Volcanoes: What They are and What They Teach

by John W. Judd

Under the influence of these several kinds of force, not only was extreme consolidation and induration produced among the rock-masses, but chemical affinity and crystalline action had the fullest play among the materials of which they were composed. In many cases we find the originally soft muds, sands, and shell-banks converted into the most highly crystalline rocks, which retain their primary chemical composition, but have entirely lost all their other original features.

FORMATION OF ALPINE GEANTICLINAL.

To the mass of folded, crumpled, and altered strata, formed from a geosynclinal by lateral pressure, geologists have given the name of a 'geanticlinal.' The formation of the Alpine geanticlinal was due to movements which commenced in the Oligocene period, attained their maximum in the Miocene, and appear to have declined and almost altogether died out in the Pliocene period.

The movements which resulted in the crushing and crumpling of the thickened mass of sediments along the Alpine line of weakness, also gave rise to the formation of a series of fissures from which volcanic action took place. These fissures were not, however, formed along the original line of weakness, for this had been strengthened and repaired by the deposition of ten-miles' thickness of sediments upon it, but along new fissures opened in directions parallel to the original lines of weakness, and in areas where a much less considerable amount of deposition had taken place since Permian times.

We have abundant evidence that, just at the period when those great movements were commencing which resulted in the formation of the great Alpine and Himalayan geanticlinal, earth-fissures were being opened upon either side of the latter from which volcanic outbursts took place. At the period when the most violent mountain-forming movements occurred, these fissures were in their most active condition, and at this time two great volcanic belts stretched east and west, on either side of, and parallel to, the great Alpine chain. The Northern volcanic band was formed by the numerous vents, now all extinct, in Auvergne, Central Germany, Bohemia, and Hungary, and was probably continued in the volcanoes of the Thian Shan and Mantchouria. The Southern volcanic band was formed by the numerous vents of the Iberian and Italian peninsulas, and the islands of the Mediterranean, and were continued to the eastward by those of Asia Minor, Arabia, and the North Indian Ocean. As the earth-movements which produced the geanticlinal died away, the volcanic energy along these parallel volcanic bands died away at the same time. In studying the geology of Central and Southern Europe, no fact comes out more strikingly than that of the synchronism between the earth-movements by which the geanticlinal of the Alps was formed, and the volcanic manifestations which were exhibited along lines of fissure parallel to that geanticlinal. The earth-movements and the volcanic outbursts both commenced in the Oligocene period, gradually attained their maximum in the Miocene, and as slowly declined in the Pliocene.

SCULPTURING OF ALPS BY DENUDATION.

The fourth stage in the great work of mountain-building in the case of the Alps consisted in the operation of the denuding forces, the disintegrating action of rain and frost, the transporting action of rivers and glaciers, by which the Alpine peaks were gradually sculptured out of the indurated and altered masses constituting the geanticlinal. The action of this fourth stage went on to a great extent side by side with that of the third stage. So soon as the earth-movements had brought the submerged sedimentary masses of the geosynclinal under the action of the surface tides and currents of the ocean, marine denudation would commence; and, as the work of elevation went on, the rock-masses would gradually be brought within the reach of those more silently-working but far more effective agents which are ever operating in the higher regions of the atmosphere. It is impossible to say what would have been the height of the Alpine chain if the work of denudation had not to a great extent kept pace with that of elevation. Only the harder and more crystalline masses have for the most part escaped destruction, and stand up in high craggy summits; while flanking hills, like the well-known Rigi, are Been to be composed of conglomerates thousands of feet in thickness, composed of their disintegrated materials. It is a remarkable fact, as showing how enormous was the work of elevation daring the formation of the geanticlinal, that some of the youngest and least consolidated rocks of the Nummulitic period are still found at a height of 11,000 feet in the Alps, and of 16,000 feet in the Himalaya.

From what has been said, it will be seen that mountain-chains may be regarded as cicatrised wounds in the earth's solid crust. A line of weakness first betrays itself at a certain part of the earth's surface by fissures, from which volcanic outbursts take place; and thus the position of the future mountain-chain is determined. Next, subsidence during many millions of years permits of the accumulation of the raw materials out of which the mountain-range is to be formed; subsequent earth-movements cause these raw materials to be elaborated into the hardest and most crystalline rock-masses, and place them in elevated and favourable positions; and lastly, denudation sculptures from these hardened rock-masses all the varied mountain forms. Thus the work of mountain-making is not, as was formerly supposed by geologists, the result of a simple upheaving force, but is the outcome of a long and complicated series of operations.

ORIGIN OF OTHER MOUNTAIN-CHAINS.

The careful study of other mountain-chains, especially those of the American continent, has shown that the series of actions which we have described as occurring in the Alps, took place in the same order in the formation of all mountain-masses. It is doubtful whether the line of weakness is always betrayed in the first instance by the formation along its course of volcanic fissures. But in all cases we have evidence of the production of a geosynclinal, which is afterwards, by lateral pressure, converted into a geanticlinal, and from this the mountain-chains have been carved by denudation. Professor Dana has shown that the geosynclinal of the Appalachian chain was made up of sediments attaining a thickness of 40,000 feet, or eight miles; while Mr. Clarence King has shown that a part of the geosynclinal of the Rocky Mountains was built up of no less than 60,000 feet, or twelve miles of strata.