auvergne, Vivarais, and Cervennes, more than a hundred conical mountains, composed of lava, scoriæ, and volcanic ashes heaped up, many of them still retaining their ancient craters, and in some cases currents of lava may be traced to great distances. The evidences of volcanic action in the Rocky Mountains we have already alluded to.
How long a period of repose may be necessary to constitute an extinct volcano, is of course undetermined. We include as such, those which show indabitable evidence of former activity, but which have not had eruptions within the historic era. It is by no. means necessary that volcanoes, to be considered active, should incessantly emit flames, they may remain for ages choked up,
and again suddenly resume all their former character. Thus Vesuvius, which had been extinct from time immemorial, although its crater was clearly formed by some ancient volcanic action; suddenly rekindled in the reign of Titus, and buried the cities of Herculaneum, Pompeii, and Stabiæ, under its ashes. After this effort it again slumbered, the memory of its former power faded away; trees and grass grew on its summit, when suddenly in 1630, it renewed its action. . At this time, the crater, according to the account of Bracini, who visited Vesuvius not long before the eruption of that year, “was five miles in circumference, and about a thousand paces deep; its sides were covered with brushwood, and at the bottom there was a plain on which cattle grazed. In the woody parts wild boars frequently liarbored. In one part of the plain, covered with ashes, were three small pools, one filled with hot and bitter water, another salter than the sea, and a third hot but tasteless.” Suddenly, in December 1630, these forests and grassy plains were blown into the air, and their ashes scattered to the winds ; seven streams of lava poured at the same time from the crater, and overflowed several villages at the foot, and on the side of the mountain; since that time there has been a constant series of eruptions. Etna after slumbering for ages, burst forth and destroyed the city of Catania; the accounts of its previous eruptions having been considered by the inhabit. ants as fables.
Subterranean noises, and the appearance, or increase of smoke, are the first symptoms of approaching volcanic action. This is soon 'accompanied by a trembling of the earth, and louder noises; the air darkens, and the smoke, thick with fine ashes, increases.. The stream of smoke rises like an immense black shaft, high up into the air, and arriving at a point where its density is the same as the atmosphere, spreads out like a vast umbrella, overshadowing the whole country with its dark gloom. Such was the appearance as described by Pliny, the Elder, who witnessed the eruption of Vesuvius which overwhelmed Pompeii, in A. D. 79. Occasionally, lightning flashes illuminate the dark cloud, and streams of red hot sand, like flames, shoot up into the sky, attended with loud explosions. The shocks, and tremblings of the
ground, increase, and the whole neighborhood gives evidence of the immense pressure which is being exerted; presently the molten lava, is by the immense force raised into the crater, and filling it up, or melting its passage through the side, flows in a red hot stream down the flanks of the mountain in a river, or rather . a torrent of fire. The eruption is sometimes attended with enormous currents of water, mud, and noxious gasses. A period of rest succeeds, generally of short duration; again the same phenomena are repeated, and thus the action continues for a variable length of time, until finally, the crisis is past and the volcano resumes its original quiet.
The substances principally ojected by volcanoes are smoke, ashes, sand, scoriæ, volcanic glass and bombs, and masses of rock. The ashes thrown out in volcanic eruptions appear to be the substance of the lava very finely divided. These ashes are raised so high that they are carried by the winds to almost incredible distances. Ashes from the eruption of a volcano in St. Vincent in 1812, were carried twenty leagues, and fell in Barbadoes, and from the eruption of Hecla in 1766, they fell in Glaumba, a distance of fifty leagues; and it is said that ashes from Vesuvius have fallen in Constantinople, a distance of four hundred and fifty leagues. The volcanic sand, is cornposed of particles somewhat larger, but of the same character as the ashes, being comminated particles of lava, and forming a principal part of the ejected matter of volcanic eruptions. Scoriæ, and pumice stone, are caused by the gasses, which bursting through the melted lava, carry up with them certain portions into the atmosphere, which becoming consolidated, present the appearance so well known under the name of slag and cinders. Volcanic glass or obsidian, is often ejected in small melted masses ; sometimes, the winds catching this, spin it into the finest threads. We have seen many specimens of this kind from the eruptions of Kirauea, in the Sandwich Islands. Among the extinct volcanoes of France, drops, tears, and elongated spheroids, being drops of lava thrown out, and consolidated in the air, are continnally found, they are called yolcanic boinb. Masses of rock are always ejected in severe eruptions; in many cases these are undoubtedly torn off from the interior of the mountain by the immense power exerted; and they are ejected without having been melted. A stone of 109 cubic yards in volume, was ejected by Cotopaxi, and thrown to a distance of nine miles.
The force which is exerted, to cause the eruptions of lavas, or liquid masses of stone, is almost beyond belief, varying according to the height of the crater. The force of Vesuvius in some of its eruptions has been estimated as equivalent to a pressure of at least 6000 pounds on every square inch ; and of Etna, about 17,000 pounds on the square inch ; the amount of force roquisite to raise melted lava to the crater of Cotopaxi, would be at least 30,000 pounds on each square inch. The masses of melted matter ejected, are equally incredible ; the amount thrown out by Vesuvius in 1737, was estimated at 11,839,168 cubic yards, and about twice this amount in 1794. In 1660, tho mass of matter disgorged by Etna, according to Mr. Lyell, was twenty times greater than the whole mass of the mountain, and in 1669, when 77,000 persons were destroyed, the lava covered 84 square miles. The greatest eruption of modern times, was from Skaptar Jokul, in Iceland, in 1783. Two streams of lava, one fifty miles long and twelve broad, the other forty miles long, and seven bioad; both avaraging 100 feet in thickness, and sometimes 500 or 600 feet, flowed in opposite directions, destroying twenty villages, and 9000 inhabitants. The velocity with which the melted lavas move varies with the slope of the mountain, and the nature of the ground, as well as the viscidity and quantity of the lava. In general, a velocity of 400 yards an hour is considered quick, although sometimes the stream flows much quicker; in flat grounds it sometimes occupies whole days in moying a few yards. Lavas cool extremely slow, the surface becomes soon consolidated, and is such a poor conducter of heat, that the interior remains heated and melted for whole years; and currents have been mentioned which were flowing ten years after emerging from the crater, and they have been seen smoking twenty years after an eruption of Etna. The currents of lava thrown out by successive eruptions being placed one above the other, alternating with beds of sand, scoriæ, &c., form a series of inclined beds that give rise to the cone of the mountain,