Volcanoes: What They are and What They Teach

by John W. Judd

A third series of volcanoes starts from near the last in the neighbourhood of Behring's Straits, and stretches along the whole western coast of the American continent. In this great range there are about eighty active volcanoes.

LINEAR ARRANGEMENT OF VOLCANOES.

In considering the facts connected with the distribution of volcanoes upon the globe, the one which, by its striking character, seems to demand our attention in the first instance is that of the remarkable linear arrangement of volcanic vents. We have already seen that small scoria-cones are often thrown up on the flanks, or at the base, of a great volcanic mountain, along lines which are manifestly lines of fissure. In the eruption of Etna, in 1865, and again in that of 1874, Professor Silvestri, of Catania, witnessed the actual opening of great fissures on the north-east and north sides of the mountain: and along the bottom of these cracks the glowing lava was clearly visible ([fig. 84], page 194). In the course of a few days, there were thrown up a number of small scoria-cones along these lines of fissure—those formed on the fissure of 1865 being seven in number, and those on the fissure of 1874 being no less than thirty-six in number. Precisely familiar phenomena were witnessed upon the slopes of Vesuvius, in 1760, when a fissure opened on the south side of the mountain, and fifteen scoria-cones, which are still visible, were thrown up along it.

We have already considered the evidence pointing to the conclusion that systems of volcanoes, like that of the Lipari Islands, are similarly ranged along lines of fissures, and there is equally good ground for believing that the great linear bands of volcanoes, which, as we have seen, stretch for thousands of miles, have had their positions determined by great lines of fissure in the earth's crust. While, however, the smaller fissures, upon which rows of scoria-cones are thrown up, seem to have been in many cases opened by a single effort of the volcanic forces, the enormous fissures, which traverse so large a portion of the surface of the globe, are doubtless the result of numerous manifestations of energy extending over vast periods of time.

The greatest of these bands along which the volcanic forces are so powerfully exhibited at the present day, is the one which stretches from near the Arctic circle at Behring's Straits to the Antarctic circle at South Victoria. The line followed by this volcanic band, which, as we have seen, includes more than one half of the active volcanoes of the globe, is a very sinuous one, and it gives off numerous offshoots upon either side of it. The great focus of this intense volcanic action may be regarded as lying in the district between the islands of Borneo and New Guinea. From this centre there radiate a number of great lines, along which the volcanic forces are exhibited in the most powerful manner. The first of these extends northwards through the Philippine Isles, Japan, the Kurile Islands, and Kamtschatka, giving off a branch to the east, which passes through the Aleutian Islands and the peninsula of Alaska. This band, along which the volcanic forces are very powerfully active, is continued towards the south-east in the New Britain, the Solomon Islands, Santa Cruz, the New Hebrides, New Zealand, and South Victoria. East and west from the great central focus there proceed two principal branches. The former of these extends through the Navigator Islands and Friendly Islands as far as Elizabeth Islands. The latter passes through Java, and then turns north-westward through Sumatra, the Nicobar Islands, the Andaman Islands up to the coast of Burmah.

The great band which we have been describing exhibits the most striking examples of volcanic activity to be found upon the globe. Besides the 150 or more volcanoes which are known to have been in a state of activity during the historical period, there are several hundred very perfect volcanic cones, many of which appear to have but recently become extinct, if indeed, they are not simply in a dormant condition. For long distances these chains of volcanic cones are almost continuous, and the only very considerable breaks in the series are those between New Zealand and the New Hebrides on the one hand, and between the former islands and South Victoria on the other.

GREAT VOLCANIC BANDS OF THE GLOBE.

Much less continuous, but nevertheless very important, is the great band of volcanoes which extends along the western side of the great American continent, and contains, with its branches, nearly a hundred active volcanoes. On the north this great band is almost united with the one we have already described by the chain of the Aleutian and Alaska volcanoes. In British Columbia about the parallel of 60° N. there exist a number of volcanic mountains, one of which, Mount St. Elias, is believed to be 18,000 feet in height, and several of these have certainly been seen in a state of eruption. Farther south in the part of the United States, territories drained by the Columbia River, a number of grand volcanic mountains exist, some of which are probably still active, for geysers and other manifestations of volcanic activity abound. From the southern extremity of the peninsula of California an almost continuous chain of volcanoes stretches through Mexico and Guatemala, and from this part of the volcanic band a branch is given off which passes through the West Indies, and forms a connection with the great volcanic band of the Atlantic Ocean. In South America the line is continued by the active volcanoes of Ecuador, Bolivia and Chili, but at many intermediate points in the chain of the Andes extinct volcanoes occur, which to a great extent fill up the gaps in the series. A small offshoot to the westward passes through the Galapagos Islands. The great band of volcanoes which stretches through the American continent is second only in importance, and in the activity of its vents, to the band which divides the Pacific from the Indian Ocean.

The third volcanic band of the globe is that which traverses the Atlantic Ocean from north to south. This series of volcanic mountains is much more broken and interrupted than the other two, and a greater proportion of its vents are extinct. This chain, as we shall show in a future chapter, attained its condition of maximum activity during the distant period of the Miocene, and now appears to be passing into a state of gradual extinction. Beginning in the north with the volcanic rocks of Greenland and Bear Island, we pass southwards, by way of Jan Mayen, Iceland, and the Faroe Islands, to the Hebrides and the north of Ireland. Thence by way of the Azores, the Canaries and the Cape de Verde Islands, with some active vents, we pass to the ruined volcanoes of St. Paul, Fernando de Noronha, Ascension, St. Helena, Trinidad and Tristan d'Acunha. From this great Atlantic band two branches proceed to the eastward, one through Central Europe, where all the vents are now extinct, and the other through the Mediterranean to Asia Minor, the great majority of the volcanoes along the latter line being now extinct, though a few are still active. The vol canoes on the eastern coast of Africa may be regarded as situated on another branch from this Atlantic volcanic band. The number of active volcanoes on this Atlantic band and its branches, exclusive of those in the West Indies, does not exceed fifty.

LENGTH OF THE VOLCANIC BANDS.