Fig. 125.—View looking toward the summit of Etna from a position upon the southern flank near the village of Nicolosi. The two breached parasitic cones seen behind this village are the Monti Rossi which were thrown up in 1669 and from which flowed the lava which overran Catania (after a photograph by Sommer).
It is generally the case that a single eruption makes but a relatively small contribution to the bulk of the mountain. From each new cone or bocca there proceeds a stream of lava spread in a relatively narrow stream extending down the slopes ([Fig. 126]).
Fig. 126.—Sketch map of Etna, showing the individual surface lava streams (in black) and the tuff covered surface (stippled).
The caldera of composite cones.—Because of the varied episodes in the history of composite cones, they lack the regular lines characteristic of the two simpler types. The larger number of the more important composite cones have been built up within an outer crater of relatively large diameter, the Somma cone or caldera, which surrounds them like a gigantic ruff or collar. This caldera is clearly in most cases at least the relic of an earlier explosive crater, after which successive eruptions of lesser violence have built a more sharply conical structure. This can only be interpreted to mean that most larger and long-active volcanoes have been born in the grandest throes of their life history, and that a larger or smaller lateral migration of the vent has been responsible for the partial destruction of the explosion crater. Upon Vesuvius we find the crescent-like rim of Monte Somma; on Etna it is the Val del Bove, etc. It is this caldera of composite cones which gave rise to the theory of the “elevation crater” of von Buch (see ante, [p. 95], and [Fig. 127]).
Fig. 127.—Panum crater, showing the caldera and the later interior cones (after Russell).
The eruption of Vesuvius in 1906.—The volcano Vesuvius rises on the shores of the beautiful bay of Naples only about ten miles distant from the city of Naples. The mountain consists of the remnant of an earlier broad-mouthed explosion crater, the Monte Somma, and an inner, more conical elevation, the Monte Vesuvio. Before the eruption of 1906 this central cone was sharply conical and rose to a height of about 4300 feet above the surface of the bay, or above the highest point of the ancient caldera. The base of this inner cone is at an elevation of something less than half that of the entire mass, and is separated from the encircling ring wall of the old crater by the atrio, to which corresponds in height a perceptible shelf or piano upon the slope toward the bay of Naples ([Fig. 128]).