Volcanoes: What They are and What They Teach

by John W. Judd

Fig. 77.—Outlines of various Volcanoes, illustrating the different relations of the craters to cones.
Click on image to see original negative view.

SUBMARINE VOLCANOES.

While speaking of the varieties of form assumed by volcanic cones and craters, we must not forget to notice the effects which are produced by denuding forces upon them. In the case of submarine volcanoes, like the celebrated island called by the English Graham Isle, by the French Isle Julie, and by the Germans the Insel Ferdinandez ([fig. 78]), which was thrown up off the coast of Sicily in 1831, it was evident that volcanic outbursts taking place at some depth below the level of the sea gradually piled up a cone of scoriæ with a crater in its midst. By constant accessions to its mass, this scoria-cone was eventually raised above the sea-level, but the action of the waves upon the loose materials soon destroyed the crater-walls and eventually reduced the island to a shoal. It is evident that in all cases in which eruptions take place beneath the sea-level, and the loose materials are exposed during their accumulation to the beating of the sea-waves, the form of the volcanic cone so produced will be greatly modified by the interaction of the two sets of opposed causes, the eruptive forces from below and the distributive action of the sea-waves.

Fig. 78.—Island thrown up in the Mediterranean Sea in July and August 1881.
(The view was taken in the month of September, after the sides of the crater had been washed away by the waves.)

Craters when once formed are often rent across, along the line of the fissure above which they are thrown up. Thus the crater of Vesuvius was in 1872 rent completely asunder on one side, so that it was possible to climb through the fissure thus produced and reach the bottom of the crater. Streams flowing down the sides of the crater, and escaping through such a rent, may in the end greatly modify the form and disguise the characters of a volcanic crater. Of this kind of action we have a striking example in the Val del Bove of Etna.

Volcanoes, as we shall point out in the sequel, are after their extinction frequently submerged beneath the waters of the ocean. The sea entering the craters, eats back their cliff-like sides and enlarges their areas. Such denuded waters are called 'calderas,' the channels into them 'barrancos.'

Sometimes the action of the waves upon a partially submerged volcano has led to the cutting back of its slopes into steep cliffs, at the same time that the crater-ring is enlarged. In such cases we have left a more or less complete rocky ring, composed of alternating lavas and fragmentary materials. Of such a ruined crater-ring, the Island of St. Paul in the South Atlantic affords an admirable example.

When the action of denudation has gone still further, all the lavas and tuffs composing the cone may be completely removed and nothing left but masses of the hard and highly-crystalline rocks which have cooled down slowly in the heart of the volcano. An example of this kind is afforded to us by St. Kilda, the remotest member of the British Archipelago.

But although the majority of volcanic craters are clearly formed by explosive action, there are some craters, like those of Kilauea in Hawaii, which probably owe their origin to quite a different set of causes. In this case the explosive action at the vent is but slight, and the crater, which is of very irregular form, appears to have originated in a fissure, which has been slowly enlarged by the liquid lavas encroaching upon and eating away its sides. Such craters as these, however, appear to be comparatively rare.