Popular Science Monthly/Volume 26/November 1884/Chilian Volcanoes, Active and Extinct
|CHILIAN VOLCANOES, ACTIVE AND EXTINCT.|
THE products of the extinct and the still active volcanoes of Chili, of which Pissis enumerates not less than seventy, are of contemporary origin with the diluvial and alluvial strata of the country. Of the gaseous emissions of their craters, it need only be mentioned that, as in all American volcanoes, chlorine is weakly represented. The vapors consist chiefly of carbonic acid, sulphur compounds, and water. The present solid products do not differ greatly in composition from the trachytes of the past. Audesite, or a feldspathic mineral very nearly like it, is an important constituent of the porphyritic lavas of both active and quiet volcanoes. Olivine is found in the older and in the more recent lavas of Descobezado, Antuco, and Osorno, as well as in those of Juan Fernandez, and very probably in the liquid outflows of all the other craters. Other lavas occur, among them obsidian, pearl-stone, pitch-stone, and pumice, the last being quite abundant in the Cordilleras of Talea and Chilian. Lapilli cover the eastern flank of Osorno to a depth of about sixteen feet, and through it rose as late as 1851 the great, strong-limbed trunks of dead trees, whose thickness indicated an age of about one hundred and fifty years, while it had been about fifty years since the last eruption of the mountain. But it was a laborious task to trace the lava-stream under the flourishing new growth that had taken root in the weathered surface and in the crevices of the hard deposit.
Of the still active volcanoes, we may say that Atacama emitted smoke after the earthquake of May, 1877. The group of San José was active in 1833, threw rocks into the valley of Pinquenes in 1843, and has been again active since the 2d of March, 1881. Numerous crater-openings, with ancient lava-flows, are found in the same region. Tinguiririca, to the south of this region, consists almost wholly of trachyte, and has several solfataras about five thousand feet below its summit, whence issue vapors having a temperature of 194°; its thick deposit of sulphur has caused it to be given the name of Morro de Azufre, or Sulphur Mountain. More important still is the volcano of Petesoa, at the outbreak of which, on the 3d of December, 1762, the district was desolated with lava and ashes, and the Rio Lontue was dammed up for ten days. Its last eruption, in February, 1837, was followed by destructive floods in the lower-lying regions, caused by the sudden melting and precipitation of the snows from its summit. An immense horizontal ice-cap now lies in the crater, whence rise vertically isolated columns of smoke that can be seen from a considerable distance. Beyond this is the volcanic center of Descobezado, in the southern part of which a solfatara opened in 1847 that kept the district trembling for many years.
The present region of active commotion begins at 36° 50' with the volcano of Chilian. A new crater opened northeast of the principal peak of this mountain on the 2d of August, 1861, the flow from which melted the snow and caused great floods. The streams, saturated with ashes, became rivers of mud, and covered the plains with a coating of black. The crater did not become quiet for a year, and then only to break out again in 1864 with increased violence. The dark column of smoke that rose from the crater was visible for miles around, the ash-rain was more formidable than in 1861, and the detonations were heard and window-sashes were shaken at Concepcion. After a short period of rest, in January, 1865, its activity was again resumed. Antuco is only a few miles south of Chilian. It was visited by Pöppig in 1827, and by Domeyko in 1845, while it was in full activity, and it still sends up faint columns of smoke. The Imperial, or Yaima, in Araucania, was in action in 1852 and 1864, but has since not given any sign of an eruption. An eruption was observed in Yillarica about 1860, but nothing since; and the fact that the snow on the top of the mountain does not exhibit any marks of change indicates that its forces are weak. Next to this volcanic center comes Osorno, to which may be added others farther south that have not yet been accurately observed. Among these is one at the southern end of Middle Island, in the Strait of Magellan, which the men of the English ship-of-war Penguin saw at the end of 1877 in full activity. Heavy subterranean rumblings are no rarity in any part of Chili.
In all only ten known Chilian craters can be pronounced with certainty to be now active volcanoes. Obviously the neighborhood of these subterranean furnaces can not be regarded as belonging to the quiet regions of the earth. No part of the earth's surface is so prolific of earthquakes as the western half of South America; and here they are more frequent and severe on the Pacific coast than on the eastern side of the Andes. On this coast they are often accompanied with scenes of horror and woe that surpass description. To the direct consequences in the loss of life and the destruction of buildings are frequently added the ravages of fire breaking out in the ruins and consuming all that has not been already crushed. The seaport towns are exposed to a still further danger of destruction by the rushing tidal-wave which follows the extraordinary retreat of the waters with which the earthquake phenomena are usually accompanied. These evils and more were suffered in their worst form during and after the earthquakes of March, 1881, with which Mendoza was visited, and August, 1868, which laid waste a considerable stretch of the coast, with many towns.
With these volcanic and seismic phenomena is associated a steady elevation of the Chilian coast, which has amounted, according to the indications of the shore-terraces, to from six metres at Cape Three Mountains, to three hundred and ninety-seven metres at Concepcion, within the present geological period. Darwin has averaged the rate at about seventeen centimetres a year. The Island of Santa Maria, in the northwest of the Bay of Arauco, rose three metres during the earthquake of 1835, but afterward sank to its old level. Depressions also seem to have taken place in former periods. The elevating force is more intensive in the Chilian Andes than in the neighboring countries, and, as it is still in operation, it is destined probably to carry the loftiest peaks of the Cordilleras to a still greater height. The frequent occurrence of the ending huapi—Indian for island—in the names of promontories, indicates that many former islands are now connected with the mainland. The islands of Imeleb and Quehui, in Chiloe, are at present separated only at high water, and appear to be approaching a permanent union.—Translated for the Popular Science Monthly from Die Natur.