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).
FIG. 128.—View of Mount Vesuvius as it appeared from the Bay of Naples shortly before the eruption of 1906. The horn to the left is Monte Somma.]
An active composite cone like that of Vesuvius is for the greater part of the time in the Strombolian condition; that is to say, light crater explosions continue with varying intensity and interval, except when the mountain has been excited to the periodic Vulcanian outbreaks with which its history has been punctuated. The Strombolian explosions have sufficient violence to eject small fragments of hot lava, which, falling about the crater, slowly built up a rather sharp cone. The period of Strombolian activity has, therefore, been called the _cone-producing period_. Just before each new outbreak of the Vulcanian type, the altitude of the mountain has, therefore, reached a maximum, and since the larger explosive eruptions remove portions of this cone at the same time that they increase the dimensions of the crater, the Vulcanian stage in contrast to the other has been called the _crater-producing period_. In this period, then, the material ejected during the explosions does not consist solely of fresh lava cakes, but in part of the older débris derived from the crater walls, whence it is avalanched upon the chimney after each larger explosion. The overhanging cloud, which during the Strombolian period has consisted largely of steam and is noticeably white, now assumes a darker tone, the “smoke” which characterizes the Vulcanian eruption.