Volcanoes: What They are and What They Teach

by John W. Judd

RELATIONS TO MOUNTAIN-CHAINS.

Another fact concerning the distribution of volcanoes which is worthy of remark is their relation to the great mountain-ranges of the globe.

Many of the grandest mountain-chains have bands of volcanoes lying parallel to them. This is stinkingly exhibited by the great mountain-masses which lie on the western side of the American continent. The Rocky Mountains and the Andes consist of folded and crumpled masses of altered strata which, by the action of denuding forces, have been carved into series of ridges and summits. At many points, however, along the sides of these great chains, we find that fissures have been opened and lines of volcanoes formed, from which enormous quantities of lava have flowed and covered great tracts of country. At some parts of the chain, however, the volcanoes are of such height and dimensions as to overlook and dwarf the mountain-ranges by the side of which they lie. Some of the volcanoes lying parallel to the great American axis appear to be quite extinct, while others are in full activity.

In the Eastern continent we find still more striking examples of the parallelism between great mountain-chains and the lands along which volcanic activity is exhibited. Stretching in a more or less continuous chain from east to west, through Europe and Asia, we find the mountain-masses known in different parts of their course as the Pyrenees, the Alps, the Balkan, the Caucasus, which form the axis of the Eastern continent. These chains consist of numerous parallel ridges, and give off branches on either side of them. They are continued to the eastward by the Hindoo Koosh and the Himalaya, with the four parallel ranges that cross the great Central-Asian plateau. Now, on either side of this grand axial system of mountains, we find a great parallel band of volcanoes. The northern volcanic band is constituted by the eruptive rocks of the Auvergne, the Eifel, the Siebengebirge, Central Germany, Bohemia, Hungary, and Transylvania, few, if any, of the vents along this northern band being still active. The remarkable volcanoes of the Thian Shan range and of Mantchouria may not improbably be regarded as a continuation of the same great series.

The southern band of volcanoes, lying parallel to the great mountain axis of the Old World, also consists for the most part of extinct volcanoes, but includes not a few vents which are still active. In this band we include the extinct volcanoes of Spain and Sardinia, the numerous extinct and active vents of the Italian peninsula and islands, and those of the Ægean Sea and Asia Minor. We may, perhaps, consider the scattered volcanoes of Arabia and the northern part of the Indian Ocean as a continuation of the same series. Both of these bands may be regarded as offshoots from the great mid-Atlantic volcanic chain, and the condition of the vents, both in the principal band and its offshoots, is such as to indicate that they form parts of a system which is gradually sinking into a state of complete extinction.

There are some other volcanic bands which exhibit a similar parallelism with mountain chains; but, on the other hand, there are some volcanoes between which and the nearest mountain axes no such connection can be traced.

RELATION TO AREAS OF UPHEAVAL.

There is yet one other fact concerning the mode of distribution of volcanoes upon the surface of the globe, to which we must allude. It was first established by Mr. Darwin as one of the conclusions derived from the valuable series of observations made by him during the voyage of H.M.S. 'Beagle,' and relates to the position of active volcanoes with respect to the portions of the earth's crust which are undergoing upheaval or subsidence.

From the relative position of the different kinds of coral-reefs, and the fact that reef-forming corals cannot live at a depth of more than twenty fathoms beneath the sea-level, or above tide-mark, we are led to the conclusion that certain areas of the earth's surface are undergoing slow elevation, while other parts are as gradually subsiding. This conclusion is confirmed by the occurrence of raised beaches, which are sometimes found at heights of hundreds, or even thousands, of feet above the sea-level, and of submerged forests, which are not unfrequently found beneath the waters of the ocean.

By a study of the evidences presented by coral-reefs, raised beaches, submerged forests, and other phenomena of a similar kind, it can be shown that certain wide areas of the land and of the ocean-floor are at the present time in a state of subsidence, while other equally large areas are being upheaved. And the observations of the geologist prove that similar upward and downward movements of portions of the earth's crust have been going on through all geological times. Now, as Mr. Darwin has so well shown in his work on 'Coral-Reefs,' if we trace upon a map the areas of the earth's surface which are undergoing upheaval and subsidence respectively, we shall find that nearly all the active volcanoes of the globe are situated upon rising areas, and that volcanic phenomena are conspicuously absent from those parts of the earth's crust which can be proved at the present day to be undergoing depression.