Volcanoes: What They are and What They Teach

by John W. Judd

VOLCANIC FISSURES OF PERMIAN PERIOD.

From the great fissures opened in Permian times along this line of weakness, great quantities of lava, scoriæ, and tuff were poured out, and these accumulated to form great volcanic mountains, which we can now only study at a few isolated spots, as in the Tyrol, Carinthia, and about Lake Lugano. Everywhere else, these Permian volcanic rocks appear to be deeply buried under the later-formed sediments, from which the Alpine chains have been carved. Few and imperfect, however, as are the exposures of these ancient rhyolite and quartz-andesite lavas and agglomerates formed at the close of the Palæozoic epoch, their greatly denuded relics form masses which are in places more than 9,000 feet in thickness. From this fact we are able to form some slight idea of the scale upon which the volcanic outbursts in question must have taken place during Permian times.

The second stage in the series of operations by which the Alpine chains have been formed, consisted in a general sinking of the surface along that line of weakness in the earth's crust, the existence of which had been betrayed by the formation of fissures and the eruption of volcanic rocks. We have already had occasion to remark how frequently such subsidences follow upon the extrusion of volcanic masses at any part of the earth's surface; and we have referred these downward movements in part to the removal of support from below the portion of the crust affected, and in part to the weight of the materials piled upon its surface by the volcanic forces.

The volcanic energy which had been manifested with such violence during the Permian period, does not appear to have died out altogether during the succeeding Triassic period. A number of smaller volcanic vents were opened from time to time, and from these, lavas, tuffs, and agglomerates, chiefly of basic composition, were poured out. The relics of these old Triassic volcanoes are found at many points along the Alpine chain, but it is evident that the igneous forces were gradually becoming exhausted during this period, and before the close of it they had fallen into a state of complete extinction.

But the great subsidence which had commenced in the Triassic period, along what was to become the future line of the Alpine chain, was continued almost without interruption during the Rhætic, the Jurassic, the Tithonian, the Neocomian, the Cretaceous and the Nummulitic periods. With respect to the strata formed during all these periods, it is found that their thiknesses, which away from the Alpine axis may be measured by hundreds of feet, is along that axis increased to thousands of feet. The united thickness of sediments accumulated along this great line of subsidence between the Permian and Nummulitic periods probably exceeds 60,000 feet, or ten miles. The subsidence appears to have been very slow and gradual, but almost uninterrupted, and the deposition of sediments seems to have kept pace with the sinking of the sea-bottom, a fact which is proved by the circumstance that nearly the whole of these sediments were such as must have been accumulated in comparatively shallow water.

FORMATION OF ALPINE GEOSYNCLINAL.

By the means we have described there was thus formed a 'geosynclinal,' as geologists have called it, that is, a trough-like hollow filled with masses of abnormally thickened sediments, which had been piled one upon another during the long periods of time in which almost uninterrupted subsidence was going on along the Alpine line of weakness in the earth's crust. In this way was brought together that enormous accumulation of materials from which the hard masses of the Alpine chains were subsequently elaborated, and out of which the mountain-peaks were eventually carved by denudation.

The third stage in this grand work of mountain-making commenced in the Oligocene period. It consisted of a series of movements affecting the parts of the earth's crust on either side of the line of weakness which had first exhibited itself in Permian times. By these movements a series of tangential strains were produced, which resulted in the violent crushing, folding, and crumpling of the sedimentary materials composing the geosynclinal.

One effect of this action was the violent flexure and frequent fracture of these stratified masses, which are now found in the Alpine regions assuming the most abnormal and unexpected positions and relations to one another. Sometimes the strata are found tortured and twisted into the most complicated folds and puckerings; at others they are seen to be completely inverted, so that the older beds are found lying upon the newer; and in others, again, great masses of strata have been traversed by numerous fractures or faults, the rocks on either side of which are displaced to the extent of thousands of feet.

Another effect of the great lateral thrusts by which the thick sedimentary masses of the geosynclinal were being so violently disturbed, was the production of a great amount of induration and chemical change in these rocks. Masses of soft clay, of the age of that upon which London is built, were by violent pressure reduced to the condition of roofing-slate, similar to that of North Wales. One of the most important discoveries of modern times is that which has resulted in the recognition of the fact of the mutual convertibility of different kinds of energy. We now know that mechanical force may be transformed into heat-force or chemical force; and of such transformations we find abundant illustrations in the crushed and crumpled rock-masses of the Alpine chains.