Volcanoes: What They are and What They Teach

by John W. Judd

Fig. 71.—The crater-lake called Lago del Bagno, in Ischia, converted into a harbour.

Fig. 72.—Lake of Gustavila, in Mexico.
(The terraces round the lake have been artificially formed.)

DIMENSIONS OF CRATER-LAKES.

Two of the largest crater-rings in the world are found in Central Italy, and are both occupied by lakes, the circular forms of which must strike every observer. The Lago di Bracciano, which lies to the north-west of Rome, is a circular lake six and a half miles in diameter, surrounded by hills which at their highest point rise to the height of 1,486 feet above the sea, while the surface of the waters of the lake is 640 feet above the sea-level. The Lago di Bolsena is somewhat less perfectly circular in outline than the Lago di Bracciano; it has a length from north to south of ten-and-a-quarter miles and a breadth from east to west of nine miles; the surface of the waters of this lake is 962 feet above that of the waters of the Mediterranean. The lake of Bolsena, like that of Bracciano, is surrounded by hills composed of volcanic materials; the highest points of this ring of hills rise to elevations of 684, 780, and 985 feet respectively above the waters of the lake.

In these great circular lakes of Bolsena and Bracciano, as well as in the smaller ones of Albano, Nemi, and the lakes of Frascati in the same district, the vast circular spaces enclosed by them, the gradual outer slope of the ring, and the inner precipices which bound the lake, all afford evidence of the explosive action to which they owe their origin.

But while the vast crater-rings we have mentioned are frequently found to be occupied by lakes, there are many other similar crater-rings which remain dry, either from the materials of which they are composed being of more pervious character, or from rivers having cut a channel through the walls of the crater, in this way draining off its waters.

Thus in the Campi Phlegræi, while we have the craters of Agnano and Avernus forming complete circular lakes, Astroni has only a few insignificant lakelets on its floor, and the Pianura, the Piano di Quarto, which have each a diameter of three or four miles, with many others, remain perfectly dry. In the vicinity of the great crater-lakes of Central Italy we find the crater-ring of the Vallariccia, which has evidently once been a lake but is now drained, its floor being covered with villages and vineyards.

CRATER-RINGS SURROUNDING CONES.

A comparison of these vast crater-rings leads us to the conclusion that in the majority of cases, if not in every instance, they are composed almost entirely of volcanic tuff and dust. In the case of the more solidly-built composite volcanic cones, the volcanic forces, as we have seen, produce fissures in the mass, and along these fissures parasitic cones are thrown up, the tension of the mass of imprisoned vapours below the mountain being thus from time to time relieved. But in the case of a volcanic cone composed of loose fragmentary materials, such temporary relief is impossible. The cracks, as soon as they originate, will be filled up and choked by the falling in of materials from above and at their sides. In this way the eruptive action will be continually repressed, till at last the imprisoned vapours acquire such a high state of tension that the outburst, when it occurs, is of the most terrible character, and the whole central mass of the volcano is blown into the air. It may often seem surprising that the ejection of such vast masses of material from the centre of a volcanic cone does not effect more in the way of raising the height of the crater-walls. But it must be remembered that, in the case of craters of such vast area, the majority of the ejected materials must fall back again within its circumference. By repeated ejections these materials will at last be reduced to such an extreme state of comminution that they can be borne away by the winds, and spread over the country to the distance of hundreds or thousands of miles. After great volcanic outbursts enormous areas are thus found covered with fine volcanic dust to the depth of many inches or feet.