Volcanoes: What They are and What They Teach

by John W. Judd

Another striking phenomenon which was exhibited in the great eruption of Vesuvius in 1872 was the vivid display of lightning accompanied by thunder. The uprushing current of steam and rock-fragments forms a vertical column, but as the steam condenses it spreads out into a great horizontal cloud which is seen to be made up of the great globes of vapour emitted at successive explosions. When there is little or no wind the vertical column with a horizontal cloud above it bears a striking resemblance to the stone-pine trees which form so conspicuous a feature in every Neapolitan landscape. Around this column of vapour the most vivid lightning constantly plays and adds not a little to the grand and awful character of the spectacle of a volcanic eruption, especially when it is viewed by night.

In the eruption of 1872 a strong wind blowing from the north-west destroyed the usual regular appearance of this 'pine-tree appendage' to the mountain, which is so well known to, and dreaded by the inhabitants of Naples; the cloud, as will be seen from the photograph ([fig. 5], facing p. 24), was blown on one side, and most of the falling fragments took the same direction.

It is well known that when high-pressure steam IS allowed to escape through an orifice, electricity is abundantly generated by the friction, and Sir William Armstrong's hydro-electric machine is constructed on this principle. Every volcano in violent eruption is a very efficient hydro-electric machine, and the uprushing column is in a condition of intense electrical excitation. This result is probably aided by the friction of the solid particles as they are propelled upwards and fall back into the crater. The restoration of the condition of electrical stability between this column and the surrounding atmosphere is attended with the production of frequent lightning-flashes and thunder-claps, the found of the latter being usually, however, drowned in the still louder roar of the uprushing steam-column.

The discharge of Buch large quantities of steam into the atmosphere soon causes the latter to be saturated with watery vapour, and there follows an excessive rainfall; long-continued rain and floods were an accompaniment of the great Vesuvian outbreak of 1872, as they have been of almost all great volcanic eruptions. The Italians, indeed, dread the floods which follow an eruption more than the fiery streams of lava which accompany it—for they have found the mud-streams (lave di fango), formed by rain-water sweeping along the loose volcanic materials, to be more widely destructive in their effects than the currents of molten rock (lave di fuoco).

Besides the phenomena which we have now described as accompanying a great volcanic outburst, many others have undoubtedly been recorded by apparently trustworthy authorities. But, in dealing with the descriptions of these grand and terrible events, we must always be on our guard against accepting as literal facts, the statements made by witnesses, often writing at some distance from the scene of action, and almost always under the influence of violent excitement and terror. The desire to administer to the universal love of the marvellous, and the tendency to exaggeration, will usually account for many of the wonderful statements contained in such records; and, even where the witness is accurately relating events which he thinks passed before his eyes, we must remember that it is probable he may have had neither the opportunity nor the capacity for exact observation.

The more carefully we sift the accounts which have been preserved of great volcanic outbursts, the more are we struck by the fact that the appearances described can be resolved into a few simple operations, the true character of which has been distorted or disguised by the want of accurate observation on the part of the witnesses.

SIMILARITY OF FEEBLE AND VIOLENT ERUPTIONS.

We are thus led to the conclusion that the grand and terrible appearances displayed at Vesuvius and other volcanoes in a state of violent eruption do not differ in any essential respect from the phenomena which we have witnessed accompanying the miniature outbursts of Stromboli. And we are convinced, by the same considerations, that the forces which give rise to the feeble displays in the latter case would produce, if acting with greater intensity and violence, all the magnificent spectacles presented in the former.

In Vesuvius and Stromboli alike, the active cause of all the phenomena exhibited is found to be the escape of steam from the midst of masses of incandescent liquefied rock. The violence, and therefore the grandeur and destructive effects of an eruption, depend upon the abundance and tension of this escaping steam.

There is one respect in which volcanic phenomena are especially calculated to excite the fear and wonder of beholders—namely, in the sudden and apparently spontaneous character of their manifestations. Eclipses were regarded as equally portentous with volcanic eruptions till astronomers learned not only to explain the causes which gave rise to them, but even to predict to the second the times of their occurrence. If we were able in like manner to warn the inhabitants of volcanic regions of the approach of a grand eruption, the fear and superstition with which these events are now regarded would doubtless be in great part dispelled. The power of prediction is alike the crucial test and the crowning triumph of a scientific theory.