Volcanoes: What They are and What They Teach

by John W. Judd

Fig. 84.—Fissure formed on the flanks of Etna during the eruption of 1865.
a. Monte Frumento, an old parasitic cone. b. Line of fissure. c, c, c. New scoria-cones thrown up on line of fissure. d. Lava from same.

Fig. 85.—Plan of the Island of Vulcano, based on the map of the Italian Government.

SHIFTING OF ERUPTIONS ALONG FISSURES.

Even in the case of great composite cones, however, we sometimes find proofs of the centre of eruption having shifted its place along the line of fissure. No better example of this kind could possibly be adduced than that of the Island of Vulcano, with the peninsula of Vulcanello, which is joined to it by a narrow isthmus (see the map, [fig. 81], p 192). In [fig. 85] we have given an enlarged plan of this island which will make its peculiar structure more intelligible (see also the section given in [fig. 77], No. 6, facing p. 178).

The south-eastern part of the island consists of four crater-rings, one half of each of Which has been successively destroyed, through the shifting of the centre of eruption towards the north-west, along the great line of fissure shown in the general map ([fig. 81]). The last formed of these four crater-rings is the one which is now most complete, and culminates in Monte Saraceno (1581 ft.), a in the plan, the highest point in the island. The older crater-rings have been in part removed by the inroads of the waters of the Mediterranean on the shores of the island. In the centre of the great crater, b, which we have just described, rises the present active cone of Vulcano, 1,266 feet high, and having a crater, c, about 600 yards in diameter and more than 500 feet in depth. From this cone, a great stream of obsidian, e, flowed in the year 1775, and a small crater, d, the Fossa Anticha, has been opened in the side of the cone. The continuation of the same line of fissure is indicated by a ruined tuff-cone, f, known as the Faraglione, and the three scoria-cones of Vulcanello, g, h, which have been thrown up so close to one another as to have their lower portions merged in one common mass, as shown in [fig. 86].

SYSTEMS OF VOLCANIC FISSURES.

Even in volcanoes of the largest dimensions we sometimes find proofs of the centre of eruption having shifted along the line of fissure. Lyell showed that such a change in the position of the central axis of the volcano had taken place in Etna, and the same phenomenon is exhibited in the clearest manner' by some of the ancient volcanoes of the Inner Hebrides, which have been dissected by the denuding forces.