Volcanoes: What They are and What They Teach

by John W. Judd

Fig. 3.—Section through the Island of Stromboli from n.w. to s.e.

a. Highest summit of the mountain, c. Cratère del Fossa, b. Point overlooking the crater, d. Steep slope known as the Sciarra del Fuoco. e. Continuation of the same slope beneath the level of the sea. f. Steep cliffs of the Punta dell' Omo.

FORM AND FUNCTION OF THE CRATER.

If we climb up to this scene of volcanic activity, we shall be able to watch narrowly the operations which are going on there. On the morning of the 24th of April, 1874, I paid a visit to this interesting spot in order to get a near view of what was taking place. On reaching a point upon the side of the Sciarra, from which the crater was in full view before me, I witnessed, and made a sketch of, an outburst which then took place, and this sketch has been reproduced in [fig. 4]. Before the outburst, numerous light curling wreaths of vapour were seen ascending from fissures on the sides and bottom of the crater. Suddenly, and without the slightest warning, a sound was heard like that produced when a locomotive blows off its steam at a railway-station; a great volume of watery vapour was at the same time thrown violently into the atmosphere, and with it there were hurled upwards a number of dark fragments, which rose to the height of 400 or 500 feet above the crater, describing curves in their course, and then falling back upon the mountain. Most of these fragments tumbled into the crater with a loud, rattling noise, but some of them fell outside the crater, and a few rolled down the steep slope of the Sciarra into the sea. Some of these falling fragments were found to be still hot and glowing, and in a semi-molten condition, so that they readily received the impression of a coin thrust into them.

Fig. 4.—The Crates of Stromboli as viewed from the side of the Sciarra during an eruption on the morning of April 24, 1874.

APERTURES AT THE BOTTOM OF THE CRATER.

But on the upper side of the crater, at the point marked 6, on the section [fig. 3], there exists a spot from which we can look down upon the bottom of the crater, and view the operations taking place there. This is the place where Spallanzani and other later investigators have carried on their observations, and, when the wind is blowing from the spectator towards the crater, he may sit for hours watching the wonderful scene displayed before him. The black slaggy bottom of the crater is seen to be traversed by many fissures or cracks, from most of which curling jets of vapour issue quietly, and gradually mingle with and disappear in the atmosphere. But besides these smaller cracks at the bottom of the crater, several larger openings are seen, which vary in number and position at different periods; sometimes only one of these apertures is visible, at others as many as six or seven, and the phenomena presented at these larger apertures are especially worthy of careful investigation.

These larger apertures, if we study the nature of the action taking place at them, may be divided into three classes. From those of the first class, steam is emitted with loud, snorting puffs, like those produced by a locomotive-engine, but far less regular and rhythmical in their succession. In the second class of apertures masses of molten material are seen welling out, and, if the position of the aperture be favourable, flowing outside the crater; from this liquid molten mass steam is seen to escape, sometimes in considerable quantities. The openings of the third class present still more interesting appearances. Within the walls of the aperture a viscid or semi-liquid substance is seen slowly heaving up and down. As we watch the seething mass the agitation within it is observed to increase gradually, and at last a gigantic bubble is formed which violently bursts, when a great rush of steam takes place, carrying fragments of the scum-like surface of the liquid high into the atmosphere.

If we visit the crater by night, the appearances presented are found to be still more striking and suggestive. The smaller cracks and larger openings glow with a ruddy light. The liquid matter is seen to be red- or even white-hot, while the scum or crust which forms upon it is of a dull red colour. Every time a bubble bursts and the crust is broken up by the escape of steam, a fresh, glowing surface of the incandescent material is exposed. If at these moments we look up at the vapour-cloud covering the mountain, we shall at once understand the cause of the singular appearances presented by Stromboli when viewed from a distance at night, for the great masses of vapour are seen to be lit up with a vivid, ruddy glow, like that produced when an engine-driver opens the door of the furnace and illuminates the stream of vapour issuing from the funnel of his locomotive.