Volcanoes: What They are and What They Teach

by John W. Judd

Let us now endeavour to analyse the phenomena so admirably displayed before us in the crater of Stromboli. The three essential conditions on which the production of these phenomena seems to depend are the following: first, the existence of certain apertures or cracks communicating between the interior and the surface of the earth; secondly, the presence of matter in a highly heated condition beneath the surface; and thirdly, the existence of great quantities of water imprisoned in the subterranean regions—which water, escaping as steam, gives rise to all those active phenomena we have been describing.

CAUSE OF THE GLOWING LIGHT.

We have said, at the outset, that there exists no analogy whatever between the action which takes place in volcanoes and the operation of burning or combustion. Occasionally, it is true, certain inflammable substances are formed by the action going on within the volcano, and these inflammable substances, taking fire, produce real flames. Such flames are, however, in almost all cases only feebly luminous, and do not give rise to any conspicuous appearances. What is usually taken for flame during volcanic eruptions is simply, as we have already pointed out, the glowing red-hot surface of a mass of molten rock, reflected from a vapour-cloud hanging over it. The red glow observed over a volcano in eruption is indeed precisely similar in its nature and origin to that which is seen above London during a night of heavy fog, and which is produced by the reflection of the gas-lights of the city from the innumerable particles of water-vapour diffused through the atmosphere. Fires, of course, occur when the molten and incandescent materials poured out from a volcano come in contact with inflammable substances, such as forests and houses, but in these cases the combustion is quite a secondary phenomenon.

There is another popular delusion concerning volcanic action, which it may be necessary to refer to and to combat. From the well-known fact that sulphur or brimstone is found abundantly in volcanic regions, the popular belief has arisen that this highly inflammable substance has something to do with the production of the eruptions of volcanoes. In school-books which were, until comparatively recent years, in constant use in this country, the statement may be found that by burying certain quantities of sulphur, iron-pyrites, and charcoal in a hole in the ground, we may form a miniature volcano, and produce all the essential phenomena of a volcanic eruption. No greater mistake could possibly be made. The chemical reactions which take place when sulphur and other substances are made to act upon each other differ entirely from the phenomena of volcanic action. The sulphur which is found in volcanic regions is the result and not the cause of volcanic action. Among the most common substances emitted from volcanic vents along with the steam are the two gases, sulphurous acid and sulphuretted hydrogen. When these two gases come into contact with one another, chemical action takes place, and the elements contained in them—oxygen, hydrogen, and sulphur—are free to group themselves together in an entirely new fashion; the consequence of this is that water and sulphuric acid (oil of vitriol) are formed, and a certain quantity of sulphur is set free. The water escapes into the atmosphere, the sulphuric acid combines with lime, iron, or other substances contained in the surrounding rocks, and the sulphur builds up crystals in any cavities which may happen to exist in these rocks.

VOLCANIC ACTION RESEMBLES BOILING.

If, however, careful and exact observations, like those carried on at Stromboli, compel us to reject the popular notions concerning the supposed resemblance between volcanic action and the combustion of sulphur or other substances, they nevertheless suggest analogies with certain other simple and well-known operations. And in pursuing these analogies, we are led to the recognition of some admirable illustrations both of the attendant phenomena and of the true cause of volcanic outbursts.

No one can look down on the mass of seething material in violent agitation within the fissures at the bottom of the crater of Stromboli, without being forcibly reminded of the appearances presented by liquids in a state of boiling or ebullition. The glowing material seems to be agitated by two kinds of movements, the one whirling or rotatory, the other vertical or up-and-down in its direction. The fluid mass in this way appears to be gradually impelled upwards, till it approaches the lips of the aperture, when vast bubbles are formed upon its surface, and to the sudden bursting of these the phenomena of the eruption are due.

Now if we take a tall narrow vessel and fill it with porridge or some similar substance of imperfect fluidity, we shall be able, by placing it over a fire, to imitate very closely indeed the appearances presented in the crater of Stromboli. As the temperature of the mass rises, steam is generated within it, and in the efforts of this steam to escape, the substance is set in violent movement. These movements of the mass are partly rotatory and partly vertical in their direction; as fresh steam is generated in the mass its surface is gradually raised, while an escape of the steam is immediately followed by a fall of the surface. Thus an up-and-down movement of the liquid is maintained, but as the generation of steam goes on faster than it can escape through the viscid mass, there is a constant tendency in the latter to rise towards the mouth of the vessel. At last, as we know, if heat continues to be applied to the vessel, the fluid contents will be forced up to its edge and a catastrophe will occur; the steam being suddenly and violently liberated from the bubbles formed on the surface of the mass, and a considerable quantity of the material forcibly expelled from the vessel. The suddenness and violence of this catastrophe is easily accounted for, if we bear in mind that the escaping steam acts after the manner of a compressed spring which is suddenly released. Steam is first formed at the bottom of the vessel which is in contact with the fire; but here it is under the pressure of the whole mass of the liquid, and moreover, the viscidity of the substance tends to retard the union of the steam bubbles and their rise to the surface of the mass. But when the pressure is relieved by the bursting of bubbles at the surface, the whole of the generated steam tends to escape suddenly.

ESCAPE OF STEAM-BUBBLES FROM LAVA.

Now within the crater of Stromboli we have precisely the necessary conditions for the display of the same series of operations. In the apertures at the bottom there exists a quantity of imperfectly fluid materials at a higher temperature, containing water entangled in its mass. As this water passes into the state of steam it tends to escape, and in so doing puts the whole mass into violent movement. When the steam rises to the surface, bubbles are formed, and the formation of these bubbles is promoted by the circumstance that the liquid mass, where exposed to the atmosphere, becomes chilled, and thereby rendered less perfectly fluid. By the bursting of these bubbles the pressure is partially relieved, and a violent escape of the pent-up steam takes place through the whole mass. Equilibrium being thus restored, there follows a longer or shorter interval of quiescence, during which steam is being generated and collected within the mass, and the series of operations which we have described then recommences.