Volcanoes: What They are and What They Teach

by John W. Judd

While Spallanzani was engaged in investigating the nature of the action going on at Stromboli and other Italian volcanoes, his contemporary Dolomieu was laying the foundation of another important branch of vulcanology by studying the characters of the different materials of which volcanoes are built up. Since the publication of Dolomieu's admirable works on the rocks of the Lipari and Ponza Islands, science has advanced with prodigious strides. The chemist has taught us how to split up a rock into its constituent elements and to determine the proportions of these to one another with mathematical precision; the mineralogist has done much in the investigation of the characters and mode of origin of the crystalline minerals which occur in these rocks; and the microscopist has shown how the minute internal structure of these rocks may be made clearly manifest. We shall proceed to give a sketch of the present state of knowledge obtained by these different kinds of investigations, concerning the materials which are ejected from volcanic vents.

The most abundant of the substances which are ejected from volcanoes is steam or water-gas, which, as we have seen, issues in prodigious quantities during every eruption. But with the steam a great number of other volatile materials frequently make their appearance. The chief among these are the add gases known as hydrochloric acid, sulphurous acid, sulphuretted hydrogen, carbonic add, and boracic acid; and with these acid gases there issue hydrogen, nitrogen, ammonia, the volatile metals arsenic, antimony, and mercury, and some other substances. In considering the nature of the products which issue from volcanic fissures, it must be remembered that many substances which under ordinary circumstances do not exhibit marked volatility are nevertheless easily carried away in fine particles when a current of steam is passed over them. As we shall have to point out in the sequel, different volatile substances have a tendency to make their appearance at volcanic vents according as the intensity of the action going on within it varies.

The volatile substances issuing from volcanic fissures at high temperatures react upon one another, and many new compounds are thus formed. We have already seen how, by the action of sulphurous acid and sulphuretted hydrogen on each other, the sulphur so common in volcanic districts has been separated and deposited. The hydrochloric acid acts very energetically on the rocks around the vents, uniting with the iron in them to form the yellow ferric-chloride. The rocks all round a volcanic vent are not unfrequently found coated with this yellow substance, which is almost always mistaken by casual observers for sulphur. In many volcanoes the constant passage through the rocks of the various acid gases has caused nearly the whole of the iron, lime, and alkaline materials of the rocks to be converted into soluble compounds known as sulphates, chlorides, carbonates, and borates; and, on the removal of these by the rain, there remains a white, powdery substance, resembling chalk in outward appearance, but composed of almost pure silica. There are certain cases in which travellers have visited volcanic islands where chemical action of this kind has gone on to such an extent, that they have been led to describe the islands as composed entirely of chalk.

GASES EMITTED FROM VOLCANOES.

Some of the substances issuing from volcanic vents, such as hydrogen and sulphuretted hydrogen, are inflammable, and when they issue at a high temperature, these gases burst into flame the moment that they come into contact with the air. Hence, when volcanic fissures axe watched at night, faint lambent flames are frequently seen playing over them, and sometimes these flames are brilliantly coloured, through the presence of small quantities of certain metallic oxides. Such volcanic flames, however, are scarcely ever strongly luminous and, as we have already seen, the red, glowing light which is observed over volcanic mountains in eruption is due to quite another cause. The study by the aid of the spectroscope of the flames which issue from volcanic vents promises to throw much new light on the rarer materials ejected by volcanoes. Spectroscopic observations of this kind have already been commenced by Janssen, at Stromboli and Santorin.

Some of the volatile substances issuing from volcanic vents, are at once deposited when they come in contact with the cool atmosphere, others form new compounds with one another and the constituents of the atmosphere, while others again attack the materials of the surrounding rocks and form fresh chemical compounds with some of their ingredients. Thus, there are continually accumulating on the sides and lips of volcanic fissures deposits of sulphates, chlorides, and borates of the alkalies and alkaline earths, with sal-ammoniac, sulphur, and the oxides and sulphides of certain metals. The lips of the fissures from which steam and acid gases issue in volcanoes are constantly seen to be coated with yellow and reddish-brown incrustations, consisting of mixtures, in varying proportions, of these different materials, and these sometimes assume the form of stalactites and pendent masses.

DEPOSITS AROUND VOLCANIC VENTS.

Some of these products of volcanic action are of considerable commercial value. At Vulcano regular chemical works have been established in the crater of the volcano, by an enterprising Scotch firm, a great number of workmen being engaged in collecting the materials which are deposited around the fissures, and are renewed by the volcanic action almost as soon as they are removed. In [fig. 6], I have given a sketch of this singular spot, taken from the high ground of the neighbouring Island of Lipari. From the village at the foot of the volcano, where the workmen live, a zig-zag road has been constructed leading up the side, and down into the crater of the volcano. On this road, workmen and mules, laden with the various volcanic materials, may be seen constantly passing up and down.

Fig. 6.—View of Vulcano, with Vulcanello in the foreground taken from the south end of the Island of Lipari.