Volcanoes: What They are and What They Teach

by John W. Judd

This argument in favour of the production of volcanic outbursts by the irruption of sea-water into subterranean reservoirs, involves, as Mr. Scrope long ago pointed out, a curious example of reasoning in a circle. It is assumed, on the one hand, that the heaving subterranean movements, which give rise to the fissures by which steam and other gases escape to the surface, are the result of the passage of water to heated masses in the earth's crust. But, on the other hand, it is supposed that it is the production of these fissures which leads to the influx of water to the heated materials. If it is the passage of water through these fissures which produces the eruptions, it may be fairly asked, what is it that gives rise to the fissures? And if, on the other hand, there exist subterranean forces competent to produce the fissures, may they not also give rise to the eruptions through the openings which they have originated? Nor does the chemical argument appear to rest upon any surer ground. It is true that many of the volatile substances emitted from volcanic vents are such as might be produced by the decomposition of sea-water, but, upon the other hand, there are not a few substances which cannot possibly be regarded as so produced, and, all the materials may equally well be supposed to have been originally imprisoned in the masses of subterranean lava.

CAUSE OF PROXIMITY OF VOLCANOES TO SEA.

The problem before us is this. Granting that it is proved that active volcanoes are always in close proximity to the ocean, are we to explain the fact by supposing that the agency of sea-water is necessary to volcanic outbursts, or by regarding the position of the coast-lines as to some extent determined by the distribution of volcanic action upon the surface of the globe? The first supposition is the one which perhaps most readily suggests itself, but the latter, as we shall hereafter show, is one in favour of which not a few weighty arguments may be advanced.

Another problem which suggests itself in connection with the distribution of volcanoes is the following. Are the great depressed tracts which form the bottom of the oceans, like the elevated tracts which constitute the continents, equally free from exhibitions of volcanic energy?

When we remember the fact that the area of the ocean beds is two and three-quarter times as great as that of the continents, it will be seen how important this question of the existence of volcanoes at the bottom of the ocean really is.

The fact that recent deep-sea soundings have shown the deepest parts of the ocean to be everywhere covered with volcanic débris is by no means conclusive upon this question; for, as we have seen, the ejections of sub-aerial volcanoes are by the wind and waves distributed over every part of the earth's surface.

SUBMARINE ERUPTIONS.

Submarine volcanic outbursts have occurred in many parts of the globe, but it may well be doubted whether any such outburst has ever commenced at the bottom of a deep ocean, and has succeeded in building up a volcanic cone reaching to the surface. Most, if not all, of the recorded submarine outbursts have occurred in the midst of volcanic districts, and the volcanic cones have been built up in water of no great depth. Indeed, when it is remembered that the pressure of each 1,000 fathoms of water is equivalent to a weight of more than one ton on every square inch of the ocean-bottom, it is difficult to imagine the ordinary explosive action of volcanic vents taking place at abysmal depths. If, however, fissures were opened in the beds of the ocean, quiet outwellings of lava might possibly occur.

The solution of this problem of the probable existence of volcanic outbursts on the floor of the ocean can only be hoped for from the researches of the geologist. The small specimens of the ocean-beds brought up by deep-sea sounding-lines, taken at wide distances apart, and including but a few inches from the surface, can certainly afford but little information upon the question. But the geologist has the opportunity of studying the sea-bottoms of various geological periods which have been upheaved and are now exposed to his view. It was at one time supposed by geologists that in the so-called 'trap-rocks' we have great lava-sheets which must have been piled upon one another, without explosive action. But the more accurate researches of recent years have shown that between the layers of 'trap-rock,' in every part of the globe, traces of terrestrial surfaces and freshwater deposits are found; and the supposed proofs of the absence of explosive action break down no less signally upon re-examination; for the loose, scoriaceous materials would either be removed by denudation, or converted into hard and solid rocks by the infilling of their vesicles and air-cavities with crystalline minerals. It is not possible, among the representatives of former geological periods, to point to any rocks that can be fairly regarded as having issued from great submarine fissures, and it is therefore fair to conclude that no such great outbursts of the volcanic forces take plane at the present day on the deep ocean-floors.

In connection with the question of the relation between the position of the volcanic bands of the globe and the areas covered by the ocean, we may mention a fact which deep-sea soundings appear to indicate, namely, that the deepest holes in the ocean-floor are situated in volcanic areas. Near Japan, the soundings of the U.S. ship 'Tuscarora' showed that at two points the depth exceeded 4,000 fathoms; and the deepest sounding obtained by H.M.S. 'Challenger,' amounting to 4,575 fathoms, was taken in the voyage from New Gruinea to Japan, in the neighbourhood of the Ladrone Islands. Depths nearly as great were found in the soundings carried on in the neighbourhood of the volcanic group of the West Indian Islands. It must be remembered, however, that at present our knowledge of the depths of the abysmal portions of the ocean is very limited. A few lines of soundings, often taken at great distances apart, are all we have to guide us to any conclusions concerning the floors of the great oceans, and between these lines are enormous areas which still remain altogether unexplored. It may be wise, therefore, to suspend our judgment upon such questions till more numerous facts have been obtained.