Volcanoes: What They are and What They Teach

by John W. Judd

In the year 1772, there occurred a volcanic eruption in the Island of Java, which is perhaps the most violent and terrible that has happened within the historical period. A lofty volcanic cone, called Papandayang, 9,000 feet high, burst into eruption, and, in a single night, 30,000,000,000 cubic feet of materials were thrown into the atmosphere, falling upon the country around the mountain where no less than forty villages were buried. After the eruption, the volcano was found to have been reduced in height from 9,000 to 5,000 feet, and to present a vast crater in its midst, which had been formed by the ejection of the enormous mass of materials.

Many similar cases might be cited of the removal of a great part of a mountain-mass by a sudden, paroxysmal outburst. In some cases, indeed, the whole mass of a mountain has been blown away during a terrific eruption, and the site of the mountain is now occupied by a lake. This is said to have been the case with the Island of Timor, where an active volcano, which was visible from a distance of 300 miles at sea, has entirely disappeared.

The removal of the central portion of great volcanic mountains by explosive action, gives rise to the formation of those vast, circular, crater-rings of which such remarkable examples occur in many volcanic districts. These crater-rings present a wall with an outer slope agreeing with that of the volcanic cone of which they originally formed a part, but with steep inner cliffs, which exhibit good sections of the beds of tuff, ash, and lava with the intersecting dykes of which the original volcano was built up. Near Naples, one of these crater-rings, with sloping outer sides and steep inner ones, is employed to form the royal game-preserve of Astroni, the only entrance to the crater being closed by gates.

FORMATION OF CRATER-LAKES.

As these crater-rings are usually composed of materials more or less impervious to water, they often become the site of lakes. The beautiful circular lake of Laach, in the Rhine Provinces, with the numerous similar examples of Central Italy—Albano, Nemi, Bracciano, and Bolsena—the lakes of the Campi Phlegræi (Agnano, Avernus, &c.), and some similar lakes in the Auvergne, may be adduced as examples of crater-rings which have become the site of lakes.

Fig. 70.—Lac Paven, in the Auvergne.
a. Scoriæ. b. Basalt.

One of the most beautiful of the crater-lakes in the Auvergne is Lac Paven ([fig. 70]), which lies at the foot of a scoria-cone, Mont Chalme, and is itself surrounded by masses of ejected materials. The crater-lake of Bagno, in Ischia ([fig. 71]), has had a channel cut between it and the sea, so that it serves as a natural harbour. The lake of Gustavila, in Mexico ([fig. 72]), is an example of a crater-lake on a much larger scale.

In many of these crater-rings the diameter of the circular space enclosed by them is often very great indeed as compared with the height of the walls.