Volcanoes: What They are and What They Teach

by John W. Judd

There is one other consideration which must be borne in mind in connection with this subject. It is well known that if water be subjected to sufficiently great pressure it may be raised to a very high temperature and still retain its liquid condition. When this pressure is removed, however, the whole mass passes at once into the condition of steam or water-gas; and the gas thus formed at high temperatures has a proportionably high tension. In a Papin's digester water confined in a strong vessel is raised to temperatures far above its ordinary boiling-point, and from any opening in such a vessel the steam escapes with prodigious violence. Now, at considerable depths beneath the earth's surface, and under the pressure of many hundreds or thousands of feet of solid rock, water still retaining its liquid condition may become intensely heated. When the pressure is relieved by the formation of a crack or fissure in the superincumbent mass of rock, the escape of the superheated steam will be of very violent character, and may be attended with the most striking and destructive results. In the existence of high temperatures beneath the earth's surface, and the presence in the same regions of imprisoned water capable of passing into the highly elastic gas which we call steam, we have a cause fully competent to produce all the phenomena which we have described as occurring at Stromboli.

It may at first sight appear that the grand and terrible displays of violence witnessed during a great volcanic eruption differ fundamentally in their character and their origin from those feeble outbursts which we are able to examine closely and analyse rigorously at Stromboli. But that such is not the case a few simple considerations will soon convince us.

STROMBOLI COMPARED WITH VESUVIUS.

Although Stromboli usually displays the subdued and moderate activity which we have been describing, yet the intensity of the action going on within it is subject to considerable variation. Occasionally the violence of the outbursts is greatly increased—the roaring of the steam-jets may be heard for many miles around, considerable streams of incandescent liquefied rock flow down the Sciarra into the sea, and the explosions in the crater are far more frequent and energetic, cinders and fragments of rock being scattered all over the island and the surrounding seas.

On the other hand, volcanoes like Vesuvius, which are sometimes the scene of eruptions on the very grandest scale, at others subside into a temporary state of moderate activity quite similar in character to that which is the normal condition of Stromboli. Thus, shortly before the great eruption of Vesuvius in April 1872, a small cone was formed near the edge of the crater, and during some months observers could watch, in ease and safety, a series of small explosions taking place, quite similar in their character and attendant phenomena to those which we have described as occurring at Stromboli. French geologists are in the habit of defining the condition of activity in a volcano by speaking of the more quiet and, regular state as the 'Strombolian stage,' and the more violent and paroxysmal as the 'Vesuvian stage'; but the two conditions are, as we have seen, presented by the same volcano at different periods, and pass into one another by the most insensible gradations.

We must now proceed to compare the grand and terrible appearances presented during a great eruption with those more feeble displays which we have been describing, to show that in all their essential features these different kinds of outbursts are identical with one another, and must be referred to the action of similar causes.

The volcanic eruption which has been most carefully studied in recent times is that which we have already referred to as occurring at Vesuvius, in the month of April 1872. With the exception, perhaps, of that which took place in October 1822, this eruption was the grandest which has broken out at Vesuvius during the present century. Owing to the circumstance of its proximity to the great city of Naples, Vesuvius has always been the most carefully watched of all volcanoes, and in recent years the erection of an observatory, provided with instruments for recording the smallest subterranean tremors affecting the mountain, has facilitated the carrying on of those continuous and minute observations which are so necessary for exact scientific inquiry.

Fig 5. Vesuvius in Eruption, as seen from Naples, April 26, 1872. (From a photograph)

VESUVIUS ERUPTION OF 1872.